2013 Vessels, flow and pulsations 3 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.
2013 Vessels, flow and pulsations 4 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.
2013 Vessels, flow and pulsations 5 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.
2013 Vessels, flow and pulsations 6 Exchange vessels. Capillaries. Only endothelium and basement membrane. Precapillary sphincters- respond to humeral and local factors. Only about 25 % open at rest.
2013 Vessels, flow and pulsations 7 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.
2013 Vessels, flow and pulsations 8 Shunt vessels. Arterio-venus anastamosis. Found in fingers, palms and earlobs. Controlled by sympathetic nerves.
2013 Vessels, flow and pulsations 9 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.
2013 Vessels, flow and pulsations 10 Peripheral resistance. R = 8ηL / πr 4. Viscosity and length: directly proportional. Radius: inversely proportional [slight change in radius can bring large change in resistance].