27. FLUID COMPARTMENTS AND FLUID BALANCE

27. FLUID COMPARTMENTS AND FLUID BALANCE

In lean adults, 

• body fluids constitute between 55% and 60% of total body mass in females and males, respectively 
• Body fluids are present in two main “compartments”—

1. inside cells and 
2. outside cells. 

• About two-thirds of body fluid is intracellular fluid (ICF) (intra- within) or cytosol, the fluid within cells. 
• The other third, called extracellular fluid (ECF) (extra- outside) is outside cells 
• and includes all other body fluids. 

• About 80% of the ECF is interstitial fluid (inter- between), which occupies the microscopic spaces between tissue cells, 
• and 20% of the ECF is plasma, the liquid portion of the blood. 

Other extracellular fluids that are grouped with interstitial fluid include 

• lymph in lymphatic vessels; 
• cerebrospinal fluid in the nervous system;
• synovial fluid in joints; 
• aqueous humor and vitreous body in the eyes;
• endolymph and perilymph in the ears; and 
• pleural, pericardial, and peritoneal fluids between serous membranes.

Two general “barriers” separate intracellular fluid, interstitial fluid, and blood plasma.

1. The plasma membrane of individual cells separates intracellular fluid from the surrounding interstitial fluid. 

•  the plasma membrane is a selectively permeable barrier: 
• It allows some substances to cross 
• but blocks the movement of other substances. 
• In addition, active transport pumps work continuously to maintain different concentrations of certain ions in the cytosol and interstitial fluid.

2. Blood vessel walls 

• divide the interstitial fluid from blood plasma. 
• Only in capillaries, the smallest blood vessels, are the walls thin enough and leaky enough to permit the exchange of water and solutes between blood plasma and interstitial fluid.

• The body is in fluid balance when the required amounts of water and solutes are present and are correctly proportioned among the various compartments. 

• Water is by far the largest single component of the body, making up 45–75% of total body mass, depending on age and gender.

• The processes of filtration, reabsorption, diffusion, and osmosis allow continual exchange of water and solutes among body fluid compartments . 
• Yet the volume of fluid in each compartment remains remarkably stable. 

• Because osmosis is the primary means of water movement between intracellular fluid and interstitial fluid, the concentration of solutes in these fluids determines the direction of water movement. 

• Because most solutes in body fluids are electrolytes, inorganic compounds that dissociate into ions, fluid balance is closely related to electrolyte balance.

• Because intake of water and electrolytes rarely occurs in exactly the same proportions as their presence in body fluids, the ability of the kidneys to excrete excess water by producing dilute urine, or to excrete excess electrolytes by producing concentrated urine, is of utmost importance in the maintenance of homeostasis.

Sources of BodyWater Gain and Loss

• The body can gain water by ingestion and by metabolic synthesis. The main sources of body water are 

1. ingested liquids (about 1600 mL) 
2. and moist foods (about 700 mL) absorbed from the gastrointestinal (GI) tract, 

• which total about 2300 mL/day. 

3. The other source of water is metabolic water 

• that is produced in the body mainly when electrons are accepted by oxygen during aerobic cellular respiration 
•  and to a smaller extent during dehydration synthesis reactions. 
• Metabolic water gain accounts for only 200 mL/day. 
• Daily water gain from these two sources totals about 2500 mL.

• Normally, body fluid volume remains constant because water loss equals water gain. 
• Water loss occurs in four ways. 

1. Each day the kidneys excrete about 1500 mL in urine,
2. the skin evaporates about 600 mL (400 mL through insensible perspiration, sweat that evaporates before it is perceived as moisture, and 200 mL as sweat), 
3. the lungs exhale about 300 mL as water vapor, and
4.  the gastrointestinal tract eliminates about 100 mL in feces. 
5. In women of reproductive age, additional water is lost in menstrual flow. 

• On average, daily water loss totals about 2500 mL. 
• The amount of water lost by a given route can vary considerably over time. 

For example, 

• water may literally pour from the skin in the form of sweat during strenuous exertion.
• In other cases, water may be lost in diarrhea during a GI tract infection.

Regulation of BodyWater Gain

• The volume of metabolic water formed in the body depends entirely on the level of aerobic cellular respiration, which reflects the demand for ATP in body cells. 
• When more ATP is produced, more water is formed. 

• Body water gain is regulated mainly by 

1. the volume of water intake, 
2. or how much fluid you drink. 

