Essentials of Anatomy and Physiology

fluids. Osmolarityis the concentration of dissolved
materials present in a fluid. Dehydration raises the
osmolarity of the blood; that is, there is less water in
proportion to the amount of dissolved materials.
Another way to express this is to simply say that the
blood is now a more concentrated solution. When
dehydrated, we feel the sensation of thirst, character-
ized by dryness of the mouth and throat, as less saliva
is produced. Thirst is an uncomfortable sensation, and
we drink fluids to relieve it. The water we drink is
readily absorbed by the mucosa of the stomach and
small intestine and has the effect of decreasing the
osmolarity of the blood. In other words, we can say
that the water we just drank is causing the blood to
become a more dilute solution, and, as the serum

osmolarity returns to normal, the sensation of thirst

diminishes.

As you may recall, the hypothalamus is also

involved in water balance because of its production

of antidiuretic hormone (ADH), which is stored in the

posterior pituitary gland. In a state of dehydration, the

hypothalamus stimulates the release of ADH from

the posterior pituitary. Antidiuretic hormone then

increases the reabsorption of water by the kidney

tubules. Water is returned to the blood to preserve

blood volume, and urinary output decreases.

The hormone aldosterone, from the adrenal cortex,

also helps regulate water output. Aldosterone in-

creases the reabsorption of Naions by the kidney

tubules, and water from the renal filtrate follows the

444 Fluid–Electrolyte and Acid–Base Balance

Metabolic water

Food

Liquids

Output Input

Urine

Sweat

Exhaled air

Feces

Figure 19–2. Water intake

and output. See text and

Table 19–1 for description.

QUESTION: On a very hot

day, how might the output

volumes change?