Blood cells form and develop mostly in the bone marrow, that is, the tissue located in the cavities of bones. Blood performs a variety of important functions as it circulates throughout the body. It delivers oxygen and vital nutrients (such as vitamins, minerals, fats, and sugars) to the tissues. It carries carbon dioxide to the lungs to be exhaled and waste products to the kidneys to be eliminated from the body. It transports hormones, which are chemical messengers, to various parts of the body, allowing those parts to communicate with each other. Blood also includes cells that fight infection and platelets that control bleeding.

Blood is a complex mixture of plasma (the liquid component), red blood cells, white blood cells, and platelets.

There are 3 cellular elements of blood: red blood cells, white blood cells, and platelets. Basically, red blood cells supply the body with oxygen, white blood cells protect against infection, and platelets start the formation of blood clots.

Types of Blood Cells*
*See chapter text for descriptions and functions.

Last full review/revision July 2011 by Peter H. Holmes, BVMS, PhD, Dr HC, FRCVS, FRSE, OBE; David J. Waltisbuhl, BASc, MSc; Michael Bernstein, DVM, DACVIM; Karen L. Campbell, MS, DVM, DACVIM, DACVD; Nemi C. Jain, MVSc, PhD; Wayne K. Jorgensen, BSc, PhD; Sarah E. Payne, DVM, DACVIM