Blood

Blood is a specialized connective tissue that circulates through the body's arteries, veins, and capillaries. It consists of a liquid component, plasma, and suspended cells known as the formed elements. Plasma accounts for about 55% of blood and is mostly water, with dissolved proteins, electrolytes, nutrients, hormones, and waste products. The formed elements are red blood cells, white blood cells, and platelets.

Red blood cells carry oxygen from the lungs to tissues via hemoglobin and also help transport carbon dioxide back to the lungs for exhalation. They have a lifespan of about 120 days and are produced in the bone marrow in a process called erythropoiesis, regulated in part by the hormone erythropoietin.

White blood cells defend the body against infection and participate in immune responses. Platelets, together with a cascade of coagulation factors, promote blood clotting to stop bleeding. The plasma contains key proteins such as albumin, globulins, and fibrinogen that help maintain blood pressure, immunity, and clot formation.

Blood types are determined by surface antigens, most notably the ABO group and the Rh factor. Compatibility is essential for transfusions. Type O negative is a universal donor in emergencies, while AB positive is a universal recipient in terms of ABO typing.

Hematopoiesis is the production of blood cells, mainly in the bone marrow. Blood circulates through the cardiovascular system, delivering oxygen and nutrients, removing wastes, and helping regulate body temperature and pH balance.

Clinical relevance includes anemia, polycythemia, leukemias, and bleeding disorders. Laboratory tests such as the complete blood count measure components like hematocrit and hemoglobin. Treatments include transfusion, iron or vitamin therapy, and medications addressing clotting disorders.