HUMAN BIOLOGY

CirCulation: the heart and blood vessels 127

Jose Luis Pelaez, Inc./Bridge/Corbis

What are the tWo circuits in Which blood

floWs through the body?

• Blood flows through a pulmonary circuit and a systemic circuit.


• The pulmonary circuit carries blood through the lungs for gas
  exchange. The systemic circuit transports blood to and from
  tissues.


• After meals, the blood in capillary beds in the digestive tract is
  diverted to the liver for processing. Blood then returns to the
  general circulation.

taKe-hoMe Message

Figure 7.8 Blood from the digestive tract detours to the liver.

Arrows show the direction in which blood flows. (© Cengage Learning)

inferior

vena cava

hepatic vein

liver capillary

beds

liver stomach

gallbladder

spleen

pancreas

large

intestine

hepatic

portal

vein

large

intestine

(cut away)

small

intestine

produced. For example, in a resting person, each minute
a fifth of the blood pumped into the systemic circula-
tion enters the kidneys (Figure 7.6C) via renal arteries.
Deoxygenated blood returns to the right half of the heart,
where it enters the pulmonary circuit. Notice that in both
the pulmonary and the systemic circuits, blood trav-
els through arteries, arterioles, capillaries, and venules,
finally returning to the heart in veins. Blood from the
head, arms, and chest arrives through the superior vena
cava. The inferior vena cava collects blood from the lower
part of the body.
Because the heart pumps constantly, the volume of
flow through the entire system each minute is equal to the
volume of blood returned to the heart each minute.

subsets of systemic vessels serve
the heart and liver

Arteries and veins that serve only the heart provide what
is called the coronary circulation. Two coronary arteries
service most of the cardiac muscle (Figure 7.7). They branch
off the aorta, the major artery carrying blood away from the
heart. Coronary veins empty blood into the right atrium.
Blood passing through capillary beds in the digestive
tract travels to another capillary bed in the liver (Figure
7.8). After a meal, the hepatic portal vein brings nutrient-
laden blood to this capillary bed. As blood seeps through
it, the liver can remove impurities and process absorbed
substances. The vessels involved in this detour collectively
are called the hepatic portal system. You will read more
about this topic in Chapter 11.
Blood leaving the liver’s capillary bed enters the
general circulation through a hepatic vein. The liver
receives oxygenated blood via the hepatic artery.

superior

vena cava

inferior vena cava

right

coronary

artery

cardiac vein

aorta

(left

pulmonary

artery)

left coronary

artery

(left pulmonary

veins)

cardiac vein

Figure 7.7 Coronary arteries and veins

serve the heart. The upper (superior) vena

cava returns blood from the upper body to the

right atrium. The lower (inferior) vena cava returns

blood to the right atrium from areas below the heart. “Vena

cava” means hollow vessel. The inset at right shows a resin cast of

coronary circulation vessels.

© Cengage Learning

© Lester V. Bergman/Corbis

lungs

all other regions

cardiac muscle

bone

skin

brain

skeletal muscle

kidneys

liver

digestive tract

heart’s right half heart’s left half

100%

21%

6%
20%
15%
13%
9%
5%
3%
8%
C

© Cengage Learning

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