General Physiology

2. General Physiology

2.1 The Circulatory System

The circulatory system transports blood from the heart to the cells and back again to the heart in a never-ending stream. The red blood cells contain hemoglobin, a complex protein arranged around iron. They have the important role of taking up large quantities of oxygen as it passes through the lungs and then passing this oxygen to the body cells. As the blood in the lungs picks up oxygen, the hemoglobin becomes bright red (arterial blood). When the hemoglobin reaches its destination, the individual cell, it releases oxygen and picks up carbon dioxide, a waste product of metabolism. The color of the blood carrying carbon dioxide to the lungs for elimination from the body is a bluish red (venous blood). Hemoglobin has another important function, that is, helping to maintain the acid balance of the blood.  Each day the carbon dioxide resulting from cellular metabolism is equivalent to a little more than one-half gallon (2.6 liters) of concentrated hydrochloric acid. That high concentration of acid would injure the cells, so nature provides a neutralizing base, half of which comes from the food consumed each day and the remainder from the body’s built-in buffering agents, one of which is hemoglobin.

2.2 Structure of the Heart

The heart is completely enclosed by a thin sac called the pericardium. This tough tissue protects the heart from rubbing against the lungs and the wall of the chest. The inside of the pericardium has a smooth lining that secretes a lubricating fluid. The heart beats smoothly and with little friction against the moistened lining of the pericardium.

A muscular wall called the septum divides the heart lengthwise. Two chambers, one above the other, are on each side of the septum. The upper chamber on each side is called an atrium. The thin-walled atria collect the blood flowing into the heart from the veins. Below each atrium is another chamber called a ventricle. The two ventricles pump the blood into the arteries.

The walls of the ventricles are made of thick, strong muscles. The right ventricle pumps blood through the pulmonary artery into the lungs. The left ventricle pumps blood to the entire body through the aorta. The left ventricle has walls three times as thick as those of the right ventricle because it has to pump the blood so much farther. Valves control the flow of the blood through the heart. The tricuspid valve is between the right atrium and the right ventricle. The mitral valve is between the left atrium and the left ventricle. The semilunar valves control the flow of blood from the ventricles to the arteries. The semilunar valve that controls blood-flow from the left ventricle to the aorta is also called the aortic valve.

Arteries carry blood from the heart to other parts of the body, but the heart itself must also receive nourishment. Blood flows to the heart muscle through coronary arteries. The coronary arteries lie over the walls of the heart in a complicated network, and carry oxygen to all parts of the heart muscle.

The interior of the heart is lined with a smooth membrane, a single layer of cells called the endocardium. The same kind of membrane also lines the valves and the blood vessels. The lining prevents damage to the blood cells by reducing friction and by minimizing the danger of blood-clot formation either inside the heart or inside the blood vessels themselves. Between the endocardium and the outer layer of the heart (epicardium) is the muscular wall itself, the myocardium, consisting of muscle cells surrounded by connective tissue.

The many connections between muscle cells in the atria and the ventricles form the basis for the conduction of impulses from cell to cell. Near the entrance of the two veins that bring blood from the upper and lower parts of the body into the top of the right atrium (the superior and the inferior venae cavas) is a small bundle of highly-specialized muscle fibers that generate impulses necessary for the coordinated contraction of the heart. This bundle is called the sinoatrial node, or pacemaker. The impulses from the sinoatrial node are collected and conveyed to the ventricles by another group of cells in the connective tissue between the left and right sides of the heart. This group is called the atrioventricular node, and from it other specialized muscle fibers, called the bundle of His, after the German anatomist who discovered them, run from the upper to the lower chambers, splitting to left and right as they descend. The two main branches of the bundle of His connect with a network of smaller impulse-carrying fibers (Purkinje fibers), which run to all parts of the ventricles. It
is by means of this intricate system of specialized fibers and cells that the heart receives the electrical impulses necessary to maintain a rhythmic, effective and concerted beat.