Cardiovascular System

Part 1. Blood Vessels

There are three major types of blood vessels in the body. These are called arteries, veins, and capillaries.

Arteries are the large blood vessels which lead blood away from the heart. Their walls are made of connective tissue, elastic fibers, and an innermost layer of epithelial cells. Arteries are strong enough to withstand the high pressure of the pumping action of the heart. Their elastic walls allow them to expand as the heartbeat forces blood into the arterial system throughout the body. Smaller and thinner branches of arteries are called arterioles. They carry the blood to the tiniest of blood vessels, the capillaries.

Capillaries are delicate, microscopic vessels with only one epithelial cell in thickness. They carry nutrient-rich, oxygenated blood from the arteries and arterioles to the body cells. Their walls are thin enough to allow passage of oxygen and nutrients out of the bloodstream and into the tissue fluid surrounding the cells. At the same time, waste products such as carbon dioxide and water pass out of the cells and into the thin-walled capillaries. The waste-filled blood then flows back to the heart in small veins called venules which branch to form larger vessels called veins.

Veins are thinner-walled than arteries. They conduct waste-filled blood toward the heart from the tissues. Veins have little elastic tissue and less connective tissue than arteries, and the blood pressure in veins is low. In order to keep blood moving back toward the heart, veins have valves which prevent the backflow of blood. Muscular action also helps the movement of blood in veins.

Part 2. Circulation of Blood

Blood vessels together with the heart form a circulatory system for the flow of blood.

Blood deficient in oxygen (deoxygenated) flows through two large veins, the venae cavae, on its way from the tissue capillaries to the heart. The blood became deoxygenated at the tissue capillaries when oxygen left the blood and entered the body cells.

Deoxygenated blood enters the right side of the heart and travels through that side and into the pulmonary artery, a vessel which divides in two, one branch leading to the left lung, the other to the right lung. The arteries continue dividing and subdividing within the lungs, forming smaller and smaller vessels (arterioles) and finally reaching the lung capillaries. The pulmonary artery is the only artery in the body which carries deoxygenated blood.

While passing through the lung capillaries, blood absorbs the oxygen. The newly oxygenated blood next immediately returns to the heart through the pulmonary vein, the only vein in the body which carries oxygen-rich blood. The circulation of blood through the vessels from the heart to the lungs and then back to the heart again is known as the pulmonary circulation.

Oxygenated blood enters the left side of the heart from the pulmonary veins. The muscles in the left side of the heart pump the blood out of the heart through the largest artery in the body, the aorta. The aorta moves up at first (ascending aorta) but then arches over dorsally and runs downward (descending aorta) just in front of the vertebral column. The aorta branches into numerous arteries which carry the oxygenated blood to all parts of the body. The names of some of these arterial branches are: brachiocephalic, intercostal, esophageal, celiac, renal, and iliac arteries.

Arterial vessels branch further to form the smaller arterioles. The oxygen-rich arterioles branch into smaller tissue capillaries which are near the body cells. Oxygen leaves the blood as it passes through the capillary walls to enter the body cells. There it combines with food to release needed energy. Waste product of this process is carbon dioxide. It must pass out of the cells and into the capillary bloodstream, at the same time that oxygen is entering the cell. As the blood makes its way back from the tissue capillaries toward the heart in venules and veins, it is full of carbon dioxide and deoxygenated.

The circuit is thus completed when deoxygenated blood enters the heart from the venae cavae. This circulation of blood from the body organs (except the lungs) to the heart and back again is called the systemic circulation.

Part 3. The Heart

The heart lies in the thoracic cavity, just behind the breastbone and between the lungs.

The heart is a pump consisting of four chambers: two upper chambers called atria, and two lower chambers called ventricles. It is a double pump synchronized very carefully.

Deoxygenated blood enters the heart through the venae cavae, superior and inferior. They bring blood to the right atrium. It contracts to force blood through the tricuspid valve into the right ventricle. The cusps of the tricuspid valve form a one-way passageway designed to keep the blood flowing only in one direction. As the right ventricle contracts to pump deoxygenated blood to the lungs through the pulmonary artery, the tricuspid valve stays shut, thus preventing blood from pushing back into the right atrium.

The newly oxygenated blood enters the left atrium of the heart from the pulmonary vein. The walls of the left atrium contract to force blood through the mitral valve into the left ventricle.

The left ventricle which has the thickest walls of all four heart chambers must pump blood with great force so that the blood travels through arteries to all parts of the body. The blood is pumped out of the left ventricle through the aortic valve and into the aorta which branches to carry blood all over the body. The aortic valve prevents the return of aortic blood to the left ventricle.

The four chambers of the heart are separated by muscular partitions called septa. They are interatrial septum and interventricular septum.

The heart wall is composed of three layers. The endocardium is a smooth layer of cells which lines the interior of the heart, and also is the material of which the valves of the heart are formed. The myocardium is the middle, muscular layer of the heart wall and is the thickest layer. The epicardium is a thin layer and forms the outermost layer of the heart wall. The pericardium is a delicate, double-folded membrane which surrounds the heart like a sac. It is attached to the breastbone in front and to the diaphragm below, while an inner portion of the membrane adheres to the heart.

There are two phases of the heartbeat, diastole and systole. Diastole is the relaxation phase of the heartbeat when the atria and ventricles fill with blood. Systole begins as diastole ends. Systole is the active contraction phase of the heartbeat when the ventricles pump blood out of the heart.

Primary responsibility for initiating the heartbeat (systole and diastole) rests with a small region of specialized muscle tissue in the posterior portion of the right atrium, where an electrical impulse originates. The region is called the sinoatrial node. It is also called the pace-maker of the heart.

The wave of electricity passes from the pace-maker to another region of the myocardium. This region is at the posterior portion of the interatrial septum and is called the atrioventricular node. This node sends the excitation wave along to a region deep in the ventricle wall, called the bundle of His. From there the electrical wave passes to all parts of the ventricles and stimulates them to contract, pumping blood from the heart.

The heartbeat can be regulated by nervous impulses from the autonomic nervous system (parasympathetic and sympathetic nerves).