Blood

Blood is a liquid connective tissue that circulates continuously through the body's blood vessels, carrying oxygen, nutrients, hormones and many other substances needed by cells. An adult usually has about 5 liters of blood, although the exact amount depends on body size. Around 55% of blood is plasma, a pale yellow liquid made mostly of water that transports proteins, salts, nutrients, hormones and waste products.

The remaining part contains blood cells: red blood cells, which carry oxygen thanks to the protein hemoglobin; white blood cells, which help defend the body against infections; and platelets, tiny cell fragments that help stop bleeding by forming blood clots. The heart pumps blood through two connected circuits.

In the pulmonary circulation, blood travels to the lungs to release carbon dioxide and collect fresh oxygen. It then returns to the heart before being sent through the systemic circulation, which delivers oxygen and nutrients to tissues throughout the body.

Blood also helps regulate body temperature, maintain the balance of water and dissolved substances, and carry waste products to organs such as the kidneys and liver for elimination or processing. Doctors can learn a great deal about a person's health from blood tests, which measure blood cells and many dissolved substances to detect disease, monitor treatments or check how well organs are working.