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Lecture IV - The Cardiac Cycle, Regulation of Cardiac Output.docx

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University of Toronto Scarborough
Biological Sciences
Stephen Reid

Thiruvarangan Cardiac Cycle Aortic Pressure (I) – arteries are pressure resovoirs Phase 1 and 2 – Aortic valve is closed but there is still flow of blood out of the aorta: ↓ aorta In the early and late stages of diastole, aortic pressure is decreasing  The first (slow) decrease of aortic pressure occurs when the semi-lunar valves are close (i.e. there is no blood being pumped from the ventricles into the aorta or pulmonary artery).  This is because even though the heart is no longer pumping blood through the aortic valve into the aorta, blood is still flowing to the systemic circulation.  The is because arteries function as pressure reservoirs – the larger the artery, the greater this function The aorta under healthy conditions is relatively plastic 1. The blood flows through the aorta and forms a bulge (when the heart is contracting) 2. When the heart is relaxed (valve close) the aorta returns to its normal (pre- contraction) diameter and forces the blood in the bulge down the aorta and into the systemic circulation (when the heart is its relaxation phase) Aortic Pressure (II) Phase 3 – the aortic valve opens  flow into aortaflow out aortaherefore, ↑ P (aorta).  Aorta pressure continues to fall from the maximum until the point between ventricular ejection phase and isovolumetric relaxation phase  Here ventricular pressure is less than the aortic pressure and this causes the semi-lunar valve to close Aortic Pressure (III) The smooth decrease in aortic pressure is interrupted by a very abrupt but slight upward pressure or deflection followed by a linear decrease in aortic after the upswing.  The dichrotic notch is a pressure reverberation associated with the closing of the aortic valve – this closing (when the ventricle pressure is less than aortic Thiruvarangan pressure) momentarily interrupts the smooth flow of blood which causes a small pressure wave upwards. o P (ventricle) < P (aorta) o aortic (semi-lunar) valve closes o produces small increase in P aorta Systolic and Diastolic Pressure o Minimum Aortic Pressure = Diastolic Pressure (DP) o 80 mm Hg o It is called diastolic pressure even though it is not achieved until slightly into systole. o Diastolic pressure is an important indicator of stroke volume o Maximum Aortic Pressure = Systolic Pressure (SP) o 120 mm Hg Blood Pressure – measured in brachial artery. o 120/80 – used to calculate mean arteriole pressure o Blood Pressure (BP) = CO X Total Peripheral Resistance (TPR) o TPR is the resistance to blood flow in the circulation. There are five factors that influence resistance and blood flow in the next couple of lectures Pulse Pressure (PP) Pulse Pressure (PP) = Systolic Pressure – Diastolic Pressure = = 120 mm Hg – 80 mm Hg = 40 mm Hg (normal) It can be used to identify cardiovascular diseases including hardening of the arteries. o The pulse pressure is in part determined by how well arteries can contract and expand when filled with blood o But as arteries age and acquire fatty deposits, the ability to expand and contract is reduced (lose elasticity) o In the aorta, this has the consequence of inhibiting blood flow during diastole. Additionally, leads to increases in blood pressure. o The hardening causes increases in systolic and diastolic pressure (but much greater in systolic) and therefore the pulse pressure increases. Mean Arterial Pressure (MAP) – the driving force for blood flow The mean arterial pressure isn’t simply equal to (SP + DP) / 2. This is because Aortic pressure is closer to the minimum value for longer than it is close the maximum value. It can be calculated in two ways: o MAP = 1/3 SP + 2/3 DP or MAP = PP/3 + DP o 40 mmHg / 3 + 80 mmHg = 13 mmHg + 80 mmHg = 93 mmHg Thiruvarangan In a clinical setting, blood pressure is measured at the arm and it is a good approximation of aortic blood pressure. o Mild Hypertension – SP > 140 mmHg or DP > 90 mmHg o Moderate Hypertension – SP > 160 mmHg or DP > 100 mmHg o Severe Hypertension – SP > 180 mmHg or DP>110 mmHg o Hypotension – SP < 90 mmHg or DP < 60 mmHg Stroke Volume – is the amount of blood pumped in one heartbeat Stroke Volume (SV) = EDV (mL) – ESV (mL) o End Diastolic Volume (EDV) – Maximum ventricular volume o It is the amount of blood in the heart just as the heart finishes the relaxation phase and is beginning to contract (blood available to be pumped) o End Systol
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