Origins of Pressure in the Circulation
These are the factors that generate pressure in the circulation i.e. blood pressure. They are:
Gravity
Compliance of Blood Vessels
Viscosity of the Blood
Inertia of Blood and Vessels
Gravity
Blood in the blood vessels makes a continuous column of fluid
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Gravitation pull on this column generates pressure on blood in the lower parts of the body
At the level of heart
The reference level to measure blood pressure is the level of heart.
Artierial pressure ≈ 90 mmHg
Venous pressure ≈ 5 mmHg
In lying position → the entire body is almost at the level of heart → gravity does not produce much change in pressure.
In standing position → height difference leads to different pressure at different levels in body as explained next.
Below the Level of Heart
As we go lower, the pressure increases.
For an average 180 cm tall person
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the foot lies 130 cm below the heart
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This tall column of blood creates extra 95 mmHg pressure at the foot
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Arterial pressure at foot ≈ 185 mmHg
Venous pressure at foot ≈ 100 mmHg
Above the Level of Heart
As we go higher, the pressure decreases.
For an average 180 cm tall person
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the top of the head about 50 cm above the heart
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The pressure here is decreased by about 37 mmHg
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Arterial pressure ≈ 53 mmHg
Venous pressure = -32 mmHg
Despite the variation in absolute pressure, the driving pressure for blood flow (i.e. difference in atrial and venous pressure) remains uniform across all levels e.g.
At the level of the heart: 90 - 5 = 85 mmHg
At the foot: 185 - 100 = 85 mmHg
At the top of the head: 52 - (-32) = 85 mmHg
Compliance of the Vessels
Compliance is basically the distensibility of the blood vessels.
Veins (are like balloons)
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Have high compliance
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With increase in volume
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more distension,
less rise in pressure
Arteries (are like tubes of tyres)
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Have low compliance
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with increase in volume
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less distension,
more rise in pressure
Viscosity of the Blood
Viscosity is basically the resistance of fluid to movement.
When there is a flow, the blood resists moving due to its viscosity
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This resistance generates pressure
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The higher the viscosity
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The higher the pressure.
Inertia of Blood
Pressure is generated in blood from conversion from other forms of energy in blood.
e.g. conversion from kinetic energy into pressure energy. The kinetic energy is imported to blood by heart during pumping.