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By the end of this section, you will be able to:
- Explain the importance of water in the body
- Contrast the composition of the intracellular fluid with that of the extracellular fluid
- Explain the importance of protein channels in the movement of solutes
- Identify the causes and symptoms of edema
The chemical reactions of life take place in aqueous solutions. The dissolved substances in a solution are called solutes. In the human body, solutes vary in different parts of the body, but may include proteins—including those that transport lipids, carbohydrates, and, very importantly, electrolytes. Often in medicine, a mineral dissociated from a salt that carries an electrical charge (an ion) is called and electrolyte. For instance, sodium ions (Na+) and chloride ions (Cl–) are often referred to as electrolytes.
In the body, water moves through semi-permeable membranes of cells and from one compartment of the body to another by a process called osmosis. Osmosis is basically the diffusion of water from regions of higher concentration to regions of lower concentration, along an osmotic gradient across a semi-permeable membrane. As a result, water will move into and out of cells and tissues, depending on the relative concentrations of the water and solutes found there. An appropriate balance of solutes inside and outside of cells must be maintained to ensure normal function.
Body Water Content
Human beings are mostly water, ranging from about 75 percent of body mass in infants to about 50–60 percent in adult men and women, to as low as 45 percent in old age. The percent of body water changes with development, because the proportions of the body given over to each organ and to muscles, fat, bone, and other tissues change from infancy to adulthood. Your brain and kidneys have the highest proportions of water, which composes 80–85 percent of their masses. In contrast, teeth have the lowest proportion of water, at 8–10 percent.
Body fluids can be discussed in terms of their specific fluid compartment, a location that is largely separate from another compartment by some form of a physical barrier. The intracellular fluid (ICF) compartment is the system that includes all fluid enclosed in cells by their plasma membranes. Extracellular fluid (ECF) surrounds all cells in the body. Extracellular fluid has two primary constituents: the fluid component of the blood (called plasma) and the interstitial fluid (IF) that surrounds all cells not in the blood.
The ICF lies within cells and is the principal component of the cytosol/cytoplasm. The ICF makes up about 60 percent of the total water in the human body, and in an average-size adult male, the ICF accounts for about 25 liters (seven gallons) of fluid. This fluid volume tends to be very stable, because the amount of water in living cells is closely regulated. If the amount of water inside a cell falls to a value that is too low, the cytosol becomes too concentrated with solutes to carry on normal cellular activities; if too much water enters a cell, the cell may burst and be destroyed.
The ECF accounts for the other one-third of the body’s water content. Approximately 20 percent of the ECF is found in plasma. Plasma travels through the body in blood vessels and transports a range of materials, including blood cells, proteins (including clotting factors and antibodies), electrolytes, nutrients, gases, and wastes. Gases, nutrients, and waste materials travel between capillaries and cells through the IF. Cells are separated from the IF by a selectively permeable cell membrane that helps regulate the passage of materials between the IF and the interior of the cell.
The body has other water-based ECF. These include the cerebrospinal fluid that bathes the brain and spinal cord, lymph, the synovial fluid in joints, the pleural fluid in the pleural cavities, the pericardial fluid in the cardiac sac, the peritoneal fluid in the peritoneal cavity, and the aqueous humor of the eye. Because these fluids are outside of cells, these fluids are also considered components of the ECF compartment.
Composition of Body Fluids
The compositions of the two components of the ECF—plasma and IF—are more similar to each other than either is to the ICF. Blood plasma has high concentrations of sodium, chloride, bicarbonate, and protein. The IF has high concentrations of sodium, chloride, and bicarbonate, but a relatively lower concentration of protein. In contrast, the ICF has elevated amounts of potassium, phosphate, magnesium, and protein. Overall, the ICF contains high concentrations of potassium and phosphate ( ), whereas both plasma and the ECF contain high concentrations of sodium and chloride.
Watch this video to learn more about body fluids, fluid compartments, and electrolytes. When blood volume decreases due to sweating, from what source is water taken in by the blood?
[reveal-answer q=”812774″]Show Answer[/reveal-answer]
[hidden-answer a=”812774″]The interstitial fluid (IF).[/hidden-answer]
Most body fluids are neutral in charge. Thus, cations, or positively charged ions, and anions, or negatively charged ions, are balanced in fluids. As seen in the previous graph, sodium (Na+) ions and chloride (Cl–) ions are concentrated in the ECF of the body, whereas potassium (K+) ions are concentrated inside cells. Although sodium and potassium can “leak” through “pores” into and out of cells, respectively, the high levels of potassium and low levels of sodium in the ICF are maintained by sodium-potassium pumps in the cell membranes. These pumps use the energy supplied by ATP to pump sodium out of the cell and potassium into the cell.
Fluid Movement between Compartments
Hydrostatic pressure, the force exerted by a fluid against a wall, causes movement of fluid between compartments. The hydrostatic pressure of blood is the pressure exerted by blood against the walls of the blood vessels by the pumping action of the heart. In capillaries, hydrostatic pressure (also known as capillary blood pressure) is higher than the opposing “colloid osmotic pressure” in blood—a “constant” pressure primarily produced by circulating albumin—at the arteriolar end of the capillary. This pressure forces plasma and nutrients out of the capillaries and into surrounding tissues. Fluid and the cellular wastes in the tissues enter the capillaries at the venule end, where the hydrostatic pressure is less than the osmotic pressure in the vessel. Filtration pressure squeezes fluid from the plasma in the blood to the IF surrounding the tissue cells. The surplus fluid in the interstitial space that is not returned directly back to the capillaries is drained from tissues by the lymphatic system, and then re-enters the vascular system at the subclavian veins.
Watch this video to see an explanation of the dynamics of fluid in the body’s compartments. What happens in the tissue when capillary blood pressure is less than osmotic pressure?
[reveal-answer q=”672493″]Show Answer[/reveal-answer]
[hidden-answer a=”672493″]Fluid enters the capillaries from interstitial spaces.[/hidden-answer]
Hydrostatic pressure is especially important in governing the movement of water in the nephrons of the kidneys to ensure proper filtering of the blood to form urine. As hydrostatic pressure in the kidneys increases, the amount of water leaving the capillaries also increases, and more urine filtrate is formed. If hydrostatic pressure in the kidneys drops too low, as can happen in dehydration, the functions of the kidneys will be impaired, and less nitrogenous wastes will be removed from the bloodstream. Extreme dehydration can result in kidney failure.
Fluid also moves between compartments along an osmotic gradient. Recall that an osmotic gradient is produced by the difference in concentration of all solutes on either side of a semi-permeable membrane. The magnitude of the osmotic gradient is proportional to the difference in the concentration of solutes on one side of the cell membrane to that on the other side. Water will move by osmosis from the side where its concentration is high (and the concentration of solute is low) to the side of the membrane where its concentration is low (and the concentration of solute is high). In the body, water moves by osmosis from plasma to the IF (and the reverse) and from the IF to the ICF (and the reverse). In the body, water moves constantly into and out of fluid compartments as conditions change in different parts of the body.
