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Flat bodies or animals with only two layers of cells help minimize diffusion timeOpen and Closed Circulatory Systems
•Circulatory systems minimize the distance that substances must diffuse to enter or leave a cell•The system connects the aqueous environment of the body cells to organs that exchange gases•Components of the circulatory system: 1) fluid (e.g. blood) 2) a set of interconnecting tubes (e.g. blood vessels) and 3) the heart•Heart: powers circulation by using energy to elevate the pressure of the blood, which flows through the blood vessels and back to the heart.•Open circulatory system: circulatory fluid bathes the organs directly (hemolymph, the circulatory fluid, is also the interstital fluid).oLess costly in terms of energy than a closed system (lower blood pressure)•Closed circulatory system: blood is confined to vessels and is distinct from the interstital fluid. Heart(s) pump blood through vessels; materials are exchanged between the smallest vessels and the interstital fluid. Annelids, cephalopods, and all vertebrates have closed circulatory systems.oHigher blood pressure allows nutrients and O2 to be transported in larger, active animalsOrganization of Vertebrate Circulatory Systems•Cardiovascular system: the closed circulatory system of humans and other vertebrates•Arteries: carry blood away from the heart to organs throughout the bodyoArterioles: arteries branch into these small vessels that convey blood to the capillaries•Capillaries: microscopic vessels with very thin, porous wallsoCapillary beds: networks of capillaries that infiltrate every tissue of the body•Veins: carry blood back to the heart; converging venuleslead into the veins (and converging capillaries lead into the venules)•Portal veins: carry blood between pairs of capillary beds. For example, hepatic portal vein carries blood from capillary beds in the digestive system to capillary beds in the liver.