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Blood is a specialized connective tissue that circulates through the cardiovascular system, delivering essential substances to tissues and removing waste. In mammals and many other animals, it consists of a liquid matrix called plasma and a population of formed elements, including red blood cells, white blood cells, and platelets.

Plasma makes up about 55% of blood and is mainly water with dissolved proteins, electrolytes, nutrients, hormones, and waste products. Major plasma proteins include albumin (which helps maintain blood volume and pressure), globulins (including antibodies), and fibrinogen (involved in clotting).

Formed elements include erythrocytes (red blood cells), leukocytes (white blood cells), and platelets (thrombocytes). Erythrocytes carry hemoglobin, which binds oxygen and carbon dioxide; they are typically anucleate in mammals and live about 120 days. Leukocytes defend against infection and coordinate immune responses; platelets participate in hemostasis with the coagulation cascade. All formed elements originate in the bone marrow (erythropoiesis, leukopoiesis, thrombopoiesis).

Functions of blood include transport of oxygen, nutrients, hormones, and waste; regulation of pH and body temperature; and immune defense. Blood also participates in hemostasis, wound healing, and maintaining fluid balance.

Blood circulates in two main circuits (systemic and pulmonary) through arteries, capillaries, and veins, propelled by the heart. Blood groups, such as ABO and Rh, are important for transfusion safety and organ compatibility. Normal adult blood volume is about 5 liters, and blood tests (for example, the complete blood count or blood typing) are common tools in medicine.