Pulmonary and Systemic Circuits. Path of Blood Flow. Heart Anatomy. Heart valves

Transcription

1 Pulmonary and Systemic Circuits Heart: double pumps pulmonary circuit short, low-pressure circulation systemic circuit long, high-friction circulation Trace the flow of blood to & away from the heart P:4 6X Path of Blood Flow Atria: Receiving Chambers R. atrium Vena cava & Coronary sinus L atrium Pulmonary veins Ventricles: Discharging Chambers Papillary muscles Chordae tendinae R. ventricle Pulmonary trunk L. ventricle aorta Heart Anatomy Heart valves Open and close in response to pressure changes atrioventricular (AV) valves: Tricuspid & mitral Open: P.atrium > P.ventricle Close: semilunar (SL) valves: aortic & pulmonary Open: P.ventricle > P.great vessels Close: 1

2 Cardiac Muscle Fibers Anatomy of Cardiac Muscle striated, short, branched, 1 central nucleus T tubules: wide, less numerous large mitochondria 25 35% of cell volume Aerobic respiration Anatomy of Cardiac Muscle Intercalated discs Desmosomes: anchoring cells Gap junctions: electrically couple adjacent cells Allows heart to be functional syncytium (single coordinated unit) Similarities between skeletal and cardiac muscles Skeletal muscle 1) Depolarization (-90mV => +30mV) Opening VG fast Na+ channels Cardiac muscle 2) Depolarization waves travel down T-tubules Ca2+ release 3) Excitation coupling Differences between skeletal and cardiac muscles Source of stimulation Contraction Contraction duration absolute refractory period Skeletal muscle Somatic nervous system Motor unit ms Brief: 1-2ms Cardiac muscle Auto-rhythmic All cardiomyocytes contract as unit, or none do 200ms blood ejection from heart Long: 250ms Prevents tetanic s 2

3 Differences between skeletal and cardiac muscles Ca2+ source Action potential Depolarization Skeletal muscle Sarcoplasmic reticulum Brief: 1-2ms Fast VG Na+ channels Repolarization Opening VG K+ channels Cardiac muscle Extracellular Ca % Depolarization: slow Ca2+ channels in sarcolemma SR 80% Long: 200ms Due to plateau phase Fast VG Na+ channels Slow Ca2+ channels Inactivation Ca2+ channels Opening VG K+ channels Action Potential of contractile muscle cells Cardiac Contractile Cells: Excitation-Contraction Coupling Excitation Contraction T-tubules Ca2+ : PLATEAU PHASE Ca2+-induced Ca2+ release Relaxation SR: Ca2+ Plasma membrane 3Na+/1Ca2+-antiport 3Na+/2K+-ATPase Digitalis (Digoxin) Setting the Basic Rhythm Intrinsic cardiac conduction system Network of autorhythmic cells gap junctions Allow coordinated heartbeat -- synchrony APs Initiation by Pacemaker Cells APs Initiation by Pacemaker Cells unstable RMPs (pacemaker potentials or prepotentials) gradual depolarization Open slow Na+ channels Close K+ channels At threshold: Depolarization Open Ca2+ channels Repolarization Close Ca2+ channels Open K+ channels 3

4 Conduction System Extrinsic Innervation of the Heart cardiac centers in medulla oblongata Cardioacceleratory center Sympathetic Cardioinhibitory center Parasympathetic lub-dup LUB: louder, longer P.ventricle > P.atria Ventricular systole begins DUP: Short, sharp Ventricular diastole begins Heart murmurs incompetent or stenotic valves Heart Sounds Cardiac Cycle Mechanical Events occurring during one complete heartbeat 2 phases Systole = contractile phase of cardiac cycle Diastole = relaxation phase of cardiac cycle Series of pressure and blood volume changes during 1 heartbeat Ventricular filling Phase of Cardiac Cycle AV valves open; semilunar valves - closed passively flows into ventricles -- 80% of blood pressure low Atrial systole -- 20% End diastolic volume (): volume of blood in ventricle at end of ventricular diastole Ventricular systole Phase of Cardiac Cycle Ventricles contract (while Atria relax) ventricular pressure: AV valves closed; semilunar valves - open Isovolumetric phase: Volume ejection phase: volume End systolic volume (ESV): volume of blood remaining in each ventricle after systole 4

