Blood

Blood serves many useful purposes in regulating normal body function. The three functions of primary importance are to exercise and sport are:

• Transportation,
• Temperature regulation,
• Acid-base(pH) balance.

We are most familiar with blood’s transporting functions. In addition, blood is critical in temperature regulation during physical activity; it picks up heat from the exercising muscle and delivers it to the skin where it can be dissipated to the environment. Blood also buffers the acids produced by anaerobic metabolism and maintains proper pH for metabolic processes.

Blood volume and composition

The total volume of blood in the body varies considerably with an individual’s size and state of training. Larger blood volumes are associated with greater lean body mass and higher levels of endurance training. The blood volume of people of average body size and normal physical activity generally ranges from 5 to 6 L in men and 4 to 5 L in women.

Blood is composed of plasma and formed elements. Plasma normally constitutes about 55% to 60% of total blood volume but can decrease by 10% of its normal amount or more with intense exercise in heat, or can increase by 10% or more with endurance training or acclimation to heat. Approximately 90% of the plasma volume is water; 7% consists of plasma proteins; and the remaining 3% includes cellular nutrients, electrolytes, enzymes, hormones, antibodies and wastes.

The formed elements, which normally constitute the other 40% to 45% of total blood volume, are the red blood cells(erythrocytes), white blood cells(leukocytes), and platelets(thrombocytes). Red blood cells constitute more than 99% of the formed-element volume; white blood cells and platelets together account for less than 1%. The percentage of the total blood volume composed of cells or formed elements is reffered to as the hematocrit.

White blood cells protect the body from infection either by directly destroying the invading agents through phagocytosis(ingestion) or by forming antibodies to destroy them. Adults have about 7,000 white blood cells per cubic millimeter of blood.

The remaining formed elements are the blood platelets. These are cell fragments that are required for blood coagulation(clotting), which prevents excessive blood loss. Exercise physiologists are most concerned with red blood cells.

Red blood cells

Mature red blood cells(erythrocytes) have no nucleus, so they cannot reproduce as other cells can. They must be replaced with new cells on a reoccurring basis, a process called hematopoiesis. The normal life span of a red blood cell is about four months. Thus, these cells are continuously produced and destroyed at about equal rates. This balance is very important, because adequate oxygen delivery to tissues depends on having a sufficient number of red blood cells to transport oxygen. Decreases in their number of function can hinder oxygen delivery and thus affect exercise performance.

Red blood cells transport oxygen, which is primarily bound to hemoglobin. Hemoglobin is composed of a protein(globin) and a pigment(heme). Heme contains iron, which binds oxygen. Each red blood cell contains approximately 250 million hemoglobin molecules, each able to bind four oxygen molecules – so each red blood cell can bind up to a billion molecules of oxygen! There is an average of 15g of hemoglobin per 100ml of whole blood. Each gram of hemoglobin can combine with 1.33ml of oxygen, so as much as 20ml of oxygen can be bound for each 100ml of blood. Therefore, when arterial blood is saturated with oxygen, it has an oxygen-carrying capacity of 20 ml of oxygen per 100ml of blood.

Blood viscosity

Viscosity refers to the thickness of the blood. Recall from our discussion of vascular resistance that the more vicious a fluid, the more resistant it is to flow. The viscosity of blood is normally about twice that of water. Blood viscosity, and thus resistance to flow, increases with higher hematocrits.

Because of oxygen transport by the red blood cells, an increase in their number would be expected to maximize oxygen transport. But if an increase in red blood cell count is not accompanied by a similar increase in plasma volume, blood viscosity and vascular resistance will increase, which could result in reduced blood flow. This generally is not a problem unless the hematocrit reaches 60% or more.

Conversely, the combination of a low hematocrit with a high plasma volume, which decreases the blood’s viscosity, appears to have certain benefits for the blood’s transport function because the blood can flow more easily. Unfortunately, a low hematocrit frequently results from a reduced red blood cell count, as in diseases such as anemia. Under these circumstances, the blood can flow easily, but it contains fewer carriers, so oxygen transport is impeded. For optimal physical performance, a low hematocrit with a normal or slightly elevated number of red blood cells is desirable. This combination facilitates oxygen transport. Many endurance athletes achieve this combination as part of their cardiovascular system’s normal adaptation to training.