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THE CARDIOVASCULAR SYSTEM LECTURE 2: THE HEART AND BLOOD VESSELS Eamonn O’Connor Applied Health Sciences Lecture Outline 1 Electrical activity of the heart Anatomy and function of blood vessels (arteries, arterioles, capillaries, venules and veins) AHS Physiology - Cardiovascular System 11-12 1 Electrical Activity of the Heart 2 The Conduction System of the Heart Spread of Excitation Through the Heart Muscle Recording the Electrical Activity of the Heart with an Electrocardiogram AHS Physiology - Cardiovascular System 11-12 Conduction System 3 Cardiac muscle doesn’t require commands from the CNS to contract Contractile activity of cardiac muscle: myogenic Autorhythmicity is the ability to generate its own rhythm Autorhythmic 2 cells provide rhythm to the heartbeat types: Pacemaker cell: initiate AP’s Conduction fibers: transmit AP’s AHS Physiology - Cardiovascular System 11-12 2 Conduction System 4 Pacemaker cells Spontaneously depolarizing membrane potentials to generate action potentials Coordinate and provide rhythm to heartbeat Conduction fibers Rapidly conduct action potentials initiated by pacemaker cells to myocardium Conduction velocity = 4 meters/second Ordinary muscle fibers, CV = 0.4 meter/second AHS Physiology - Cardiovascular System 11-12 Pacemaker Cells of the Myocardium 5 Sinoatrial node (SA node): 70-80 APs.min-1 Pacemaker of the heart Atrioventricular node (AV node): 40-60 APs.min-1 Conduction fibres of the pericardium Internodal pathways Bundle of His: 20-40 APs.min-1 Purkinje fibres: 20-40 APs.min-1 AHS Physiology - Cardiovascular System 11-12 3 Spread of Excitation Between Cells 6 Atria contract first followed by ventricles (fibrous skeleton) Coordination due to presence of gap junctions and conduction pathways AHS Physiology - Cardiovascular System 11-12 Figure 13.9 Anatomy of the Conduction System 7 Figure 13.10 AHS Physiology - Cardiovascular System 11-12 4 Spread of Excitation 8 Interatrial Pathway Node right atrium left atrium Simultaneous contraction right and left atria SA Internodal Pathway: SA Node AV Node Slow conduction - AV Nodal Delay = 0.1 sec Atria contract before ventricles Ventricular Excitation (fast conduction) Down Bundle of His Up Purkinje Fibers Purkinje Fibers contact ventricle contractile cells contracts from apex up Ventricle AHS Physiology - Cardiovascular System 11-12 Spread of Excitation 9 Figure 13.11 AHS Physiology - Cardiovascular System 11-12 5 Electrocardiogram 10 ECG is used to look at some aspects of cardiac electrical activity Non-invasive technique Used to test for clinical abnormalities in conduction of electrical activity in the heart Body fluids are conductors Currents in the body can spread to surface AHS Physiology - Cardiovascular System 11-12 Standard ECG Trace 11 P wave: atrial depolarization QRS complex: vent. depolarization T wave: vent. repolarization P-R: AV node conduction time R-T: vent. contraction (systole) T-Q: vent. relaxation (diastole) R-R: time between heart beats Figure 13.16b AHS Physiology - Cardiovascular System 11-12 6 ECG Arrhythmias 12 AHS Physiology - Cardiovascular System 11-12 Overview of the Vasculature 13 Arteries – carry blood away from heart Microcirculation (microscopic) Arterioles Capillaries Venules Veins – return blood to heart Differences between blood vessels: Structure: diameter composition of walls Function AHS Physiology - Cardiovascular System 11-12 Figure 14.6 7 Arteries 14 Carry blood away from heart Thick, elastic walls Large diameter, therefore low resistance to blood flow Pressure remains relatively constant through arteries. Pressure reservoirs because of stretch during systole elastic recoil helps maintain blood pressure during diastole AHS Physiology - Cardiovascular System 11-12 Arteries: a Pressure Reservoir 15 Storage site for pressure Thick elastic arterial walls Expand as blood enters arteries during systole Recoil during diastole Low compliance AHS Physiology - Cardiovascular System 11-12 Figure 14.7 8 Arteries & Disease 16 Atherosclerosis - ‘hardening of the arteries’ A plaque composed of cholesterol, calcium and other substances builds up in an artery Plaques reduce blood flow They can rupture and cause clots Heart Often attacks or strokes can result occurs with age Smoking, diabetes and obesity are other risk factors Angioplasty or stent implantation can be used as treatments AHS Physiology - Cardiovascular System 11-12 Arterioles 17 Arterioles are resistance vessels Walls contain smooth muscle: regulation of radius, and thus, resistance (>60% TPR) Functions: Regulate blood pressure Effect of diameter. Wider diameter – vasodilation (vasodilatation) Narrower diameter - vasoconstriction Greater diameter, greater blood flow Influenced by nerves, hormones and local effects in tissues eg. during exercise (later lecture) AHS Physiology - Cardiovascular System 11-12 Figure 14.17 9 Capillaries 18 Site of exchange between blood and tissue 5-10 micrometer diameter - small diffusion distance Walls - 1 endothelial cell layer plus basement membrane (small diffusion barrier) 10-40 billion per body Total SA = 600 m2 1 mm long AHS Physiology - Cardiovascular System 11-12 Types of Capillaries 19 Continuous capillaries Most common Small gaps (water filled) between endothelial cells Allows small water soluble molecules to move through (Na, K, glucose…) Figure 14.16a AHS Physiology - Cardiovascular System 11-12 10 Types of Capillaries 20 Fenestrated capillaries (located in kidneys, liver, intestines, bone marrow) Large gaps between endothelial cells forming pores or fenestrations Allows proteins and in some cases blood cells to move through Figure 14.16b AHS Physiology - Cardiovascular System 11-12 Exchange Across Capillary Walls 21 Mechanisms: Diffusion – most common mechanism Lipophilic – across membrane – through pores Lipophobic Transcytosis – exchangeable proteins Mediated transport – in brain Materials exchanged: Gases - O2, CO2 Glucose & Fatty acids Hormones etc. AHS Physiology - Cardiovascular System 11-12 Figure 14.18 11 Venules 22 Smaller than arterioles Connect capillaries to veins Thin walls Little smooth muscle in walls Some exchange of material between blood and interstitial fluid AHS Physiology - Cardiovascular System 11-12 Veins 23 Large diameter, but thin walls, which contain blood and elastic tissue Valves allow unidirectional blood flow (peripheral veins) Volume reservoirs at rest, systemic veins contain 60% of total blood volume High compliance Return of blood to heart from veins is called venous return AHS Physiology - Cardiovascular System 11-12 Figure 14.20 12