Your systemic circulation is the vast highway system that carries blood from your heart to every part of your body, and then returns it to the heart. The vessels that carry blood away from the heart are the arteries; the vessels that carry blood back to the heart are veins. Like a system of roads, your circulatory system keeps branching off into successively smaller vessels that carry blood to and from the smallest structures and ?nally individual cells in body tissues. At a cellular level, single red blood cells exchange oxygen and nutrients with single body cells through the walls of microscopic capillaries.

The Arteries and the Capillaries

The aorta, the largest artery in your body, emerges from the left side of your heart. About 1 inch in diameter, it ascends from your left ventri- cle engorged with oxygen-rich blood, then arches down the chest into the abdomen. Major arteries branch off it to supply different areas of your body. The carotid and vertebral arteries travel to your head and neck. The subclavian arteries supply the arms. The abdominal (descending) aorta provides branches to your stomach, liver, kidneys, and intestinal tract. The aorta then divides into the iliac arteries and then the femoral arteries of the legs.
The pulmonary artery carries blood from your heart to your lungs. Exiting from your right ventricle, it transports oxygen-depleted blood into your lungs to replenish the oxygen. This pulmonary circulation functions similarly to your systemic circulation but is limited to the lungs, where oxygen exchange occurs at a cellular level.
The arteries subdivide into smaller vessels called arterioles. The arteries and arterioles have flexible muscular walls that can dilate (widen) and contract, with a critical impact on directing blood ?ow. Blood ?ows more easily to areas where there is less resistance, so arter- ies that widen increase the circulation to that area, while a constricted artery reduces blood ?ow. Branching off from the arterioles are the smallest vessels, the capillaries. Most capillary walls are only one cell thick. Specialized capillaries in different types of body tissue allow the passage of different types of molecules through their walls. In the lungs, for example, molecules of carbon dioxide (a waste product) pass into the tissue to be breathed out, while molecules of oxygen pass into the blood cells. In your intestinal system, nutrients from digested food pass through the capillary walls into the blood.

The Veins

At the level of individual cells throughout your body, the capillaries receive spent blood from body tissue that has a lower level of oxygen. The capillaries ?ow into larger vessels called venules, which converge and form still larger veins. The pressure in veins is signi?cantly lower than the pressure in arteries, and the walls are thinner, which is why blood samples are typically taken from a vein. As with arteries, the walls of veins can expand or contract. Any tensing of your muscles squeezes the veins, helping to counteract gravity and keep blood ?owing toward your heart. Larger veins also have a system of one-way valves that keep the returning blood ?owing the right way.
Venous blood from the body enters the heart via two major vessels: the superior vena cava, bringing blood from the upper part of the body, and the inferior vena cava, returning blood from the lower part. These large veins enter the right atrium, where the blood is sent into the pul- monary circulation for oxygen pickup.

Blood

Blood is the ?uid vehicle by which oxygen, enzymes (proteins that pro- mote body processes), and other life-sustaining nutrients are brought to body cells in order to maintain an optimal environment for growth. Blood is composed of specialized blood cells—red blood cells, white blood cells, and platelets—and of plasma, the ?uid in which the blood cells are suspended.
The vast majority of blood cells are red blood cells, also called ery- throcytes or red corpuscles, which do the work of oxygen transport. An individual red blood cell is saucer-shaped to maximize its surface area for ef?cient oxygen exchange. Chemically, a red blood cell contains large quantities of hemoglobin, an iron-rich protein that is the body’s oxygen transport carrier molecule. As red blood cells travel through the lungs, where oxygen levels are high, the hemoglobin readily combines with oxygen. When the blood cells reach body tissues where oxygen levels are relatively low, the hemoglobin just as effectively releases oxy- gen. The red blood cells also pick up the waste product carbon dioxide and carry it back to the lungs, where it is released and then exhaled out of the body. Red blood cells are formed in the bone marrow at the rate of about 8 million a second, or many billion in a single day. They live from 3 to 4 months.
White blood cells, or leukocytes, play a critical role in protecting the body against infection. One type of white blood cell, called a lympho- cyte, identi?es invading microorganisms or other harmful substances in the body and triggers the body’s immune response. The number of white blood cells increases when your body is ?ghting infection. Also suspended in the plasma are cell fragments called platelets, which initi- ate a blood-clotting response when you are injured or a blood vessel is damaged. White blood cells and platelets make up about 1 to 2 percent of blood volume.
About 55 percent of the blood volume is plasma, a yellowish, watery substance that contains proteins, glucose (sugar), cholesterol, and other components. Proteins in the plasma perform varied roles such as carrying nutrients, contributing to the clotting factor, and acting as infection-?ghting antibodies in an immune response.