Excess Water Intake

1. Extracellular fluid becomes osmotically more concentrated.

2. Osmoreceptors in the hypothalamus are stimulated by the increase in the osmotic pressure of body fluids.

3. The hypothalamus signals the posterior pituitary gland to release ADH into the blood.

4. Blood carries ADH to the kidneys.

5. ADH causes the distal convoluted tubules and collecting ducts to increase water reabsorption.

6. Urine output decreases, and further water loss is minimized.

1. Extracellular fluid becomes osmotically less concentrated.

2. This change stimulates osmoreceptors in the hypothalamus.

3. The posterior pituitary gland decreases ADH release.

4. Renal tubules decrease water reabsorption.

5. Urine output increases, and excess water is excreted.


Water Balance Disorders

Among the more common disorders involving an imbalance in the water of body fluids are dehydration, water intoxication, and edema.


In 1994, thousands of starving people died in the African nation of Rwanda. It wasn't lack of food that killed most of these people, but cholera, a bacterial infection that cripples the ability of intestinal lining cells to reabsorb water. The severe diarrhea that develops can kill in days, sometimes even hours. Dehydration is deadly.

Dehydration is a deficiency condition that occurs when output of water exceeds intake. It is a great problem for athletes, military personnel, and certain industrial workers. This condition may develop following excessive sweating or as a result of prolonged water deprivation accompanied by continued water output. In either case, as water is lost, the extracellular fluid becomes increasingly more concentrated, and water tends to leave cells by osmosis (fig. 21A). Dehydration may also accompany illnesses in which prolonged vomiting or diarrhea depletes body fluids.

During dehydration, the skin and mucous membranes of the mouth feel dry, and body weight drops. Severe hyperthermia may develop as the body temperature-regulating mechanism falters due to lack of water for sweat. In severe dehydration, as waste products accumulate in the extracellular fluid, symptoms of cerebral disturbances, including mental confusion, delirium, and coma, may develop.

Because the kidneys of infants are less able to conserve water than are those of adults, infants are more likely to become dehydrated. Elderly people are also especially susceptible to developing water imbalances because the sensitivity of their thirst mechanisms decreases with age, and physical disabilities may make it difficult for them to obtain adequate fluids.

The treatment for dehydration is to replace the lost water and electrolytes. If only water is replaced, the extracellular fluid will become more dilute than normal. This may produce a condition called water intoxication.

^Vater Intoxication

Babies rushed to emergency rooms because they are having seizures sometimes are suffering from drinking too much water, a rare condition called water intoxication. This can occur when a baby under six months of age is given several bottles of water a day or very dilute infant formula. The hungry infant gobbles down the water, and soon its tissues swell with the excess fluid.

When the serum sodium level drops, the eyes begin to flutter, and a seizure occurs. As extracellular fluid becomes hypotonic, water enters the cells rapidly by osmosis (fig. 21B). Coma resulting from swelling brain tissues may follow unless water intake is restricted and hypertonic salt solutions given to draw water back into the extracellular fluid. Usually, recovery is complete within a few days.

Water intoxication in infancy sometimes occurs when caregivers dilute formula to make it last longer. Another source of water intoxication in infants is bottled water products that are sold alongside infant formula. Their placement on grocery shelves leads some parents to believe that these products are adequate nutritional supplements. Used this way, they can be dangerous.


Edema is an abnormal accumulation of extracellular fluid within the interstitial spaces. A variety of factors can cause it, including decrease in the plasma protein concentration (hy-poproteinemia), obstructions in lym-

Cell membrane


(1) Water loss from extracellular fluid compartment

(2) Solute concentration increases in extracellular fluid compartment

Cell membrane


(3) Water leaves intracellular fluid compartment by osmosis

Figure 21A

If excess extracellular fluids are lost, cells dehydrate by osmosis.

(1) Water loss from extracellular fluid compartment

(2) Solute concentration increases in extracellular fluid compartment phatic vessels, increased venous pressure, and increased capillary permeability.

Hypoproteinemia may result from liver disease causing failure to synthesize plasma proteins; kidney disease (glomerulonephritis) that damages glomerular capillaries, allowing proteins to escape into the urine; or starvation, in which the intake of amino acids is insufficient to support synthesis of plasma proteins.

In each of these instances, the plasma protein concentration is decreased, which decreases plasma

(2) Solute concentration of extracellular fluid compartment decreases

Cell membrane

osmotic pressure, reducing the normal return of tissue fluid to the venular ends of capillaries. Consequently, tissue fluid accumulates in the interstitial spaces.

As discussed in chapter 16 (p. 655), lymphatic obstructions may result from surgery or from parasitic infections of lymphatic vessels. Back pressure develops in the lymphatic vessels, which interferes with the normal movement of tissue fluid into them. At the same time, proteins that the lymphatic circulation ordinarily removes accumulate in the interstitial spaces, raising osmotic pressure of the intersti-

(1) Excess water is added to extracellular fluid compartment

(2) Solute concentration of extracellular fluid compartment decreases

(3) Water moves into intracellular fluid compartment by osmosis

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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