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The Circulatory System The Structure and Function of the Heart and Blood Vessels Blood Vessels • Arteries: thick-walled blood vessels that ALWAYS carry blood AWAY from the heart. • Veins: thin-walled blood vessels that ALWAYS carry blood TOWARD the heart. Arteries Arterioles Capillaries Venules Veins Blood Vessels Arteries • Transports blood under pressure. • Blood moves in a pulse-like wave throughout the circulatory system. • Arterioles have smaller diameter than arteries and are less elastic, BUT the contraction and relaxation of arterioles is the major determinant of the overall blood pressure. Capillaries • Narrowest of all blood vessels. • RBCs travel in single file. • Branching of the capillaries increases the surface area available for diffusion. • Connects the arterial & venous systems. Veins • Thinner walls, larger diameters & less muscle than arteries. • Contains 70% of total blood volume. • Most veins must work against gravity. – Valves allow one-way flow of blood. – Contraction of skeletal muscles pushes blood toward heart. – If veins are constantly stretched, they will lose their elasticity & varicose veins will form. The Heart • Size of your fist. • Hardest-working muscle in the body. • Contains four chambers: – Left and right atria (receiving chambers). – Left and right ventricles (delivery chambers). • Left and right sides of the heart are separated by a muscular septum. The Heart • Protected by the ribcage, sternum & spine. Pathways of Blood • Pulmonary Circuit – Right side of heart. – Low-pressure system. • Systemic Circuit – Left side of heart. – High-pressure system. Pulmonary Circuit • The right atrium receives oxygen-poor blood from the: – Superior vena cava. – Inferior vena cava. • Blood enters the right atrium & flows through the tricuspid valve or the right atrioventricular valve (AV-valve). Pulmonary Circuit • The blood passes through the pulmonary semilunar valve & enters the pulmonary trunk, which divides into the left & right pulmonary arteries. • The pulmonary arteries divide into capillaries at the lungs where external gas exchange occurs. • Oxygenated blood travels from the lung capillaries to the left & right pulmonary veins, which return the blood to the left atrium. Systemic Circuit • Blood passes from the left atrium through the left AVvalve or bicuspid (mitral) valve to the left ventricle. – Left ventricular walls are two times thicker than in the right ventricle. • Blood travels through the aortic semi-lunar valve to the aorta. Systemic Circuit • The aorta branches into smaller systemic arteries, which branch into arterioles and then capillaries. • At the capillaries, internal gas exchange occurs with the body cells. • Capillaries rejoin as venules & then as veins. • Deoxygenated blood returns to the right atrium via the superior & inferior vena cava. Systemic Circuit Coronary Circulation • Delivers oxygenated blood directly to the heart muscle. • Consists of the left & right coronary artery. Cardiac Contractions • Sinoatrial (SA) node or “pacemaker” (in the right atrium) maintains the heart’s intrinsic pumping rhythm. – Nerves influence the rate & strength of the heart’s contractions. • This signal travels to the atrioventricular (AV) node, where it is delayed for 0.1 seconds. • It travels to the ventricles via the Purkinje fibers and the Bundle of His. • The delay at the AV node causes the atria to contract simultaneously before the ventricles. Pacemaker Specialization of Cardiac Muscle • The rhythmic contractions of the heart are due to special characteristics of the cardiac muscle cells. The Heart Cycle • The cardiac cycle is a continuous cycle of contraction and relaxation. – Systole • Heart contraction. – Diastole • Heart relaxation. Systole 1. Atria contract to further fill the ventricles. 2. Tricuspid & bicuspid valves forced closed due to an increase in ventricular pressure. 3. Ventricles contract to force blood from the heart. 4. Right ventricle (open pulmonary semi-lunar valve) pulmonary trunk & pulmonary arteries; and 5. Left ventricle (open aortic semi-lunar valve) aorta. Blood pressure is increased (120 mm Hg). Diastole Blood enters all four chambers: 1. Pulmonary veins Left atrium; and 2. Vena cava (superior & inferior) Right atrium. 3. Tricuspid & bicuspid valves (AV valves) open, allowing blood to flow into the left and right ventricles. 4. Pulmonary & aortic semi-lunar valves close due to a decrease in ventricular pressure. Blood pressure is reduced (80 mm Hg). Systole & Diastole Heart Valves & Heartbeat Sounds • Heart valves open and close at different times to ensure blood flows in the proper direction. • “Lub”: tricuspid and bicuspid valves close (beginning of systole). • “Dub”: pulmonary and aortic semi-lunar valves close (end of systole). Blood Pressure