 Dear students, in this topic we shall discuss the laminar and turbulent flow of blood in blood vessels. The blood flow is affected by the smoothness or roughness of the inner lining of blood vessels. The texture of blood vessels results in either the turbulent or the laminar, that is smooth flow. First we shall discuss laminar flow in smaller blood vessels. In smaller blood vessels, the flow is streamlined and continuous. It is characterized by a parabolic velocity profile across the vessel. When blood is moving, it is a parabolic movement in a vessel. Flow also occurs in layers which are moving at different velocities. Flow is zero at the wall and maximum at the center. This is how these flow characters are in smaller vessels or laminar flow characters. So different layers are being formed, the force provided by the pressure difference which supplies the force required to slide the layers to slide past each other. Dear students, in comparison to laminar flow in smaller vessels, the laminar flow in larger vessels is pulsatile. It has more complex velocity profile than the continuous laminar flow of smaller vessels. In larger vessels, blood is first accelerated and then slowed down with each heartbeat. The reason of pulsatile laminar flow is the elasticity of larger blood vessels which expand and relax with pressure oscillation. Larger blood vessels expand and when pressure is less, these blood vessels become relaxed. Another character of the pulsatile laminar flow is that when the heart's aortic walls are closed, the direction of flow in these larger vessels is reversed. At that time, blood tries to go back and there are some turbulence. Dear students, Now we shall discuss the turbulent flow. In a turbulent flow, direction of fluid movement is not aligned. It is haphazard and a turbulent flow requires higher energy to move blood through a vessel. First turbulence occurs in the proximal portions of aorta and pulmonary artery. The highest turbulence occurs at the time of ventricular contraction and during the back flow of blood when pulmonary and aortic walls close. This also happens if the smoothness of the blood vessel is reduced by any type of obstruction in the blood vessel. For example, on the aorta, fat deposits are accumulated in the blood vessels that narrow the rta walls and they are not smooth. This creates turbulence in the blood flow. Dear students, the turbulent flow or turbulence is uncommon in peripheral undivided smaller vessels with smooth walls. However, turbulence also occurs in these vessels in some situations. For example, when the blood velocity is very high, if we have exercised a lot of blood velocity, then in such a situation, the peripheral smaller vessels or smaller arteries that have a normal laminar flow can also have turbulent flow.