Cardiovascular Physiology Introductory Lecture PDF

Cardiovascular physiology 1 Functional anatomy, action potentials, conduction and ECG’s Objectives • Describe the cells type in the heart • Describe the cardiac conducting system and autorhythmicity • Draw and explain the features of a normal ECG trace • Describe cardiac output • Describe the com...

Cardiovascular physiology 1 Functional anatomy, action potentials, conduction and ECG’s Objectives • Describe the cells type in the heart • Describe the cardiac conducting system and autorhythmicity • Draw and explain the features of a normal ECG trace • Describe cardiac output • Describe the components of blood pressure and its regulation Introduction • Cardiovascular system consists of – Heart – Blood vessels • 2 circuit system • Transports blood around body for: – – – – – Oxygen diffusion CO2 removal Transport of nutrients Removal of waste Immune function Cardiac muscle • Chamber muscles contract as one: functional syncytium Heart muscle has: • Unique cell structure • Its own signal generator • Its own conducting system Contractile muscle cell membrane • Intercalated • Desmosomes: Fuse cells together • Gap junctions: share ions between cells Cardiac contractile muscle action INVOLVES CALCIUM. ABSOLUTE potential CAUSES REFRACTORY PERIOD. ALLOWS FOR COMPLETE STOP OF CONTRACTION 4 4 Summary • Contractile muscle – Contract as one = Syncitium – Are Intercalated but have Gap Junctions to pass electrical signals – Long action potential (Calcium influx – plateau), long refractory period • • • • • Phase 0 Fast sodium channels open Phase 1 Sodium channels close and potassium channels open Phase 2 calcium channels open, potassium efflux Phase 3 calcium channels close, slow potassium channels open Phase 4 resting membrane potential – One AP = one contraction – Draw it Cardiac pacemaker cells • Set rhythm of heart • SA node usually dominates • Sequence – SA mode – AV node – AV bundle (bundle of His) – Bundle branches – Endocardial network (Purkinje fibres) • Contraction: atria then ventricles Pacemaker cell action potential Summary • Pacemaker – Master controller = SA NODE – Sequence of signal spread is: SA, AV AV bun, Bun Branches, Prukinje network – Passes electrical signal to contractile muscle – Sequence of ion movements in action potential: Na slow leak in, Ca Influx – Continuously generates AP’s – Draw it Electrocardiograms • Currents from heart detected with electrodes • Number of leads determines sensitivity of signal (minimum 3, standard 11-12) • Normal heart rhythm has a distinct sequence • Abnormal sequence = heart problem! The normal sequence Divides into • P wave • QRS complex • T wave Note this relates to electrical activity, NOT contraction Intervals and segments P-R interval • Atrial excitation (depolarization) to ventricular excitation • Can be P-Q interval S-T segment • Ventricular myocardium is depolarised Q-T interval • Ventricular depolarisation to ventricular repolarisation R-R interval • Ventricular cycle • Used for heart rate Questions • What activity does ECG reveal in the heart? Name the regions of the heart depolarising/repolarising during: • The P wave: Depolarization of the atria • The QRS wave: Ventricular depolarization • The ST segment: Ventricles depolarized • The T wave: Ventricles repolarising Cardiac output • Cardiac output = stroke volume x heart rate • Cardiac output = venous return • Cardiac output = arterial pressure total peripheral resistance • The amount of blood pumped out depends on: – – – – How many times the heart beats How much blood is in the ventricles How much pressure is developed in the ventricles How much pressure there is in the arteries Venous return • Most important factor in stroke volume • How much blood comes in through major veins (vena cava/pulmonary veins) • Most blood in circulation is in veins (see later) Frank-Starling Law of the Heart (preload) • More blood in the heart means more stretch • More stretch = stronger contraction • Stronger contraction = more blood ejected Stroke volume • Amount of blood pumped out of the ventricle in 1 beat (50-120mL) Calculated from the end diastolic volume (EDV) MINUS the end systolic volume (ESV) of the ventricle Nervous input - Sympathetic simulation (+ adrenaline) • Sympathetic simulation: • Direct connections • Noradrenaline Adrenaline • Increases IC calcium → ↑ contraction β1 • Also increases heart rate β1 • Effects via cyclic AMP Parasympathetic stimulation • • • • Via vagus Slows heart M2 Hyperpolarises cells Little effect on contraction Summary, cardiac output Blood pressure • Blood in vessels is flowing – creates pressure • Pressure = driving force for blood to move • Blood pressure = force blood applies blood vessels • Vessels create a resistance to flow • Need pressure to overcome resistance to get blood to flow Ohm’s Law F = ∆P R Pouiseuille's law F = π ∆Pr4 8vl F = flow v = viscosity ∆P = pressure difference l = length (of vessel) r = radius (of vessel) Diastolic pressure • When the heart is not pumping blood: – Lowest pressure – Diastolic pressure – Diameter of all arteries feeds back to PA’s and aorta – Total peripheral resistance Systolic pressure • When heart is pumping blood: – Systolic pressure – Sum of diastolic and extra from heart • Difference between systolic and diastolic = PULSE PRESSURE Summary questions What is the definition of blood pressure? Which blood pressure is higher, systolic or diastolic? What is pulse pressure and mean arterial pressure? To increase flow, how must the following change: pressure resistance To increase pressure, how must resistance and flow change? The most important factor controlling flow is: Typical flows • Can redirect flow according to need • Variation is mostly skeletal muscle, skin, gut • Autonomics and local events • But what about increasing cardiac output? What controls overall blood pressure? • The body maintains a control of arterial blood pressure • Measured at baroreceptors • Feedback to the – cardioinhibitory – Cardioacceleratory – vasomotor centres • Moderated by higher brain centres Response to low pressure Response to high pressure Key points • Arteriole dilation/constriction determines which organs get blood • Venoconstriction and skeletal contraction increase venous return → ↑stroke volume • Sympathetic stimulation → – Increase cardiac contraction, heart rate – selective constriction (blood vessels)