11. Biophysics of circulation and respiration.

Fluids

a fluid is a substance that continually deforms (flows) under an applied shear stress, no matter how small the stress is

Hydrostatic pressure

The pressure exerted by a fluid at a given point within the fluid, due to the force of gravity

Pascal’s principle

a change in pressure applied to an enclosed fluid is transmitted undiminished to every point of the fluid and to the walls of the container

Viscosity

a measure of the resistance of a fluid which is being deformed by either shear stress or tensile stress

kinematic viscosity

the ratio of the dynamic viscosity and the density of the fluid

fluidity

the reciprocal of the dynamic viscosity

velocity

the velocity (speed) of a given particle

volumetric flow rate

the volume of fluid which passes through a given surface per unit time

relationship between the volumetric flow rate and the flow velocity

flow velocity and the cross-section of the tube are inversely proportional

Bernoulli’s law

states that the sum of the pressure, kinetic energy per unit volume, and the potential energy per unit volume has the same value at all points along a streamline in ideal liquids

Bernoulli’s law II

consequence for blood flow: more cells travel with the axis of the vessel

(static pressure is the lowest there, the pressure difference drives them from the walls towards the ‘mainstream’)

Function of respiratory organs

Gas exchange
Filtering out air pollution
Thermoregulation
• Setting air temperature
• Air humidification
Immunological defense

Work done on lungs must overcome the following

Elastic forces
Viscous forces
Physical and biophysical factors influencing respiratory work

Elastic forces

lung recoil (collagen/elastin, surface tension)

Static: before breathing
Dynamic: during breathing

Viscous forces

80% due to AWR (especially at medium-sized pulmonary airways)
20% due to tissue resistance (friction)

Physical and biophysical factors influencing respiratory work

Respiratory resistance of the lung and its relation to the dilatation of the lung, interpretation of compliance.
The role of surfactant based on the interpretation of surface tension.

Compliance

Change in lung volume for applied unit pressure

Hysteresis

different pathways of compliance changes between inspiration and expiration. Effect of surfactant

Surfactant

Compounds with both hydrophilic and hydrophobic regions to decrease surface tension in the lungs