January 13, 2012
Each time our heart beats, a specific volume of blood is pumped in and out of the body. Blood gets circulated by systematic opening and closing of the valves, before it finally returns to the heart. Blood circulation takes place under certain pressure and its velocity is dependent on our physical activities and cardiac health. If the rate of blood flow deviates from the normal, then there's a probability of underlying body ailment. We explain you the nature of blood flow and the factors affecting its velocity over here.
Blood Flow Velocity ~ An Overview
The definition of blood flow velocity as given in the medical dictionary is, "a value equal to the total volume flow divided by the cross-sectional area of vascular bed". Velocity of blood is measured in cm/s. Blood flows at zero velocity through the vessel walls. Movement of blood inside the body slows down if a person has undergone considerable volume of blood loss. Decrease in rate of flow might also lead to de-oxygenation or starve the vessels of oxygen. People who have suffered heart strokes experience immense decrease in velocity of blood flow. Brain cell death is one of the most severe consequences of extremely retarded blood flow. On the other hand, when high blood velocity is high, it gives rise to several health problems. Vessels under high velocity are more susceptible to breakage and rupture.
Velocity is also dependent on blood pressure. A healthy cardiovascular system has well-balanced blood pressure and velocity. Under such circumstances, a steady state of blood flow is maintained in the body. Changes in blood pressure can alter the rate of flow. Blood velocity is dependent on its surface area of flow and the resistance offered to its path. Larger the surface area of the vessels, greater is the flow rate. Resistance is offered by closing of valves, which leads to decrease in velocity.
The pattern of blood flow is laminar (blood flows linearly and smoothly) under normal conditions. Linear flow is observed in adjacent layers of blood vessels while it flows turbulently in ascending aorta, branch points, stenotic arteries and stenotic heart valves. Turbulence or chaotic flow is felt clearly under a stethoscope. Physiological murmurs generated due to the turbulence is usually perceived during pregnancy and when an individual exercises or performs strenuous physical activities. The cardiac output gets extremely elevated as a consequence of turbulent blood flow. Research on blood velocity has revealed that there is a significant decrease in the velocity when a person is under the influence of alcohol. Studies show that rate of blood flow in the middle cerebral artery is less in heavy drinkers and smokers.
Determining Blood Flow Velocity
Velocity of blood flow is determined by a factor known as Reynolds number (Re). The formula is stated below:
Re = (mean velocity× vessel diameter× blood density)/blood viscosity.
The value of Re is directly proportional to the velocity of blood flow and inversely proportional to the viscosity of blood. Under ideal conditions, i.e., in long, straight and smooth blood vessels, critical Re is relatively high, whereas in branching vessels, critical Re is much lower.
Velocity of blood flow can be measured by two methods, viz., Laser Doppler flow measurement and Ultrasonic (Doppler, transit time) flow meters. In the first process, a monochromatic laser diode is used to measure the rate of blood flow while in ultrasonic techniques, the flow meters measure the difference of the transit time of ultrasonic pulses moving in and against the direction of blood flow.
We can also infer that the velocity of blood defines our health status. Thus, it becomes necessary to maintain a consistency in blood flow and blood pressure for having perfectly sound cardiovascular health.