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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, - PowerPoint PPT Presentation
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Cardiovascular system
L-4 Blood pressure & special circulation
Dr Than Kyaw27 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 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.
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 140/90 110
chicken 175/145 160
Man 120/70 100
Structure of blood vessels
Arteries – strong and thick walled, smaller diameterVeins – 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 pressurePif = interstitial fluid hydrostatic pressureif = interstitial colloidal fluid osmotic pressurec = 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:1. High capillary pressure2. Low blood protein concentration3. Lymphatic blockage4. 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
AortaSmall arteries Arterioles Capillaries Venules Small veinsVenae cavae
2.5 2040
2500250808
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
Regulation of blood flow
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
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