For example, if you are sweating, you will lose water through your skin. Sweating depletes your tissues of water and increases the solute concentration in those tissues. As this happens, water diffuses from your blood into sweat glands and surrounding skin tissues that have become dehydrated because of the osmotic gradient. Additionally, as water leaves the blood, it is replaced by the water in other tissues throughout your body that are not dehydrated. If this continues, dehydration spreads throughout the body. When a dehydrated person drinks water and rehydrates, the water is redistributed by the same gradient, but in the opposite direction, replenishing water in all of the tissues.
Solute Movement between Compartments
The movement of some solutes between compartments is active, which consumes energy and is an active transport process, whereas the movement of other solutes is passive, which does not require energy. Active transport allows cells to move a specific substance against its concentration gradient through a membrane protein, requiring energy in the form of ATP. For example, the sodium-potassium pump employs active transport to pump sodium out of cells and potassium into cells, with both substances moving against their concentration gradients.
Passive transport of a molecule or ion depends on its ability to pass through the membrane, as well as the existence of a concentration gradient that allows the molecules to diffuse from an area of higher concentration to an area of lower concentration. Some molecules, like gases, lipids, and water itself (which also utilizes water channels in the membrane called aquaporins), slip fairly easily through the cell membrane; others, including polar molecules like glucose, amino acids, and ions do not. Some of these molecules enter and leave cells using facilitated transport, whereby the molecules move down a concentration gradient through specific protein channels in the membrane. This process does not require energy. For example, glucose is transferred into cells by glucose transporters that use facilitated transport.
Disorders of the Fluid Balance: Edema
Edema is the accumulation of excess water in the tissues. It is most common in the soft tissues of the extremities. The physiological causes of edema include water leakage from blood capillaries. Edema is almost always caused by an underlying medical condition, by the use of certain therapeutic drugs, by pregnancy, by localized injury, or by an allergic reaction. In the limbs, the symptoms of edema include swelling of the subcutaneous tissues, an increase in the normal size of the limb, and stretched, tight skin. One quick way to check for subcutaneous edema localized in a limb is to press a finger into the suspected area. Edema is likely if the depression persists for several seconds after the finger is removed (which is called “pitting”).
Pulmonary edema is excess fluid in the air sacs of the lungs, a common symptom of heart and/or kidney failure. People with pulmonary edema likely will experience difficulty breathing, and they may experience chest pain. Pulmonary edema can be life threatening, because it compromises gas exchange in the lungs, and anyone having symptoms should immediately seek medical care.
In pulmonary edema resulting from heart failure, excessive leakage of water occurs because fluids get “backed up” in the pulmonary capillaries of the lungs, when the left ventricle of the heart is unable to pump sufficient blood into the systemic circulation. Because the left side of the heart is unable to pump out its normal volume of blood, the blood in the pulmonary circulation gets “backed up,” starting with the left atrium, then into the pulmonary veins, and then into pulmonary capillaries. The resulting increased hydrostatic pressure within pulmonary capillaries, as blood is still coming in from the pulmonary arteries, causes fluid to be pushed out of them and into lung tissues.
Other causes of edema include damage to blood vessels and/or lymphatic vessels, or a decrease in osmotic pressure in chronic and severe liver disease, where the liver is unable to manufacture plasma proteins. A decrease in the normal levels of plasma proteins results in a decrease of colloid osmotic pressure (which counterbalances the hydrostatic pressure) in the capillaries. This process causes loss of water from the blood to the surrounding tissues, resulting in edema.
Mild, transient edema of the feet and legs may be caused by sitting or standing in the same position for long periods of time, as in the work of a toll collector or a supermarket cashier. This is because deep veins in the lower limbs rely on skeletal muscle contractions to push on the veins and thus “pump” blood back to the heart. Otherwise, the venous blood pools in the lower limbs and can leak into surrounding tissues.
Medications that can result in edema include vasodilators, calcium channel blockers used to treat hypertension, non-steroidal anti-inflammatory drugs, estrogen therapies, and some diabetes medications. Underlying medical conditions that can contribute to edema include congestive heart failure, kidney damage and kidney disease, disorders that affect the veins of the legs, and cirrhosis and other liver disorders.
Therapy for edema usually focuses on elimination of the cause. Activities that can reduce the effects of the condition include appropriate exercises to keep the blood and lymph flowing through the affected areas. Other therapies include elevation of the affected part to assist drainage, massage and compression of the areas to move the fluid out of the tissues, and decreased salt intake to decrease sodium and water retention.
Your body is mostly water. Body fluids are aqueous solutions with differing concentrations of materials, called solutes. An appropriate balance of water and solute concentrations must be maintained to ensure cellular functions. If the cytosol becomes too concentrated due to water loss, cell functions deteriorate. If the cytosol becomes too dilute due to water intake by cells, cell membranes can be damaged, and the cell can burst. Hydrostatic pressure is the force exerted by a fluid against a wall and causes movement of fluid between compartments. Fluid can also move between compartments along an osmotic gradient. Active transport processes require ATP to move some solutes against their concentration gradients between compartments. Passive transport of a molecule or ion depends on its ability to pass easily through the membrane, as well as the existence of a high to low concentration gradient.
Answer the question(s) below to see how well you understand the topics covered in the previous section.
Critical Thinking Questions
- Plasma contains more sodium than chloride. How can this be if individual ions of sodium and chloride exactly balance each other out, and plasma is electrically neutral?
- How is fluid moved from compartment to compartment?
[reveal-answer q=”114248″]Show Answers[/reveal-answer]
- There are additional negatively charged molecules in plasma besides chloride. The additional sodium balances the total negative charges.
- Fluid is moved by a combination of osmotic and hydrostatic pressures. The osmotic pressure results from differences in solute concentrations across cell membranes. Hydrostatic pressure results from the pressure of blood as it enters a capillary system, forcing some fluid out of the vessel into the surrounding tissues.
extracellular fluid (ECF): fluid exterior to cells; includes the interstitial fluid, blood plasma, and fluids found in other reservoirs in the body
fluid compartment: fluid inside all cells of the body constitutes a compartment system that is largely segregated from other systems
hydrostatic pressure: pressure exerted by a fluid against a wall, caused by its own weight or pumping force
interstitial fluid (IF): fluid in the small spaces between cells not contained within blood vessels
intracellular fluid (ICF): fluid in the cytosol of cells
1.2: Body Fluids and Fluid Compartments - Biology
A significant percentage of the human body is water, which includes intracellular and extracellular fluids.
Describe the characteristics of water content in the body
- On average, body water can account for 50% of the total human body weight and it is significantly higher in newborns. Obesity decreases the percentage of water in the body.
- Body water is regulated by hormones, including anti-diuretic hormone (ADH), aldosterone, and atrial natriuretic peptide.
- Water content in the body can be evaluated using bioelectrical impedance and mass spectrometry.
