The blood is a highly complex medium of transport with many specialized cells performing immune, clotting, nutritional, waste removal, oxygen:carbon dioxide exchange, and temperature regulation functions. Many systems influence the blood. Plasma is the transport medium, transporting erythrocytes (red blood cells), which carry oxygen and carbon dioxide, and leukocytes (white blood cells), which are involved in regulatory and immune system functions. With age, comes a reduction in albumin levels resulting in reduced osmotic pressure and potential for edema ( ) and ascites (abdominal fluid formation).

As the kidneys age, there may be higher levels of wastes in the blood. In older dogs with clotting disorders, such as von Willebrand's Disease, minor injuries can become major problems. Some older dogs, especially those which have had heartworm infections may be susceptible to pulmonary thromboemboloic (PTE) disease when thrombi (blood clots) block the arteries, especially to the lungs.

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