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Chapter 42 Circulation and Gas Exchange The Mammalian Heart: A Closer Look • A closer look at the mammalian heart provides a better understanding of double circulation LE 42-6 Pulmonary artery Aorta Anterior vena cava Pulmonary artery Right atrium Left atrium Pulmonary veins Pulmonary veins Semilunar valve Semilunar valve Atrioventricular valve Atrioventricular valve Posterior vena cava Right ventricle Left ventricle • The heart contracts and relaxes in a rhythmic cycle called the cardiac cycle • The contraction, or pumping, phase is called systole • The relaxation, or filling, phase is called diastole LE 42-7 Atrial systole; ventricular diastole Semilunar valves closed 0.1 sec AV valves open 0.3 sec Semilunar valves open 0.4 sec Atrial and ventricular diastole AV valves closed Ventricular systole; atrial diastole • The heart rate, also called the pulse, is the number of beats per minute • The cardiac output is the volume of blood pumped into the systemic circulation per minute Maintaining the Heart’s Rhythmic Beat • Some cardiac muscle cells are self-excitable, meaning they contract without any signal from the nervous system • The sinoatrial (SA) node, or pacemaker, sets the rate and timing at which cardiac muscle cells contract • Impulses from the SA node travel to the atrioventricular (AV) node • At the AV node, the impulses are delayed and then travel to the bundle of His then to the Purkinje fibers that make the ventricles contract • Impulses that travel during the cardiac cycle can be recorded as an electrocardiogram (ECG or EKG) Pacemaker generates wave of signals to contract. SA node (pacemaker) Signals pass to heart apex. Signals are delayed at AV node. AV node Bundle branches ECG Signals spread throughout ventricles. Heart apex Purkinje fibers • The pacemaker is influenced by nerves, hormones, body temperature, and exercise Concept 42.3: Physical principles govern blood circulation • The physical principles that govern movement of water in plumbing systems also influence the functioning of animal circulatory systems Blood Vessel Structure and Function • The “infrastructure” of the circulatory system is its network of blood vessels • All blood vessels are built of similar tissues and have three similar layers LE 42-9 Vein Artery 100 µm Endothelium Valve Basement membrane Endothelium Endothelium Smooth muscle Capillary Connective tissue Smooth muscle Connective tissue Vein Artery Arteriole Venule • Structural differences in arteries, veins, and capillaries correlate with functions • Arteries have thicker walls that accommodate the high pressure of blood pumped from the heart • In the thinner-walled veins, blood flows back to the heart mainly as a result of muscle action LE 42-10 Direction of blood flow in vein (toward heart) Valve (open) Skeletal muscle Valve (closed) Blood Flow Velocity • Physical laws governing movement of fluids through pipes affect blood flow and blood pressure • Velocity of blood flow is slowest in the capillary beds, as a result of the high resistance and large total cross-sectional area Venae cavae Veins Venules Capillaries Arterioles Arteries 120 100 80 60 40 20 0 Aorta Pressure (mm Hg) Velocity (cm/sec) Area (cm2) LE 42-11 5,000 4,000 3,000 2,000 1,000 0 50 40 30 20 10 0 Systolic pressure Diastolic pressure Blood Pressure • Blood pressure is the hydrostatic pressure that blood exerts against the wall of a vessel • Systolic pressure is the pressure in the arteries during ventricular systole; it is the highest pressure in the arteries • Diastolic pressure is the pressure in the arteries during diastole; it is lower than systolic pressure • Blood pressure is determined by cardiac output and peripheral resistance due to constriction of arterioles LE 42-12_4 Blood pressure reading: 120/70 Pressure in cuff above 120 120 Rubber cuff inflated with air Artery Pressure in cuff below 120 120 Pressure in cuff below 70 70 Artery closed Sounds audible in stethoscope Sounds stop Capillary Function • Capillaries in major organs are usually filled to capacity • Blood supply varies in many other sites • Two mechanisms regulate distribution of blood in capillary beds: – Contraction of the smooth muscle layer in the wall of an arteriole constricts the vessel – Precapillary sphincters control flow of blood between arterioles and venules LE 42-13ab Precapillary sphincters Arteriole Thoroughfare channel Venule Capillaries Sphincters relaxed Arteriole Sphincters contracted Venule LE 42-13c Capillaries and larger vessels (SEM) 20 µm • The critical exchange of substances between the blood and interstitial fluid takes place across the thin endothelial walls of the capillaries • The difference between blood pressure and osmotic pressure drives fluids out of capillaries at the arteriole end and into capillaries at the venule end LE 42-14 Tissue cell Capillary Red blood cell Net fluid movement out Net fluid movement in 15 µm Direction of blood flow Blood pressure Osmotic pressure Inward flow Pressure Capillary INTERSTITIAL FLUID Outward flow Arterial end of capillary Venous end Fluid Return by the Lymphatic System • The lymphatic system returns fluid to the body from the capillary beds • This system aids in body defense • Fluid reenters the circulation directly at the venous end of the capillary bed and indirectly through the lymphatic system