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Circulatory System Why the need? Why do organisms need a circulatory system? To exchange materials with their environment – nutrients, wastes, O2, CO2, etc., • Organisms need a system that will do this. • The more complex the organism, the more complex this system must be. • Unicellular organisms, and those that have few layers of cells - rely on diffusion and exocytosis. Two Types of Circulatory Systems 1. Open Circulatory Systems • Arthropods (insects, spiders lobsters etc.,) – blood is pumped by the heart through simple open ended tube – it flows through the body cavity, bathing the internal organs. 2. Closed circulatory system More efficient system Found in higher invertebrates (ex earthworm) and all invertebrates Cardiovascular Systems of Vertebrates • Composed of a heart, arteries and veins. • blood stays in the circulatory system • chemicals are exchanged by diffusion at capillaries. Heart • A specialized muscle that contracts regularly and continuously, pumping blood to the body • May be two, three or four chambered. – Two - fish – Three – frogs – Four – humans Heart • Chambers are called atria (receive blood from veins) and ventricles (send blood to the arteries) Arterial to Venule System • Arteries carry blood away from the heart – Aorta divides into arteries which divide to form arterioles. Arterial to Venule System • Veins carry blood to the heart – Venules become Veins, which become the Vena cava • The finest (smallest) divisions of the vascular system are capillaries • Where arterioles transition to venules. The distinction between arteries and veins is by direction of blood flow, not oxygen content. • Veins carry blood toward the heart • Arteries carry it away from the heart. • Because of this, not all arteries carry oxygenated blood. • Two major arteries do not carry oxygenated blood. 1. The pulmonary artery carries deoxygenated blood from the heart to the lungs (to pick up oxygen there) 2. The umbilical arteries which carry deoxygenated blood away from the baby’s body to the placenta (to pick up oxygen there). • Higher vertebrates have double circulation: – Right side a pump for the pulmonary circuit (to the lungs) – Left side is pump for the systemic circuit (to the body) – Because the left side has to pump to a larger area the muscles of the left ventricle are larger than those of the right Pulse • Absent in veins • Blood is moved by: 1. The contraction of the muscles surrounding the veins 2. Gravity 3. The contracting heart pulls blood along the veins towards it. Blood is prevented from flowing back by valves.