5 Isovolumetric relaxation Phase of Cardiac Cycle Ventricles relax Isovolumetric: volume AV valves: closed Semilunar valves: closed atria relaxed Atrial filling Wigger s Diagram: Summary of events during cardiac cycle Left heart QRS P T P EKG 1st 2nd Heart sounds Dicrotic notch Aorta Pressure (mm Hg) Left ventricle 40 Atrial systole Left atrium Ventricular volume (ml) SV 50 ESV Atrioventricular valves Aortic and pulmonary valves Phase Open Closed Open Closed Open Closed 1 2a 2b 3 1 Ventricular Atrial Isovolumetric Ventricular Isovolumetric filling phase ejection phase relaxation 1 2a 2b 3 Ventricular filling Ventricular systole Early diastole (mid-to-late diastole) (atria in diastole) Ventricular filling EKG vs. Heart Sounds Pressure Ventricular ejection Isovolumetric Isovolumetric relaxation Ventricular volume Cardiac Output (CO) Cardiac output varies to meet metabolic demands CO = HR SV At rest CO = HR (75 beats/min) SV (70 ml/beat) CO = 5.25 L/min HR (+) - chronotropic factors: HR (-) - chronotropic factors: HR 5

6 Stroke Volume SV = ESV = factors affect SV Preload = degree of stretch (tension) of cardiac muscle cells before they contract (Frank-Starling law of heart) Venous return => (preload) Slow HR => ventricular filling (filling time) => Afterload = pressure ventricles must overcome to eject blood (resistance) Loss of arterial elasticity => Afterload Afterload => SV & ESV Contractility Frank Starling s Law of the Heart The heart pumps all the blood that returns to it MAP = CO x TPR End-diastolic volume () Larger Stronger Residual SV Stroke volume Larger SV Contractility Contractility: the amount of force produced at a given preload ONLY sympathetic NO parasympathetic influence Increased by Positive inotropic agents Thyroxine, glucagon, epinephrine, digitalis, high extracellular Ca2+ Decreased by negative inotropic agents Acidosis, increased extracellular K+, calcium channel blockers Atrial (Bainbridge) reflex sympathetic reflex initiated by venous return => atrial filling Stretch of atrial walls stimulates SA node HR stimulates atrial stretch receptors, activating sympathetic reflexes Figure Factors involved in determining cardiac output. Exercise (by Heart rate Exercise, epinephrine, sympathetic activity, (allows more thyroxine, fright, anxiety skeletal muscle and time for excess Ca2+ respiratory pumps; ventricular see Chapter 19) filling) Venous Sympathetic Parasympathetic return Contractility activity activity ESV (preload) Initial stimulus Stroke Heart volume rate Physiological response Principles of Blood Pressure and Flow Blood Flow= pressure/ resistance pressure gradient: driving force for blood flow Large gradient: from aorta to capillaries At aorta: 2.5 cm diameter and 100 mm Hg pressure At capillaries: 8 µm diameter and 25 mm Hg pressure Resistance opposes flow Result Cardiac output 6

7 Changes in vessel diameter blood pressure velocity of blood flow Blood pressure and flow Arterial pressure is variable Example: 120/90 systolic pressure diastolic pressure Pulse pressure (difference between systolic and diastolic) Example: = 30 mm Hg Mean arterial pressure (MAP) Mean arterial pressure the average arterial pressure during a single cardiac cycle perfusion pressure = driving force that pushes blood through the systemic circuit MAP = 1/3(PSyst) + 2/3(PDiast) MAP = CO x TPR Total peripheral resistance: the force that opposes the flow of blood Peripheral resistance Resistance opposes flow (R=8nl/πr4) Vasomotor vessel radius: principal method of blood flow control blood in direct contact w/ vessel wall Congestive heart failure (CHF) Progressive condition; CO is so low that blood circulation inadequate to meet tissue needs Pulmonary congestion LH failure blood backs up in lungs Peripheral congestion RH failure blood pools in body organs edema Failure of either side ultimately weakens other Treat by removing fluid, reducing afterload, increasing contractility 7