Page 359 - Anatomy and Physiology of Farm Animals, 8th Edition

344 / Anatomy and Physiology of Farm Animals

as a low‐pressure reservoir of blood and combination of these determinants. Any
chemical regulator (e.g., hormone or par­
are also considered the capacitance vessels.
VetBooks.ir Constriction of venous smooth muscle acrine agent) or neural reflex that affects
cardiac activity or the smooth muscle of
promotes an increase in blood flowing
back to the heart and an increase in cardiac arterioles has the potential to alter blood
filling pressure. This contributes to pressure. The large number of therapeutic
increases in cardiac output and the ability agents used to treat high blood pressure in
to perfuse capillaries for exchange. humans and animals, and the different
mechanisms by which these agents have
their effects, illustrate the diversity of
Regulation of Arterial Blood factors that can alter blood pressure.
Pressure and Blood Volume In normal animals, blood volume and
arterial blood pressure are directly related.
Arterial blood pressure is a function of Increases or decreases in blood volume
cardiac output (CO) and total peripheral tend to produce similar changes in cardiac
vascular resistance (TPR), usually written output and therefore in arterial blood pres­
as MAP = CO × TPR, where MAP is mean sure. In light of this relation and the goal of
arterial pressure. To understand the basis biologic systems to maintain homeostasis,
for this function, recall that arteries are it is predictable that decreases in blood
relatively noncompliant and function as a pressure elicit physiologic responses
reservoir of blood under pressure. The designed to increase blood volume and
pressure depends on the amount of blood increases in blood pressure elicit responses
being pumped into the reservoir (CO) and designed to reduce blood volume. The
the rate at which blood is permitted to flow organs primarily responsible for bringing
out of the reservoir (Fig. 18‐10). Recall also about changes in blood volume are the
that the resistance of the arterioles, which kidneys. Thus, blood volume regulation by
contribute the most to the total vascular the kidneys is one factor in the ultimate
resistance, regulates the rate of blood flow determination of arterial blood pressure.
out of the arteries. Thus, the level of car­ Many of the chemical agents and neural
diac function and the degree of arteriolar reflexes that regulate mean arterial pres­
constriction are the two determinants of sure by their action on the heart and blood
MAP. Changes in MAP can be brought vessels also affect the ability of the kidneys
about by changes in cardiac function, the to regulate blood volume by altering the
degree of arteriolar constriction, or some rate of urine formation.

Arterial pressure
Pump

Venous pressure

Veins Capillaries Arteries
Resistance

Figure 18-10. Simplified model of the circulatory system with pump supplying low‐compliance vessels
(arteries), off‐on valve vessels regulating outflow from arteries (arterioles), and low‐pressure compliant
vessels (veins) returning fluid to the pump. Height of fluid columns in arteries and veins indicates relative
pressures in these vessels. Filling of the pump depends in part on the venous pressure.