1. Cardiovascular system L-4 Blood pressure & special circulation
Dr Than Kyaw
27 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 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?

3. 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.

4. 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.

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

7. 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

8. 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

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

11. 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

13. 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.

14. 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)

15. 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

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 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

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 cells
Reduced O2 concentration dilation of vessels
more blood permitted to flow
O2 replenished

19. 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

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 the
heart