Gutter=17> 15. 1 Introduction
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The heart and circulation  15.1 IntroductionIn unicellular organisms and simple animals such as sponges, the exchange of nutrients and waste products between the cells and the environment can be accomplished by simple diffusion across the cell membranes. Since diffusion is a random process, the greater the distance to be traversed, the greater the time required for equilibration. Consequently, in more complex animals where cells may be located at some distance from the external environment, diffusion by itself would not suffice to permit adequate exchange. Animals have evolved a circulatory system and this allows the cells of the body to exchange the products of their metabolism for oxygen and other nutrients with the circulating blood. The properties of the blood itself have been discussed in Chapter 13. The circulation serves three primary functions:
In what follows it will be useful to keep in mind a number of key questions:
• How is exchange between the blood and the tissues accomplished? The consequences of circulatory failure and other disorders of the cardiovascular system will be discussed in Chapters 28 and 31. 15.2 The organization of the circulation The circulation consists of a pump (the heart) and a series of interconnected pipes (the blood vessels). As the blood is pumped from the right side of the heart through the lungs {the pulmonary circulation) and then from the left side of the heart to the body {the systemic circulation) the overall arrangement is of two circulations in series (Fig. 15.1). The pumping activity of the heart raises the pressure in the aorta above that of the large veins, which are at atmospheric pressure. It is this pressure (the arterial blood pressure or, more commonly, the blood pressure) that causes the flow of blood around the systemic circulation. Equally, blood flows through the lungs because the pressure in the pulmonary arteries is greater than that in the pulmonary veins. The gross anatomy of the heart The heart is about the size of a clenched fist and consists of four muscular chambers—two atria and two ventricles—which lie side by side separated by a muscular sheet called the septum (Fig. 15.2). The atria are thin-walled chambers that receive blood from the large veins and deliver it to the ventricles which provide the force required to pump the blood round the pulmonary and systemic circulations. The walls of the ventricles are much thicker than those of the atria—the wall of the left ventricle being thickest. The atria are separated from the ventricles by a fibrous septum which contains the four heart valves—the atrioventricular valves and the aortic and pulmonary valves. The muscle of the heart is called the myocardium. The heart lies in a tough fibrous sac known as the pericardium. which prevents the heart from expanding excessively due to overfilling with blood. The pericardium is attached to the diaphragm so that the apex of the heart is relatively fixed. When the ventricles contract, the atria move towards the apex. This has the effect of expanding the atria as the ventricles contract. 278 15 The heart and circulation   Fig. 15.1 A schematic drawing of the circulation. The arrows indicate the direction of blood flow. Note that the blood returning to the heart enters the right atrium. It then enters the right ventricle which pumps the blood through the lungs. After leaving the lungs, the blood enters the left atrium and then passes to the left ventricle which pumps it through the rest of the body via the systemic circulation. Thus the pulmonary circulation is in series with the systemic circulation. PA, PV, pulmonary artery and-pulmonary vein; RA, LA, right and left atria; RV, LV, right and left ventricles.   Fig. 15.2 A simple diagram to illustrate the arrangement of the chambers and the direction of blood flow through the heart. The blood passing through the right side of the heart contains deoxygenated blood while that passing through the left side of the heart contains oxygenated blood. ^ 279  The structure of the blood vesselsThe blood vessels are divided into four broad categories: arteries, arterioles, capillaries, and veins (Table 15.1). The walls of the larger blood vessels consist of three layers:
The arteries are the primary distribution vessels and may be subdivided into two groups: 1. The elastic arteries which, in humans, are large vessels of 1—2 cm in diameter. They include the aorta and pulmonary arteries together with their major branches. The walls of the elastic arteries are very distensible because their tunica media contains a high proportion of elastin (up to 40 per cent, compared to about 10 per cent for a muscular artery). Table 15.1 The total cross-sectional area of the different types of blood vessel in the systemic and pulmonary circulations relative to that of the aorta
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