Blood Volume

As a general matter of health, optimum blood volume is essential to the proper function of the body's cardiopulmonary and circulatory systems. Blood is a significant mass within the body, with an average volume for an adult of 51. liters (1); blood typically represents approximately 8% of body mass. For an elite-level endurance athlete, the blood volume may be 30% greater than that of the average adult.

Blood is composed of solid and liquid elements. Plasma, composed of 90% water, is the liquid that transports various types of dissolved particles as well as blood cells. The cells contained the human blood stream are erythrocytes, leukocytes, and platelets. Erythrocytes are the red blood cells that contain hemoglobin, the protein that transports oxygen within the body, as well as carry the waste carbon dioxide produced by the body. Leukocytes are the white blood cells that are responsible for attacking infection and other hostile organisms within the body. There are five different types of white blood cells operating within the bloodstream. Platelets are the colorless bodies that adhere to one another to create blood clots that stop bleeding both in exterior wounds as well as internal bleeding.

The body regulates blood volume through the operation of the kidneys. When the kidneys determine that the blood volume is too high, water and sodium are excreted into the urine. To reverse this process, the kidney will retain sodium and increase water levels within the body. As blood, through plasma, is 90% water, this process has an immediate impact upon blood volume.

The relationship between blood volume and blood pressure is an important one. Blood pressure is a function of the amount of blood pumped by the heart and the degree of resistance to the flow of blood made by the arteries. The circulatory system, the interconnected series of arteries, veins, and capillaries through which blood flows in the body, has a number of internal mechanisms by which blood pressure is controlled. When blood pressure is too great, the heart is required to work harder than is desirable, among other effects. When blood pressure is too low, it may be indicative of low blood volume, or other conditions. Increased blood volume can cause a corresponding high blood pressure.

Blood volume is of particular interest to an endurance athlete. One of the first physiological changes observed in an athlete who begins an endurance training program is an increase in blood volume. This volume increase is the body's response to the demands placed upon it by this type of training. Greater amounts of oxygen are required to be transported by the red blood cells, along with greater demands internally for fluid, due to increased needs to cool the body through sweating.

The legitimate increases in the blood volume of an endurance athlete will be the result of determined and focused training. The natural expansion of blood volume can be enhanced through altitude acclimatization, or altitude training, as well as through heat training. The decreased amount of oxygen in high altitudes stimulates the body into the production of the hormone erythropoietin (EPO) made in the kidneys. EPO then triggers the production of more red blood cells to transport sufficient oxygen to make up the deficiencies at altitude. Significant blood volume benefits from altitude training occur within one to three months of commencement; the benefits are retained on a declining basis for up to three months.

When endurance athletes train in temperatures in excess of 77°F (25°C), the body will become accustomed to retaining the mineral sodium, which is commonly excreted through the skin' pores when the body is exposed to significant heat (sweating). The greater the level of sodium remaining in the body, the greater the amount of water that will be retained by the body, which has the effect of maintaining blood volumes. Blood volumes will be increased through hot weather training within 14 days.

Since the relationship between blood volume, oxygen transport ability, and endurance performance became well established, athletes have used illegal methods to increase blood volume. In recent years, these efforts have centered on the injection of a synthetic form of EPO, to increase red blood cell production. A related class of pharmaceuticals, known as plasma expanders, also achieved the goal of greater blood volume. EPO and plasma expanders are prohibited substances in international competitions and world championships; detection of such substances in the urine samples of athletes will disqualify them, as well as subject them to a suspension from competition.

Unless the person illegally ingesting these substances possessed a high baseline of physical conditioning, such as a very fit, highly trained endurance athlete, the additional red blood cell and blood volume capabilities these techniques afforded would be negligible. Further, artificial increases to the red blood cell levels in the bloodstream expose the user to greater risk of stroke, due to the denser, more viscous nature of the modified blood.

see also Acclimatization; Blood doping; Cardiovascular system; EPO; World Anti-Doping Agency (WADA).