The Body Fluid Compartments

The Body Fluid Compartments:  Extracellular and Intracellular Fluids; Interstitial Fluid and Edema

The maintenance of a relatively constant volume  and a stable composition of the body fluids is essential for homeostasis.

Fluid Intake and Output Are Balanced During Steady-State Conditions

The relative constancy of the body fluids is remarkable because there is continuous exchange of fluid and solutes with the external environment as well as within the different compartments of the body.

Daily Intake of Water

Water is added to the body by two major sources:

 (1) it is ingested in the form of liquids or water in the food, which together normally add about 2100 ml/day to the body fluids.

 (2) it is synthesized in the body as a result of oxidation of carbohydrates, adding about 200 ml/day. This provides a total water intake of about 2300 ml/day.

Daily Loss of Body Water

Insensible Water Loss. Some of the water losses cannot be precisely regulated. For  example, there is a continuous loss of water by evaporation from the respiratory tract and diffusion through the skin, which together account for about 700 ml/day of water loss under normal conditions. This is termed insensible water loss because we are not consciously aware of it.

The insensible water loss through the skin occurs independently of sweating,  the average  water loss by diffusion through the skin is about 300 to 400 ml/day.

Fluid Loss in Sweat.

The amount of water lost by sweating is highly variable, depending on physical activity and environmental temperature. The volume of sweat normally is about    100 ml/day, but in very hot weather  or during heavy exercise, water loss in sweat occasionally increases to 1 to 2 L/hour. This would rapidly deplete the body fluids if intake were not also increased by activating the thirst mechanism.

Water Loss in Feces.

Only a small amount of water (100 ml/day) normally is lost in the feces. This can  increase to several liters a day in people with severe diarrhea.

Water Loss by the Kidneys.

The remaining water loss from the body occurs in the urine excreted by the kidneys. There are multiple mechanisms that control the rate of urine excretion. In fact, the most important means by which the body maintains a balance between water intake and output, as well as a balance between intake

and output of most electrolytes in the body, is by controlling the rates at which the kidneys excrete these substances. For example, urine volume can be as low

as 0.5 L/day in a dehydrated person or as high as  20 L/day  in a person who has been drinking tremendous  amounts of water. This variability of intake is also true for most of the electrolytes of the body, such as sodium, chloride, and potassium. In some people, sodium intake may be as low as 20 mEq/day, whereas in others, sodium intake may be as high as 300 to 500 mEq/day.The kidneys are faced with the task of adjusting the excretion rate of water and electrolytes to match precisely the intake of these substances, as well as compensating for excessive losses of fluids and electrolytes that occur in certain disease states.

Body Fluid Compartments

The total body fluid is distributed mainly between two compartments:

the extracellular fluid and the intracellular fluid.

Fig: Summary of body fluid regulation, including the major body fluid compartments and the membranes that separate these compartments. The values shown are for an average 70-kilogram person

The extracellular fluid is divided into the interstitial fluid and the blood plasma.In the average 70-kilogram adult human, the total body water is about 60 per cent of the body weight, or about 42 liters.This percentage can change, depending on age, gender, and degree of obesity.

Intracellular Fluid Compartment

About 28 of the 42 liters of fluid in the body are inside the 75 trillion cells and are collectively called the intracellular fluid. Thus, the intracellular fluid constitutes about 40 per cent of the total body weight in an “average” person.

the intracellular fluid of all the different cells together is considered to be one large fluid  compartment.

Extracellular Fluid  Compartment

All the fluids outside the cells are collectively called the extracellular fluid. Together these fluids account for about 20 per cent of the body weight, or about 14 liters in a normal 70-kilogram adult. The two largest compartments

of the extracellular fluid are:

 The interstitial fluid: which makes up more than three fourths of the extracellular fluid.

 The plasma: which makes up almost one fourth of the extracellular fluid, or about 3 liters. The plasma is the non cellular part of the blood; it exchanges substances continuously with the interstitial fluid through the pores of the capillary membranes. These pores are highly permeable to almost all solutes in the extracellular fluid except the proteins. Therefore, the extracellular fluids are constantly  mixing, so that the plasma and interstitial fluids have about the same composition except for proteins, which have a higher concentration in the plasma.

Blood Volume

Blood contains both extracellular fluid (the fluid in plasma) and intracellular fluid (the fluid in the red blood cells). However, blood is considered to be a separate fluid compartment because it is contained in a chamber of its own, the circulatory system. The blood volume is especially important in the control of cardiovascular dynamics. The average blood volume of adults is about 7 per

cent of body weight, or about 5 liters. About 60 per cent of the blood is plasma and 40 per cent is red blood cells, but these percentages can vary considerably in different people, depending on gender, weight, and other factors.

