The circulatory system is responsible for transporting essential substances to tissues and removing metabolic byproducts. It also plays a role in maintaining homeostasis by regulating body temperature, fluid balance, and the supply of oxygen and nutrients in different physiological states. The cardiovascular system, which includes the heart, blood vessels, and capillaries, is responsible for carrying out these functions. The heart acts as two pumps in series: one pumps blood through the lungs for oxygen and carbon dioxide exchange (pulmonary circulation) and the other pumps blood to all other tissues of the body (systemic circulation). The flow of blood through the heart is unidirectional, achieved through the arrangement of flap valves. Despite intermittent cardiac output, continuous flow to body tissues occurs through the distention of the aorta and its branches during ventricular contraction and the elastic recoil of the walls of large arteries during ventricular relaxation. In the intact circulation, the total volume of blood remains constant, and any increase in blood volume in one area must be accompanied by a decrease in another. The distribution of blood to different regions of the body is determined by the output of the left ventricle and the contractile state of the resistance vessels (arterioles) in these regions. The circulatory system consists of conduits arranged in series and in parallel, with important implications for resistance, flow, and pressure in blood vessels. Blood enters the right ventricle through the right atrium and is then pumped through the pulmonary arterial system. The blood passes through lung capillaries, where carbon dioxide is released and oxygen is taken up. The oxygen-rich blood returns to the left atrium via the pulmonary veins and is then pumped from the ventricle to the periphery, completing the cycle. Blood flows rapidly through the aorta and its arterial branches, which narrow and have thinner walls as they approach the periphery. The histological composition also changes along these branches. While the aorta is predominantly elastic, the peripheral arteries become more muscular, and the arterioles have a predominance of smooth muscle. The large arteries have relatively small frictional resistance and slightly lower pressures than the aorta. The small arteries offer moderate resistance to blood flow, and the greatest pressure drop occurs across the terminal segment of the small arteries and the arterioles. The arterioles, often referred to as the "stopcocks" of the vascular system, provide maximal resistance to blood flow. The degree of contraction of the circular muscle in these small vessels can be adjusted to regulate tissue blood flow and control arterial blood pressure. In addition to the pressure reduction along the arterioles, there is a transition from pulsatile to steady blood flow.
he circulatory system transports and distributes essential substances to tissues and removes metabolic byproducts. This system also participates in homeostatic mechanisms such as regulation of body temperature, maintenance of fluid balance, and adjustment of O2 and nutrient supply in various physiological states. The cardiovascular system, which accomplishes these tasks, is composed of a pump (the heart), a series of distributing and collecting tubes (blood vessels), and an extensive system of thin vessels (capillaries) that enable rapid exchange between the tissues and vascular channels. Blood vessels throughout the body are filled with a heterogeneous fluid (blood) that is essential for the transport processes performed by the heart and blood vessels. This chapter is a general, functional overview of the heart and blood vessels, whose functions are analyzed in much greater detail in subsequent chapters.
The Heart
The heart consists of two pumps in series: one pump propels blood through the lungs for exchange of O2 and CO2 (the pulmonary circulation) and the other pump propels blood to all other tissues of the body (the systemic circulation). Flow of blood through the heart is in one direction (unidirectional). Unidirectional flow through the heart is achieved by the appropriate arrangement of flap valves. Although cardiac output is intermittent, continuous flow to body tissues (periphery) occurs by distention of the aorta and its branches during ventricular contraction (systole) and by elastic recoil of the walls of the large arteries with forward propulsion of the blood during ventricular relaxation (diastole).
The Cardiovascular Circuit
In the normal intact circulation, the total volume of blood is constant, and an increase in the volume of blood in one area must be accompanied by a decrease in another. However, the distribution of blood circulating to the different regions of the body is determined by the output of the left ventricle and by the contractile state of the resistance vessels (arterioles) of these regions. The circulatory system is composed of conduits arranged in series and in parallel (
Fig.
15.1 ). This arrangement, which is discussed in subsequent chapters, has important implications in terms of resistance, flow, and pressure in blood vessels.
Blood entering the right ventricle via the right atrium is pumped through the pulmonary arterial system at a mean pressure about one seventh that in the systemic arteries. The blood then passes through the lung capillaries, in which CO2 in the blood is released and O2 is taken up. The O2-rich blood returns via the pulmonary veins to the left atrium, where it is pumped from the ventricle to the periphery, which thus completes the cycle.
Blood moves rapidly through the aorta and its arterial branches. As these branches approach the periphery, the branches narrow, and their walls become thinner. They also change histologically. The aorta is a predominantly elastic structure, but the peripheral arteries become more muscular until, at the arterioles, the muscular layer predominates (
Fig. 15.2
In the large arteries, frictional resistance is relatively small, and pressures are only slightly less than those in the aorta. The small arteries, on the other hand, offer moderate resistance to blood flow. This resistance reaches a maximal level in the arterioles, which are sometimes referred to as the “stopcocks” of the vascular system. Hence, the pressure drop is greatest across the terminal segment of the small arteries and the arterioles (
Fig. 15.3 ). Adjustment in the degree of contraction of the circular muscle of these small vessels allows regulation of tissue blood flow and aids in the control of arterial blood pressure.
In addition to the reduction in pressure along the arterioles, there is a change from pulsatile to steady blood flow (see