15 16 Heart and Circulation III and IV

P mean aortic pressure vena cava pressure mean aortic

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P = (Mean Aortic Pressure – Vena Cava Pressure) Mean Aortic Pressure ~100 mmHg Vena Cava Pressure ~0 mmHg Flow = Cardiac Output~5000 ml/min at rest Therefore, Total Peripheral Resistance = P / Flow = (100 – 0) mmHg / 5000 ml/min = 0.02 mmHg/ml/min Aorta  Total Peripheral Resistance  Vena Cava
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Pulmonary Vascular Resistance Resistance = P / Flow P = (Mean Pulmonary Arterial P. – Pulmonary Venous P.) Mean Pulmonary Arterial Pressure ~15 mmHg Pulmonary Venous Pressure ~3 mmHg Flow = Cardiac Output ~5000 ml/min at rest Therefore, Pulmonary Vascular Resistance = (15 – 3) mmHg/5000 ml/min = 0.0024 mmHg/ml/min
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Blood Pressure Along the Vascular System a Indicator of Vascular Resistance 1. Pressure drop (Pressurein – Pressureout) in each segment of the circulation reflects vascular resistance of the segment, because P = Flow x Resistance, where flow (cardiac output) is the same for all segments of the circulation. 2. Greatest pressure drop occurs in the arterioles - implying that arterioles is the site of greatest resistance in the circulation. 3. Arteriolar resistance is regulated by sympathetic nervous system and local mechanisms to provide sufficient blood flow to meet the metabolic demand of organs.
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Local Control of Organ Blood Flow Local Control - Independent of autonomic nervous system Function : To provide adequate flow to meet oxygen demand of an organ Basic Mechanism : Regulation of vascular resistance to regulate organ blood flow Examples of Local Control of Organ Blood Flow Exercise Hyperemia : Exercise-induced increase in skeletal muscle blood flow to support the increase in O2 consumption Hypoxic Vasodilation : Hypoxia-induced vasodilation to increase blood flow to compensate for hypoxia Autoregulation : Maintenance of organ blood flow despite changes in arterial pressure
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Arteriolar Resistance Determines Organ Blood Flow Pressure Reservoir (Arteries) Variable Arteriolar Resistances 1 2 3 4 5 1 2 3 4 5 Flow α 1/Resistance
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Autoregulation Arterial Pressure (mmHg) Organ Blood Flow 70 175 Maintenance of Blood Flow Despite Changes in Pressure Maximal Vasoconstriction Maximal Vasodilation Autoregulatory Range
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Local Control of Organ Blood Flow in Active Hyperemia and Autoregulation
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Organ Vasodilator Hypothesis for Local Control of Organ Blood Flow Cellular Oxygen Consumption Accumulation of Metabolite (Vasodilator) Arteriolar Vasodilation Artery Arterioles Blood Flow (Oxygen Supply) Concept : Imbalance between oxygen consumption and supply leads to
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