DEHYDRATION, THIRST, AND WATER INTAKE

1. In dehydration, what is the immediate source (compartment) of water lost from the body?

2. For most animals, what is considered to be a severe loss of body water?

3. With a continuing loss of water (dehydration), is there a proportionate loss of electrolytes?

4.

5. Where is the thirst center located?

6. How does dehydration stimulate thirst?

7. How does hypovolemia stimulate thirst?

8. How can thirst be temporarily relieved?

When water losses exceed water gains, a condition known as dehydration develops. The extent is variable and, when mild, physiologic mechanisms may be adequate to reestablish water balance via the thirst mechanism if water is available. Therapeutic measures (fluid replacement, treatment of underlying cause) may be necessary when water losses,are moderate to severe and related to a disease condition.

Dehydration

In dehydration, the immediate source of water lost from the body is the ECF, followed by a shift from the intracellular to the extracellular fluid. A loss of water equal to 10% of body weight is considered to be severe for most animals. The concentrations of electrolytes (ions) in the body fluids do not continue to increase during dehydration, but are excreted by the kidney in proportion to the water loss. With continuing dehydration, water and electrolytes are depleted. Therefore, rehydration requires not only water but also appropriate electrolytes.

Stimulus for Thirst

When water losses exceed water gain, there is an effort on the part of the kidneys to conserve water. Also, animals are provided with a thirst mechanism to recognize the need for water intake greater than that provided by food and metabolic water. Thirst is the conscious desire for water. Central to the thirst mechanism is a thirst center located in the hypothalamus (see Chapter 4) of the brain and represented by thirst cells.

Another stimulus of thirst is the kidney hormone angiotensin II (see Chapter 11). This is formed in response to low blood pressure to bring about changes to increase blood pressure (e.g., salt retention, peripheral vasoconstriction, water ingestion). Loss of blood volume (hypovolemia), as in hemorrhage (an isotonic fluid loss), results in lowered blood pressure and angiotensin II is formed. The thirst stimulation previously described causes an animal to drink water, which is subsequently absorbed, and blood volume and blood pressure are restored toward normal.

Relief of Thirst

An experiment can be performed with a dog to show the effect of dehydration on thirst stimulation. A hypertonic NaCl solution is slowly injected intravenously, which increases the osmoconcentration of plasma and subsequently that of the thirst cells in the hypothalamus. After a few minutes, water that was previously offered to the dog and ignored is now consumed. The amount consumed is approximately equal to the amount that would have been needed to make the hypertonic plasma isotonic. Even though there was insufficient time for the water ingested to be absorbed, the dog’s thirst was temporarily relieved because temporary relief can occur when the mouth and pharynx are wetted and when the stomach is distended following the ingestion of water. Both of these temporary relief methods help to prevent overingestion that would otherwise occur because a brief time is required after either method for water to be absorbed and to lower the osmoconcentration of thirst cell or to increase blood pressure, depending on what stimulus produced the thirst. Thirst is an important mechanism for maintaining water balance. Water must be adequately provided for animals or ill health, discomfort, and loss of production will be observed.

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