The amount of tissue fluid formed is slightly
greater than the amount returned to the capillaries. If
this were to continue, blood volume would be gradu-
ally depleted. The excess tissue fluid, however, enters
lymph capillaries. Now called lymph, it will be
returned to the blood to be recycled again as plasma,
thus maintaining blood volume. This is discussed fur-
ther in Chapter 14.
PATHWAYS OF CIRCULATION
The two major pathways of circulation are pulmonary
and systemic. Pulmonary circulation begins at the
right ventricle, and systemic circulation begins at the
left ventricle. Hepatic portal circulation is a special
segment of systemic circulation that will be covered
separately. Fetal circulation involves pathways that are
present only before birth and will also be discussed
separately.
PULMONARY CIRCULATION
The right ventricle pumps blood into the pulmonary
artery (or trunk), which divides into the right and left
pulmonary arteries, one going to each lung. Within
the lungs each artery branches extensively into smaller
arteries and arterioles, then to capillaries. The pul-
monary capillaries surround the alveoli of the lungs; it
is here that exchanges of oxygen and carbon dioxide
take place. The capillaries unite to form venules,
which merge into veins, and finally into the two pul-
monary veins from each lung that return blood to the
left atrium. This oxygenated blood will then travel
through the systemic circulation. (Notice that the pul-
monary veins contain oxygenated blood; these are the
only veins that carry blood with a high oxygen con-
tent. The blood in systemic veins has a low oxygen
content; it is systemic arteries that carry oxygenated
blood.)
SYSTEMIC CIRCULATION
The left ventricle pumps blood into the aorta, the
largest artery of the body. We will return to the aorta
and its branches in a moment, but first we will sum-
marize the rest of systemic circulation. The branches
of the aorta take blood into arterioles and capillary net-
works throughout the body. Capillaries merge to form
venules and veins. The veins from the lower body take
blood to the inferior vena cava; veins from the upper
body take blood to the superior vena cava. These two
caval veins return blood to the right atrium. The major
arteries and veins are shown in Figs. 13–3 to 13–5, and
their functions are listed in Tables 13–1 and 13–2.
The aorta is a continuous vessel, but for the sake of
precise description it is divided into sections that are
named anatomically: ascending aorta, aortic arch, tho-
racic aorta, and abdominal aorta. The ascending aorta
is the first inch that emerges from the top of the left
ventricle. The arch of the aorta curves posteriorly over
the heart and turns downward. The thoracic aorta
continues down through the chest cavity and through
the diaphragm. Below the level of the diaphragm, the
abdominal aorta continues to the level of the 4th lum-
bar vertebra, where it divides into the two common
iliac arteries. Along its course, the aorta has many
branches through which blood travels to specific
organs and parts of the body.
The ascending aorta has only two branches: the
right and left coronary arteries, which supply blood to
the myocardium. This pathway of circulation was
described previously in Chapter 12.
The aortic arch has three branches that supply
blood to the head and arms: the brachiocephalic
artery, left common carotid artery, and left subclavian
artery. The brachiocephalic (literally, “arm-head”)
artery is very short and divides into the right common
carotid artery and right subclavian artery. The right
and left common carotid arteries extend into the neck,
where each divides into an internal carotid artery and
external carotid artery, which supply the head. The
right and left subclavian arteries are in the shoulders
behind the clavicles and continue into the arms. As the
artery enters another body area (it may not “branch,”
simply continue), its name changes: The subclavian
artery becomes the axillary artery, which becomes the
brachial artery. The branches of the carotid and sub-
clavian arteries are diagrammed in Figs. 13–3 and
13–5. As you look at these diagrams, keep in mind that
the name of the vessel often tells us where it is. The
facial artery, for example, is found in the face.
Some of the arteries in the head contribute to an
important arterial anastomosis, the circle of Willis
(or cerebral arterial circle), which is a “circle” of arter-
ies around the pituitary gland (Fig. 13–6). The circle
of Willis is formed by the right and left internal
carotid arteries and the basilar artery, which is the
union of the right and left vertebral arteries (branches
of the subclavian arteries). The brain is always active,296 The Vascular System
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