The Major Components of Blood

Among all the body's systems, the blood is unique: it is the only tissue in the body that flows. This flowing tissue, endlessly making its course from the heart to the remotest parts of the body and returning, is a sea in which the body is bathed. Blood has two distinct parts (Figure 2). Plasma, the liquid part of the blood, makes up about 55% of the blood volume. Since there is a total of 5 liters of blood in the body of an average adult, the plasma volume (PV) in the body is about 2.75 liters. Plasma is a yellowish solution consisting of about 91% water, and the other 9% is a host of substances indispensable to life. Among them are: nutrients such as glucose, fats, and amino acids; chemicals important to the body, such as sodium, potassium, and calcium; special proteins, such as fibrinogen, albumin, and various globulins that produce antibodies, which fight off viruses and other unwelcome intruders in the body; and hormones, which are regulatory substances such as insulin, and epinephrine, more familiarly known as adrenaline, which speeds up the heart rate whenever some emergency requires a greater blood flow to the muscles.

The role of plasma in the body is to help transport food and oxygen to the cells of the body and to carry wastes away from the cells. In addition, with its potent arsenal to draw upon, plasma plays a crucial role in maintaining the body's chemical balance, water content, and temperature at a safe level. That is, the plasma serves the body by helping to maintain homeostasis, or a stable internal environment in the body. In fact, essentially all the organs, tissues, and fluids of the body perform functions that help to maintain the body as a stable system. By analyzing plasma, medical doctors can find out what types of nutrients are circulating throughout the body, and they can measure the levels of hormones and other constituents that plasma helps to transport.

The cellular portion of blood normally makes up about 45% of the blood volume and it consists primarily of three cellular components (Table 1): white blood cells (WBCs, also known as leukocytes), platelets, and red blood cells (RBCs, also known as erythrocytes). The WBCs constitute the blood's mobile security system. Some WBCs are endowed with the curious ability to wiggle out of the bloodstream and back in again. The WBCs can move like an amoeba, slipping through thin walls of capillaries and wandering among cells and tissues. They converge together in great numbers wherever invading bacteria, viruses, fungi, or parasites gain entry into the body, destroying them by swallowing them or by synthesizing antibodies, which are complex proteins that react with and destroy these foreign substances. Whenever white cells mobilize for action, the body compensates by manufacturing more. Double the usual number may appear in the blood within hours. Often this rising white cell count, as physicians describe it, serves as an early tip-off that a dangerous infection has entered the body.

Table I. Comparison of some characteristics of the cellular components of blood.

The smallest of blood's three cellular components are the platelets, named for their resemblance to tiny plates. Their main function was discovered when doctors observed that people with low platelet counts were especially vulnerable to bleeding. It is now well known that platelets are vital to blood clotting. When they touch the roughened surface of a torn blood vessel, they burst apart, releasing chemicals that set off a reaction in the blood leaking out. The result is that they convert one of the plasma's proteins, fibrinogen, into a network of fibers that trap RBCs - thereby forming a clot which seals the leak.

The main focus of this chapter will be the examination of the most abundant of all the cells of the body, the red blood cells, or RBCs. These RBCs outnumber the WBCs about 700 to 1. Theirs is the exclusive and all- important job of picking up oxygen in the lungs, carrying it to the rest of the body, and carrying waste carbon dioxide back the other way. Their life is hectic and brief: after about three or four months they grow old, are eaten, and then replaced by new recruits sent into the bloodstream from the bone marrow. As mentioned previously, the study of the activity of the RBCs is called erythrokinetics. Erythrokinetics involves looking at the entire lifetime of a RBC from its "birth" (it is born in the bone marrow), through its passage around the body (each RBC travels around the body in about a minute), all the way through its destruction. A normal lifetime for each RBC is 90 to 120 days.

The RBC's effectiveness as an oxygen-carrier is due to its content of hemoglobin, a compound of protein and iron which gives blood its red color. Each RBC contains nearly 300 million hemoglobin molecules! And, when a person lives in an area at sea level where atmospheric pressure equals 760 torr each hemoglobin molecule can carry four oxygen molecules, which means that each RBC can carry 1.2 billion oxygen molecules! Hemoglobin has a chemical way of latching onto oxygen as the blood passes through the lungs and holding it in its grip until the destination is reached. As the blood passes through the tissue capillaries, the hemoglobin will not release oxygen into the tissues if too much oxygen is already there, but, if the oxygen concentration is too low, sufficient oxygen will be released by the hemoglobin to re-establish an adequate tissue oxygen concentration. (This is another example of a homeostatic control mechanism.) When, for any reason, the hemoglobin content in the bloodstream dips below the minimum for body needs, the result is anemia, meaning (although not literally) "no blood." Anemia is a reduction in the number or volume of RBCs. This reduction in RBCs results, obviously, in a reduction in the amount of hemoglobin and, therefore, in a reduction in the body's oxygen-carrying capacity. Another cause of anemia could be a diet deficient in iron-rich foods.

Normal RBCs are biconcave disks that are capable of changing their shape as they- pass through capillaries. Actually, the RBC is a "bag" that can be deformed into almost any shape without rupturing the cell. They are remarkably flexible and remarkably small. In normal men (if there are any!!), the average number of RBCs per cubic millimeter is 5,200,000 ( 300,000) and in normal women 4,700,000 ( 300,000). The number of RBCs varies in the two sexes and at different ages. Also, if a person moves to a higher altitude, for instance, to the mountains), the number of RBCs present in that person may not be enough to supply the body with oxygen. This is because as you go into higher altitudes, the atmospheric pressure becomes lower. For each liter of air you breathe at lower atmospheric pressures, there are fewer molecules of air, including fewer molecules of oxygen. With fewer molecules of oxygen available, your lungs are not able to supply each RBC with the amount of oxygen to which it is accustomed. In this condition, the RBCs cannot deliver enough oxygen to the body (including the brain) and you will become dizzy. Therefore, the body responds by increasing the production of RBCs so that the oxygen needs of the body can be met.