Biomedical importance of surface tension & Viscosity

Biomedical importance of surface tension & Viscosity 26.6.12

Surface Tension

Factors affecting surface tension • Temperature • Solute Solutes can have different effects on surface tension depending on their structure: Little or no effect, for example sugar Increase surface tension, inorganic salts Decrease surface tension progressively, alcohols Decrease surface tension and, once a minimum is reached, no more effect: surfactants

Gibbs-Thomsan Principle • According to this principle substances which lower the surface tension becomes concentrated in the surface layer whereas substances which increase surface tension are distributed in the interior of the liquid • Lipids and proteins effective in lowering surface tension are found concentrated in the cell wall • Soaps and bile salts reduce the surface tension of water while sodium chloride and most inorganic salts increase the surface tension

Laplace law • Laplace’s law states that the pressure inside an inflated elastic container with a curved surface, e.g., balloon is inversely proportional to the radius as long as the wall (of balloon) tension is presumed to change little. • A common illustration of this phenomenon is that the effort required to blow up a balloon is greatest when the diameter of the balloon is least

Biomedical Importance of surface tension • Surface tension of plasma • Bile salts • Hay’s test • Respiration

Biomedical Importance of surface tension • Surface tension of plasma is slightly less than water • Emulsifying action of Bile salts Bile salts are surface active. They facilitate action of pancreatic lipase and hence absorption of free fatty acids. Bile salts lowers surface tension of fat droplets • Hay’s test This test is based on the surface tension and is employed for detecting the presence of bile salts in urine (an indication of jaundice). If urine contains bile salts, the fine sulfur powder sprinkled on its surface settles down due to lowering of surface tension. Fine sulfur continues to float on the surface if urine does not contain bile salts

Alveolus • Alveoli can be compared to gas in water, as the alveoli are wet and surround a central air space. The surface tension acts at the air-water interface and tends to make the alveolus smaller (by decreasing the surface area of the interface). • Surfactant is a lipoprotein mixture secreted by special surfactants secreting cells i. e. type II granular pneumocytes present in alveolar epithelium

Surface tension of water prevents expansion of alveolus • Surface tension at the air-water interface prevents expansion of alveoli • Air taken in by respiration pushes on the water lining the alveoli • The normal force due to surface tension of water pushes the air out reducing the size of the alveoli

Alveolar Instability • On the basis of Laplace law, air would be displaced from the smaller alveolus into the larger one and the smaller alveolus would thus become still smaller • This process would continue until the smaller alveolus would collapse entirely while displacing all of its air into the larger one • This process would lead to instability of alveoli

Alveolar Stability • Instability is prevented by surfactant. Amount of surfactant per alveolus is constant • As an alveolus becomes smaller the surfactant becomes more concentrated at the surface of alveolar lining fluid. Hence surface tension becomes progressively more less. This helps alveolus expand during inhalation • On the other hand as an alveolus becomes bigger, the surfactant is spread more thinly on the fluid surface. This increases the surface tension • This property of surfactant stabilizes the sizes of the alveoli, causing the larger alveoli to contract more and smaller ones to contract less.

The absence of surfactant in the alveolar membrane of some premature infants causes the respiratory stress syndrome in them • Factors reducing production of surfactant or increasing its rate of destruction may contribute to adult respiratory distresss syndrome

Compliance and hysteresis • Compliance is the ability of lungs and thorax to expand. Lung compliance is defined as the volume change per unit of pressure change across the lung. • Measurements of lung volume obtained during the controlled inflation/deflation of a normal lung show that the volumes obtained during deflation exceed those during inflation, at a given pressure. • This difference in inflation and deflation volumes at a given pressure is called hysteresis and is due to the air-water surface tension that occurs at the beginning of inflation.

However, surfactant decreases the alveolar surface tension to very low, near-zero levels • At the end of inhalation, surfactant distributes thinly on the surface limiting the volume of the alveoli • Pulmonary surfactant thus greatly reduces surface tension increasing compliance allowing the lung to inflate much more easily, thereby reducing the work of breathing. It reduces the pressure difference needed to allow the lung to inflate.

