Pulmonary and Systemic Circulations

Blood whose oxygen content has become partially depleted and whose carbon dioxide content has increased as a result of tissue metabolism returns to the right atrium. This blood then enters the right ventricle, which pumps it into the pulmonary trunk and pulmonary arteries. The pulmonary arteries branch to transport blood to the lungs, where gas exchange occurs between the lung capillaries and the air sacs (alveoli) of the lungs. Oxygen diffuses from the air to the capillary blood, while carbon dioxide diffuses in the opposite direction.

The blood that returns to the left atrium by way of the pulmonary veins is therefore enriched in oxygen and partially depleted of carbon dioxide. The path of blood from the heart (right ventricle), through the lungs, and back to the heart (left atrium) completes one circuit: the pulmonary circulation.

Oxygen-rich blood in the left atrium enters the left ventricle and is pumped into a very large, elastic artery—the aorta. The aorta ascends for a short distance, makes a U-turn, and then descends through the thoracic (chest) and abdominal cavities. Arterial branches from the aorta supply oxygen-rich blood to all of the organ systems and are thus part of the systemic circulation.

As a result of cellular respiration, the oxygen concentration is lower and the carbon dioxide concentration is higher in the tissues than in the capillary blood. Blood that drains into the systemic veins is thus partially depleted of oxygen and increased in carbon dioxide content. These veins ultimately empty into two large veins—the superior and inferior venae cavae—that return the oxygen-poor blood to the right atrium. This completes the systemic circulation: from the heart (left ventricle), through the organ systems, and back to the heart (right atrium). The systemic and pulmonary circulations are illustrated in figure 13.9, and their characteristics are summarized in table 13.8.

The numerous small muscular arteries and arterioles of the systemic circulation present greater resistance to blood flow than that in the pulmonary circulation. Despite the differences in resistance, the rate of blood flow through the systemic circulation must be matched to the flow rate of the pulmonary circulation. Since the amount of work performed by the left ventricle is greater (by a factor of 5 to 7) than that performed by the right ventricle, it is not surprising that the muscular wall of the left ventricle is thicker (8 to 10 mm) than that of the right ventricle (2 to 3 mm).

380 Chapter Thirteen

Table 13.8

Summary of the Pulmonary and Systemic Circulations

Source

Arteries

O2 Content of Arteries

Veins

O2 Content of Veins

Termination

Pulmonary Circulation

Right ventricle

Pulmonary arteries

Low

Pulmonary veins

High

Left atrium

Systemic Circulation

Left ventricle

Aorta and its branches

High

Superior and inferior venae cavae and their branches*

Low

Right atrium

*Blood from the coronary circulation does not enter the venae cavae, but instead returns directly to the right atrium via the coronary sinus.

*Blood from the coronary circulation does not enter the venae cavae, but instead returns directly to the right atrium via the coronary sinus.

Bicuspid valve (into left ventricle)

Pulmonary semilunar valve

Aortic semilunar valve

Tricuspid valve (into right ventricle)

Aorta

Superior vena cava

Right atrium

Tricuspid valve

Papillary muscles Inferior — (b) vena cava

Pulmonary semilunar valve

Aortic semilunar valve

Tricuspid valve (into right ventricle)

Aorta

Superior vena cava

Pulmonary trunk

Pulmonary semilunar valve

Left atrium

Mitral (bicuspid) valve

Chordae tendineae

Interventricular septum has three flaps, and is therefore called the tricuspid valve. The AV valve between the left atrium and left ventricle has two flaps and is thus called the bicuspid valve, or, alternatively, the mitral valve (fig. 13.10).

The AV valves allow blood to flow from the atria to the ventricles, but they normally prevent the backflow of blood into the atria. Opening and closing of these valves occur as a result of pressure differences between the atria and ventricles. When the ventricles are relaxed, the venous return of blood to the atria causes the pressure in the atria to exceed that in the ventricles. The AV valves therefore open, allowing blood to enter the ventricles. As the ventricles contract, the intraventricular pressure rises above the pressure in the atria and pushes the AV valves closed.

There is a danger, however, that the high pressure produced by contraction of the ventricles could push the valve flaps too much and evert them. This is normally prevented by contraction of the papillary muscles within the ventricles, which are connected to the AV valve flaps by strong tendinous cords called the chordae tendineae (fig. 13.10). Contraction of the papillary muscles occurs at the same time as contraction of the muscular walls of the ventricles and serves to keep the valve flaps tightly closed.

One-way semilunar valves (fig. 13.11) are located at the origin of the pulmonary artery and aorta. These valves open during ventricular contraction, allowing blood to enter the pulmonary and systemic circulations. During ventricular relaxation, when the pressure in the arteries is greater than the pressure in the ventricles, the semilunar valves snap shut, thus preventing the backflow of blood into the ventricles.

■ Figure 13.10 The heart valves. (a) A superior view of the heart valves. (b) A sagittal section through the heart, showing both AV valves and the pulmonary semilunar valve (the aortic semilunar valve is not visible in this view).

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