Ganong's Review of Medical Physiology, 23rd Edition

276
SECTION III
Central & Peripheral Neurophysiology

hypothalamus are part of more complex phenomena such as eat-
ing, and emotions such as rage. For example, stimulation of vari-
ous parts of the hypothalamus, especially the lateral areas,
produces diffuse sympathetic discharge and increased adrenal
medullary secretion, the mass sympathetic discharge seen in ani-
mals exposed to stress (the flight or fight reaction; see Chapter 17).
It has been claimed that separate hypothalamic areas con-
trol epinephrine and norepinephrine secretion. Differential
secretion of one or the other of these adrenal medullary cate-
cholamines does occur in certain situations (see Chapter 22),
but the selective increases are small.
Body weight depends on the balance between caloric intake
and utilization of calories. Obesity results when the former
exceeds the latter. The hypothalamus and related parts of the
brain play a key role in the regulation of food intake. Obesity
is considered in detail in Chapter 27, and the relation of obe-
sity to diabetes mellitus is discussed in Chapter 21.
Hypothalamic regulation of sleep and circadian rhythms
are discussed in Chapter 15.

THIRST

Another appetitive mechanism under hypothalamic control is
thirst. Drinking is regulated by plasma osmolality and extra-
cellular fluid (ECF) volume in much the same fashion as vaso-
pressin secretion. Water intake is increased by increased
effective osmotic pressure of the plasma (Figure 18–3), by de-
creases in ECF volume, and by psychologic and other factors.
Osmolality acts via
osmoreceptors,
receptors that sense the
osmolality of the body fluids. These osmoreceptors are located in
the anterior hypothalamus.

Decreases in ECF volume also stimulate thirst by a pathway

independent of that mediating thirst in response to increased

plasma osmolality (Figure 18–4). Thus, hemorrhage causes

increased drinking even if there is no change in the osmolality

of the plasma. The effect of ECF volume depletion on thirst is

mediated in part via the renin–angiotensin system (see Chap-

ter 39). Renin secretion is increased by hypovolemia and

results in an increase in circulating angiotensin II. The angio-

tensin II acts on the

subfornical organ,

a specialized receptor

area in the diencephalon (see Figure 34–7), to stimulate the

neural areas concerned with thirst. Some evidence suggests

that it acts on the

organum vasculosum of the lamina termi-

nalis (OVLT)

as well. These areas are highly permeable and

are two of the circumventricular organs located outside the

blood–brain barrier (see Chapter 34). However, drugs that

block the action of angiotensin II do not completely block the

thirst response to hypovolemia, and it appears that the barore-

ceptors in the heart and blood vessels are also involved.

The intake of liquids is increased during eating

(prandial

drinking).

The increase has been called a learned or habit

response, but it has not been investigated in detail. One factor

is an increase in plasma osmolality that occurs as food is

absorbed. Another may be an action of one or more gas-

trointestinal hormones on the hypothalamus.

When the sensation of thirst is obtunded, either by direct dam-

age to the diencephalon or by depressed or altered states of con-

sciousness, patients stop drinking adequate amounts of fluid.

Dehydration results if appropriate measures are not instituted to

maintain water balance. If the protein intake is high, the products

of protein metabolism cause an osmotic diuresis (see Chapter

38), and the amounts of water required to maintain hydration are

large. Most cases of

hypernatremia

are actually due to simple

dehydration in patients with psychoses or hypothalamic disease

who do not or cannot increase their water intake when their

thirst mechanism is stimulated. Lesions of the anterior commu-

nicating artery can also obtund thirst because branches of this

artery supply the hypothalamic areas concerned with thirst.

FIGURE 18–3
Relation of plasma osmolality to thirst in
healthy adult humans during infusion of hypertonic saline.
The
intensity of thirst is measured on a special analog scale.
(Reproduced
with permission from Thompson CJ et al: The osmotic thresholds for thirst and
vasopressin release are similar in healthy humans. Clin Sci Lond 1986;71:651.)

10

8

6

4

2

0

280 300 320

Plasma osmolality (mosm/kg)

Intensity of thirst

FIGURE 18–4

Diagrammatic representation of the way in

which changes in plasma osmolality and changes in ECF volume

affect thirst by separate pathways.

Hypertonicity

Osmoreceptors

Hypovolemia

Hypothalamus

Thirst

Baroreceptors

Angiotensin II