Constituents of Extracellular and Intracellular Fluids:

Ionic Composition of Plasma and Interstitial Fluid Is Similar

Because the plasma and interstitial fluid are separated only by highly permeable capillary membranes, their ionic composition is similar. The most important difference between these two compartments is the higher concentration of protein in the plasma; because the capillaries have a low permeability to the plasma proteins, only small amounts of proteins are leaked into the interstitial spaces in most tissues. Because of the Donnan effect, the concentration of positively charged ions (cations) is slightly greater (about 2 per cent) in the plasma than in the interstitial fluid. The plasma proteins have a net negative  charge and, therefore, tend to bind cations, such as sodium and potassium ions, thus holding extra amounts of these cations in the plasma along with the  plasma proteins. Conversely, negatively charged ions (anions) tend to have a slightly higher concentration in the interstitial fluid compared with the plasma, because the negative charges of the plasma proteins repel the negatively charged anions. For practical purposes, however, the concentration of ions in interstitial fluid and in the plasma is considered to be

about equal;  the extracellular fluid, including the plasma and the interstitial fluid, contains large amounts of sodium and chloride ions, reasonably large amounts of bicarbonate ions, but only small quantities of potassium, calcium, magnesium, phosphate, and organic acid ions. The composition of extracellular fluid is carefully regulated by various mechanisms, but especially by the kidneys. This allows the cells to remain continually bathed in a fluid that contains the proper concentration of electrolytes and nutrients for optimal cell function.

Fig: Chemical compositions of extracellular and intracellular fluids

Important Constituents of the Intracellular Fluid

The intracellular fluid is separated from the extracellular fluid by a cell membrane that is highly permeable to water but not to most of the electrolytes in the body. In contrast to the extracellular fluid, the intracellular

fluid contains only small quantities of sodium and chloride ions and almost no calcium ions. Instead, it contains large amounts of potassium and phosphate

ions plus moderate quantities of magnesium and sulfate ions, all of which have low concentrations in the extracellular fluid. Also, cells contain large amounts of  protein, almost four times as much as in the plasma.

Regulation of Fluid Exchange and Osmotic Equilibrium Between Intracellular and  Extracellular Fluid:

 The relative amounts of extracellular fluid distributed between the plasma and interstitial spaces are determined mainly by the balance of hydrostatic and colloid osmotic forces across the capillary membranes. The distribution of fluid between intracellular and extracellular compartments, in contrast, is determined mainly by the osmotic effect of the smaller solutes— especially sodium, chloride, and other electrolytes—acting across the cell membrane.

The reason for this is that the cell membranes are highly permeable to water but relatively impermeable to even small ions such as sodium and chloride. Therefore, water moves across the cell membrane rapidly, so that the intracellular fluid remains isotonic with the extracellular fluid.

Basic Principles of Osmosis and Osmotic Pressure

Osmosis is the net diffusion of water across a selectively permeable membrane from a region of high water concentration to one that has a lower water concentration. When a solute is added to pure water, this reduces the concentration of water in the mixture. Thus, the  higher the solute concentration in a solution, the lower the water concentration. Further, water diffuses from a region of low solute concentration (high water concentration)

to one with a high solute concentration (low water concentration). Because cell membranes are relatively impermeable to most solutes but highly permeable to water  (i.e., selectively permeable),  whenever  there  is  a  higher

concentration of solute on one side of the cell membrane, water diffuses across the membrane toward the region of higher solute concentration. Thus, if a solute such as sodium chloride is added to the extracellular fluid, water rapidly diffuses from the cells through the cell membranes into the extracellular fluid until the water concentration on both sides of the membrane becomes equal. Conversely, if a solute such as sodium chloride is removed from the extracellular fluid, water diffuses from the extracellular fluid through the cell membranes and into the cells. The rate of diffusion of water is called the rate of osmosis.

The osmotic pressure of a solution is directly proportional to the concentration of osmotically active particles in that solution. This is true regardless of whether  the solute is a large molecule or a small molecule. For  example, one molecule of albumin with a molecular weight of 70,000 has the same osmotic effect as one molecule of glucose with a molecular weight of 180. One molecule of sodium chloride, however, has two osmotically active particles, Na+ and  Cl–, and therefore has twice the osmotic effect of either an albumin molecule or a glucose molecule.

