Blood is composed of formed elements (cells and cell fragments) which are suspended in the liquid fraction known as plasma.

Blood has three general functions:

1. Transportation: e.g., oxygen, carbon dioxide, nutrients, wastes, and hormones;
2. Regulation: e.g., pH, temperature, and 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 the various organ systems within the body.

The elements in blood are formed from common bone marrow stem cells and include:

1. red blood cells (erythrocytes),
2. white blood cells (leukocytes) and
3. cell fragments known as platelets.

In a healthy individual, red blood cells (RBCs) make up ~99% of cells which plays a primary role in O2 and CO2 exchange. Hemoglobin, the iron-containing heme protein which binds oxygen, is concentrated within these cells; hemoglobin allows blood to transport 40 to 50 times the amount of oxygen that plasma alone could carry. White cells are required for immune processes; i.e., to protect against infections and also cancers. The platelets play a primary role in blood clotting. In a healthy cardiovascular system, the constant movement of blood helps keep these cells well dispersed throughout the plasma of the larger diameter vessels. The total volume of blood in an average sized individual (70 kg) is approximately 5.5 liters.

Approximately 90% of plasma is water which acts:

1. as a solvent;
2. to suspend the components of blood;
3. in absorption of molecules and their transport; and
4. in the transport of thermal energy.

Proteins make up 7% of the plasma (by weight) and produce a colloid 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 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).