A single drop of blood contains more than 250 million separate blood cells floating in a straw-colored fluid called plasma. Blood cells constitute about 40 percent of the total volume of blood, which, in a mature adult of average size, amounts to between 1 and 1.3 gallons (3.8 and 4.9 liters), roughly 7 percent of body weight.

You have three types of blood cells, each of which performs a different function. Red blood cells, or erythrocytes, transport oxygen and carbon dioxide; white blood cells, or leukocytes, defend the body against disease and other hostile intrusions; and your platelets, or thrombocytes, are key elements in blood clotting.

Plasma, which makes up about 55 percent of the blood, is over 90 percent water. Yet plasma contains thousands of different substances, including proteins, glucose, salts, vitamins, hormones, antibodies, and wastes – almost everything the body uses. Thanks to plasma, blood flows freely and distributes materials to all parts of the body that need them for nourishment and protection.


Why is Blood Red?

Interestingly enough, if you look at a tiny smear of blood under a microscope, it doesn’t look red at all, but yellowish. The red color shows only when great masses of cells are seen together. It comes from hemoglobin, an iron-containing red pigment that is the main component of red cells. The redness of any blood sample varies according to the amount of oxygen it is carrying. Oxygen-laden arterial blood is bright scarlet. As oxygen is removed, venous blood becomes a darker, bluish purple.

What Do Red Cells Do?

One of the most important properties of hemoglobin is its unique ability to combine loosely with oxygen when the two substances come together. That happens in the lungs, where each passing hemoglobin molecule picks up as many as four oxygen molecules and transports them via the bloodstream to the body’s tissues. Red blood cells also are important in the transport of carbon dioxide, the gas that is produced by the breakdown of nutrients.

The red blood cells are small, thin, and disk-shaped, with depressions on both sides. They are by far the most numerous of the solid elements in the bloodstream. At any given time, the body contains perhaps 25 trillion of them, enough, if spread out, to cover four tennis courts. The red blood cells work extraordinarily hard, circuiting the system some 300,000 times before they finally wear out and disintegrate after a life of about 120 days. Replacements are made, at a rate of 3 million new cells every second, in the bone marrow; there they are taken up by the capillary network and sent on their way through the bloodstream.

Are White Blood Cells Really White?

The body’s defenders – the soldiers that fight bacteria and other enemies – are the so-called white cells. Their name, like that of the red cells, is somewhat misleading: they actually look not white but colorless.

While there is only one kind of red cell, white blood cells come in many varieties, each type capable of fighting the body’s battles in a different way. One kind, for instance, destroys dead cells. Other kinds produce antibodies against viruses, detoxify foreign substances, or literally eat up and digest bacteria.

The life span of white cells varies, depending on the challenges they encounter as they travel through the bloodstream to the sites where they are needed. Despite their essential role in keeping the body alive and well, white cells are few compared with the red cells, numbering only 1 for every 700 red cells. The white cells are manufactured in several places. Some are made in the bone marrow; others originate in the lymph nodes, the spleen, the thymus, the tonsils, and other parts of the lymph system.

What Makes Blood Clot?

At times of sudden danger, when a blood vessel has been cut or damaged and blood is escaping, platelets come swiftly to the rescue – and die in the process. Platelets stick to the edges of a wound, secrete a substance that summons other platelets to take part in the life-saving mission, clump together, and, if the wound is minor, close it with a platelet plug. If the wound is really bad, the platelets set off a series of chemical reactions that cause a clot to form and seal the hole.

The body has more platelets (15 million in each drop of blood) than white cells, but fewer than red ones. They are formed in the bone marrow. And they get their name from their shape: under a microscope, they look like round or oval plates. Platelets have a life span of five to eight days.

How does Altitude Affect the Composition of Blood?

As the many people who go mountain climbing know quite well, the higher the altitude they reach the lower the level of oxygen available to them. The human body has fairly fixed oxygen requirements, and in this situation, a built-in form of a adaptation comes into play.

When oxygen is in short supply, the kidneys, which monitor blood as it flows through them, step up their production of a hormone called eryth-ropoietin. (The liver produces this hormone as well.) When the hormone reaches the bone marrow, the manufacture, of red blood cells is accelerated. If people remain for several weeks at an altitude of 14,000 feet (4,267 meters) or higher, the production of red blood cells in their bodies may rise as much as 30 to 40 percent. The greater the number of red blood cells produced, the more oxygen will ultimately reach its destination in body tissues.