We’ve seen that the blood vessels of the body are arteries, veins and capillaries. Each kind of blood vessel is adapted to carry out its function.

The arteries carry blood away from the heart. As you might expect, they are thick-walled and muscular to cope with the high pressure of the blood that is pumped from the ventricles.

The major arteries of the body also have layers of elastic tissue around them. Arteries branch into smaller arteries called arterioles.

Veins are thin-walled, have few muscle fibers, and unlike arteries they collapse when empty. Veins also have a series of valves that prevent blood from moving in the wrong direction, as the blood pressure in veins is low. Capillaries join to small vessels called venules that drain into veins

Capillaries have epithelial walls that are only a single cell layer in thickness. Capillaries need thin walls because that is where all exchanges of gases and nutrients, as well as removal of wastes, occurs between the bloodstream and the surrounding tissues. The network of capillaries that infiltrates a tissue is called a capillary bed.

Blood is a fluid tissue, with cells suspended in a fluid matrix of plasma. Plasma is made up of about 90 percent water in which inorganic salts, important in maintaining the osmotic balance of the blood, are dissolved..

Plasma also contains clotting factors that prevent blood from being lost through injury when blood vessels are severed. Substances are also transported in the plasma from one part of the body to another. These substances include hormones, waste products, and nutrients.

The cells suspended in blood plasma fall into two groups: red blood cells and white blood cells. The white blood cells play a major role in the immune response of the body against disease and foreign cells. You’ll learn about these cells elsewhere in this course.

Red blood cells contain the oxygen-carrying protein hemoglobin. The structure of hemoglobin makes it very efficient at transporting oxygen, as you’ll find out when we discuss the respiratory system.

Now that we’ve examined the components of animal circulatory systems, we’ll move on to look at the anatomy and physiology of the human circulatory system.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education