Blood is a specialized connective tissue that circulates through the body, transporting oxygen, nutrients, hormones, and waste while helping regulate body temperature and protect against infection. It is made up of plasma, the liquid portion of blood, and formed elements, which include erythrocytes (red blood cells) that carry oxygen using hemoglobin, leukocytes (white blood cells) that defend the body against pathogens, and platelets (thrombocytes) that help blood clot. Blood cells are produced in the red bone marrow through a process called hematopoiesis, and red blood cell production is regulated by the hormone erythropoietin released by the kidneys. Blood types are determined by the ABO blood group system and the Rh factor, which are based on the presence or absence of specific antigens on the surface of red blood cells. Several diseases affect blood, including anemia (low red blood cell levels), sickle cell disease (a genetic disorder that alters hemoglobin and causes sickle-shaped cells), and leukemia (a cancer of white blood cells that disrupts normal blood cell production).

Term	Definition
Plasma	Liquid portion of blood
Hemoglobin	Oxygen-carrying protein in red blood cells
Leukocytes	White blood cells involved in immunity
Platelets	Cell fragments involved in clotting
Hematopoiesis	Production of blood cells
Erythropoietin	The hormone that stimulates red blood cell production
Erythrocyte	A red blood cell responsible for transporting oxygen.
Homeostasis	The maintenance of stable internal conditions in the body.
Anemia	A condition characterized by low red blood cell levels or hemoglobin.
Sickle Cell Disease	A genetic disorder that affects hemoglobin and alters red blood cell shape.
Leukemia	A cancer affecting white blood cells.