• An area in the hypothalamus known as the thirst center governs the urge to drink.
• When water loss is greater than water gain, dehydration—a decrease in volume and an increase in osmolarity of body fluids—stimulates thirst . 

• When body mass decreases by 2% due to fluid loss, mild dehydration exists. 
• A decrease in blood volume causes blood pressure to fall. 
• This change stimulates the kidneys to release renin, 
• which promotes the formation of angiotensin II. 

1. Increased nerve impulses from osmoreceptors in the hypothalamus,
2. triggered by increased blood osmolarity, and
3. increased angiotensin II in the blood 

• both stimulate the thirst center in the hypothalamus. 

• Other signals that stimulate thirst come from 

(1) neurons in the mouth that detect dryness due to a decreased flow of saliva and 

(2) baroreceptors that detect lowered blood pressure in the heart and blood vessels. 

• As a result, the sensation of thirst increases, 
• which usually leads to increased fluid intake (if fluids are available) 
• and restoration of normal fluid volume. 

• Overall, fluid gain balances fluid loss. 
• Sometimes, however, the sensation of thirst does not occur quickly enough or access to fluids is restricted, and significant dehydration ensues.

This happens most often 

• in elderly people, 
• in infants, and
• in those who are in a confused mental state. 

• When heavy sweating or fluid loss from diarrhea or vomiting occurs, it is wise to start replacing body fluids by drinking fluids even before the sensation of thirst occurs 

Regulation of Water and Solute Loss


Even though the loss of water and solutes through sweating and exhalation increases during exercise, elimination of excess body water or solutes occurs

•  mainly by control of their loss in urine.

• The extent of urinary salt (NaCl) loss is the main factor that determines body fluid volume. 
• The reason for this is that “water follows solutes” in osmosis, 
• and the two main solutes in extracellular fluid (and in urine) are 

1. sodium ions (Na ) and
2. chloride ions (Cl ). 

• In a similar way, the main factor that determines body fluid osmolarity is the extent of urinary water loss.
• Because our daily diet contains a highly variable amount of NaCl, urinary excretion of Na and Cl must also vary to maintain homeostasis. 
• Hormonal changes regulate the urinary loss of these ions, which in turn affects blood volume. 

• The increased intake of NaCl produces an increase in plasma levels of Na and Cl (the major contributors to osmolarity of extracellular fluid). 
• As a result, the osmolarity of interstitial fluid increases, 
• which causes movement of water from intracellular fluid into interstitial fluid and then into plasma. 
• Such water movement increases blood volume.

• The three most important hormones that regulate the extent of renal Na and Cl reabsorption (and thus how much is lost in the urine) are 

1.  angiotensin II, 
2. aldosterone, 
3. and atrial natriuretic peptide (ANP). 

When your body is dehydrated, angiotensin II and aldosterone 

• promote urinary reabsorption of Na and Cl (and water by osmosis with the electrolytes),
• conserving the volume of body fluids by reducing urinary loss.

• An increase in blood volume, as might occur after we finish one or more supersized drinks, 
• stretches the atria of the heart and
• promotes release of atrial natriuretic peptide. 

ANP 

• promotes natriuresis, 
• elevated urinary excretion of Na (and Cl )
• followed  by water excretion, 
• which decreases blood volume. 

An increase in blood volume 

• also slows release of renin from juxtaglomerular cells of the kidneys. 
• When renin level declines, less angiotensin II is formed. 
• Decline in angiotensin II from a moderate level to a low level increases glomerular filtration rate
• and reduces Na , Cl , and water reabsorption in the kidney tubules. 

• In addition, less angiotensin II leads to lower levels of aldosterone, 
• which causes reabsorption of filtered Na and Cl  to slow in the renal collecting ducts. 
• More filtered Na and Cl thus remain in the tubular fluid to be excreted in the urine. 

The osmotic consequence of excreting more Na and Cl  

• is loss of more water in urine,
•  which decreases blood volume and blood pressure.

• The major hormone that regulates water loss is antidiuretic hormone (ADH). 
• This hormone, also known as vasopressin, 
• is produced by neurosecretory cells that extend from the hypothalamus to the posterior pituitary. 

• In addition to stimulating the thirst mechanism, an increase in the osmolarity of body fluids stimulates release of ADH 
•  ADH promotes the insertion of water-channel proteins (aquaporin-2) into the apical membranes of principal cells in the collecting ducts of the kidneys. 
• As a result, the permeability of these cells to water increases. 
• Water molecules move by osmosis from the renal tubular fluid into the cells 
• and then from the cells into the bloodstream. 

