Physiology Illustration Laminar versus turbulent flow in blood vessels.
Laminar versus turbulent flow in blood vessels.
Laminar flow refers to streamline movement of blood. In laminar flow, blood flows in layers which move parallel to the long axis of the blood vessel (straight arrows parallel with the vessel long axis). Close to the vessel wall, an infinitely thin layer of blood in contact with the wall is stationary (i.e., does not flow). The next layer in contact with this layer has a low velocity. As the layers extend toward the vessel interior, their velocity increases. Velocity is highest for the layer at the center of the vessel lumen. Therefore, blood flow velocity is zero for the layer in contact with the vessel wall and highest at the center of the vessel lumen. Blood flow in most vessels of the body is laminar. Despite the pulsatile nature of flow in arteries, laminar blood flow is silent. Thus, no sound is normally heard via a stethoscope placed over arteries. Constriction of the vessel, or obstruction of the vessel lumen, disrupts laminar flow and leads to turbulent blood flow. This is shown in the illustration by curved arrows and short straight arrows showing flow in directions other than along the long axis of the blood vessel. At the point of constriction, blood flow velocity increases, but small eddies lead to flow in directions other than parallel to the long axis of the vessel. Such current eddies lead to turbulence. Turbulent blood flow is noisy and can be heard by using a stethoscope placed over the artery at or distal to the point of constriction or obstruction. Laminar and turbulent flow provide the physical basis for the auscultation method of blood pressure measurement using a pressure cuff placed over the upper arm (to cause vessel constriction) and a stethoscope placed over the brachial artery (to listen for the noise of turbulent flow when the vessel is constricted). The sounds heard by this method are referred to as the Korotkoff's sounds.
Posted: Friday, October 30, 2015 Last updated: Friday, October 30, 2015