General principals of circulation

Contin……..

1-Blood flow Relationship with Pressure gradient Relationship with Resistance2-velocity of blood Relationship with Cross sectional area Relationship with Pressure

Q = ∆P/R

• Depends on:– Pressure gradient –

difference in pressure between the beginning and ending of a vessel

– Vascular resistance – hindrance or opposition to blood flow through a vessel

Blood flow and pressure gradient relationship

Pressure gradient: aortic pressure – central venous pressure

Flow of blood through out body

= pressure gradient within vessels X resistance to flow

In hemodynamic,

difference in two Pressure is comparedPB and pressure inside blood vessels

Pressure difference b/w two points separated

by some distance

So, pressure gradient is expressed as

= F/A

= ∆P/ ∆x

Considering this we can define three different

kinds of pressure differences in the circulation-

1-Driving pressure

-axial pressure difference,

2-Transmural pressure-

Radial pressure difference

3- Hydrostatic pressure

-

1-Driving pressure-axial pressure difference, In Circulation it is arterial and venous end pressure difference, in systemic or pulmonary circulationIt governs the flow of blood

2-Transmural pressure- Radial axis pressure difference It is pressure difference b/w intravascular and tissue pressureIt governs vessel diameter and major determinate of resistance

3- Hydrostatic pressure- density of blood and gravitational force when blood lies in vertical column

Blood flow and pressure gradient relationship

Linear in rigid vessels

blood vessels are distensible

Critical Closing Pressure

Equilibrium of factors

Critical Closing Pressure

1 - vasomotor tone2 - Intramural pressureThese factors equilibrate and maintain

the blood flow

Can be understand by a physics law called as Laplace Law

Pascal's principle states that the pressure is everywhere

same inside the balloon at equilibrium.

But examination immediately reveals that there are great

differences in wall tension on different parts of the balloon. The

variation is described by Laplace's Law.

Acc. to Laplace law tension in cylinder wall

T- Tension, P- Transmural pressure R- Radius, W- wall thickness

In sphere r1=r2

So, P=2T/R

But in blood vessels P=T/R

This law is applicable for all the hollow viscous organBlood vesselsHeartLungsKidney

LaPlace's Law The larger the vessel radius, the larger the wall

tension required to withstand a given internal fluid pressure.

For a given vessel radius and internal pressure, a spherical vessel will have half

the wall tension of a cylindrical vessel.Why does the wall tension increase with radius?

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Why does wall tension increase with radius?

If the upward part of the fluid pressure remains the same, then the downward component of the wall tension must remain the same. But if the curvature is less, then the total tension must be greater in order to get that same downward component of tension.

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Tension in Arterial Walls The tension in the walls of arteries and veins in

the human body is a classic example of LaPlace's law.

This geometrical law applied to a tube or pipe says that for a given internal fluid pressure, the wall tension will be proportional to the radius of

the vessel.

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The implication of this law for the large arteries,

which have comparable blood pressures, is that the

larger arteries must have stronger walls since an

artery of twice the radius must be able to withstand

twice the wall tension.

Arteries are reinforced by fibrous bands to

strengthen them against the risks of an aneurysm.

The tiny capillaries rely on their small size.

The walls of the capillaries of the human circulatory system are so thin as to appear transparent under a microscope, yet they withstand a pressure up to about half of the full blood pressure.

LaPlace's law gives insight into how they are able to withstand such pressures: their small size implies that the wall tension for a given internal pressure is much smaller than that of the larger arteries.

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Capillary Walls

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Blood flow and resistance relationship

The larger arteries provide much less resistance to flow than the smaller vessels according to Poiseuille's law, and thus the drop in pressure across them is only about half the total drop.

The capillaries offer large resistances to flow, but don’t required much strength in their walls

According mathematical calculation in Principles of physics, Resistance is represented as - R = 8ηl/∏r4

After replacing these values in Poiseuille’s law by R Blood flow Q will be

Q = ∆P/R

In vascular system, resistance to flow is represented by the total peripheral resistance and is expressed as peripheral resistance unit

vasodilation resistance decreases

vasoconstriction resistance increases

Factors promoting total peripheral resistance (TPR) -- combined resistance of all vessels

– As resistance increases flow rate decreases

Resistance (R) α Viscosity (η)Viscosity described by Newton in 1713 as an internal friction to flow in a fluid or lack

of slipperiness.

– Radius the main determinant of resistance

– Increased surface area exposed to blood increases resistance

– Flow is faster in larger vessels than smaller

• Vascular resistance opposition to blood flow due to friction between blood and the walls of blood vessels– Increase in vascular resistance = increase in BP– Decrease in vascular resistance = decease in BP

• Vascular resistance is dependent upon:– Size of the blood vessel (lumen)

• Smaller means greater resistance to blood flow; alternates between vasoconstriction and vasodilation

– Blood viscosity• Ratio of RBCs to plasma volume• Higher viscosity = higher resistance

– Total blood vessel length• Resistance increase with total length• Longer the length = greater contact between vessel wall and

blood

2-velocity of blood Relationship with Cross sectional area

Relationship with Pressure

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Relationship with Cross sectional area

Blood Flow, Velocity, and Pressure

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Lymphatic circulation• Driven by factors similar to

venous circulation:- muscle activity- valves- respiration

• Lymph = plasma-proteins

• Lymphatic circulation collects fluid not reabsorbed by the capillaries

• Lymph is filtered in nodes before return to blood circulation