Role of Surfactant in Respiration, Viscosity and Viscous force 6.12.12

Alveolus

If the alveoli were lined with water alone the surface tension would be too strong so the alveoli would collapse If we regard the alveoli as spherical bubles, then: P = inward directed collapsing pressure T = Surface Tension r = radius of the buble According to the law of LaPlace: the smaller alveoli (with smaller radius - r) have a higher tendency to collapse

Importance of Surfactant Reduces the tendency of alveoli to collapse by reducing surface tension Reduces work of breathing by increasing lung compliance Prevents pulmonary oedema Responsible for alveolar stabilization

Surfactant Increases Lung Compliance 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. Surfactant increases lung compliance

Prevention of Pulmonary Oedema Surface tension not only pulls alveolar wall to the center of alveolus but also pulls fluid from capillaries into the interstitial space surrounding the alveolus and into the alveoli leading to pulmonary oedema Surfactant prevents this phenomenon by lowering the surface tension

Alveolar Stabilization

Alveolar Stabilization Surfactant prevent this happening

Alveolar Stability In the presence of surfactant, the surface tension developed in the alveoli is inversely proportional to the concentration of surfactant per unit area Amount of surfactant per alveolus is constant. In the smaller alveoli, surfactant molecules become concentrated at the surface while in a large alveolus surfactant molecules are scattered over the surface Thus in a smaller alveolus the tendency to develop more pressure due to smaller radius is neutralized by tendency to develop less pressure due to more concentration of surfactant per unit area (causing lower 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.

Alveolar Stability

Respiratory distress syndrome (RDS) Occurs in newborns (especially premature) due to inadequate formation of surfactant resulting in elevated alveolar surface tension, increased work of breathing and inadequate exchange of gases.

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 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

Types of Fluid Flow Laminar flow: In Laminar flow, the central axis of flow moves with the highest velocityand the successive cylindrical laminae moves progressively more slow as one moves from central axis to the wall Laminar flow has a parabolic front Turbulent flow: A flow in which the elements of the fluid move irregularly in axial, radial and circumferential directions. It occurs at different velocities which fluctuates randomly. The velocity difference across the container changes erratically

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

Viscous force for Newtonian Fluids Viscosity is the property of flowing fluid (liquid/gas) by virtue of which relative motion between layers in contact is opposed Relative motion causes internal friction between layers in contact. This internal friction is called viscous force For Newtonian fluids, the viscous force is proportional to the surface area (A) of the layer and the velocity gradient between layers (in the direction perpendicular to the layer)

Viscous force and viscosity of Newtonian Fluids η is the constant of proportionality and is the viscosity. Negative sign shows that F acts in a direction opposite to the one in which the layers move

Viscous Force of a Newtonian Fluid Relative to the wall, layer closest to it will experience maximum viscous force and moves with slowest velocity compared to other layers of fluid. Farther the layer of fluid is from wall, faster it moves and minimum viscous force it experiences This implies that F « 1/Δx In tubes of larger diameter, role of viscous force is not significant and only layers closer to the walls will experience it Role of viscous force in fluid flow becomes significant in tubes with smaller diameter Viscous force therefore depends on the radius of the tube

Viscous Force of a Newtonian Fluid

Shear rate of Newtonian Fluids When fluid moves along a tube, layers in a fluid slip over one another and move at different speeds thereby creating a velocity gradient in a direction perpendicular to the wall of the tube. This velocity gradient is termed the shear rate Shear rate is expressed as the change of velocity between two neighboring fluid layers divided by their distance measured perpendicularly

Reference books Chapter 3: Human Physiology: From Cells to Systems By Lauralee Sherwood Chapter 1 :An Introduction to Med. Biophysics  by Prakash Chapter 5: Textbook Of Medical Physiology by Khurana