American Medical Association Family Medical Guide - American Medical Association [315]
The arteries carry oxygen-rich blood away from the heart to the tissues. The main arteries branch into smaller arteries called arterioles, which branch to form tiny capillaries, the smallest blood vessels in the body. Capillary walls are only one cell thick, allowing oxygen and other vital nutrients in the blood to pass through the walls to nourish the tissues and allowing waste to be taken up by the capillaries for removal. The capillaries returning to the heart gradually become larger, forming tiny blood vessels called venules, which gradually become larger and form veins. The veins carry oxygen-depleted blood through organs such as the kidneys and liver, which remove waste, and through the lungs, which provide a fresh supply of oxygen. From the lungs, the veins carry the blood back to the heart, where the cycle of circulation begins again.
Heart Disorders
To function properly, your heart requires a constant supply of blood, which provides oxygen and other nutrients to the heart muscle. Three major coronary arteries and a network of smaller blood vessels on the surface of the heart supply blood to the heart muscle. The walls of healthy arteries are muscular, smooth, and elastic to allow blood to flow through them easily. Heart disease, also called coronary artery disease, develops when the coronary arteries become narrowed, reducing or blocking the flow of blood (and oxygen and other nutrients) to the heart. This narrowing of the arteries is usually caused by atherosclerosis, a gradual thickening and hardening of artery walls caused by buildup of a fatty substance called plaque. Over a lifetime, plaque can grow larger and the artery walls can become thicker, making the channel inside the artery narrower and potentially blocking blood flow.
In atherosclerosis, the heart has to pump harder and faster to push blood through the narrowed arteries, causing high blood pressure (see page 574). The reduced supply of oxygen to the heart muscle can cause pain in the chest called angina. In some people, the heart muscle is damaged and its pumping action weakened, causing heart failure (see page 570).
The Heart and Blood Vessels
The Heart
The heart is a muscular organ made up of two pumps, each divided into two sections that are linked by valves. The larger section is the left ventricle, which pumps oxygen-rich blood through the aorta to all parts of the body. The blood then returns to the heart, entering the right atrium through two large channels (superior vena cava and inferior vena cava). From the right atrium the blood passes through the tricuspid valve into the right ventricle. It is then pumped through the pulmonary artery into the lungs, where it eliminates carbon dioxide and takes up oxygen. The oxygen-rich blood flows back to the left atrium of the heart through the pulmonary veins. From the left atrium it passes through the mitral valve into the left ventricle, where the cycle begins again.
Coronary arteries
Oxygen-rich blood is pumped out of the aorta and flows into the coronary arteries, a branching network of blood vessels on the surface of the heart that supply oxygen- and nutrient-rich blood to the heart muscle.
The Blood Vessels
Blood vessels transport blood to and from the heart and throughout the body. Arteries carry fresh, oxygen-rich blood away from the heart to the tissues; veins return used, oxygen-depleted blood from the tissues back to the heart and lungs for a new supply of oxygen.
Artery walls are very strong because blood is pumped through them under pressure from the heart. Arteries have three layers: a fibrous outer layer, a middle layer of strong muscle and tough elastic tissue, and a membrane inner lining.
Veins run parallel