Lecture 2.2a - Control of Cardiac Output PDF

Cardiac ouput: ◦Amount of blood pumped from right or left ventricles per minute ◦Cardiac output = heart rate x stroke volume ◦Cardiac output ~ 5L/minute ◦With exercise ~25L/minute ◦Supply should meet demand Heart rate: ◦Average heart rate in healthy adult is 60-80bpm...

Cardiac ouput: ◦Amount of blood pumped from right or left ventricles per minute ◦Cardiac output = heart rate x stroke volume ◦Cardiac output ~ 5L/minute ◦With exercise ~25L/minute ◦Supply should meet demand Heart rate: ◦Average heart rate in healthy adult is 60-80bpm ◦SA node sets the sinus rhythm ◦Factors that increase heart rate (positive chronotropic): ‣ Sympathetic nervous system (adrenaline/noradrenaline) - signals from beta 1 adrenoceptors to SA and AV nodes to increase heart rate ‣ Hormones, (T3,T4 - increasing metabolism, therefore increasing heart rate) ‣ Body temperature ‣ Increased Ca2+ levels - generates action potential for heart rate contraction ‣ Hypoxia and hypercapnia (low oxygen and high carbon dioxide) - receptors sense these changes and signal brain to reverse these effects. Increases heart rate even during respiratory failure ‣ Age - increasing age reduces heart rate ◦If HR > 100bpm: tachycardia ◦If HR < 60bpm: Bradycardia ◦Factors that decrease heart rate (negative chronotropic): ‣ Parasympathetic nervous system (acetylcholine) - muscarinic receptors ‣ Hyperkalaemia (increased K+ levels) ‣ Decreased Ca2+ levels Stroke volume: ◦Stroke volume = amount of blood pumped from left or right ventricles per beat ◦Stroke volume = EDV-ESV ‣ 120ml-50ml = 70ml per beat ◦ Stroke volume is influenced by three factors: ‣ Preload: Amount of stretch of the ventricles at end of diastole - EDV Factors that alter the preload: ◦Venous return ◦Skeletal muscle pump ◦Respiratory pump ◦Venous tone ◦Gravity ‣ Contractility: The more the heart stretches, the more it contracts (Frank-Starling Law) As the heart fills up with more blood during diastole, it contracts harder and pumps out more blood in systole Factors that increase the contractility (positive inotropic) ◦Sympathetic nervous system (adrenaline, noradrenaline) - increases stroke volume ◦Hormones (T3,T4), glucagon - increases expression of Beta-1 receptors  ◦Drugs (e.g. dopamine) Factors that decrease the contractility (negative inotropic) ◦Beta blockers (block action of sympathetic NS) ◦Ca2+ channel blockers ◦Increase in K+, Na+, H+ ‣ After load: After load is the amount of resistance that needs to be overcome by the ventricles to pump blood to aorta Increase in after load decreases stroke volume Factors that increase after load: ◦Semilunar valve damage ◦Increased vascular resistance Factors that decrease after load: ◦Decreased vascular resistance