- Important functions of water in the body include supporting the cellular metabolism, molecular transport, biochemical reactions, and the physical properties of water, such as surface tension.
- hydrolysis: A biochemical reaction in which water molecules are used to break down a molecule into smaller molecules.
- bioelectrical impedance analysis: A commonly used method for estimating body composition, by measuring resistance to the flow of electricity in the body, which is associated with hydration levels.
In physiology, body water is the water content of the human body. It makes up a significant percentage of the total composition of a body. Water is a necessary component to support life for many reasons. All cells in the human body are made mostly of water content in their cytoplasm.
Water molecule: A 3-dimensional model of hydrogen bonds (labeled 1) between molecules of water.
Water also provides a fluid environment for extracellular communication and molecular transport throughout the body. Water itself is also a key component of biochemical reactions involved in physiology, such as hydrolysis. Many organ systems depend on the physical properties of water, such as the surface tension of water in the alveoli of the lungs.
Overall Water Content
The total amount of water in a human of average weight (70 kilograms) is approximately 40 liters, averaging 57 percent of his total body weight. In a newborn infant, this may be as high as 79 percent of the body weight, but it progressively decreases from birth to old age, with most of the decrease occurring during the first 10 years of life.
Also, obesity decreases the percentage of water in the body, sometimes to as low as 45 percent. The water in the body is distributed among various fluid compartments that are interspersed in the various cavities of the body through different tissue types. In diseased states where body water is affected, the fluid compartments that have changed can give clues to the nature of the problem.
Water Content Regulation and Measurement
Body water is regulated largely by the renal and neuro-endocrine systems. Water content regulation is one of the most important parts of homeostasis due to its influence on blood pressure and cardiac output. Much of this regulation is mediated by hormones, including anti-diuretic hormone (ADH), renin, angiotensin II, aldosterone, and atrial natriuretic peptide (ANP).
These hormones act as messengers between the kidneys and the hypothalamus however, the lungs and heart are also involved in the secretion of some of these hormones, such as angiotensin converting enzyme (ACE) and ANP respectively.
There are many clinical methods to determine body water. One way to get an uncertain estimate is by calculation based on body weight and urine output. Another way to measure body water is through dilution and equilibration using mass spectrometry, which measures the abundance of water in breath samples from an individual.
In bioelectrical impedance analysis, a person’s hydration level is calculated from high-precision measurements of the opposition to the flow of an electric current through body tissues. Since water conducts electricity, a lower hydration level will cause a greater amount of resistance to electrical flow through the body.
The Movement of Solutes Between Compartments
The ICF has higher amounts of potassium, phosphate, magnesium, and protein compared to the ECF. The plasma has high concentrations of sodium, chloride, and bicarbonate, but lower levels of protein as compared to the ICF. While water moves passively via osmosis, sodium and potassium ions move in and out of cells using active transport ion pumps. The pumps are powered by adenosine triphosphate (ATP) to provide the energy to move the ions against their concentration gradients (i.e. sodium moves out of the cell and potassium is pumped in) and maintain the gradients inside and outside the cell.
The image above shows the composition of the cell membrane that separates the extracellular fluid of a cell from the intracellular fluid (cytoplasm).
Distribution of Fluids in the Human Body
In this article we will discuss about the Distribution of Fluids in the Human Body:- 1. Fluid Concentration in Human Body 2. Extracellular Fluid 3. Intracellular Fluid 4. Exchanges between Fluid Compartments.
Fluid Concentration in Human Body:
1. Water makes up 50 to 70 per cent of the weight of the adult human body and var­ies inversely as the fat content.
2. This mentioned amount of water is dis­tributed throughout the body as the major component of the intracellular and extra­cellular fluids.
3. The intracellular fluids amount to about 50 per cent of the body weight in a lean individual and much less in an obese per­son.
4. The extracellular fluids represent about 20 per cent of the body weight.
5. Of the extracellular fluids, interstitial flu­ids amount to some 15 per cent and blood plasma about 5 per cent of the body weight.
6. Relatively small volumes are represented by specialized fluids such as cerebrospi­nal fluid, ocular fluid, lymph, and syno­vial fluids, etc.
1. All body cells exist in an environment of fluid collectively designated extracellu­lar fluid. This includes the blood plasma, interstitial fluid and lymph.
2. 7 per cent protein is present in plasma, slightly less in hepatic lymph and 0.1 per cent protein in subcutaneous interstitial fluid.
3. They are solutions mainly of Nacl and NaHCO3, with small amounts of Ca, Mg, K, H, phosphate, sulphate and organic acid ions, some nonelectrolytes (glucose, urea, lipids, etc.) and with pH values ranging from 7.35 to 7.45 under normal conditions.
4. The total concentration of the ionic con­stituents is about 310 m mol per litre of plasma.
5. Much of the intracellular magnesium is not in the ionic form but is bound to pro­tein and other smaller organic molecules.
6. Several components of the extracellular fluid are important in preserving osmotic, anion-cation balance and hydrogen ion regulation.
7. The cations (K + , Ca ++ , Mg ++ and H + ), al­though present in comparatively very low- concentrations, exert profound influences on physiological processes.
8. The interstitial fluid contains a higher total concentration of diffusible anion and a lower concentration of cation than does the plasma.
1. Less amounts (5-10 m mol/litre) of sodium are present in the intracellular fluid which also contains little but extremely biologi­cally important calcium.
2. The chief cations of this fluid are potas­sium (about 160 m mol/litre) in muscle and magnesium (about 26 m mol/litre) in muscle.
3. The intracellular fluids contain much more phosphate and sulphate ions as well as proteins than the extracellular fluid. Chlo­ride ion is practically absent from this fluid except in the case of erythrocytes, and cells of the kidney tubules, stomach and intestines. Both Na and K ions are able to cross the membrane more freely under cer­tain physiological and pathological con­ditions.
4. Much of the magnesium is present as un-dissociated compounds of protein and organic phosphate and, therefore, is not in ionic form.
Exchanges between Fluid Compartments:
Many of the substances entered into the body and produced by the cells are distributed to other tis­sues or excreted.
The exchange systems are out­lined as follows:
1. Alveolar Air – Blood Plasma:
This system provides for entrance of oxygen into and loss of CO2 and water from the body.
2. Plasma – Erythrocytes:
This system pro­vides for ready exchange of oxygen, CO2, water and certain anions (particularly CI – and HCO3 – ) in both directions. Cations are exchanged very slowly.
3. Plasma – Interstitial Fluid:
These two me­dia are separated by the capillary walls, which are permeable to water, inorganic ions and small organic molecules (glucose, amino acids, urea, etc.) but not to large organic molecules such as proteins.
4. Interstitial Fluid – Intracellular Fluid:
These two compartments are separated by the cell membranes across which gases in solu­tion, water and small unchanged mol­ecules can diffuse. The small molecules, e.g., glucose, is not subjected to simple dif­fusion but is carried across cell membrane by active transport processes.
