Cardiovascular system L-4 Blood pressure & special circulation Dr Than Kyaw 27 February 2012

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 and Low pressure in veins. Blood pressure (BP) – generally refers to arterial blood pressure Blood flows from the left ventricle through the vessels and back to the right atrium Blood flow - continuous rather than intermittent, considering that the ventricles contract intermittently. Why?

Pressure generation and flow Greatest pressure – within aorta when the left ventricle contracts After complete relaxation of ventricle – BP in aorta does not diminish entirely Large arteries: higher number of elastic C/T fibers than muscle permit expansion of arteries when the blood advances into them Stretched elastic fibers rebound and exert pressure on the blood in the large vessels after the heart ceases to exert the pressure This continuous pressure in the arteries permits a continuous rather than an intermittent blood flow through the body.

Generation of systemic pressure during left ventricular systole and maintenance of blood flow and pressure during diastole A: Contraction of ventricle and stretching of aorta B: 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.

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 calibration Direct measurement - cannulation in the artery - electronically measured with transducer Human - Sphygmomanometer

Blood pressure of adult resting animals Species Mm Hg Mean (mm Hg) Horse 130/95 115 Cow 140/95 120 Swine 140/80 110 Sheep 140/90 114 Dog 120/70 100 Cat chicken 175/145 160 Man

Structure of blood vessels Arteries – strong and thick walled, smaller diameter Veins – thin and larger diameter, C/T, smooth m/s Function 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 substances Capillary bed – 4% of total blood volume But the vast number of capillaries provides a large total cross-sectional area that leads to slow rate of blood flow favouring transcapillary exchange

Endothelial wall of a muscle capillary Diameter = 5 – 10 m

Filtration (arterial end) and absorption (venous end) of a capillary Pc = capillary hydrostatic pressure Pif = interstitial fluid hydrostatic pressure if = interstitial colloidal fluid osmotic pressure c = plasma colloidal osmotic pressure

Capillary imbalances Filtration 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 pressure Low blood protein concentration Lymphatic blockage Increased porosity of the capillaries Edema = resulted from accumulation of fluid in interstitial cellular spaces due to imbalance of filtration and absorption.

Systolic and Diastolic Pressure Peak left ventricular (systole) – high point of arterial pressure - systolic blood pressure Relaxed left ventricle (diastole) – lowest pressure in the artries - diastolic pressure BP usually given two values; e.g. 130/70 mm Hg upper value – systolic pressure lower value – diastolic pressure Pulse pressure – the difference between systolic & diastolic pressure - above example = 60 mm Hg Mean blood pressure – diastolic pressure + 1/3 of pulse pressure (Note: it is not the half way between systolic and diastolic pressures)

Vessel cross-sectional area Blood vessels Cm2 Aorta Small arteries Arterioles Capillaries Venules Small veins Venae cavae 2.5 20 40 2500 250 80 8

Graphic illustration of pressure changes Decreasing pressures from major arteries to major veins. Note the sharp decrease in pressure in arterioles.

Regulation of blood flow Blood flows from a point of high mean pressure to a point of low mean pressure Mean blood pressure - higher in arteries than capillaries - higher in capillaries than veins Driving force of blood pressure needs to overcome vascular resistance provided by blood vessels Resistance 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

Autoregulation of blood flow An auto-regulatory mechanism affecting blood flow to a body part by the amount of O2 being received by the cells Reduced O2 concentration dilation of vessels more blood permitted to flow O2 replenished

Cardiac output and blood diversion Resting condition: - body organs and muscle receives rather constant amount - muscle receives 20 – 25% of output blood Under 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

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 the heart