MedSci 205: Lecture 19 - Mechanics of Breathing Part II

Question 1

Poiseuille’s Law

Answer 1

Gives the relationship for laminar flow in a cylindrical tube.

States that rate of flow is due to the difference in pressure between 2 ends.

Flow is proportional to the pressure difference.

V = (change in P x pi x r to the 4) / 8 x n x l

Where
r = radius
n = viscosity
l = length of cylinder

Question 2

Dynamic Mechanical Properties

Answer 2

Poiseuille’s Law.

Airway Resistance.

Expiratory Flow Limitation.

Flow Volume Curves.

Question 3

Airway Resistance

Answer 3

Intermediate sized airways contribute most of the resistance.

Total cross sectional area increases towards the periphery BUT total airflow is constant.

Flow is therefore more laminar in the small airways.

Question 4

Note to self

Answer 4

Make hand written flash cards for expiratory flow limitation and flow volume curves

Question 5

The Work of Breathing

Answer 5

Work = Pressure x Volume

VE = fR x VT
rapid shallow breathing decreases elastic work but increases frictional (viscous) work.
slow, deep breathing decreases frictional work but increases elastic work.

NOTE: during increased levels of ventilation of ~50L/min, Total cost of breathing = Total VO2 - Resting VO2

Question 6

The Work of Inspiration

Answer 6

Consists of three different fractions:

Compliance (or elastic) work.

Tissue resistance work.

Airway resistance work.

Question 7

Compliance (Elastic) Work

Answer 7

Work required to expand the lungs against elastic forces.

Question 8

Tissue Resistance Work

Answer 8

Work required to overcome the viscosity of the lung and chest wall structures.

Question 9

Airway Resistance Work

Answer 9

Work required to move air through the airways into the lungs.

Question 10

Control of Breathing

Answer 10

Is a negative feedback system:
Central control affects effectors.
Effectors affect Sensors.
Sensors affect Central control.

Influences from higher centres have and influence directly as well as indirectly by influencing the cycle of inspiration and expiration which eventually has an effect on the muscles of breathing. Other influences on ins and exp cycle include Reflexes from: lungs, airways, CV system, muscles, joints and skin as well as reflexes from both arterialchemoreceptors and central chemoreceptors.