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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 Arterial System • Transports blood under pressure. • Blood moves in a pulse-like wave. • Contraction & 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. Venous System • Thinner walls, larger diameters & less muscle than arteries. • Contains 70% of total blood volume. • Most veins work against gravity. • Valves allow one-way flow. • Contraction of skeletal muscles pushes blood toward heart. Blood Vessels The Heart • Size of fist. • Hardest-working muscle. • Contains four chambers: • Left & right atria (receiving chambers). • Left & right ventricles (delivery chambers). • Septum separates left & right sides. 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 • Right atrium receives oxygen-poor blood from: • Superior vena cava. • Inferior vena cava. • Blood flows through the tricuspid valve or the right atrioventricular (AV) valve to the right ventricle. Pulmonary Circuit • Blood flows through the pulmonary semi-lunar valve & enters the pulmonary trunk, which divides into the left & right pulmonary arteries. • Divides into capillaries where external gas exchange occurs. • Oxygenated blood enters the left & right pulmonary veins, returning blood to the left atrium. Systemic Circuit • Blood flows through the left AV-valve 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 • Aorta branches into smaller systemic arteries, which branch into arterioles & then capillaries. • Internal gas exchange occurs. • Capillaries rejoin as venules & then veins. • Deoxygenated blood returns to the right atrium via the superior & inferior vena cava. Systemic Circuit Coronary Circulation • Delivers oxygenated blood 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 Purkinje fibers & Bundle of His. • The delay at the AV node causes the atria to contract simultaneously before the ventricles. Cardiac Contractions The Heart Cycle • The cardiac cycle is a continuous cycle of relaxation & contraction. • Diastole • Heart relaxation • Systole • Heart contraction. Diastole (80 mm Hg) • Blood enters all four chambers. • Pulmonary veins left atrium. • Inferior & superior vena cava right atrium. • Left & right AV valves are open. • Pulmonary & aortic semi-lunar valves are closed. Systole (120 mm Hg) • Aria contract to further fill the ventricles. • Tricuspid & bicuspid valves forced closed due to an increase in ventricular pressure. • Ventricles contract to force blood from the heart. • Pulmonary & aortic semi-lunar valves are open. Diastole & Systole Blood Pressure • Pressure exerted on the arterial walls. • The elastic property of vessels allows for continuous blood flow throughout the circulatory system. • The difference between your systolic pressure & your diastolic pressure is called your pulse pressure. Blood Pressure • Highest in the aorta. • Blood pressure & velocity both decline as blood enters the arterioles. • The drop in pressure results from the resistance (friction) to blood flow. • Low blood pressure reduces your capacity to transport blood. • High blood pressure weakens & ruptures arterial walls. Blood Pressure Heart Valves & Heart Sounds • “Lub”: Tricuspid & bicuspid valves close (beginning of systole). • “Dub”: Pulmonary and aortic semilunar valves close (end of systole). Heart Valves & Heart Sounds