Left and Right Sides of the Heart
Figure 4-1 is a schematic diagram of the circuitry of the cardiovascular system. The left and right sides of the heart and the blood vessels are shown in relation to each other. Each side of the heart has two chambers, an atrium and a ventricle, connected by one-way valves, called atrioventricular (AV) valves. The AV valves are designed so that blood can flow only in one direction, from the atrium to the ventricle.
Figure 4–1 A schematic diagram showing the circuitry of the cardiovascular system. The arrows show the direction of blood flow. Percentages represent the percent (%) of cardiac output. See the text for an explanation of the circled numbers.
The left heart and right heart have different functions. The left heart and the systemic arteries, capillaries, and veins are collectively called the systemic circulation. The left ventricle pumps blood to all organs of the body except the lungs. The right heart and the pulmonary arteries, capillaries, and veins are collectively called the pulmonary circulation. The right ventricle pumps blood to the lungs. The left heart and right heart function in series so that blood is pumped sequentially from the left heart to the systemic circulation, to the right heart, to the pulmonary circulation, and then back to the left heart.
The rate at which blood is pumped from either ventricle is called the cardiac output. Because the two sides of the heart operate in series, the cardiac output of the left ventricle equals the cardiac output of the right ventricle in the steady state. The rate at which blood is returned to the atria from the veins is called the venous return. Again, because the left heart and the right heart operate in series, venous return to the left heart equals venous return to the right heart in the steady state. Finally, in the steady state, cardiac output from the heart equals venous return to the heart.
Blood Vessels
The blood vessels have several functions. They serve as a closed system of passive conduits, delivering blood to and from the tissues where nutrients and wastes are exchanged. The blood vessels also participate actively in the regulation of blood flow to the organs. When resistance of the blood vessels, particularly of the arterioles, is altered, blood flow to that organ is altered.
Circuitry
The steps in one complete circuit through the cardiovascular system are shown in Figure 4-1. The circled numbers in the figure correspond with the steps described here.
1. Oxygenated blood fills the left ventricle. Blood that has been oxygenated in the lungs returns to the left atrium via the pulmonary vein. This blood then flows from the left atrium to the left ventricle through the mitral valve(the AV valve of the left heart).
2. Blood is ejected from the left ventricle into the aorta. Blood leaves the left ventricle through the aortic valve (the semilunar valve of the left side of the heart), which is located between the left ventricle and the aorta. When the left ventricle contracts, the pressure in the ventricle increases, causing the aortic valve to open and blood to be ejected forcefully into the aorta. (As noted previously, the amount of blood ejected from the left ventricle per unit time is called the cardiac output.) Blood then flows through the arterial system, driven by the pressure created by contraction of the left ventricle.
3. Cardiac output is distributed among various organs. The total cardiac output of the left heart is distributed among the organ systems via sets of parallel arteries. Thus, simultaneously, 15% of the cardiac output is delivered to the brain via the cerebral arteries, 5% is delivered to the heart via the coronary arteries, 25% is delivered to the kidneys via the renal arteries, and so forth. Given this parallel arrangement of the organ systems, it follows that the total systemic blood flow must equal the cardiac output.
The percentage distribution of cardiac output among the various organ systems is not fixed, however. For example, during strenuous exercise, the percentage of the cardiac output going to skeletal muscle increases, compared with the percentage at rest. There are three major mechanisms for achieving such a change in blood flow to an organ system. In the first mechanism, the cardiac output remains constant, but the blood flow is redistributed among the organ systems by the selective alteration of arteriolar resistance. In this scenario, blood flow to one organ can be increased at the expense of blood flow to other organs. In the second mechanism, the cardiac output increases or decreases, but the percentage distribution of blood flow among the organ systems is kept constant. Finally, in a third mechanism, a combination of the first two mechanisms occurs in which both cardiac output andthe percentage distribution of blood flow are altered. This third mechanism is used, for example, in the response to strenuous exercise: Blood flow to skeletal muscle increases to meet the increased metabolic demand by a combination of increased cardiac output and increased percentage distribution to skeletal muscle.
4. Blood flow from the organs is collected in the veins. The blood leaving the organs is venous blood and contains waste products from metabolism, such as carbon dioxide (CO2). This mixed venous blood is collected in veins of increasing size and finally in the largest vein, the vena cava. The vena cava carries blood to the right heart.
5. Venous return to the right atrium. Because the pressure in the vena cava is higher than in the right atrium, the right atrium fills with blood, the venous return. In the steady state, venous return to the right atrium equals cardiac output from the left ventricle.
6. Mixed venous blood fills the right ventricle. Mixed venous blood flows from the right atrium to the right ventricle through the AV valve in the right heart, the tricuspid valve.
7. Blood is ejected from the right ventricle into the pulmonary artery. When the right ventricle contracts, blood is ejected through the pulmonic valve (the semilunar valve of the right side of the heart) into the pulmonary artery, which carries blood to the lungs. Note that the cardiac output ejected from the right ventricle is identical to the cardiac output that was ejected from the left ventricle. In the capillary beds of the lungs, oxygen (O2) is added to the blood from alveolar gas, and CO2 is removed from the blood and added to the alveolar gas. Thus, the blood leaving the lungs has more O2 and less CO2 than the blood that entered the lungs.
8. Blood flow from the lungs is returned to the heart via the pulmonary vein. Oxygenated blood is returned to the left atrium via the pulmonary vein to begin a new cycle.