Human Physiology, 14th edition (2016)

460 Chapter 14

the absence of dilution) can reduce ADH secretion. This is

because an increased blood volume mechanically stimulates

stretch receptors in the left atrium, aortic arch, and carotid

sinus, which in turn cause increased firing of sensory neurons

(in cranial nerves IX and X). ADH secretion is inhibited by this

sensory input, so that more water is eliminated from the blood

by the kidneys.

Conversely, a lowering of blood volume by about 10%

reduces stimulation of these stretch receptors, reducing the

firing of their associated sensory neurons. This produces an

increase in ADH secretion, which stimulates the kidneys to

retain more water in the blood. These negative feedback loops

thereby help to maintain homeostasis of blood volume.

When blood volume rises, the stimulation of stretch recep-

tors in the atria of the heart has an additional effect: it stimu-

lates the atria to secrete a hormone known as atrial natriuretic

peptide. This hormone increases the excretion of salt and water

in the urine, thereby working, like a decrease in ADH secre-

tion, to lower blood volume. Atrial natriuretic peptide is dis-

cussed in a separate section.

specific hormones on the kidneys. Through their effects on the
kidneys and the resulting changes in blood volume, these hor-
mones serve important functions in the regulation of the car-
diovascular system.
The sympathetic nervous system is also involved in the
homeostasis of blood volume. An increase in blood volume is
detected by stretch receptors in the atria of the heart, which
selectively regulate sympathetic nerve activity. The activity of
sympathetic fibers to the heart is increased, while the sympa-
thetic nerve activity to the kidneys is reduced (there is little
change in sympathetic nerve activity to other organs). Reduced
sympathetic nerve stimulation of renal arteries produces vaso-
dilation and increased blood flow, thereby promoting increased
urine production to lower the blood volume and complete the
negative feedback loop.

Regulation by Antidiuretic Hormone (ADH)

One of the major hormones involved in the regulation of blood
volume is antidiuretic hormone (ADH), also known as vaso-
pressin. This hormone is produced by neurons in the hypo-
thalamus, transported by axons into the posterior pituitary, and
released from this storage gland in response to hypothalamic
stimulation (chapter 11, section 11.3). The release of ADH from
the posterior pituitary occurs when neurons in the hypothalamus
called osmoreceptors detect an increase in plasma osmolality
( fig. 14.11 ).
An increase in plasma osmolality occurs when the plasma
becomes more concentrated. This can be produced either by
dehydration or by excessive salt intake. Stimulation of osmore-
ceptors produces sensations of thirst, leading to increased water
intake and an increase in the amount of ADH released from the
posterior pituitary. Through mechanisms that will be discussed in
conjunction with kidney physiology (chapter 17, section 17.3),
ADH stimulates water reabsorption from the filtrate. A smaller
volume of urine is thus excreted as a result of the action of ADH
( fig. 14.11 ).
A person who is dehydrated or who consumes excessive
amounts of salt thus drinks more and urinates less. This raises
the blood volume and, in the process, dilutes the plasma to
lower its previously elevated osmolality. The rise in blood vol-
ume that results from these mechanisms is extremely impor-
tant in stabilizing the condition of a dehydrated person with
low blood volume and pressure.
Drinking excessive amounts of water without excessive
amounts of salt does not result in a prolonged increase in blood
volume and pressure. The water does enter the blood from the
intestine and momentarily raises the blood volume; at the same
time, however, it dilutes the blood. Dilution of the blood decreases
the plasma osmolality and thus inhibits the release of ADH. With
less ADH there is less reabsorption of water in the kidneys—a
larger volume of more-dilute urine is excreted. Water is therefore
a diuretic —a substance that promotes urine formation—because
it inhibits the release of antidiuretic hormone.
Dilution of the blood (decreased blood osmolality) low-
ers ADH secretion, but a rise in blood volume itself (even in

Figure 14.11 The negative feedback control

of blood volume and blood osmolality. Thirst and

ADH secretion are triggered by a rise in plasma osmolality.

Homeostasis is maintained by countermeasures, including

drinking and conservation of water by the kidneys.

Stimuli Salt ingestion

Osmoreceptors in

hypothalamus

Posterior

pituitary

Thirst

Water retention

by kidneys Drinking

Blood volume

Blood osmolality

Negative feedback

responses

Blood osmolality

ADH

Dehydration

( Blood volume)

Sensor

Integrating center

Effector