Presentation on theme: "Cardiovascular system L-4 Blood pressure & special circulation"— Presentation transcript:
1 Cardiovascular system L-4 Blood pressure & special circulation Dr Than Kyaw27 February 2012
2 Blood Pressure (BP) Pressure generation and flow Blood is under pressure within its closed system.Pressure varies in different parts of the system.High pressure in arteries,Moderate pressure in capillaries andLow pressure in veins.Blood pressure (BP) – generally refers to arterial blood pressureBlood flows from the left ventricle through the vessels and back to the right atriumBlood flow - continuous rather than intermittent, considering that the ventricles contract intermittently. Why?
3 Pressure generation and flow Greatest pressure – within aorta when the left ventricle contractsAfter complete relaxation of ventricle – BP in aorta does not diminish entirelyLarge arteries: higher number of elastic C/T fibers than musclepermit expansion of arterieswhen the blood advances into themStretched elastic fibersrebound and exert pressure on the bloodin the large vessels after the heart ceases toexert the pressureThis continuous pressure in the arteries permits a continuous rather than an intermittent blood flow through the body.
4 Generation of systemic pressure during left ventricular systole and maintenance of blood flow and pressure during diastoleA: Contraction of ventricle and stretching of aortaB: Followed by retention of system blood in vessels by the closed aortic semilunar valve. Continued blood flow is provided by the elastic recoil of the aorta.
5 Measuring BP Droppler flow method - transducer or cuff is fixed at an appropriate location on the fore- or hind limb or tail- ultrasonic beam is emitted into the blood vessel- the ultrasound reflected from the moving blood changes its frequency- BP is then measured by calibrationDirect measurement- cannulation in the artery- electronically measured with transducerHuman- Sphygmomanometer
8 Structure of blood vessels Arteries – strong and thick walled, smaller diameterVeins – thin and larger diameter, C/T, smooth m/sFunction of capillaries- to exchange fluid, nutrients, electrolytes, hormones, and other substances between the blood and the interstitial fluid.To serve this role, the capillary walls are very thin and have numerous minute capillary pores (intercellular cleft) permeable to water and other small molecular substancesCapillary bed – 4% of total blood volumeBut the vast number of capillaries provides a large total cross-sectional area that leads to slow rate of blood flow favouring transcapillary exchange
13 Capillary imbalancesFiltration pressure (8 mmHg) > absorption pressure (7 mmHg)This tends to accumulate interstitial fluid.Normally it does not occur as the lymph removes extra-filatratem.Imbalance of bulk flow may occur:High capillary pressureLow blood protein concentrationLymphatic blockageIncreased porosity of the capillariesEdema = resulted from accumulation of fluid in interstitial cellular spaces due to imbalance of filtration and absorption.
14 Systolic and Diastolic Pressure Peak left ventricular (systole) – high point of arterial pressure- systolic blood pressureRelaxed left ventricle (diastole) – lowest pressure in the artries- diastolic pressureBP usually given two values; e.g. 130/70 mm Hgupper value – systolic pressurelower value – diastolic pressurePulse pressure – the difference between systolic & diastolic pressure- above example = 60 mm HgMean blood pressure – diastolic pressure + 1/3 of pulse pressure(Note: it is not the half way between systolic and diastolic pressures)
15 Vessel cross-sectional area Blood vesselsCm2AortaSmall arteriesArteriolesCapillariesVenulesSmall veinsVenae cavae2.520402500250808
16 Graphic illustration of pressure changes Decreasing pressures from major arteries to major veins.Note the sharp decrease in pressure in arterioles.
17 Regulation of blood flow Blood flows from a point of high mean pressure to a point of low mean pressureMean blood pressure - higher in arteries than capillaries- higher in capillaries than veinsDriving force of blood pressure needs to overcome vascular resistance provided by blood vesselsResistance depends on the length and radius of the vessels, and nature of the blood (viscosity)In addition – constriction or dilatation of the blood vessels changes blood flow and pressure
18 Autoregulation of blood flow An auto-regulatory mechanism affecting blood flow to a body part by the amount of O2 being received by the cellsReduced O2 concentration dilation of vesselsmore blood permitted to flowO2 replenished
19 Cardiac output and blood diversion Resting condition:- body organs and muscle receives rather constant amount- muscle receives 20 – 25% of output bloodUnder extreme muscular exertion:- up to 75%- also diversion of blood flow from other organs (kidney, intestine) so that it can be used by the muscle
20 Breathing and blood flow Inspiration:- expansion of thorax- reduce pressure in mediastinal space (intrthoracic pressure)- allow expansion of volume and lowering of pressure of:- lymphatics, venae cavae, (oesophagus)- helpful for the return of venous blood and lymph to theheart