The perme­ability of electrolytes follows biological pump mechanisms. These membranes are also relatively impermeable to large mol­ecules such as proteins, except in special situations, namely the liver.
Total Body Water and Its Distribution | Human Physiology | Biology
In this article we will discuss about the concept of total body water and its distribution.
Water is the most vital and at the same times the most abundant component of the human body. It constitutes about 70 percent of the total body weight and within which the major cations like sodium, potassium, calcium, hydrogen, magnesium and anions like chloride, bicarbonate and protein of the body are dissolved.
Without water there would be no form of life and it forms the intracellular medium within which metabolic reactions characteristics of living substances take place. Water-deprivation brings about death more earlier than that of food-deprivation. If water is given instead of food, life may continue for several weeks by the loss of most of the body fat and 50 percent, of tissue protein.
Total body water in an average human being, weighing about 70 kg is 40 litres to 45 litres. In human being it is about 65% of the body weight in males and about 10% less in females. But the above values vary mostly with the relative degrees of leanness and fatness of the individual. In lean person, the value is higher than that of in obese person. In general, woman contains more fat than man. The total body water content can be deter­mined most accurately by the process of desiccation.
In 1863 Bischoff determined the water content of an executed criminal by the method of desic­cation. Mitchell and his associates (1945), Wid-dowson and his co-workers (1951) have also de­termined the water content of the human beings by direct method. The average water content in different tissues of the body has been presented in Table 5.1.
It has been observed after studying thoroughly the water content of the body in man as well as in different animal species that the total water con­tent in man is similar to that of in other animals. Besides this, the relative distribution of water in the various organs and tissues is mostly same in man as well as in other species.
The percentage of water in various tissues and the proportion of total weight of the body which each tissue represents, have been presented in Table 5.3.
The water of the body can be considered to be distributed with­in two main compartments—the extracellular and the intracellular. The distribution of body water in different compartments has been presented schematically in Fig. 5.1. The cell membrane actually provides the boundary in between the extracellular and the intra­cellular compartments.
1. Extracellular Compartment:
The extracellular fluid compartment is a compartment containing heterogenous collections of fluids and not a continuous fluid phase. Edelman and Leibman (1959) have studied thoroughly the distribution pattern of body water by dilution technique and also by tissue analysis. It is postulated that 55% of water is present in the intracellular space and the rest in the extracellular space.
The extracellular fluid phase can be divided into following sub compartments:
i. Transcellular water- 2.5 percent
ii. Dense connective tissue and cartilage water- 7.5 percent.
iii. Plasma water that is confined within the vascular system- 7.5 percent.
iv. Interstitial fluid and lymph- 20 percent.
v. Inaccessible bone water- 7.5 percent
The term transcellular was introduced by Edelman and associates (1952) in order to designate the extracellular fluid having been separated from the other extracellular fluid by an epithelial membrane.
This transcellular fluid includes:
(b) Joint or synovial fluid,
(d) Fluids of the pleural, pericardial and peritoneal cavity,
(e) Fluids within the ducts of the digestive gland,
(f) Mucous membranes of the nasorespiratory tract, gastro-intestinal tract and genitalia, and
(g) Intraluminal fluid of gastro-intestinal system.
2. Intracellular Fluid Compartment:
It is neither a continuous nor a homogeneous phase and represents the sum of the fluid contents of all the cells of the body. In a cell there are many anatomic subdivisions and for this reason there is a striking difference in water content and ionic composition in between the cytoplasm, nucleus, mitochondria and microsomes of various cell types. This intracellular fluid contains about 30-40% of the body weight and holds about 55% of the whole body water.
Measurement by Dilution Techniques:
Total body water and extracellular water can be measured by dilution technique with varying degree of precisions. Volume of water present in each compartment cannot be measured directly and thus indirect method—the dilution technique has been adopted for its determination.
In this technique the amount of dye used, the final concentration of the dye in the solution is made, are considered for determining the volume of distribution. E.g., if a known quantity of dye—Q is taken and the final concentration is achieved as C, then the volume of distribution V will be V = (Q/C).
If a beaker of unknown capacity is taken, then its volume can be determined by mixing uniformly a known amount of dye in the volume of water present in the beaker. If the final concentration of the dye is determined by the calorimeter then the volume capacity of the beaker can be determined. Suppose 35 mg of dye has been added and the final concentration that has been achieved to be 0.07 mg per ml, then the volume of the beaker will be
The result will be valid only when the drug will be mixed thoroughly.
In vivo determination of body fluid compartments by the dilution principle, certain points are generally considered. The dye injected in the body must be evenly distributed and confined to the body fluid compartment to be measured. If the dye is excreted or lodged in other compartments or metabolised then those amount should be determined and subtracted from the quantity administered.
So the equation will be:
Total Body Water:
Total body water is generally determined by using antipyrine. The antipyrine is distributed evenly throughout all the body water compartments and thus diffuses readily across the cell membrane. It is not bound to any intracellular and extracellular compartments. It is also slowly excreted and slowly metabolised.
Tritiated water (H3O or HTO) and deuterium oxide (D2O) -the two isotopes are often used for the determination of total body water. D2O and H3O are distributed in the body exactly like water. These are excreted in the urine, faeces and respiratory gases and also evaporated through the skin.
As for example of using the D2O or HTO for the measurement of total body water, suppose 100 ml of D2O in isotonic saline solution is injected intravenously to a man of weighing about 75 kg. After an equilibrium period of 2 hours, the plasma sample is analysed and D2O concentration is found to be 0.0023 ml per ml. During the period of equilibrium it is found to have a loss (through respiratory, urinary and circulatory pathways) of average 0.5% of the quantity administered.
So the volume of distribution will be:
Extracellular Fluid Volume:
The extracellular fluid volume is not determined so precisely only due to lack of substances that may diffuse to cross the capillary walls readily, enter the cell interstices easily but do not permeate through the cell membrane. Besides this, the substance must be non-toxic and the rate of excretion must be very low in comparison with the rate of distribution in extracellular compartment. There is no such ideal substance available but several substances that have been used are insulin, raffinose, sucrose, mannitol thiosulphate, radiosulphate, thiocyanate, radiochloride and radiosodium.
Intracellular Fluid Measurement:
There is no direct method has yet been developed. It can be determined by subtracting the value of the extra­cellular compartment from the value of the total body water.
The intracellular fluid (ICF) is all fluids contained inside the cells, which consists of cytosol and fluid in the cell nucleus.  The cytosol is the matrix in which cellular organelles are suspended. The cytosol and organelles together compose the cytoplasm. The cell membranes are the outer barrier. In humans, the intracellular compartment contains on average about 28 liters (6.2 imp gal 7.4 U.S. gal) of fluid, and under ordinary circumstances remains in osmotic equilibrium. It contains moderate quantities of magnesium and sulfate ions.
In the cell nucleus the fluid component of the nucleoplasm is called the nucleosol. 
