Blood is one of the connective tissues. As a connective tissue, it consists of cells and cell fragments (formed elements) suspended in an intercellular matrix (plasma). Blood is the only liquid tissue in the body that measures about 5 liters in the adult human and accounts for 8 percent of the body weight.

The body consists of metabolically active cells that need a continuous supply of nutrients and oxygen. Metabolic waste products need to be removed from the cells to maintain a stable cellular environment. Blood is the primary transport medium that is responsible for meeting these cellular demands.

Blood cells are formed in the bone marrow, the soft, spongy center of bones. New (immature) blood cells are called blasts. Some blasts stay in the marrow to mature. Some travel to other parts of the body to mature.

The activities of the blood may be categorized as transportation, regulation, and protection.

These functional categories overlap and interact as the blood carries out its role in providing suitable conditions for celluar functions.

The transport functions include:

  • Carrying oxygen and nutrients to the cells.
  • Transporting carbon dioxide and nitrogenous wastes from the tissues to the lungs and kidneys where these wastes can be removed from the body.
  • Carrying hormones from the endocrine glands to the target tissues.

The regulation functions include:

  • Helping regulate body temperature by removing heat from active areas, such as skeletal muscles, and transporting it to other regions or to the skin where it can be dissipated.
  • Playing a significant role in fluid and electrolyte balance because the salts and plasma proteins contribute to the osmotic pressure.
  • Functioning in pH regulation through the action of buffers in the blood.

The protection functions include:

  • Preventing fluid loss through hemorrhage when blood vessels are damaged due to its clotting mechanisms.
  • Helping (phagocytic white blood cells) to protect the body against microorganisms that cause disease by engulfing and destroying the agent.
  • Protecting (antibodies in the plasma) against disease by their reactions with offending agents.