Coupon Accepted Successfully!


Structure and Function of the Heart, and the Path of Circulation

The human heart consists of four chambers: the right and left atria (located in the upper region), which receive blood from veins, and the right and left ventricles (located in the lower region), which are the major pumping portions of the heart and propel blood into arteries. The wall of the heart is largely composed of cardiac muscle, often called the myocardium, and the two sides are separated by a thick structure called the septum. The entire heart is surrounded by a fluid-filled sac, the pericardium, which serves a protective role.

Figure15.1: Structure of the heart


Figure 15.2: Path of circulation


If we were to follow the path blood takes as it travels throughout the body, wewould find that it always flows in one direction, and that we can trace the major vessels through which it flows and their connections with the heart (see Figure 15.2). Arbitrarily starting in the left ventricle, the most powerful pumping region of the heart, oxygen-rich blood is forced into the aorta, the largest artery in the body, under considerable pressure. The aorta quickly divides and subdivides, ultimately supplying oxygenated blood to all of the body’s tissues via capillaries. As the deoxygenated blood returns towards the heart, venules ultimately merge into two major veins, the superior and inferior vena cava, which enter the heart’s right atrium. Blood passes into the right ventricle through the tricuspid valve, and is pumped out again into the pulmonary artery.
Notice that the blood present in the pulmonary artery is oxygen-poor, not oxygen-rich as it is in most other arteries. The pulmonary artery branches into two smaller arteries, one leading to each lung, and ultimately terminates in capillary beds surrounding the alveoli, or air sacs, of the lungs. It is here that oxygen enters the blood from the lungs, and carbon dioxide leaves, both by the process of diffusion. Venules leading from these capillaries ultimately join to form the pulmonary vein, which deposits oxygenated blood into the left atrium of the heart. Blood then enters the left ventricle through the bicuspid valve, and the cycle is ready to begin again.
In essence, as we can see from this analysis, the heart actually consists of two separate pumping systems: one which pumps blood from the heart to the tissues and back, referred to as the systemic circuit; and another, which pumps blood from the heart to the lungs and back, called the pulmonary circuit. In addition, the cells of the myocardium itself must be supplied with oxygen and nutrients if they are to continue functioning effectively. A small vessel, the coronary artery, branches off the aorta and supplies the myocardium; cardiac veins return this blood to the right atrium. If the coronary artery or its branches become blocked, the myocardium may be deprived of oxygen, potentially resulting in a myocardial infarction, or “heart attack”.
While the hearts of all mammals and birds are structurally similar, lower vertebrates have different, less efficient heart arrangements. Amphibians, for example, have a three-chambered heart with only one ventricle, but blood destined for the systemic and pulmonary circulation remains relatively unmixed due to ridges in the ventricular wall. In addition, many amphibians respire directly through their moist skin (in addition to their lungs), lessening the need for strict separation. Fish, by contrast, have the most primitive hearts of all, usually consisting of only two chambers, a single atrium and a single ventricle. The ventricle pumps deoxygenated blood directly to the gills, where gas exchange occurs, and then on to the tissues of the body directly. The blood pressure eventually dissipates, and circulation in general is rather sluggish in fish.

Test Your Skills Now!
Take a Quiz now
Reviewer Name