Cell Membrane: Viscosity

Viscosity

If two tubes, one containing water and the other containing castor oil are tilted together, the latter will flow slowly, when compared to the former. This is because of the frictional force that exists between liquid layers. This resistance for the flow of the liquid is termed as Viscosity.

Viscosity is defined as the internal resistance against the free flow of a liquid to the frictional forces between the fluid layers moving over each at different velocities. Each and every liquid has its own characteristic viscosity co-efficient. The co-efficient of viscosity of a liquid is defined as the force in dynes required to maintain the streamline flow of one fluid layer of unit area over another layer of equal area separated from one another by 1 cm at a rate of 1cm/sec. Viscosity is measured in poises or millipoises.

Factors affecting viscosity

1. Density

Density and viscosity are directly proportional to each other. They are related by Stoke’s law. If a small sphere of radius ‘r’ and density ‘r’ falls vertically through a liquid with the density ‘r’at a steady velocity ‘u’, inspite of the acceleration due to gravity (g), the co-efficient of viscosity and density are related as follows.

2. Temperature

Temperature and viscosity are inversely related to each other. As temperature increases, viscosity of the liquid decreases.

3. Solute

Dissolved substances in the pure solvent increases the viscosity of the solvent.For eg.a protein solution is highly viscous than pure water. Size and Shape of the solute particles also affect the viscosity of the solution.

Biological Applications

·              Carbohydrate and protein solutions are highly viscous in nature.

·              Blood plasma has a normal viscosity of 15 – 20 mpoises. Alterations in the viscosity is an indication of diseased condition. Viscosity increases during macroglobulinemia, retinal hemorrhages and congestive heart failure.

·              Viscosity of blood is 30 – 40 mpoises and is due to the red blood cells. Viscosity of blood decreases during anemia.

·              Blood viscosity is useful in streamlining the blood flow.

·              The lubricating property of the synovial fluid is achieved mainly by the viscous nature of the mucopolysaccharides present in the synovial fluid.