The interstitial, intravascular and transcellular compartments comprise the extracellular compartment. Its extracellular fluid (ECF) contains about one-third of total body water.
Intravascular compartment Edit
The main intravascular fluid in mammals is blood, a complex mixture with elements of a suspension (blood cells), colloid (globulins), and solutes (glucose and ions). The blood represents both the intracellular compartment (the fluid inside the blood cells) and the extracellular compartment (the blood plasma). The average volume of plasma in the average (70-kilogram or 150-pound) male is approximately 3.5 liters (0.77 imp gal 0.92 U.S. gal). The volume of the intravascular compartment is regulated in part by hydrostatic pressure gradients, and by reabsorption by the kidneys.
Interstitial compartment Edit
The interstitial compartment (also called "tissue space") surrounds tissue cells. It is filled with interstitial fluid, including lymph.  Interstitial fluid provides the immediate microenvironment that allows for movement of ions, proteins and nutrients across the cell barrier. This fluid is not static, but is continually being refreshed by the blood capillaries and recollected by lymphatic capillaries. In the average male (70-kilogram or 150-pound) human body, the interstitial space has approximately 10.5 liters (2.3 imp gal 2.8 U.S. gal) of fluid.
Transcellular compartment Edit
The third extracellular compartment, the transcellular, consists of those spaces in the body where fluid does not normally collect in larger amounts,   or where any significant fluid collection is physiologically nonfunctional.  Examples of transcellular spaces include the eye, the central nervous system, the peritoneal and pleural cavities, and the joint capsules. A small amount of fluid, called transcellular fluid, does exist normally in such spaces. For example, the aqueous humor, the vitreous humor, the cerebrospinal fluid, the serous fluid produced by the serous membranes, and the synovial fluid produced by the synovial membranes are all transcellular fluids. They are all very important, yet there is not much of each. For example, there is only about 150 milliliters (5.3 imp fl oz 5.1 U.S. fl oz) of cerebrospinal fluid in the entire central nervous system at any moment. All of the aforementioned fluids are produced by active cellular processes working with blood plasma as the raw material, and they are all more or less similar to blood plasma except for certain modifications tailored to their function. For example, the cerebrospinal fluid is made by various cells of the CNS, mostly the ependymal cells, from blood plasma.
Fluid shifts occur when the body's fluids move between the fluid compartments. Physiologically, this occurs by a combination of hydrostatic pressure gradients and osmotic pressure gradients. Water will move from one space into the next passively across a semi permeable membrane until the hydrostatic and osmotic pressure gradients balance each other. Many medical conditions can cause fluid shifts. When fluid moves out of the intravascular compartment (the blood vessels), blood pressure can drop to dangerously low levels, endangering critical organs such as the brain, heart and kidneys when it shifts out of the cells (the intracellular compartment), cellular processes slow down or cease from intracellular dehydration when excessive fluid accumulates in the interstitial space, oedema develops and fluid shifts into the brain cells can cause increased cranial pressure. Fluid shifts may be compensated by fluid replacement or diuretics.
Third spacing Edit
"Third spacing" is the abnormal accumulation of fluid into an extracellular and extravascular space. In medicine, the term is often used with regard to loss of fluid into interstitial spaces, such as with burns or edema, but it can also refer to fluid shifts into a body cavity (transcellular space), such as ascites and pleural effusions. With regard to severe burns, fluids may pool on the burn site (i.e. fluid lying outside of the interstitial tissue, exposed to evaporation) and cause depletion of the fluids. With pancreatitis or ileus, fluids may "leak out" into the peritoneal cavity, also causing depletion of the intracellular, interstitial or vascular compartments.
Patients who undergo long, difficult operations in large surgical fields can collect third-space fluids and become intravascularly depleted despite large volumes of intravenous fluid and blood replacement.
The precise volume of fluid in a patient's third spaces changes over time and is difficult to accurately quantify.
Third spacing conditions may include peritonitis, pyometritis, and pleural effusions.  Hydrocephalus and glaucoma are theoretically forms of third spacing, but the volumes are too small to induce significant shifts in blood volumes, or overall body volumes, and thus are generally not referred to as third spacing.
Total body potassium content
- You have 40mmol/Kg of potassium. A 70kg male has about 2800mmol, or about 109g of potassium.
- Of this, 90% is in the intracellular fluid. This is the only exchangeable potassium.
- Extracellular fluid contains 2% of the total body potassium, and bone contains 8%
Potassium equilibrates freely and rapidly across the extracellular fluid
There isn’t enough of it to matter seriously in any of the Gibbs-Donnan effects, or to contribute significantly to the various osmolar forces. It merely exists.
Na+/K+ ATPase activity
Potassium concentration inside the cell is kept artificially high by the action of Na+/K+ ATPase, which exchanges 3 sodium atoms for every 2 potassium.
One's intracellular stores of potassium are not distributed as homogeneously as the shiny purple-filled cylinder would have you believe. According to Vander's Renal physiology the skeletal muscle contains most of one's potassium stores, because it contributes the largest intracellular volume to the overall count. Cells differ somewhat in their potassium content, and this gives rise to a confusing plethora of values one can find quoted in the physiology textbooks (from 120 to 150mmol/L)
FLUID RESUSCITATION STRATEGIES
Although there has been various different strategies defined in literature in decades, none has been adopted alone by most of the clinicians as the superior strategy. We think that many clinicians tend to keep their accustomed strategy, despite the evidences in the literature. There are studies that compare outcomes of different strategies of fluid management. Lately, 𠇌rystalloids vs colloids” debates are fading, while recent studies mostly focus on the amount of fluid given perioperatively.
Traditional approach to determine the fluid amounts is more likely to generate formulas based on parameters such as patients’ body weights and duration of surgeries. However, there is an evidence that each patient has his/her own body fluid status depending on the type of surgery, comorbid conditions, fluid already administered before, and various other factors. In addition, each patient should be considered as unique and his/her unique status should be monitored closely in the correct ways. As stated before, the main goal of fluid management is to maintain adequate tissue perfusion, with minimized risks of complications of over-hydration, such as pulmonary edema, cerebral edema, and intestinal edema. Both inadequate and excessive fluid administration may increase the stress on the circulatory system, and can affect tissue healing after surgery. From this perspective, without decent monitoring of patient’s current status, any strategy may fail.
Debates about fluid management strategies are gathered around liberal strategy, restricted (conservative) strategy and goal-directed strategy so far. Liberal and restricted strategies are defined by different authors with variable volume ranges. For example, in one study, restricted fluid volume is defined as 1000 mL plus loss through drains, while in another study, patients in restricted fluid volume group were subjected to over 2000 mL fluid on the day of surgery. These variances make it difficult to consider these studies as a whole. Still, majority of authors studying this subject point out that restrictive strategy has positive effects on gastrointestinal function, wound healing and pulmonary function[44,70,72-74]. Brandstrup et al stated that, excessive hydration with crystalloids is related with increased major complications, such as leakage, peritonitis, sepsis, pulmonary edema and bleeding in patients who underwent elective colorectal surgery. Also, intestinal edema is known to be related with increased bacterial translocation and multiple organ dysfunction syndrome rates[75,76]. It can be concluded that, staying closer to the dehydration level is more reasonable, because it is safer and more efficient than administering large volumes to avoid dehydration. On the other hand, the liberal strategy is superior to the restricted strategy for reducing postoperative nausea, headache, dizziness and vomiting[77,78].
