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Physiology Lecture 57 Tanveer Raza MD MS MBBS [email protected] Vascular distensibility • Blood vessels are distensible – As pressure increases vessel dilate • Resistance decreases • Blood Flow increases Tanveer Raza MD MS MBBS [email protected] Vascular distensibility • Veins are most distensible – Stores large quantities of extra blood • Arteries are less distensible than veins – Arterial walls are stronger than veins – Same pressure causes more increase in venous blood than in a comparable artery – Pulmonary arteries are more distensible than systemic arteries Tanveer Raza MD MS MBBS [email protected] Vascular Compliance • Vascular Compliance (Vascular Capacitance) – Total quantity of blood that can be stored in a given portion of the circulation for each millimeter of mercury pressure rise • Compliance = Distensibility X Volume Tanveer Raza MD MS MBBS [email protected] Vascular Compliance • Volume-Pressure Curves Tanveer Raza MD MS MBBS [email protected] Vascular Compliance • Sympathetic stimulation increases vascular tone – Decreases dimension of one segment of circulation • Transferring blood to other segments – Shifts blood to heart, increases Stroke Volume Output Tanveer Raza MD MS MBBS [email protected] Vascular Compliance • Pulse pressure – Difference between highest Systolic and Lowest Diastolic Pressure SBP 120mmHg DBP 80mmHg ------------Pulse Pressure 40mmHg Tanveer Raza MD MS MBBS [email protected] Vascular Compliance • Pulse pressure – Mainly affected by • Stroke volume output (SVO) of the heart – Greater the SVO, greater the amount of blood, greater the Pulse pressure • Compliance of the arterial tree – The lesser compliant, the greater pulse pressure » Example in arteriosclerosis, when arteries become hardened Tanveer Raza MD MS MBBS [email protected] Vascular Compliance Aortic pressure pulse contours in arteriosclerosis, aortic stenosis, patent ductus arteriosus, and aortic regurgitation Tanveer Raza MD MS MBBS [email protected] Vascular Compliance • Mean Arterial Pressure (MAP) – Average pressure within an artery over a complete cycle of one heartbeat (cardiac Cycle) Changes in SBP, DBP, and MAP with age. The shaded areas show approximate normal ranges Tanveer Raza MD MS MBBS [email protected] Vascular Compliance • Mean Arterial Pressure (MAP) – MAP remain nearer to DBP than SBP during greater part of the cardiac cycle – MAP is determined about 60% by the DBP and 40% by the SBP at normal resting heart rate Tanveer Raza MD MS MBBS [email protected] Vascular Compliance • Mean Arterial Pressure (MAP) – Clinical significance • MAP is the perfusion pressure • MAP greater than 60 mmHg is enough to sustain the organs Tanveer Raza MD MS MBBS [email protected] Capillary Circulation • 50% of Circulating blood in Capillaries – Most important blood as exchange takes place here • Capillary Pressure – At arteriolar end 32mmHg • Pulse pressure at arteriolar end 5mmHg – At venous end 15mmHg • Pulse Pressure at venous end 0mmHg Tanveer Raza MD MS MBBS [email protected] Capillary Circulation • Rate of blood flow 0.07 cm/s • Time taken for blood to go from arteriolar end to venous end 1-2 seconds • Factors affecting transport across capillary – Diffusion – Filtration Tanveer Raza MD MS MBBS [email protected] Capillary Circulation • Filtration across a capillary – Depends on Starling forces Fluid movement= κ [ ( Pc + πi ) - ( Pi + πc ) ] Κ Pc πi Pi Πc = = = = = Capillary filtration coefficient Capillary hydrostatic pressure Interstitial colloid osmotic pressure Interstitial hydrostatic pressure Capillary colloid osmotic pressure Tanveer Raza MD MS MBBS [email protected] Capillary Circulation • Filtration across a capillary Depends on Starling forces – Hydrostatic Pressure gradient – Osmotic pressure gradient Tanveer Raza MD MS MBBS [email protected] Capillary Circulation • Filtration across a capillary – Hydrostatic Pressure gradient • Capillary hydrostatic pressure (Pc) minus Interstitial fluid hydrostatic pressure (Pi) – (Pc - Pi) • Interstitial fluid pressure – Subatmospheric » Subcutaneous tissue -2mmHg – Positive » Brain (6 mmHg), Liver and Kidneys Tanveer Raza MD MS MBBS [email protected] Capillary Circulation • Filtration across a capillary – Osmotic pressure gradient • Colloid Osmotic pressure of plasma minus colloid osmotic pressure of interstitial fluid – (πc - πi) • πi is negligible Tanveer Raza MD MS MBBS [email protected] Capillary Circulation • Filtration across a capillary – κ= Capillary filtration coefficient • Depends on capillary wall permeability and filtration area Tanveer Raza MD MS MBBS [email protected] Tanveer Raza MD MS MBBS [email protected] VEINS • Function – Passage of blood – Storage – Venous return (Venous Pump) Tanveer Raza MD MS MBBS [email protected] Central Venous Pressure (CVP) • Central Venous Pressure (CVP) – Pressure in Right atrium • Blood flows from all the systemic veins into Rt. Atrium of the heart • Normal CVP 0 mmHg – Equal to atmospheric pressure Tanveer Raza MD MS MBBS [email protected] Central Venous Pressure (CVP) • CVP depends on – Ability of heart to pump blood out of Rt. Atrium and ventricle into lung – Venous return to Rt. Atrium • Blood volume • Peripheral venous pressure • Arteriolar dilatation Tanveer Raza MD MS MBBS [email protected] Central Venous Pressure (CVP) • CVP depends on – Venous return to Rt. Atrium • Blood volume • Peripheral venous pressure – Increased large vessel tone will increase peripheral venous pressure • Arteriolar dilatation – Decreases peripheral resistance and increases blood flow onto veins Tanveer Raza MD MS MBBS [email protected] Thank You
