Page 348 - Anatomy and Physiology of Farm Animals, 8th Edition

Physiology of the Heart and Circulation / 333

ventricle of the heart due to the great deal networks and effectively increases the
number of tubes, resulting in a relatively
of elastic tissue contained within it. At the
VetBooks.ir end of contraction (diastole), the walls of low resistance. The greatest contribution
to total vascular resistance in the sys­
the aorta recoil and help move the arterial
blood through the body via the arterial sys­ temic circulation is from arterioles that
tem. The mean blood pressure is therefore are located just prior to the capillaries
an average of systolic and diastolic blood (Fig. 18‐2). Arterioles have very muscular
pressures. Blood flows from a point of high walls and are able to shunt blood effec­
mean pressure to a point of low mean pres­ tively from one organ system to another in
sure. In the systemic circulation, mean times of high metabolic demand. For
blood pressure is higher in the arteries example, when exercising, the arterioles
than in the capillaries and higher in the adjacent to the intestines will contract
capillaries than in the veins, from which whereas those adjacent to skeletal muscle
blood re‐enters the right side of the heart will relax to shunt blood effectively to
(Fig. 18‐2). A series of one‐way valves those large muscle groups. This regulation
described in Chapter 17 regulates blood occurs via the autonomic nervous system
flow through the heart. but can be mimicked pharmacologically. In
The driving force of blood pressure that the systemic circulation, once blood has
is created during systole is necessary to moved through the capillaries into the
overcome the vascular resistance pro­ venous system, it has entered a low pres­
vided by the blood vessels. Any tube offers sure system with high capacitance.
a resistance to the flow of liquid through it. Transport is the ultimate function of
The resistance (R) to flow through a single the cardiovascular system. Blood is the
tube depends on the length (L) of the tube, transport medium; the heart provides the
the radius (r) of the tube, and the charac­ force for moving blood (i.e., pump func­
ter of the fluid flowing through the tube tion) around the circulation; and vessels
(viscosity, η). The resistance increases with provide a path for the movement and
the length of the tube, decreases as the permit exchange between blood and inter­
radius of the tube increases, and increases stitial fluids at the level of the capillaries.
with the viscosity of the fluid. While all The rate of transport and exchange is usu­
three factors can affect resistance, changes ally determined by the rate of blood flow
in the radius have the largest effect, as through the capillaries.
shown in the following formula. (Poiseuille
first described this formula and the math­
ematical relations between these factors.) Cardiac Cycle
8 L
R The cardiac cycle is one complete cycle
r 4 (heartbeat) of cardiac contraction and
According to this formula alone, the relaxation. The events of the cardiac cycle
vessels with the smallest radius, the capil­ occur in a specific sequence, and for
laries, would have the greatest resistance. descriptive purposes, the continuous cycle
This is true for a single vessel, but it is not is divided into phases or periods marked
true when considering the total combined by different events. Figure 18‐3 illustrates
resistance for the different types of vessels the changes in blood pressures and vol­
in the systemic circulation. Rather, the umes in the left atrium and left ventricle
total resistance at the level of the capillar­ and pressure changes in the aorta during
ies is less because of the extensive branch­ the cardiac cycle, and it identifies some
ing in capillary networks. While each phases of the cycle. The sequence of
vessel in the capillaries has a very small changes in blood pressures and volumes in
diameter, this branching increases the total the right atrium, right ventricle, and pul­
surface area of the capillaries in their monary trunk are similar and occur in the