Blood serves four primary functions essential to survival: transportation, protection, regulation, and temperature control. Transportation involves carrying oxygen from the lungs to tissues and returning carbon dioxide for exhalation; blood also distributes nutrients, hormones, and waste products throughout the body. Protection is provided by white blood cells that fight infections and pathogens, plus platelets and clotting factors that prevent excessive bleeding through coagulation. Regulation includes maintaining proper pH balance, electrolyte concentrations, and osmotic pressure to keep the internal environment stable—a process called homeostasis. Blood also distributes hormones that regulate bodily functions from metabolism to reproduction. Temperature control occurs as blood absorbs heat from active tissues and organs, then releases it at the skin surface through radiation and perspiration, helping maintain core body temperature around 37°C (98.6°F). These functions work together seamlessly: oxygen-rich blood delivered to muscles enables movement, while white blood cells in that same blood prevent infection, and hormonal signals carried by blood coordinate all these processes. The roughly 5 liters of blood circulating through an average adult's body continuously performs these roles through the cardiovascular system's pumping action and an extensive network of arteries, capillaries, and veins.