Control of cardiac output.pdf

4. Control of cardiac output MD3001 Dr Alun Hughes 1 Lecture overview Changes in cardiac output Control of cardiac output Pressure Resistance 2 Changes in cardiac output ~70 ml ~30 L/min ~140 ml ~5 L/min Stroke volume x heart rate = cardiac output ~70 bpm ~200 bpm Extrinsic and intrins...

4. Control of cardiac output MD3001 Dr Alun Hughes 1 Lecture overview Changes in cardiac output Control of cardiac output Pressure Resistance 2 Changes in cardiac output ~70 ml ~30 L/min ~140 ml ~5 L/min Stroke volume x heart rate = cardiac output ~70 bpm ~200 bpm Extrinsic and intrinsic control of stroke volume Intrinsic mechanisms – Self-regulation – Frank-Starling mechanism – ↑ EDV ↑ force of contrac1on Preload – Venous pressure and venous return to heart (end diastolic pressure, EDP) Afterload – Aortic / pulmonary artery pressure Extrinsic mechanisms – Sympathetic nerves Frank-Starling’s Law of the Heart Allows for automatic adjustment for small imbalances between the left ventricle and right ventricle. Automatic balancing between CO from left-side of heart to volume returning to right-side. Rhoades p254 Figure 13.4 Frank-Starling’s Law of the Heart plus inotropic stimulation Rhoades p255 Figure 13.8 Control of cardiac output Pressure in the systemic and pulmonary vessels Guyton p159 12th ed, p171 13th ed Transmission and damping of pressure pulses Guyton p170 12th ed, p 182 13th ed Vascular compliance Rhoades p218 Figure 11.3 10 Mean arterial blood pressure (MABP) MABP = Diastolic pressure + 1/3 pulse pressure (Pulse pressure = systolic pressure – diastolic pressure) Rhoades p267 Figure 14.1 Factors determining the magnitude of pulse pressure Stroke volume – Intrinsic and extrinsic factors – Remember afterload, preload, sympathetic innervation Speed of ejection of stroke volume Arterial compliance – Decreases with age • Arteriosclerosis Factors affecting flow of a fluid though a vessel - Viscosity Guyton p165 12th ed, p177 13th ed Factors affecting flow of a fluid though a vessel – Vessel length Factors affecting flow of a fluid though a vessel – Vessel radius Guyton p163 12th ed, p175 13th ed The Poiseuille Equation Blood viscosity and vessel length can be considered as constant under normal circumstances, therefore can be reduced to a constant value, “K” Viscosity and vessel length (now known as “K”) plus vessel radius are all factors that generate resistance to flow by contributing to friction between blood and the walls of vessels. As resistance increases, flow decreases. Arterial pressure = cardiac output x total peripheral resistance Guyton p163 12th ed, p175 13th ed Calculating resistance in the pulmonary and systemic circulation MABP = Diastolic pressure + 1/3 pulse pressure Systemic = 120/75 Pulmonary = 26/8 Arterial pressure = cardiac output x total peripheral resistance Main points Cardiac output depends on venous return and can be intrinsically and extrinsically altered Arterial pressure depends on cardiac output and total peripheral resistance Small changes in vessel radius have big implications on blood flow 18 Learning outcomes To explain how cardiac output can be influenced by both intrinsic and extrinsic factors. To describe how and why pressures within different parts of the cardiovascular system vary. To identify the variables within the Poiseuille equation that define mean arterial blood pressure, and how these vary between systemic and pulmonary circulations. To calculate mean arterial blood pressure based on systolic and diastolic pressures.