396
U N I T 5
Circulatory Function
volume and causes blood to accumulate in the systemic
veins and right heart.
The veins and venules are thin-walled, distensible,
and collapsible vessels. The veins are capable of enlarg-
ing and storing large quantities of blood, which can be
made available to the circulation as needed. Even though
the veins are thin walled, they are muscular. This allows
them to contract or expand to accommodate varying
amounts of blood. Veins are innervated by the sympa-
thetic nervous system. When blood is lost from the cir-
culation, the veins constrict as a means of maintaining
the circulating blood volume.
Because the venous system is a low-pressure system,
blood flow must oppose the effects of gravity. In a per-
son in the standing position, the weight of the blood in
the vascular column causes an increase of 1 mm Hg in
pressure for every 13.6 mm of distance below the level
of the heart. Were it not for the valves in the veins and
the action of the skeletal muscles, the venous pressure
in the feet would be about +90 mm Hg in the standing
adult. Gravity has no effect on the venous pressure in a
person in the recumbent position because the blood in
the veins is then at the level of the heart.
Valves in the veins of extremities counteract the effects
of gravity (Fig. 17-19), and with the help of skeletal
muscles that surround and intermittently compress the
leg veins in a milking manner, move blood forward to
the heart. This pumping action is known as the
venous
or
muscle pump
and is efficient enough that under nor-
mal circumstances, the pressure in the feet of a walking
adult remains less than 20 mm Hg. There are no valves
in the abdominal or thoracic veins, and blood flow in
these veins is heavily influenced by the pressure in the
abdominal and thoracic cavities, respectively.
Capillaries
Capillaries are microscopic, single-cell–thick vessels that
connect the arterial and venous segments of the circula-
tion. The capillary wall is composed of a single layer of
endothelial cells surrounded by a basement membrane or
basal lamina (Fig. 17-20). Intercellular junctions join the
capillary endothelial cells; these are called the
capillary
pores.
Lipid-soluble materials diffuse directly through the
capillary cell membrane. Water and water-soluble mate-
rials leave and enter the capillary through the capillary
pores. The size of the capillary pores varies with capillary
function. In the brain, the endothelial cells are joined by
tight junctions that form the blood–brain barrier. This
prevents substances that would alter neural excitability
from leaving the capillary. In organs that process blood
contents, such as the liver, capillaries have large pores
so that substances can pass easily through the capillary
wall. In the kidneys, the glomerular capillaries have small
openings called
fenestrations
that pass directly through
the middle of the endothelial cells, a feature that is con-
sistent with the filtration function of the glomerulus.
Lymphatic System
The lymphatic system, commonly called the
lymphatics,
represents an accessory route through which fluid can
flow through the tissue spaces into the blood and then
Open valve
Closed
valve
Toward heart
FIGURE 17-19.
Portion of a femoral vein opened, to show the
valves.The direction of flow is upward. Backward flow closes
the valve.
Lumen
Red blood cell
Nucleus of
endothelial cell
Basal lamina (cut)
Capillary
pores
Endothelial cell
Intercellular junctions
FIGURE 17-20.
Endothelial cells and intercellular junctions in
a section of capillary.
