2013 Vessels, flow and pulsations 1

Blood Vessels, Flow and Pulsation

Prof. K. Sivapalan

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Circulation.

• Blood ejected into aorta flows through vessels and back to heart.

• The histology of the vessels shows a common pattern.

• Structure and properties of the vessels are adapted depending on the role played by that part in circulation.

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Structure of blood vessols.

• Vasa vasorum.• Nerve.• Tunica adventitia.• Tunica media

– External elastic membrane.– Smooth muscle.

• Tunica intima.– Internal elastic membrane.– Lamina propria.– Basement membrane.– Endothelium.

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Elastic arteries [Windkessel vessels].

• Aorta and large arteries have large amount of elastic fibers and small amount of smooth muscles.

• They stretch when pressure increases and recoil when pressure reduces.

• Aorta distends when blood is ejected from ventricles and recoils during diastole.

• This keeps blood flowing to the periphery continuously.• The pressure fluctuates in the arteries depending on

ejection by the heart, elasticity of aorta, peripheral resistance and out flow.

• Highest is systolic pressure and the lowest is diastolic pressure.

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Arterial pulsation.

• The pressure changes are transmitted along the vessel as pulse wave.

• It can be felt as a distension over the arteries.• Mean pressure is diastolic pressure +1/3 of the pulse

pressure.• Velocity of the pulse : 5 M / Sec at 5 years, 8 M / Sec

at 60 years. [faster when solidifying]• Average velocity of blood is 0.5 M / Sec.• Arterial pulse indicates patency but not flow.• The character of the pulse varies depending on the

state of heart and vessels.

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Resistance vessels.

• Less elastic fibers and more muscles.• Small arteries and (mainly) arterioles.• Sympathetic nerves – Vasomotor tone.• Convert pulsatile flow into continuous flow.• Provide majority of peripheral resistance.• Arterioles determine the amount of blood

flow to any area.

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Exchange vessels.

• Capillaries.• Only endothelium and basement

membrane.• Precapillary sphincters- respond to

humeral and local factors.• Only about 25 % open at rest.

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Capacity vessels.

• Veins and venules.• Thin wall, easily distended.• Little muscles but potent vaso spasm possible.• Pressure by skeletal muscles [calf] additional

pump.• Valves prevent back flow specially in dependant

parts.• Pressure waves in the atria are transmitted

backwards to the large veins.

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Shunt vessels.

• Arterio-venus anastamosis.• Found in fingers, palms and earlobs.• Controlled by sympathetic nerves.

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Flow, pressure & resistance.

• In rigid tubes,• Flow α pressure.

– Flow 1/α resistance.• In blood vessels,

– Flow stops at critical closing pressure.

• Reason-– ? viscosity.– ? red cells larger than

capillaries.– ? vasomotor tone.

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Peripheral resistance.

• R = 8ηL / πr4.• Viscosity and length: directly proportional.• Radius: inversely proportional [slight

change in radius can bring large change in resistance].

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Blood flow.

• Laminar flow:• Smooth flow.

– Pulsatile flow.– Pulse can be felt [not heard]

• Turbulent flow:– Edi currents and vibrations

[sound]– Depends on, density, velocity,

viscosity and the diameter of the tube.

– Sounds produced.

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Summary