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Lecture VI - Regulation of Cardiac Output.docx

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Biological Sciences
Stephen Reid

Thiruvarangan Regulation of Cardiac Output; Blood Flow Regulation Coronary Artery Blood Flow Occurs Mainly During Diastole The reason for this is that during systole, when the heart is contracting, it compresses some (large portion) of the coronary arteries and completely occludes them and thus coronary blood flow decreases to the cardiac muscles.  In mammals, the heart muscle has to be supplied with blood through coronary arteries. Stroke Volume Regulation – Afterload The afterload is the pressure that the ventricles must work against during the ventricular ejection phase of the heart cycle (into systemic circulation). It is equal to the aortic blood pressure. When the afterload increases, the stroke volume decreases.  On the left side of the heart, the afterload is the pressure from the aorta and on the right side, the afterload is the pressure from the pulmonary arteries.  A decrease in arterial pressure (afterload) leads to a marginal decrease in EDV but a substantial decrease in ESV. Since SV = EDV – ESV, a decrease in the afterload means an increase in the SV. Heart Failure It is any reduction in the heart’s function that prevents it from maintaining adequate cardiac output.  Symptoms of Heart Disease o Breathing difficulties (shortness of breath, trouble breathing lying down) o Fatigue (exercise intolerance, tiring easily) o Swelling of feet, ankles, or legs o Cyanosis (blueness of blood)  Causes of Heart Failure o Heart Attack (Myocardial Infarction) – it is the death of cardiac myocytes due to blockage of the coronary arteries o High Blood Pressure, Stenosis (valve doesn’t open wide enough) of the Aorta (i.e. increases in afterload)  ↑ cardiac workload  enlargement of the heart (↑ muscle thickness)  ↓ ventricular compliance (harder to stretch thick muscle)  ↓ EDV  ↓ SV and CO  Reduction in ventricular contractility  Obstruction of an AV valve  Third Degree heart block (lecture 2 and 3) Thiruvarangan o ↓ decreases HR  ↑ increases in EDV (due to increased filling time)  chronic overstretching and weakening Heart failure can be looked at from the left and right side of the heart and can lead to different symptoms.  Left Heart Failure (congestive heart failure) o Left ventricular failure (i.e. ↓ in contractility) o ↑ Pressure in pulmonary veins as the blood returning from the lung backs up o ↑ Pressure in lung capillaries o Forces fluid into the alveoli of the lungs (edema) – the lung tissues collapse on each other and decrease gas exchanges Moreover... o ↓ Pressure in systemic arteries because of insufficient generation of CO o Fluid retention by the kidney is triggered o Veins are volume reservoirs and so venous volume and pressure increase o Increase in pressure in the pulmonary artery (edema) If the right heart isn’t functioning well, we get a build up of fluid and pressure in the systemic circulation (venous system). This leads to a diffusion of fluid from the capillaries into distal tissue which will lead to swelling which leads to tissue death. Thiruvarangan Blood Flow Measuring Blood Flow: Venous Occlusion Plethysmography  Inflation cuff is raised to prevent venous flow but not arterial flow because arterial pressure is greater than venous pressure.  Measure the increase in fluid volume resulting from arterial flow into the lower leg. o Increase in limb volume is indicative of blood flow into the limb Measuring Blood Flow: Blood Flow Probes/MRI Calculating Blood
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