However, the goal directed strategy (GDS) is totally based on patient’s current data, obtained from monitoring methods (See section: Monitoring body fluid status). Rivers and colleagues, one of the pioneers of this strategy, monitored CVP, mean arterial pressure, serum lactate, and mixed venous oxygen saturation in order to manage therapy in sepsis patients. Later studies were focused on monitoring hemodynamics, and the effects of administered fluids on patients. Now, GDS can be defined as an individualized fluid therapy, based on patient’s fluid responsiveness in other words, 𠇏luid need”. The extra volume, which won’t be able to affect the left ventricle stroke volume is regarded as unnecessary and as a matter of fact, hazardous. It makes perfect sense to totally evaluate patient’s needs and replace what is needed. Still, efficiency of GDS is limited with the power of our monitoring tools, which is determined by accessibility, applicability of the tools and the quality of information we acquire from them.
PPV and SVV are defined to monitor the fluid need of the patient dynamically as it is stated above. Esophageal Doppler monitoring of cardiac volumes and aortic flow are also one of the helpful tools in GDS. In a systematic review of esophageal Doppler guided GDS studies reduced hospital stay, fewer ICU admissions, and less inotropes usage were detected in GDS group. In a single center, blinded, prospective controlled trial, 128 patients who underwent colorectal resection were randomized into two groups. Each group was managed with esophageal Doppler or CVP guided fluid therapy during surgery. Intraoperative Doppler guided fluid management was associated with decrease in the duration of hospital stay. A randomized controlled study on 108 elective colorectal surgery patients also showed shorter hospital stay and decreased morbidity in GDS group. GDS is also advantageous in patients who undergo major surgery. A systematic review and meta-analysis studies by Hamilton on major surgery patients state that preemptive hemodynamic monitoring reduces mortality and morbidity. Similarly, Poeze et al showed that efforts to achieve an optimized hemodynamic condition resulted in a decreased mortality rate, in their meta-analysis study in 2005. Another meta-analysis also shows that GDS reduces both major and minor gastrointestinal complications after surgery.
In contrast with these studies, in a multicenter study, which included 762 high risk patients in 56 intensive care units, no significant effects of GDS were found. In this study, patients were randomly assigned to cardiac-index group, mixed venous oxygen-saturation group and standard therapy group. Predetermined hemodynamic targets were reached significantly better in the control group. There were no significant differences among the three groups, regarding mortality at six months. Even the subgroup analysis of patients, whose predetermined hemodynamic targets have been reached successfully, showed similar mortality rates among the three groups. Moreover, the number of dysfunctional organs and the duration of stay in the intensive care unit were similar in all groups.
Despite these evidences, low accessibility and applicability of esophageal Doppler are the major disadvantages of this method. This leads researchers to search for a more accessible and applicable method for common use in postoperative care unit, such as non-invasive pulse oximetry and invasive arterial pressure measurement. Thus, predictive value of pulse pressure variation, systolic pressure variation and stroke volume variation tests for fluid responsiveness are defined. All of these tests are applicable in an average postoperative care unit. However, the true value of these tests should be evaluated by larger studies. After that, optimization of patient monitoring devices should be done accordingly. Moreover, even PLR alone can provide important information about fluid responsiveness and lead the intensivists for GDS.
Since there is still insufficient number of randomized controlled trials with standardized criteria, the fluid management debates are going on. A consensus on criteria for each fluid management strategy should be made. We think that the related studies from all around the world with defined criteria are going to reveal the true value of each strategy.
Each surgeon should keep in mind that the patient is totally managed by the anesthesiologist during the surgery, so depending on the anesthesiologist’s preference on fluid strategy, patient’s fluid status after surgery may vary widely. Besides, intraoperative bleeding and other causes of surgical fluid loss should also be considered. During or after the surgery, the blood loss in patients with low hemoglobin levels is generally managed with erythrocyte suspensions. However, in patients with reasonable hemoglobin levels, appropriate fluid strategy should be chosen to avoid complications of transfusion. We think that determining the actual fluid status and the needs of a postoperative patient, by using monitoring tools and examining the report of the anesthesiologist, is of great importance.
Введение в физиологию человека
In this course, students learn to recognize and to apply the basic concepts that govern integrated body function (as an intact organism) in the body's nine organ systems.
Metabolic Pathways, Biology, Organ Systems, Medicine
Best course I've ever taken! Detailed and thorough explanations of the systems of the body. Fascinating, enlightening, and enjoyable. Highly recommend for anyone interested in how the body functions.
this course is so reasonable in price and with good assignments! At first, im annoyed if i got only 70 or below, ii will need to redo. but it does help me to consolidate my knowledge. so thank you!!
Welcome to Module 2 of Introductory Human Physiology! We begin our study of the human body with an overview of the basic concepts that underlie the functions of cells and organs within the body and their integration to maintain life. This is an important introduction to how physiologists view the body. We will return to these basic concepts again as we progress through the organs systems and consider how they respond to perturbations incurred in daily functions and in disease.
The things to do this week are to watch the 6 videos, to answer the in-video questions, to read the notes for each topic, and to complete two problem sets (homeostasis, transporters & channels, and endocrine concepts). It will be most effective if you follow the sequence of videos. The notes provide a more detailed summary of each topic. We encourage you to find which resource (videos and/or notes) works best for you.We have included a set of problems to be completed as homework exercises. We strongly encourage you to complete these problems sets. They are not graded and are for your personal feedback. It has been our experience that these exercises are helpful in increasing understanding and retention of the newly learned materials.Please use the interactive forum as a means to exchange ideas, to ask questions, to form study groups and interest groups, and to meet your community. We will monitor the forum daily.Thank you for joining us. We are excited about sharing this educational experience with you. Welcome!
