upper chamber, the ATRIUM, which receives blood from the veins, and a lower chamber, the VENTRICLE, which receives blood from the atrium and pumps it out into the arteries. The openings between the chambers on each side of the heart are separated by flaps of tissue that act as valves to prevent backward flow of the continuously forward moving column of blood. The one on the right has three flaps, or cusps, and is called the TRICUSPID VALVE. The one on the left has two flaps and is called the MITRAL, or BICUSPID, VALVE. The outlets of the ventricles are supplied with similar valves. On the right the pulmonary valve is at the origin of the pulmonary artery, and on the left the aortic valve is at the origin of the aorta.
Physiologically, the heart acts as four interrelated pumps. The right atrium receives deoxygenated blood from the body via the superior and inferior vena cavae. It pumps this blood through the tricuspid valve to the right ventricle. The right ventricle pumps the blood past the pulmonary valve through the pulmonary artery to the lungs for oxygenation. The left atrium receives the oxygenated blood from the lungs through four pulmonary veins and pumps it to the left ventricle past the mitral valve. The left ventricle pumps the blood to all areas of the body via the aortic valve and the aorta.
The heart muscle, the MYOCARDIUM, is striated like the skeletal muscles of the body, but involuntary in action, like the smooth muscles. The walls of the atria are thin with relatively little muscle fiber because the blood flows from the atria to the ventricles under low pressure. However, the walls of the ventricles, which comprise the bulk of the heart, are thick and muscular. The wall of the left ventricle is considerably thicker than that of the right, because more force is required to pump the blood into the peripheral systemic circulation than into the lungs located only a short distance from the heart.
The heart acts by contraction and relaxation. It contracts with a wringing motion, forcing blood into the arteries. Each contraction is followed by limited relaxation or dilation. Cardiac muscle never completely relaxes, it always maintains a degree of tone. Contraction of the heart is called SYSTOLE and the period of work. Relaxation of the heart with limited dilation is called DIASTOLE and the period of rest. A complete CARDIAC CYCLE is the time from onset of one contraction, or heart beat, to the onset of the next.
The contractions of the heart are stimulated and maintained by the SINOATRIAL NODE, commonly called the PACEMAKER of the heart, which is a group of hundreds of cells in the upper part of the right atrium that sets off electrical impulses, causing both atria to contract simultaneously. The normal heart rate, or number of contractions, is about 72 beats per minute.
The BLOOD PRESSURE is the pressure the blood exerts on the walls of the arteries. The highest pressure is called SYSTOLIC pressure, because it is caused when the heart is in systole, or contraction. A certain amount of blood pressure is maintained in the arteries even when the heart is relaxed. This is the DIASTOLIC pressure, because it is present during diastole, or relaxation of the heart.
Normal blood pressure can vary considerably with age, weight, and general condition of the individual. For young adults the systolic pressure is between 120 and 150 mm of mercury, and the diastolic pressure is between 70 and 90 mm of mercury. Women have a lower blood pressure than men. The difference between systolic and diastolic pressure is known as PULSE PRESSURE.
The blood vessels of the body fall into three distinct classifications:
1. Distributorsarteries and arterioles
3. Collectorsveins and venules
The ARTERIES are elastic tubes constructed to withstand high pressure. They carry blood away from the heart to all parts of the body. The smallest branches of the arteries are called arterioles.
The AORTA is the large tubelike structure arising from the left ventricle of the heart. It arches upward over the left lung and then down along the spinal column through the thorax and the abdomen, where it divides to send arteries down both legs (fig. 3-31). The CORONARY ARTERIES are branches of what is generally called the ascending aorta, and they supply the heart with blood.
There are certain branches of the aorta with which you should be familiar, since these often must be compressed to control hemorrhage. You will find a discussion of pressure points in the hemorrhage section of the chapter in this manual entitled First Aid and Emergency Procedures.