The bloodstream is the system of blood vessels that carries blood throughout the body. It includes arteries, veins, and capillaries, which transport oxygen, nutrients, and waste products to and from cells. The heart pumps blood into the arteries, which then distribute it to various tissues and organs. In addition to red blood cells, the bloodstream contains white blood cells, which help fight infections, and platelets, which assist in blood clotting. The plasma, a yellowish liquid component, makes up about 55% of blood and carries hormones, proteins, and other substances essential for maintaining health.