Blood supplies cells with water, electrolytes, nutrients, and hormones and removes waste products. The cellular elements supply oxygen (RBCs), protect against foreign organisms and antigens (WBCs), and initiate coagulation (platelets). Because of the diversity of the hematopoietic system, its diseases are best discussed from a functional perspective. Function may be classified as either normal responses to abnormal situations (eg, leukocytosis and left shift in response to inflammation) or primary abnormalities of the hematopoietic system (eg, pancytopenia from marrow failure). Furthermore, abnormalities may be quantitative (ie, too many or too few cells) or qualitative (ie, abnormalities in function). (Also see The Biology of the Immune System.)
Also see pet health information regarding an introduction to blood.