3HemodynamicsHemodynamics refers to principle that govern the blood flow in the cardiovascular system .
4PATTERN & PHYSICS OF BLOOD FLOW Blood is transported to all parts of body through blood vessels.Blood vessel bring Oxygen and nutrition.They remove waste product.We will study general principles regarding blood flow, Physics of blood flow, Role of blood vessels, then blood pressure regulation.
6Blood FlowBlood is constantly reconditioned so composition remains relatively constantReconditioning organs receive more blood than needed for metabolic needsDigestive organs, kidneys, skinAdjust extra blood to achieve homeostasisBlood flow to other organs can be adjusted according to metabolic needsBrain can least tolerate disrupted supply
7DISTRIBUTION OF CARDIAC OUTPUT Digestive organ, kidney, skin receive blood in excess of their own needs, therefore, can withstand better main blood flow is reduced.Brain suffers irreparable damage when blood supply is not there for more than 4mins. If oxygen is not supplied to brain, permanent damage occurs.
8Blood FlowFlow rate through a vessel (volume of blood passing through per unit of time):Directly proportional to the pressure gradientInversely proportional to vascular resistanceF = ΔPRF = flow rate of blood through a vesselΔP = pressure gradientR = resistance of blood vessels
9PRESSURE GRADIENTPressure Gradient is difference in pressure between the beginning and end of vessel.Blood flows from area of high pressure to an area of low pressure, down the pressure gradient.When heart contracts, it gives pressure to the blood, which is main driving force for flow through a vessel.Due to resistance in the vessel, the pressure drops as blood flows.
11RESISTANCE What is Resistance? It is measure of hindrance or opposition to blood flow through a vessel, caused by friction between the blood in the vessel wall.If resistance to flow increases, it is difficult for blood to pass through a vessel, therefore, flow rate decreases.When resistance increases, the pressure gradient must increase to maintain the same flow rate.
12APPLIED If vessels offer more resistance to flow e.g. increased peripheral resistance, which occurs in high blood pressure then heart must work harder to maintain adequate circulation.
13RESISTANCE Resistance to blood flow depends on three factors: 1. Viscosity of blood2. Vessel length3. Radius of the vessel – this is most important.
14RESISTANCE 1. Viscosity of blood We write η for Viscosity. Viscosity refers to the friction, which is developed between the molecules of fluid as they slide over each other during flow of fluid.Greater the viscosity, Greater the resistance to flow.Blood viscosity is determined by number of circulating RBC, blood viscosity is increased in Polycythemia and decreased in Anaemia.
15RESISTANCE 2. Vessel length Greater the length of a vessel, more will be the resistance.How length of a vessel affects the resistance?When blood flows through a vessel, blood rubs against the vessel wall, greater the vessel surface area in contact with the blood , greater will be the resistance to the flow.
16RESISTANCE 3. Radius of the vessel It is the most important factor to determine the resistance to flow.Fluid passes more readily through a large vessel.Slight change in radius of a vessel brings great change to flow because Resistance is inversely proportional to the fourth power of the Radius [multiplying the radius by itself four times].R α 1/r4
17RESISTANCE 3. Radius of the vessel Therefore, doubling the radius, reduces the resistance to 1/16 its original value.[r4 = 2×2×2×2 = 16 or R α 1/16]and there is increased flow through a vessel 16 fold.On the other hand, when we decrease the radius to the half, blood flow will be decreased 16 times.
18Relationship of Resistance and Flow to Vessel Radius
19APPLIEDClinically radius of arteriole can be regulated and is the most important factor in controlling resistance to blood flow throughout the vascular system.
20Vascular Tree Closed system of vessels Consists of Arteries Arterioles Carry blood away from heart to tissuesArteriolesSmaller branches of arteriesCapillariesSmaller branches of arteriolesSmallest of vessels across which all exchanges are made with surrounding cellsVenulesFormed when capillaries rejoinReturn blood to heartVeinsFormed when venules merge
22Arteries Specialized to Serve as rapid-transit passageways for blood from heart to organsDue to large radius, arteries offer little resistance to blood flowAct as pressure reservoir to provide driving force for blood when heart is relaxingArterial connective tissue containsCollagen fibersProvide tensile strengthElastin fibersProvide elasticity to arterial walls
25Blood Pressure Force exerted by blood against a vessel wall Depends onVolume of blood contained within vesselCompliance of vessel wallsSystolic pressurePeak pressure exerted by ejected blood against vessel walls during cardiac systoleAverages 120 mm HgDiastolic pressureMinimum pressure in arteries when blood is draining off into vessels downstreamAverages 80 mm Hg
26Arterioles Major resistance vessels WHY? Because their radius is small.As arteriolar resistance is high, it causes marked drop in mean pressure as blood flows through arteriole.Mean Arterial Blood Pressure [ABP] of 93mm Hg in arteries falls to mean ABP of 37mm Hg as blood leaves the arteriole and enters the capillaries.
28ARTERIOLESArteriolar Resistance converts the pulsatile systolic to diastolic pressure swings in the arteries into the non-fluctuating pressure present in the capillaries.Radius of the arteriole can be adjusted to achieve 2 functions:Distribute cardiac output among systemic organs, depending on body’s momentary needsHelp regulate arterial blood pressure
29Arterioles Mechanisms involved in adjusting arteriolar resistance VasoconstrictionRefers to narrowing of a vesselVasodilationRefers to enlargement in circumference and radius of vesselResults from relaxation of smooth muscle layerLeads to decreased resistance and increased flow through that vessel
31Capillaries Thin-walled, small-radius, extensively branched Sites of exchange between blood and surrounding tissue cellsMaximized surface area and minimized diffusion distanceVelocity of blood flow through capillaries is relatively slowProvides adequate exchange time
32Veins Venous system transports blood back to heart Capillaries drain into venulesVenules converge to form small veins that exit organsSmaller veins merge to form larger vesselsVeinsLarge radius offers little resistance to blood flowAlso serve as blood reservoir
33References Human physiology by Lauralee Sherwood, seventh edition Text book physiology by Guyton &Hall,11th editionText book of physiology by Linda .s contanzo,third edition