Welcome to the first lecture in introductory human physiology, and today, we want to talk about homeostasis. This is the basic theme for physiology. All of the organ systems are going to integrate in order to maintain homeostasis of the body. And the homeostasis of the body is to maintain conditions within the body that are compatible with the life of the cells. So the things that we want to look at today, the learning objectives are first, to explain the basic organization of the body. Secondly, we want to define the fluid compartments of the body. Third, explain how solutes such as sodium, chloride, glucose and so forth distribute within the body. And fourth, we want to explain what homeostasis is and the homeostatic mechanisms that regulate this, we're going to deal with in the very next lecture, which is coming right up next. And then last we're going to very quickly talk about mass balance, and how the body maintains mass balance. All right, so the first thing that we want to consider then, is the body components. So as you all know, the human body starts with a single fertilized egg, and this egg then undergoes division to make multiple copies as well as differentiation. The differentiation allows the specific cells to acquire specialized functions. These functions, then, these groups of cells that have the same specialized function, will work together to form what are called tissues. We have four tissue types within the body. There are muscle, nervous tissue, connective tissue, and epithelium. These four tissue types will form the organs. And the organs will work together to perform a specific function for the body. And then at that point, if we have more than one more organ functioning, that is we have several organs functioning together, then they're called an organ system. So for instance, an organ system would be called, the organ would be the kidney and the organ system, the renal system or urinary system would be the kidney with the two ureters that are taking the urine that's generated from the kidneys down to the bladder. Where the urine can be stored in the bladder, and then eventually expelled to the outside of the body through what's called the urethra, so that's our urinary system. So the organ systems that we're going to consider, there are ten organ systems of the body, we're going to consider nine of them. And they are going to perform very specific functions. So for instance the skin. The skin is the largest organ of your body. It has its specific function. It is protective, so it forms a barrier to the outside world and keeps all of the inside materials sort of organized. The skin is a very important barrier for the loss of water. So it's a hydrophobic barrier, so it allows the body to retain water even though we have conditions where we would normally become dehydrated. The second of the organs that we need to deal with then are the organs that are going to overcome the barrier of the skin, and that is organs that allow us to have entry into the body. For instance the respiratory system, which allows the entry of oxygen and the expulsion of CO2. So we have gases then that can come in and out of the body, and we have the GI tract or the gastrointestinal tract, which allows food or nutrients to enter into the body, and then solid waste to be removed from the body. We also have transport systems, and the transport system is predominantly the cardiovascular system. The cardiovascular system takes the nutrients that are entering from the GI track and delivers it to all of the cells. It takes the gases which are coming in from the lung, the respiratory system, and delivers that to all of the tissues and organs of the body. This is done by bulk flow, and we'll talk about this when we get to the cardiovascular system, but this is moving materials through a series of vessels which are the vasculature. Once they get to the tissues, then we have to move the gasses and the nutrients and the solutes out of the vasculature, and actually into the tissues themselves. And they have to cross a very small space. And this space then is between the tissues of the cells and the vasculature. And we're going to talk about that in just a few minutes. And that is going to occur by diffusion. So that's going to be a very slow process that's only a local delivery system. And then we have to be able to remove materials from the body, and this is done by the renal system as I said. So liquid waste are removed, excess ions are removed, excess water is removed from the urinary system and of course the GI tract, the gastrointestinal tract will remove solid waste products. But the physiologist looks at the bodies in a slightly different manner, and that is they divided into what are called fluid compartments. We have effectively two major fluid compartments. One is where we take all of the cytoplasm, that is the liquid components that are within cells. The cells are bounded by a plasma membrane, and this liquid component, this cytoplasm, we take all of that from all of the cells and put it into one fluid compartment. And that would be called the intracellular fluid compartment, or the ICF. And it is bounded by the plasma membrane. And that's what's shown here. And then outside of the cells, we have this extra cellular space, and this extra fluid compartment or the ECF, is what is immediately outside all of the cells. The ICF, the intracellular fluid compartment is the largest of these two fluid compartments. And it is effectively two-thirds of the total body water or the total fluid of the body, and the extracellular fluid compartment is one-third. Now these two compartments are dissimilar in content, that is is that inside cells we have very high levels of potassium and very small concentrations of sodium. We also have present within the cells, proteins, which are negatively charged. In the extracellular fluid compartment, we have very high concentrations of sodium and small concentrations of potassium. So we have completely different types of an environment. The other thing about these two environment is that extracellular fluid compartment, can be divided further into two compartments. One is the intravascular compartment, and that's within the blood vessels, and the other is this interstitial fluid space. And this interstitial fluid spaces is that little space that's between the vascular and the cells themselves. This is usually filled with connective tissues. So these two compartments actually have the same content of ions and solutes, so that the amount of sodium that is present within the vasculature is equal to the amount of sodium that's present within the interstitial space. And the amount of potassium and so forth is equal between these two compartments. So there is an equilibrium of equal distribution of these solutes between the two spaces, and this is because the barrier, that is the epithelial cells that are lining the blood vessels, are a bit leaky, and so they allow this material to move from one compartment to the other, and to form an equilibrium. The two compartments do differ in that the intravascular's fluid compartment also has proteins, which are not present within the interstitial space. Now one other thing about these two compartments then is that we have a equilibrium between the intravascular space and the interstitial space, but we have a disequilibrium between the ECF and the ICF. But that is maintained at a constant or a steady state, and this is done so by the presence of an enzyme, which is an ATPas, which cleaves ATPase. So the enzyme uses energy to move the sodium out of the cells, so 3NA is pumped out of the cells for every 2K that enter the cells. This is needed because there are little leaks between these two compartments, which allow potassium then to slowly leak out of the cells and into the extocellurar fluid space. And this pump then reorganizes the distribution of the ions and keeps the ions at a disequilibrium. So that we have a steady state that is input is equal to output, but that the amount of sodium on the outside of the cells is different from the amount of sodium that's inside the cells. And the amount of potassium inside the cells is different from the amount of potassium that's on the outside of the cells, So the fluid compartments then or the total body water is about 60% of your total body weight. So if we have an individual who is a 70 kg male, then 42 liters of that individual is fluid, is water. That means that the intracellular fluid space or the cytoplasm which is two thirds of the total body water, would be equal to 28 liters. And that the extracellular fluid space, which surrounds the cells and is this interface between the cells and the external environment, this will be equal to 14 liters. Then within the ECF or the extracellular fluid space, we have this intravascular fluid. And the intravascular fluid is actually only one twelfth of the total body water. That is, it's one fourth of the ECF. So we have one fourth of the ECF is equal to the intravascular space times one third, which is the ECF, that is of the total body water. And that gives us then one twelfth of the total body water is equal to the fluid phase of the blood, of the vasculature, that is equal to the plasma. So that's pretty amazing, if you think about it, because, when you think about the body, you think of the fluid phase of the body is the blood, that is the plasma, which is the fluid portion, the liquid portion of the blood, and not all of the other fluids that are within the body. But it's actually the smallest amount of fluid that's within the body. So we have self-regulating mechanisms then, which are active between these different fluid phases, these different fluid compartments. We have an equilibrium, which is allowing equal amounts of substance to be distributed between the intravascular space and the interstitial space. So sodium, potassium, chloride, the calcium, they equally