422 Chapter 13
13.4 Cardiac Cycle
The two atria fill with blood and then contract simultane-
ously. This is followed by simultaneous contraction of both
ventricles, which sends blood through the pulmonary and
systemic circulations. Pressure changes in the atria and
ventricles as they go through the cardiac cycle are respon-
sible for the flow of blood through the heart chambers and
out into the arteries.CLINICAL APPLICATION
In a fetus, there is an opening called the foramen ovale
( fig. 13.14 ) between the left and right atria. Blood flows
from the right atrium into the left atrium through this open-
ing, because the pressure is higher in the right side than
the left side of the heart. This pressure difference is due to
the constriction of arterioles in the fetal lungs in response to
hypoxia (low oxygen). Constriction of pulmonary arterioles
in the fetus also causes a higher pressure in the pulmonary
trunk than in the aorta, which shunts (diverts) blood from the
pulmonary trunk through a connection called the ductus
arteriosus into the aorta ( fig. 13.15 ).
After the baby is born and starts breathing, the pulmo-
nary oxygen levels rise. This causes pulmonary arterioles to
dilate and the pressure in the right side of the heart to lower
below the pressure in the left side, promoting the closing of
the foramen ovale. The rise in blood oxygen also stimulates
smooth muscle contraction in the ductus arteriosus, caus-
ing it to close. If these fetal structures remain open post-
natally, they are referred to as a patent foramen ovale or a
patent ductus arteriosus and can produce heart murmurs.| CHECKPOINT
6a. Using a flow diagram (arrows), describe the pathway
of the pulmonary circulation. Indicate the relative
amounts of oxygen and carbon dioxide in the vessels
involved.
6b. Use a flow diagram to describe the systemic
circulation and indicate the relative amounts of
oxygen and carbon dioxide in the blood vessels.
6c. List the AV valves and the valves of the pulmonary
artery and aorta. How do these valves ensure a
oneway flow of blood?
7a. Discuss how defective valves affect blood flow within
the heart and produce heart murmurs.
7b. Describe the patterns of blood flow in interatrial
and interventricular septal defects, and in a patent
foramen ovale in both a fetus and an adult.LEARNING OUTCOMESAfter studying this section, you should be able to:- Describe the cardiac cycle in terms of systole and
diastole of the atria and ventricles. - Explain how the pressure differences within the heart
chambers are responsible for blood flow during the
cardiac cycle.
Clinical Investigation CLUES
Jessica was told that she has a mitral valve prolapse.- What is a mitral valve prolapse, and where on the
chest might it best be heard? - Is it likely that Jessica’s fatigue is due to her mitral
valve prolapse?
Figure 13.15 The flow of blood through a patent
(open) ductus arteriosus. The ductus is normally open in a
fetus but closes after birth, eventually becoming the ligamentum
arteriosum. (AO 5 aorta; PA 5 pulmonary arteries.)AOPADuctus arteriosusThe cardiac cycle refers to the repeating pattern of contrac-
tion and relaxation of the heart. The phase of contraction is
called systole, and the phase of relaxation is called diastole.
When these terms are used without reference to specific cham-
bers, they refer to contraction and relaxation of the ventricles.
It should be noted, however, that the atria also contract and
relax. There is an atrial systole and diastole. Atrial contrac-
tion occurs toward the end of diastole, when the ventricles are
relaxed; when the ventricles contract during systole, the atria
are relaxed ( fig. 13.16 ).