Viscosity • Viscosity is a property of flowing fluid (liquid/gas) by virtue of which relative motion between layers in contact is opposed. • It is the internal resistance against the free flow of liquid due to frictional forces between the fluid layers moving over each other at different velocities • If velocity of layers is same then relative velocity is zero and the liquid is said to be non viscous

Factors affecting Viscosity • Density Viscosity of a liquid varies directly with its density • Temperature: An increase in temperature decreases the viscosity of liquid. This is due to an increase in the kinetic energy of molecules for overcoming the resistance due to intermolecular attractions and also for breaking intermolecular H-bonds of associated liquid • Solute concentration: Viscosity of a solution is directly proportional to the solute concentration. Suspended particles cause an increase in viscosity in proportion to the volume of suspended material relative to the total volume

Classification of fluids showing viscous properties • Newtonian fluids: An ideal or Newtonian fluid is one in which the viscosity is constant and is independent of how rapidly the fluid flows. Plasma is a Newtonian fluid even at very high protein concentrations • Non- Newtonian fluids: Non-Newtonian fluids change viscosity at variant flow velocities. Blood is a Non-Newtonian fluid Viscoelastic fluids: The fluids which have elastic as well as viscous properties are called viscoelastic. They have a memory of their original state i. e. they return to their original state after the deformation is removed

Viscosity of blood • Blood is a suspension of cells in plasma. Plasma is a water solution of salts and heavily hydrated macromolecules • The viscosity of blood thus depends on the viscosity of the plasma, in combination with the hematocrit (The hematocrit or packed cell volume (PCV) or erythrocyte volume fraction (EVF) is the volume percentage (%) of red blood cells in blood. It is normally about 45% for men and 40% for women) • Blood cells behave as suspended particles and increase the viscosity of blood. Fibrinogen due to its asymmetry and high density imparts maximum viscosity to the blood • Globulins have greater influence than albumin

Effect of Erythrocytes on viscosity of blood • Deformability of Erythrocytes • Haematocrit

Deformability of Erythrocytes • Erythrocyte deformability refers to the cellular properties of erythrocytes which determine the degree of shape change under a given level of applied force • Erythrocytes change their shape extensively under the influence of applied forces in fluid flow or while passing through microcirculation. • The extent and geometry of this shape change is determined by both the mechanical properties of erythrocytes, the magnitude of the applied forces and the orientation of erythrocytes with the applied forces

Deformability and viscosity • Erythrocyte deformability is an important determinant of blood viscosity, hence blood flow resistance in the vascular system • Deformability of erythrocytes affects viscosity of blood in small vessels where erythrocytes are forced to pass through blood vessels with diameters smaller than their size • In sickle cell anaemia, deformability of erythrocytes is reduced thereby increasing viscosity of blood

Elastic Erythrocytes • Erythrocyte when freely suspended assumes biconcave discoid shape indicative of large excess of its surface area over its volume • Shape change of erythrocytes under applied forces is reversible and the biconcave-discoid shape is maintained after the removal of the deforming forces • In other words, erythrocytes behave like elastic bodies, while they also resist to shape change under deforming forces

The viscoelasticity of blood is traceable to the elastic red blood cells, which occupy about half the volume. • When the red cells are at rest they tend to aggregate and stack together • In order for blood to flow freely, the size of these aggregates must be reduced, which in turn provides some freedom of internal motion. The forces that disaggregate the cells also produce elastic deformation and orientation of the cells

In Region 1, the cells are in large aggregates • In Region 2, the cells are disaggregated and the applied forces are forcing the cells to orient. As the shear rate increases, the applied forces deform the cells • In Region 3, increasing stress deforms the cells, and if the cells have normal deformability they will form layers that slide on layers of plasma.

Effect of haematocrit on Blood Viscosity • Viscosity of blood also varies with changing haematocrit • When the haematocrit rises, the friction between the successive layers of blood increases. Hence with increasing haematocrit the viscosity of the blood rises drastically

Effect of Temperature on the Viscosity of Blood • This is due to the presence of thermoproteins, which show physical change s at temperatures below or above 37 °C • Cryglobulin operates at temperatures below 37 °C forming reversible or irreversible precipitates, gels or crystals • Pyroglobulin gets precipitated at 56 °C • Bence-Jones proteins gets precipitated at 40 °C • At 0 °C, the viscosity of blood is increased up to three times. This reduces the circulation in the tissues exposed to cold

Effect of Inflammation and disease conditions on Viscosity of Blood • Inflammation causes loss of plasma into the tissues. This leads to slowing of blood flow, disturbance in the axial flow, rouleaux formation and increase in viscosity • Blood viscosity is increased in diabetes mellitus, multiple myloma, jaundice, leukaemia, asphyxia, vomiting and diarrhea Typical mammalian erythrocytes: (a) seen from surface; (b) in profile, forming rouleaux

Viscosity of Sinovial Fluid • Synovial fluid is highly viscous. Its viscosity varies from 50 to 200 times that of water. Its viscosity is due to the presence of hyaluronic acid • Viscosity of Synovial fluid reflects its hyaluronic acid content • In inflammatory effusions such as rheumatoid arthritis etc , the viscosity of synovial fluid is markedly decreased (down to water like) indicating the presence of a thin watery fluid containing degraded small molecular hyaluronidase particles

Biomedical importance of surface tension & Viscosity