Paul A. Iaizzo - Handbook of Cardiac Anatomy, Physiology, and Devices

Blood is composed of formed elements (cells and cell fragments) which are suspended in the liquid fraction known as plasma. Blood, often considered as the only liquid connective tissue in the body, has three general functions: (1) transportation (e.g., O2, CO2, nutrients, waste, hormones), (2) regulation (e.g., pH, temperature, osmotic pressures), and (3) protection (e.g., against foreign molecules and diseases, as well as for clotting to prevent excessive loss of blood). Dissolved within the plasma are many proteins, nutrients, metabolic waste products, and various other molecules being transported between multiple organ systems.

The formed elements in blood include red blood cells (erythrocytes), white blood cells (leukocytes), and the cell fragments known as platelets. These are all formed in bone marrow from a common stem cell. In a healthy individual, the majority of bloods cells are red blood cells (~99 %) which have a primary role in O2 exchange. Hemoglobin, the iron-containing heme protein which binds oxygen, is concentrated within the red cells; hemoglobin allows blood to transport 4050 times the amount of oxygen that plasma alone could carry. The white cells are required for the immune process, e.g., to protect against infections and also cancers. Platelets play a primary role in blood clotting. In a healthy cardiovascular system, the constant movement of blood helps keep these various cells and plasma constituents well dispersed throughout the larger-diameter vessels.

The hematocrit is defined as the percentage of blood volume that is occupied by the red cells (erythrocytes). It can be easily measured by centrifuging (spinning at high speed) a sample of blood, which forces these cells to the bottom of the centrifuge tube. The leukocytes remain on the top and the platelets form a very thin layer between the cell fractions (other more sophisticated methods are also available for such analyses). Normal hematocrit is approximately 45 % in men and 42 % in women. The total volume of blood in an average-sized individual (70 kg) is approximately 5.5 L; hence, the average red cell volume would be roughly 2.5 L. Since the fraction containing both leukocytes and platelets is normally relatively small or negligible, in such an individual, the plasma volume can be estimated to be 3.0 L. Approximately 90 % of plasma is water which acts: (1) as a solvent, (2) to suspend the components of blood, (3) in the absorption of molecules and their transport, and (4) in the transport of thermal energy. Proteins make up 7 % of the plasma (by weight) and exert a colloidc osmotic pressure. Protein types include albumins, globulins (antibodies and immunoglobulins), and fibrinogen. To date, more than 100 distinct plasma proteins have been identified, and each presumably serves a specific physiologic function. The other main solutes in plasma include: electrolytes, nutrients, gases (some O2, large amounts of CO2 and N2), regulatory substances (enzymes and hormones), and waste products (urea, uric acid, creatine, creatinine, bilirubin, and ammonia).

Blood flows throughout the bodys tissues within blood vessels via bulk flow (i.e., all constituents together and in one direction). An extraordinary degree of vascular branching exists within the human body, which ensures that nearly every cell in the body lies within a short distance from at least one of the smallest branches of this systema capillary. Nutrients and metabolic end products move between the capillary vessels and the surroundings of the cell through the interstitial fluid by diffusion. Subsequent movement of these molecules into a cell is accomplished by both diffusion and mediated transport. Nevertheless, blood flow through all organs can be considered as somewhat passive and occurs only because arterial pressure is kept higher than venous pressure via the pumping action of the heart.

In an individual at rest at any given moment, approximately 5 % of the total circulating blood is actually within the capillaries. Yet, this volume of blood can be considered to perform the primary functions of the entire cardiovascular system, specifically the supply of nutrients and removal of metabolic end products. The cardiovascular system, as reported by the British physiologist William Harvey in 1628, is a closed-loop system, such that blood is pumped out of the heart through one set of vessels (arteries) and then returns to the heart in another (veins).