The Circulatory System, the Heart, and Blood - Blood and our internal fluid environment

What makes this fairly rudimentary collection of plumbing so absolutely indispensable to life is the fluid it pumps—blood. If any part of the body—cell, tissue, or major organ—is denied circulating blood and the substances it carries with it for longer than a few minutes, that part will fail. It is the job of the heart and the blood vessels to get blood to all the body's far-flung tissues, where it both picks up and deposits substances.

Blood is really a kind of fluid tissue. About 80 percent of its volume is water, and blood's indispensable, life-sustaining power is owed in great part to its watery base, which permits it both to flow and to take up and carry materials in solution. All our tissues and organs have a kind of give-and-take arrangement with the circulating blood.

The Circulatory System

Carrier of Oxygen

Perhaps the most critical of these give-and-take transactions occurs in the lungs; it is this transaction that if interrupted by heartbeat stoppage for more than a few minutes causes death by oxygen starvation of vital tissues. Before a unit of blood is pumped out by the heart to the body, it picks up in the lungs the oxygen that we have inhaled and which every cell in the body needs to function. The blood then transports the oxygen, delivering it to other parts of the body. By the time a given unit of blood has made a tour of the blood vessels and returned to the lungs, it has given up most of its oxygen and is laden instead with carbon dioxide, the principal waste product of living processes. The venous blood releases its carbon dioxide, to be exhaled by the lungs.

Distributor of Nutrients

Food, or more accurately the nutrient molecules needed by cells, are also transported throughout the body by the blood. In the digestive tract, food is broken down into tiny submicroscopic pieces that can pass through the tract's walls (mainly along the small intestine) and be picked up by the blood for distribution around the body.

One of the specialized, small-volume transportation jobs handled by the blood is to pick up hormones from the endocrine glands and present these chemical messengers to the organs they affect.

Composition of Blood

Blood is a distinctive and recognizable type of tissue, but this does not mean it is a stable, uniform substance with a fixed proportion of ingredients. Quite the opposite is true; its composition is ever changing in response to the demands of other body systems. Other organs are constantly pouring substances into the blood, or removing things from it. Blood in one part of the body at a given moment may be vastly different in chemical makeup from blood in another part of the body.

Despite its changing makeup, blood does have certain basic components. A sample of blood left to stand for an hour or so separates into a clear, watery fluid with a yellowish tinge and a darker, more solid clump. The clear yellow liquid is called plasma , and accounts for about 55 percent of the volume of normal blood. The darker clump is made up mainly of the blood's most conspicuous and populous inhabitants, the red cells that give blood its color.


It is the plasma that enables our blood to carry out most of the transportation tasks assigned it. Being over 90 percent water, the plasma has water's property of being able to carry substances both in solution and in suspension. (A substance in solution is one, like salt, that must be removed from water by chemical or physical action, such as boiling; while a substance in suspension—such as red blood cells within whole blood in a standing test tube—separates out more readily, particularly when its watery carrier has been contained and its flow stilled.)

Red Blood Cells

Red blood cells (or erythrocytes ) numbering in the trillions are carried in suspension by the plasma. In turn, the red blood cells carry the single most important substance needed by the body's cells—oxygen. For such an important task, the red blood cell looks hardly adequate. As it matures, this cell loses its nucleus. Lacking a nucleus, it is sometimes not even called a cell but a red blood corpuscle . What gives red blood cells their special oxygen-carrying ability, and also their color, is their possession of a complex iron-protein substance called hemoglobin .


Molecules of hemoglobin have the property of loosely combining with oxygen where it is plentiful, as in the lungs. They can then hold on to oxygen until they reach an area where oxygen has been depleted by the demands of living processes. There—usually in the fine tubes of the capillaries—hemoglobin's hold on oxygen is challenged by the demands of other cells, and the red cells give up their oxygen. The hemoglobin of red cells develops an immediate affinity for carbon dioxide, the waste product of cell metabolism, and the red blood cells then carry this carbon dioxide back to the lungs for exhalation.

Hemoglobin's ability to carry oxygen is not unique. Water, and therefore plasma, also have this ability. Hemoglobin's specialness lies in how much oxygen it can carry. Hemoglobin increases by more than 50 times the oxygen-carrying capacity of our blood.

White Blood Cells

White blood cells have many different shapes and sizes, all going under the general scientific name of leukocytes . They are typically larger than red blood cells, but far less numerous. If we accept an estimate of 25 trillion as the number of living red blood cells in our body, then the number of white blood cells might be generously estimated at around 40 billion, a ratio of one white cell to about 600-700 reds.

According to their shape, size, and other characteristics, white blood cells have been divided into various categories such as lymphocytes, monocytes, and granulocytes. But as a group these blood cells are distinguished by their common propensity for attacking foreign bodies that invade our tissues, whether these invaders be sizable splinters or microscopic bacteria. White blood cells move in force to the site of an infection, do battle with the intruding agents, and frequently strew the area with the wreckage of the encounter—a collection of dismantled alien bacteria and dead white cells, which we know as pus.


Platelets , also called thrombocytes , initiate some of the first steps in the complex biochemical process that leads to the clotting of blood. They thus help to spare us from bleeding to death from a slight injury. Platelets are the most rudimentary and diminutive of the major blood components. Like mature red blood cells, they lack nuclei, but are only one-quarter as big. By no stretch of the imagination can they be called blood cells. Rather, they are blood elements—bits of cell substance with a recognizable size and shape, circulating with the blood.

The Proportions of Blood Cells

All the several types of blood cells and subcells in a healthy body occur in proportions that, though never precisely fixed and unchanging, are recognized as having normal upper and lower limits. If a particular type of cell shows a sudden increase or decrease in population, so that its proportion relative to other blood cells shows a variation markedly outside its normal range, some infection, disease, or disorder must be suspected.

In addition to occurring in certain normal-range proportions, each type of cellular blood component has a typical shape, appearance, and set of chemical and physical properties. Variations from these norms occur in many diseases.

The analysis of blood samples (usually taken from the finger or arm) and their inspection under a microscope have proved invaluable in diagnosing illness and disease, often before a person feels any symptoms whatsoever. This is why a thorough medical checkup should always include taking a sample of your blood. It is then up to the physician to decide which of the dozens of tests should be made on your blood in the medical laboratory. One common test is a blood count , in which the number of a certain type of cell in a given unit of your blood can be estimated, and then compared to the normal number in the same amount of blood.

Blood Groups and Rh Factors

The identification of blood groups and Rh factors is another aspect of blood analysis. The four most common blood groups are called A, B, AB, and O, classifications based on chemical differences that may be incompatible if one group is mixed with another. Thus, it is absolutely essential before a person receives a blood transfusion to know both his own blood type and the type of the blood he is to be given. Blood group O is considered the safety for transfusion, and people with type O blood are sometimes called “universal donors.” It is a wise practice to carry, along with your other important cards, a card giving your own blood type. The blood of a donor, however, is always cross-matched (checked for compatibility) with the blood of the person who is to receive it in order to avoid transfusion reactions.

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