distribute between these two phases, these two compartments. There's no net transfer of substance or of energy between these two compartments, and there's no barrier to movement. As I said, the epithelial cells that are dividing these two compartments are fairly leaky and there's no energy expenditure to maintain this equilibrium. In contrast, we have a steady state which is present between our exocellular fluid space and the intracellular fluid space. And here, we have a constant amount of substance within the compartments and that the input is going to be equal to the output, but that the concentrations within these two compartments can be dissimilar. And that this requires energy to maintain. We need to use ATP, the energy of the cells, in order to maintain this gradient between the two compartments. So why are we so interested in these fluid compartments? Why is it the physiologists are asking about the fluid compartments of the body? And the reason for that is that as that the cells themselves require specific factors to be within a very tight range. These factors are the amount of oxygen, the amount of CO2, the amount of hydrogen ions, the temperature, the amount of glucose which is presented to the cells. So the cells then are requiring this very tightly regulated environment and yet as you go through your daily life, you are bringing into your body a very diverse amount of material. So you're constantly changing, your environment is constantly changing. And it is the ECF that is the buffer zone. What do I mean about that, well just think about it, if you eat a large hamburger for lunch, you're bringing in glucose, fat, proteins, amino acids, into the body and that material will go from the gastrointestinal track directly into the blood. And then from the blood it will then be distributed to the cells. But the organs of the body are trying to maintain that ECF, that buffer zone, which is where all this material is being delivered within a normal range or within a very set range. And it's the maintenance of this ECF, the constituents of the ECF is relatively constant, which is the main theme of physiology. And this is what homeostasis is about. So that's our central theme, and what we're going to see is that all of the organs of the body are going to act on that ECF to try to keep the contents of the ECF under this very narrow range which is compatible with the life of the cells. So what happens if we do not maintain the ECF in this very tight range of needed factors? When we have input is equal to the output we'll have wellness. So under those conditions then, as long as the materials that are within the ECF or within the range that's compatible with life of the cells, everything is fine. But when we have input, say for instance, that says effectively is increased over output, then we can get illness or pathophysiology. And the converse can occur if we have output that is greater than input. Then again, we can have illness or pathophysiology. And so it is this balance, this very tight balance that has to be maintained at all times in order to keep the body at a constant activity. If the organ system does not perform its function then we can end up with input or output which is not equal to the opposite. Under those conditions then, we will have pathophysiology. So one of the major ways that the body is going to regulate this ECF is by using homeostatic control systems or reflex loops. And that's what's diagramed here, and that reflex loops have essentially three components. They have a Sensor which is going to detect a specific signal or stimulus, and that Sensor then sensing information to what is called the Integration Center. And this Integration Center is usually the brain. The Integration Center has within it the set points that are compatible with the life of the cells. And so it will then evaluate the incoming signal to see whether or not the incoming signal matches the set point that the body needs or whether it exceeds it or is below it. It then will decide whether or not it needs to make a response and will send out an effector path wave to the effectors. So this is an efferent path wave going out to the effectors, which will generate a response that will then bring the body back to its normal condition. This is exactly analogous to the temperature control system that you have in your house for heating. So, the integration center would be a but we set a specific temperature that we want within the room. And then the stimulus is the incoming reading that is, what is the temperature of the room. And the output would be whether we have to turn on the heat or we have to turn on the air conditioning to bring the temperature of the room back to normal. So, this is a simple reflex loop. And it's essentially the types of reflex loops that the body is going to use. So let's consider one of these systems where we have a case where we've decided that in a given week that you want to eat nothing but a high salt diet. So on Monday, the amount of sodium that's coming into your diet is equal to the amount that's being released from the body in urine. And so we have then, what is called, a neutral balance. So the mass balance then is equal. What's coming into the body is equal to what's leaving the body. But by Wednesday, with this high salt diet, you're eating a lot of sodium. You're taking in Chinese food with a lot of soy sauce on it, and so it's really salty. And so that on this diet is very high on salt we have now a positive balance where the amount is coming in from the diet exceeds that which is lost in the urine. And so this now is a positive balance for sodium. But by Friday now the amount of sodium that's coming in from the diet is equal to the amount that's lost in the urine, and so were again under neutral balance. We're under neutral balance, but look what's happened to the body. We've actually increased the amount of sodium, the content of sodium within the body, and I just finished telling you that the body wants to maintain a very tight regulated amounts of sodium within the ECF at all times. That's one of the regulated factors that the body is interested in keeping constant. And yet we have with this diet, we have increased the total amount of sodium within the body. So how could we do that? With increase the total amount of sodium within the body, but what happens when you are eating in a high salt diet. What happens when you taking a lot of salted food like a potato chips, you eat a bag of potato chips, what happens? You get thirsty, and as you get thirsty then you drink water, and as you drink water that fluid will come in to the body and dilute the content of the sodium, so that it now has a concentration which is the same as the concentration of sodium that we had on Monday. So the concentration of the sodium in the body is going to stay equal, but the content, the amount of sodium that's added to the body has increased. And where did it go? It went to the ECF. All of the sodium went into the ECF. It's not able to cross that plasma membrane that hydrophobic barrier, and instead it's staying in the ECF. So where the volume of water go that you drunk? It also goes into the ECF, so that we could dilute them the sodium concentration within the ECF. So all of the volume, all of the fluid volume is into the ECF. So we have increased the sodium content that was in the ECF, and we've increased the water content in the ECF, but we've maintained the concentration of sodium in ECF as constant. At what cost? So let's think about it. So what is within the ECF? We said that there's a vasculature, and the interstitial fluid space. The vasculature, within the vasculature, we have increased the volume of blood, and by increasing the volume of blood, we have increased the pressure within the vasculature, so by holding this extra sodium and holding this extra fluid within the body, we increase the volume of the blood. And by doing so, we then increase pressure within the cardiovascular system. So there is a cost, then, to maintaining the ECF at normal range. So what are general concepts? So the first is that the human body, then, is this interdependent set of self regulating systems whose primary function is to maintain an internal environment compatible with living cells and tissues, and this is homeostasis. And this is the primary theme of physiology, and it is what all of the organ systems of the body are trying to maintain. The second is is that we have stability of these internal variables, and it can be achieved by balancing our inputs and outputs to the body and among the organ systems. But what we need to remember is that there's a hierarchy among the organ systems, and that the two organ systems that always win out is the brain and the heart. And often, they will take dominance and allow the body to maintain the brain and the heart, say, for instance, perfusion of the brain and the heart, but then lose the perfusion to other organ systems. So there is now going to be a tradeoff where the body is going to make some decisions which may not, under difficult conditions, or pathological conditions, which may not maintain everything as a constant. Okay, so the next time we come in then let's look at all the different mechanisms that we can use to maintain this homeostasis. Okay, see you then.
Transcellular fluid is the portion of total body water contained within the epithelial-lined spaces. It is the smallest component of extracellular fluid, which also includes interstitial fluid and plasma. It is often not calculated as a fraction of the extracellular fluid, but it is about 2.5% of the total body water.
Examples of this fluid are cerebrospinal fluid, ocular fluid, joint fluid, and the pleaural cavity that contains fluid that is only found in their respective epithelium-lined spaces.
The function of transcellular fluid is mainly lubrication of these cavities, and sometimes electrolyte transport.