what does blood do in the human body
Blood Blood is a body fluid in humans and other animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells.[1]
Blood Blood accounts for 7% of the human body weight,[3][4] with an average density around 1060 kg/m3, very close to pure water's density of 1000 kg/m3.[5] The average adult has a blood volume of roughly 5 litres (11 US pt),[4] which is composed of plasma and several kinds of cells. These blood cells (which are also called corpuscles or "formed elements") consist of erythrocytes (red blood cells, RBCs), leukocytes (white blood cells), and thrombocytes (platelets). By volume, the red blood cells constitute about 45% of whole blood, the plasma about 54.3%, and white cells about 0.7%.
Vein Veins are present throughout the body as tubes that carry blood back to the heart. Veins are classified in a number of ways, including superficial vs. deep, pulmonary vs. systemic, and large vs. small.
Vein Veins are less muscular than arteries and are often closer to the skin. There are valves in most veins to prevent backflow.