Osmolarity of the Body Fluids.

 the approximate osmolarity of the various osmotically  active substances in plasma, interstitial fluid, and intracellular fluid. Note that about 80 per cent of the total osmolarity of the interstitial fluid and plasma is due to sodium and chloride ions, whereas for intracellular fluid, almost half the osmolarity is due to potassium ions, and the remainder is divided among many other intracellular substances, the total osmolarity of each of the three compartments is about 300 mOsm/L, with the plasma being about 1 mOsm/L greater than that of the interstitial and intracellular fluids.The slight difference

between plasma and interstitial fluid is caused by the osmotic effects of the plasma proteins, which maintain about 20 mm Hg greater pressure in the capillaries  than in the surrounding interstitial spaces.

Isotonic, Hypotonic, and Hypertonic Fluids.

If a cell is placed in a solution of impermeant solutes having an osmolarity of 282 mOsm/L, the cells will not shrink or swell because the water concentration

in the intracellular and extracellular fluids is equal and the solutes cannot enter or leave the cell. Such a solution is said to be isotonic because it neither shrinks

nor swells the cells. Examples of isotonic solutions include a 0.9 per cent solution of sodium chloride or a 5 per cent glucose solution. These solutions are important in clinical medicine because they can be infused into the blood without the danger of upsetting osmotic equilibrium between the intracellular and  extracellular  fluids.  

If  a  cell  is  placed  into  a hypotonic solution that has a lower concentration of impermeant solutes (less than 282 mOsm/L), water will diffuse into the cell, causing it to swell; water will continue to diffuse into the cell, diluting the intracellular fluid while also concentrating the extracellular fluid until both solutions have about the same osmolarity. Solutions of sodium chloride with a concentration of less than 0.9 per cent are hypotonic and cause cells to swell.

 If a cell is placed in a hypertonic solution having a higher concentration of impermeant solutes, water will flow out of the cell into the extracellular fluid, concentrating the intracellular fluid and diluting the extracellular fluid. In this case, the cell will shrink until the two concentrations become equal. Sodium chloride solutions of greater than 0.9 per cent are hypertonic.

Edema: Excess Fluid in the Tissues

Edema refers to the presence of excess fluid in the body tissues. In most instances, edema occurs mainly in the extracellular fluid compartment, but it can involve intracellular fluid as well.

Intracellular Edema

Two conditions are especially prone to cause intracellular swelling:

(1)  depression of the metabolic systems of the tissues.

 (2) lack of adequate nutrition to the cells.

 For example, when blood flow to a tissue is decreased, the delivery of oxygen and nutrients is reduced. If the blood flow becomes too low to maintain normal tissue metabolism, the cell membrane ionic pumps become depressed. When this occurs, sodium ions that normally leak into the interior of the cell can no longer be pumped out of the cells, and the excess sodium ions inside the cells cause osmosis of water into the cells. Sometimes this can increase intracellular volume of a tissue area—even of an entire ischemic leg, for example—to two to three times normal. When this occurs, it is usually a prelude to death of the tissue.

Intracellular edema can also occur in inflamed tissues. Inflammation usually has a direct effect on the cell membranes to increase their permeability, allowing  sodium and other ions to diffuse into the interior  of the cell, with subsequent osmosis of water into the  cells.

Extracellular Edema

Extracellular fluid edema occurs when there is excess fluid accumulation in the extracellular spaces. There are two general causes of extracellular edema:

(1) Abnormal leakage of fluid from the plasma to the interstitial spaces across the capillaries.

 (2) Failure of the lymphatic  to return fluid from the interstitium back into the blood. The most common clinical cause of interstitial fluid accumulation is excessive capillary  fluid filtration.

Fluids in the “Potential Spaces” of the Body:

 It  include the  pleural cavity, Pericardial cavity,  peritoneal cavity, and  synovial cavities, including both the joint cavities and the bursae. Virtually all these potential spaces have surfaces that almost touch each other, with only a thin layer of fluid in between, and the surfaces slide over each other. To facilitate the sliding, a viscous  proteinaceous  fluid lubricates the surfaces.

Edema Fluid in the Potential Spaces Is Called “Effusion.”

When edema occurs in the subcutaneous tissues adjacent to the potential space, this fluid is called effusion. Thus, lymph blockage or any of the multiple abnormalities that can cause excessive capillary filtration can cause effusion in the same way that interstitial edema is caused. The abdominal cavity is especially prone to collect effusion fluid and  the effusion is called ascites. In serious cases, 20 liters or more of ascitic fluid can accumulate. The other potential spaces, such as the pleural cavity, pericardial cavity, and joint spaces, can become seriously swollen when there is generalized edema.