C h a p t e r 8
Disorders of Fluid, Electrolyte, and Acid–Base Balance
169
the
effective circulating volume
, which can be described
as that portion of the ECF that fills the vascular com-
partment and is “effectively” perfusing the tissues.
1,2
The effective circulating volume is monitored by sen-
sors that are located both in the vascular system and
the kidney.
While thirst and ADH are the main regulators of
water intake and output, the sympathetic nervous
system and the renin-angiotensin-aldosterone sys-
tem function in the regulation of sodium balance by
the kidneys (see Chapter 18). The
sympathetic ner-
vous system
responds to changes in arterial pressure
and blood volume by adjusting the glomerular fil-
tration rate and the rate at which sodium is filtered
from the blood. Sympathetic activity also regulates
renal reabsorption of sodium and renin release. The
renin-angiotensin-aldosterone system
exerts its action
through angiotensin II and aldosterone. Angiotensin II
acts directly on the renal tubules to increase sodium
reabsorption. It also acts to constrict renal blood ves-
sels, thereby decreasing the glomerular filtration rate
and slowing renal blood flow so that less sodium is
filtered and more is reabsorbed. Angiotensin II is also a
powerful regulator of
aldosterone
, a hormone secreted
by the adrenal cortex. Aldosterone acts to increase
sodium reabsorption by the kidneys, while increasing
potassium elimination.
Thirst and Disorders of Thirst
Like appetite and eating, thirst and drinking are two
separate entities.
1,2,8,9
Thirst is the conscious sensation
of the need to obtain and drink fluids high in water
content. Drinking water or other fluids often occurs
as the result of habit or for reasons other than those
related to thirst. Most people drink without being
thirsty, and water is consumed before it is needed. As
a result, thirst is basically an emergency response. It
usually occurs only when the need for water has not
been anticipated.
Thirst is controlled by the thirst center in the hypo-
thalamus. There are two stimuli for true thirst based
on water need: (1) cellular dehydration caused by an
increase in ECF osmolality, and (2) a decrease in the
effective circulating volume, which may or may not be
associated with a decrease in serum osmolality. Sensory
neurons, called
osmoreceptors
, which are located in
or near the thirst center in the hypothalamus, respond
to changes in ECF osmolality by swelling or shrinking
(Fig. 8-7). Thirst normally develops when there is
as little as a 1% to 2% change in serum osmolality.
9
Stretch receptors in the vascular system that monitor the
effective circulating volume also aid in the regulation
of thirst. Thus, thirst is one of the earliest symptoms of
hemorrhage and is often present before other signs of
blood loss appear.
A third stimulus, the production of angiotensin II by
the renin-angiotensin mechanism in the kidney, func-
tions in the production of nonosmotic thirst. Angiotensin
II increases in response to low blood volume and low
blood pressure. This system is considered a backup
system for thirst should other systems fail. Because it is
a backup system, it probably does not contribute to the
regulation of normal thirst. However, elevated levels of
angiotensin II may lead to thirst in conditions such as
congestive heart failure and chronic kidney disease, in
which a decrease in renal blood flow leads to increased
renin levels.
Hypodipsia.
Hypodipsia represents a decrease in
the ability to sense thirst. Water deficit is commonly
associated with lesions in the area of the hypo-
thalamus (e.g., head trauma, meningiomas, occult
hydrocephalus, subarachnoid hemorrhage). There
is also evidence that thirst is decreased and water
intake reduced in elderly persons, despite higher
Hypothalamo-
hypophysial tract
Anterior lobe
Posterior lobe
Pituitary
gland
A
B
Supraoptic
nucleus
Osmoreceptors
Paraventricular
nucleus
Capillary
plexus
Blood volume
Secretion of
ADH
Reabsorption of
water by the kidney
Feedback
Extracellular
water volume
Thirst
Water ingestion
Serum osmolality
FIGURE 8-7.
(A)
Sagittal section through the pituitary and
anterior hypothalamus. Antidiuretic hormone (ADH) is formed
primarily in the supraoptic nucleus and to a lesser extent in
the paraventricular nucleus of the hypothalamus. It is then
transported down the hypothalamohypophysial tract and
stored in secretory granules in the posterior pituitary, where it
can be released into the blood.
(B)
Pathways for regulation of
extracellular water volume by thirst and antidiuretic hormone.
ADH, antidiuretic hormone.
