414
Chapter 12
the fl uid lost involves the control of fl uid ingestion and kidney
function. These slower-acting compensations are mediated by
hormones, including renin, angiotensin and aldosterone, and are
described in Chapter 14. Replacement of the lost erythrocytes
requires the hormone
erythropoietin
to stimulate
erythropoi-
esis
(maturation of immature red blood cells); this is described
in detail in the last section of this chapter. These replacement
processes require days to weeks in contrast to the rapidly occur-
ring refl ex compensations illustrated in Figure 12–59.
Hemorrhage is a striking example of hypotension due
to a decrease in blood volume. There is a second way, how-
ever, that hypotension can occur due to volume depletion that
does not result from loss of whole blood. In such cases the
loss is of salts—particularly sodium (along with chloride or
bicarbonate)—and water. Such fl uid loss may occur through
the skin, as in severe sweating or burns. It may occur via the
gastrointestinal tract, as through diarrhea or vomiting, or by
unusually large urinary losses. Regardless of the route, the
loss of fl uid decreases circulating blood volume and produces
symptoms and compensatory cardiovascular changes similar
to those seen in hemorrhage.
Hypotension may also be caused by events other than
blood or fl uid loss. One major cause is a depression of cardiac
pumping ability (for example, during a heart attack).
Another cause is strong emotion, which in rare cases can
cause hypotension and fainting. Somehow, the higher brain
centers involved with emotions inhibit sympathetic activity
to the cardiovascular system and enhance parasympathetic
activity to the heart, resulting in a markedly decreased arterial
pressure and brain blood fl ow. This whole process, known as
vasovagal syncope,
is usually transient. It should be noted that
the fainting that sometimes occurs in a person donating blood
is usually due to hypotension brought on by emotion, not the
blood loss, because losing 0.5 L of blood will not generally
cause serious hypotension.
Massive release of endogenous substances that relax
arteriolar smooth muscle may also cause hypotension by
reducing total peripheral resistance. An important example is
the hypotension that occurs during severe allergic responses
(Chapter 18).
Shock
The term
shock
denotes any situation in which a decrease in
blood fl ow to the organs and tissues damages them. Arterial
pressure is usually, but not always, low in shock, and the clas-
sifi cation of shock is quite similar to what we have already seen
for hypotension.
Hypovolemic shock
is caused by a decrease in
blood volume secondary to hemorrhage or loss of fl
uid other
than blood.
Low-resistance shock
is due to a decrease in total
peripheral resistance secondary to excessive release of vasodila-
tors, as in allergy and infection.
Cardiogenic shock
is due to
an extreme decrease in cardiac output from any of a variety of
factors (for example, during a heart attack).
The cardiovascular system, especially the heart, suffers
damage if shock is prolonged. As the heart deteriorates, car-
diac output further declines and shock becomes progressively
worse. Ultimately, shock may become irreversible even though
blood transfusions and other appropriate therapy may tem-
porarily restore blood pressure. See Chapter 19 for a detailed
description of a clinical case involving low-resistance shock.
The Upright Posture
A decrease in the effective circulating blood volume occurs in
the circulatory system when going from a lying, horizontal posi-
tion to a standing, vertical one. Why this is so requires an under-
standing of the action of gravity upon the long, continuous
columns of blood in the vessels between the heart and the feet.
The pressures we have given in previous sections of this
chapter are for an individual in the horizontal position, in
which all blood vessels are at approximately the same level as
the heart. In this position, the weight of the blood produces
negligible pressure. In contrast, when a person is vertical, the
intravascular pressure everywhere becomes equal to the pres-
sure generated by cardiac contraction plus an additional pres-
sure equal to the weight of a column of blood from the heart
to the point of measurement. In an average adult, for example,
the weight of a column of blood extending from the heart to
the feet amounts to 80 mmHg. In a foot capillary, therefore,
the pressure increases from 25 (the average capillary pressure
resulting from cardiac contraction) to 105 mmHg, the extra
80 mmHg being due to the weight of the column of blood.
This increase in pressure due to gravity infl uences the
effective circulating blood volume in several ways. First, the
increased hydrostatic pressure that occurs in the legs (as well
as the buttocks and pelvic area) when a person is standing
pushes outward on the highly distensible vein walls, causing
marked distension. The result is pooling of blood in the veins;
that is, much of the blood emerging from the capillaries sim-
ply goes into expanding the veins rather than returning to the
heart. Simultaneously, the increase in capillary pressure caused
by the gravitational force produces increased fi ltration of fl
uid
Table 12–6
Fluid Shifts After Hemorrhage
Normal
Immediately
After
Hemorrhage
18 h After
Hemorrhage
Total blood
volume, ml
5000
4000
4900
Erythrocyte
volume, ml
2300
1840
1840
Plasma
volume, ml
2700
2160
3060
Table 12–6
physiological
inquiry
Calculate the hematocrit before and 18 hours after the
hemorrhage, and explain the changes that are observed.
Answer can be found at end of chapter.
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