THE CARDIOVASCULAR SYSTEM REGULATION OF CARDIAC OUTPUT. Eamonn O Connor Allied Heath Sciences

Transcription

1 THE CARDIOVASCULAR SYSTEM LECTURE 4: REGULATION OF CARDIAC OUTPUT Eamonn O Connor Allied Heath Sciences Lecture Outline 1 Autonomic Input to the Heart Factors Affecting Cardiac Output: Changes in Heart Rate Factors Affecting Cardiac Output: Changes in Stroke Volume Increases in Cardiac Output: Exercise 1

2 Cardiac Output 2 Volume of blood pumped by each ventricle per minute Cardiac Output = CO = SV x HR Equal on both sides of the heart Average CO = ~5 litres.min -1 at rest (70ml.beat -1 x 70beat.min -1 ) Can increase 5-fold during exercise Regulation of Cardiac Output 3 Regulate heart rate and stroke volume Extrinsic and Intrinsic regulation Extrinsic - (outside the organ) Nerves Hormonal Intrinsic - (autoregulation or local) 2

3 Autonomic Nervous System 4 Efferent nervous system a) (involuntary) b) (voluntary) a.1) a.2) Autonomic Nervous System 5 Efferent nervous system Autonomic (involuntary) Somatic (voluntary) Parasympathetic Sympathetic Motorneurons Parasympathetic and sympathetic have opposing effects 3

4 Autonomic Input to the Heart 6 Figure Factors Affecting CO: Heart Rate 7 Heart Rate - Determined by SA Node Firing Rate SA node intrinsic firing rate = 100/min If no extrinsic control on heart, HR = 100 SA node under control of ANS & hormones Rest: Parasympathetic dominates, HR = 75 Excitement: Reduction in parasympathetic impulses Increase in sympathetic activity Result: HR increases 4

5 Factors Affecting CO: Heart Rate 8 AV Nodal Innervation Sympathetic increases conduction velocity through node (from SA to AV node) Parasympathetic decreases conduction velocity through node (from SA to AV node) Hormonal Control of HR Epinephrine (adrenaline) Same effect as sympathetic nervous system Increases frequency of SA firing rate & conduction velocity Glucagon Increases HR Factors Affecting CO: Stroke Volume 9 Primary factors affecting stroke volume Ventricular contractility End-diastolic volume Afterload 5

6 Factors Affecting Stroke Volume 10 Ventricular contractility (Extrinsic control) Ventricles never completely empty of blood If ventricles contract with more force they eject more blood SV increases Extrinsic control: Sympathetic nerves increase force of contraction Parasympathetic have no effect Adrenaline, thyroid hormones, insulin and glucagon increase force of contraction Figure Factors Affecting Stroke Volume 11 End-Diastolic Volume Under Intrinsic Control - Frank-Starling s Law Increase venous return Increased EDV stretches the myocardium (cardiac muscle) Increase strength of contraction (Starling effect) Increase stroke volume Starling s Law: Mechanisms of ensuring that venous return matches CO 6

7 Length-Tension Curve (Starling Curve) 12 Figure Optimum length of cardiac muscle greater than resting length 1) Increased EDV (length) Stretch in muscle fibres Closer to optimum length 2) Greater strength of contraction Increased SV (tension) Family of Starling Curves 13 Figure

8 Factors Affecting Stroke Volume 14 End Diastolic Volume End-diastolic pressure = preload Filling time (depends on HR) Atrial pressure (depends on venous return and atrial contraction force) Central venous pressure (pressure in large veins) Afterload Afterload = pressure in aorta during ejection This is the force that the heart must pump against Summary of Factors Influencing SV 15 Venous return Sympathetic activity or Epinephrine Ventricle End-diastolic volume Contractility Arterial pressure (afterload) Stroke volume Figure

9 Increases in CO: Exercise 16 CO can increase up to 25L.min -1 during exercise Exercise impacts on both HR and SV Influenced by Autonomic nerves Venous return Skeletal-Muscle Pump 17 One-way valves in peripheral veins Skeletal muscle contracts Squeezes on veins increasing pressure Blood moves toward heart Blood cannot move backwards due to valves Skeletal muscle relaxes Blood flows into veins between muscles 9

10 The Skeletal Muscle Pump 18 Figure Independent Regulation of Blood Flow During Exercise Cardiac output increases during exercise Distribution of blood does not increase proportionally Dilation to skeletal muscle and heart increases blood flow Constriction to GI tract and kidneys decreases blood flow 10

11 Distribution of rest & During Exercise 20 11