Blood, Composition And Function (1978)

There isn't much of it. Only about 5.6 L, about 6 qt of blood in an average adult human. But you can't get along very well without it. Blood can be thought of as an organ that carries oxygen and nutrients to body tissues and carbon dioxide and other wastes away from them. Or it can be thought of as a fluid that contains different kinds of cells in suspension. To see what kind of fluid it is, what kinds of cells are in suspension, and how the blood transport system works, let's do a little close observation through a microscope. The watery fluid is called plasma. Plasma makes up about 55% of your blood. The rest is made up of three kinds of cells. The most numerous are the red blood cells, the erythrocytes, that give your blood its reddish color. They contain the red protein pigment hemoglobin. In 1 cubic millimeter of your blood, there are from four and a half to six million red blood cells. These erythrocytes, red cells, are disc-shaped with a slight depression in their center. You can see that more clearly in this electron microphotograph. Blood cells are sometimes called corpuscles. A mature human red blood cell doesn't have a nucleus. Each is about 8/1000 of a millimeter in diameter, 8 microns, and about 2 to 3 microns thick. The red blood cells' main job is to carry oxygen to all the cells of your body. An iron complex called heme in the hemoglobin loosely unites with oxygen from your lungs and then passes the oxygen on to the cells. An average red cell lives about 120 days. Each second, about 2 million of them are destroyed and replaced by new ones. The white cells, the leukocytes, are a second type of cell in your blood. The red cells outnumber them 600 to 1. Here's a closer view. The one irregularly shaped cell is a white one. White ones vary in shape. The two largest here are white cells. As the red corpuscles stream by in this capillary, you can see several white cells tumbling along very slowly. This is typical for them. Unlike the red ones, the white cells can slip through the walls of small blood vessels and into body tissues. A white cell is about 10/1000 of a millimeter, 10 microns, in diameter. There are five types of them. Three kinds are called granulocytes. They seem to belong to one family. All three have very grainy cytoplasm. Every white cell is capable of changing its shape and moving like an amoeba. Ameboid characteristics include the ability to find and ingest particles of food. In this demonstration, the white cell is ingesting yeast cells. This is the way that cells eat and destroy harmful bacteria and other microorganisms that may enter your body. White blood cells are your body's first line of defense. Not only do they destroy bacteria, but they also produce antibodies and repair wounded tissue. A cubic millimeter of your blood contains about 8 to 10,000 white cells. At any point of infection or injury, white cells will gather in enormous numbers. Eventually, they help to form pus. A rise in your white blood count is one indication of a physiological problem. Now, let's observe the third cellular component of blood. Mixed with these white cells are small dark bodies, blood platelets. There are about 300,000 platelets in a cubic millimeter of your blood. A platelet is about 3 microns in diameter, about a third the size of a red blood cell. The main function of platelets, or thrombocytes, is to aid in the clotting of blood. We'll see how this vital function of coagulation takes place. This is fresh blood about 10 minutes after exposure to the air. Coagulation seals a wound and stops the blood flow. We can trace the process. To make this fresh blood begin to clot, we need to add nothing but time. We'll cover 12 minutes in a few seconds with time-lapse photography. Coagulation involves a series of chemical reactions. The clot forms and separates from the remaining fluid, which is blood serum. Let's examine the coagulated blood. The cell mass is clumped together, crossed by dark strands to which the blood platelets stick. In time-lapse photography, you can see the long strands precipitate among the blood platelets. These threads are fibrin, a sticky protein material forming a network from invisible fibrinogen in the blood. The fibrin strands entrap the blood cells and stop the flow of blood. How does all this actually work inside your body? We'll see a demonstration in which a capillary in a rabbit's ear is punctured. Almost instantly, repair begins. Blood platelets sweep toward the injured point. Strands of fibrin form. The red cells begin to clump. The flow begins to slow down. The blood is coagulating. Within 10 to 15 minutes, the wound is sealed and normal capillary flow resumes. In an electron microphotograph, coagulated blood looks something like curdled milk. Blood clotting is one of the several important jobs of the three kinds of cells that make up your blood. The platelets that help blood clot. The red cells that transport oxygen. And the white cells that destroy bacteria. Because blood cells die, thousands of them per second, they have to be replaced constantly. All blood cells come from the long bones of your body. The central cavity of each of these bones is filled with marrow and blood vessels pass through the bone tissue. We'll take a closer look. The marrow is packed with the primitive cells from which all three types of blood cells develop. Here they originate. And in an electron microphotograph of bone marrow, you can see the newly formed cells. But the three cell components make up only 45% of your blood. Let's investigate the other 55% the plasma. First, we'll put a few drops of a chemical into the blood, sodium oxalate or citrate. This will prevent normal coagulation. The discovery of such anticoagulant chemicals made it possible to store whole blood in blood banks and use it in transfusions. Time-lapse photography over a period of 8 hours shows a clear separation of components without coagulation. Blood plasma is at the top. The three kinds of blood cells are below. One more demonstration will show the difference between blood plasma and blood serum. Blood with an anticoagulant is on the right. Normal blood on the left. The pale liquid collecting at the right is plasma. The liquid separating from the blood clot on the left is serum. With a centrifuge, we can completely separate the blood plasma from the blood cells. Plasma is often used in transfusions. It can restore blood volume that's been lost in bleeding. Blood serum is also used for transfusions in cases where the coagulant ability of plasma isn't needed. Serum is a result of coagulation but won't produce it because the plasma proteins responsible for coagulation have been used up and are no longer available. Plasma and the cell components form the whole blood that flows in a closed circulatory system. Closely related to the blood and its function is lymph. Lymph, the yellowish fluid, flows in lymph vessels that are adjacent to the blood vessels. This illustrates how white blood cells and plasma, both green symbols, ooze out of the capillaries that retain red cells and platelets. The two components, white blood cells and plasma, make up lymph. In the lymphatic system, there are valves that regulate flow similar to valves in the veins. The lymphatic vessels bulge at lymph nodes. These contain white blood cells that remove bacteria and foreign materials. An infection may cause the nodes in your neck and armpits to swell. Lymph and blood are related through the exchange of tissue fluid. Tissue fluid is like blood plasma and it seeps through the capillary walls. The filtered plasma, tissue fluid, bathes the cells of the body. Through the capillaries of the lymphatic system, the tissue fluid returns to the blood. Altogether, the lymph, tissue fluid, and blood work to maintain homeostasis, the stable environment of your body. The ability to maintain this stable internal environment makes your body much more independent of its external environment. Your good health depends very much upon the complex functions of that marvelous stream of life, your blood.