• The result is production of a small volume of very concentrated urine. 

• Intake of water in response to the thirst mechanism decreases the osmolarity of blood and interstitial fluid. 
• Within minutes, ADH secretion shuts down, 
• and soon its blood level is close to zero. 

When the principal cells are not stimulated by ADH, 

• aquaporin-2 molecules are removed from the apical membrane by endocytosis. 
• As the number of water channels decreases, the water permeability of the principal cells’ apical membrane falls, 
• and more water is lost in the urine.

Under some conditions, factors other than blood osmolarity influence ADH secretion. 

1. A large decrease in blood volume, which is detected by baroreceptors (sensory neurons that respond to stretching) in the left atrium and in blood vessel walls, also stimulates ADH release. 

2. In severe dehydration, 

• glomerular filtration rate decreases because blood pressure falls, so that less water is lost in the urine. 
• Conversely, the intake of too much water increases blood pressure, 
• causing the rate of glomerular filtration to rise, 
• and more water to be lost in the urine. 

3. Hyperventilation (abnormally fast and deep breathing) 

• can increase fluid loss through the exhalation of more water vapor.

4. Vomiting and diarrhea 

• result in fluid loss from the GI tract.

5. Finally, fever, heavy sweating, and destruction of extensive areas of the skin from burns 

• can cause excessive water loss through the skin. 

• In all of these conditions, an increase in ADH secretion will help conserve body fluids.

Summary of Factors That Maintain Body Water Balance
FACTOR	MECHANISM	EFFECT
Thirst center in hypothalamus	Stimulates desire to drink fluids.	Water gain if thirst is quenched.
Angiotensin II	Stimulates secretion of aldosterone.	Reduces loss of water in urine.
Aldosterone	By promoting urinary reabsorption of Na+ and Cl-, increases water reabsorption via osmosis	Reduces loss of water in urine.
Atrial natriuretic peptide (ANP)	Promotes natriuresis, elevated urinary excretion of Na+ (and Cl-), accompanied by water.	Increases loss of water in urine.
Antidiuretic hormone (ADH), also known as vasopressin	Promotes insertion of water-channel proteins (aquaporin-2) into the apical membranes of principal cells in the collecting ducts of the kidneys. As a result, the water permeability of these cells increases and more water is reabsorbed.	Reduces loss of water in urine.

Movement of Water Between Body Fluid Compartments


Normally, cells neither shrink nor swell 

• because intracellular and interstitial fluids have the same osmolarity. 

• Changes in the osmolarity of interstitial fluid, however, cause fluid imbalances.

• An increase in the osmolarity of interstitial fluid draws water out of cells, and they shrink slightly. 
• A decrease in the osmolarity of interstitial fluid, by contrast, causes cells to swell. 

• Changes in osmolarity most often result from changes in the concentration of Na .
• A decrease in the osmolarity of interstitial fluid, as may occur

1. after drinking a large volume of water, 
2. inhibits secretion of ADH. 

• Normally, the kidneys then excrete a large volume of dilute urine, 
• which restores the osmotic pressure of body fluids to normal. 
• As a result, body cells swell only slightly, 
• and only for a brief period.

•  But when a person steadily consumes water faster than the kidneys can excrete it (the maximum urine flow rate is about 15 mL/min) 
• or when renal function is poor, 
• the result may be water intoxication, a state in which excessive body water causes cells to swell dangerously . 

If the body water and Na lost during blood loss or excessive sweating, vomiting, or diarrhea is replaced by drinking plain water, 

• then body fluids become more dilute. 
• This dilution can cause the Na concentration of plasma and then of interstitial fluid to fall below the normal range. 
• When the Na concentration of interstitial fluid decreases, its osmolarity also falls. 
• The net result is osmosis of water from interstitial fluid into the cytosol. 
• Water entering the cells  causes them to swell, 
• producing convulsions, coma, and possibly death. 

To prevent this dire sequence of events in cases of severe electrolyte and water loss,

• solutions given for intravenous or oral rehydration therapy (ORT) include a small amount of table salt (NaCl).

CLINICAL CONNECTION 


Enemas and Fluid Balance

• An enema (EN-e-ma) is the introduction of a solution into the rectum to draw water (and electrolytes) into the colon osmotically. 
• The increased volume increases peristalsis, 
• which evacuates feces. 

Enemas 

• are used to treat constipation. 
• Repeated enemas, especially in young children, increase the risk of fluid and electrolyte imbalances.