The mechanics of breathing

Question 1

Equation for airflow

Answer 1

Airflow (V) = (sum)Pressure (P) / Resistance (R)

Question 2

Describe resistance and how it affects airflow

Answer 2

Air generates resistance as it comes into contact with airway surface. The mores resistance present, the lower the rate of flow.

Question 3

What are the factors that affect the level of resistance in airways

Answer 3

Cross sectional area of the airway lumen, airflow pattern

Question 4

Describe the Hagen-Poiseuille equation

Answer 4

Resistance is inversely proportional to cross-sectional radius^4 (of an airway)

A small decrease in radius produces a large increase in resistance (decrease in airflow)

Question 5

Name some examples of respiratory diseases that impact the cross-sectional area of the lumen

Answer 5

Excessive mucus secretion, oedema, damage to airway structure

Question 6

Describe how pattern of airflow can affect airflow

Answer 6

Airflow linear -> turbulent, increased airway resistance. Decrease in airflow

Question 7

What is a ‘loss of patency’

Answer 7

Patency refers to the state of being open/unobstructed. Loss of patency refers to a closing or obstruction of the airway

Question 8

What two things maintain the open structure of the airways

Answer 8

Elastic fibres within the wall of the airway and radial traction

Question 9

What is transpulmonary pressure

Answer 9

Difference between the pressure within the alveoli and intrapleural space

This determines the level of force acting to expand or compress the lungs

Question 10

What is lung compliance

Answer 10

Relationship between change in lung volume produced by a particular change in transpulmonary pressure, describes how easily the lungs can be distended

Question 11

Equation for lung compliance

Answer 11

Compliance (CL) = (sum)Volume / (sum)Pressure

Question 12

How do diseases such as fibrosis and emphysema affect static lung compliance

Answer 12

Emphysema degreades elastin fibres, lung is more complaint (reduces recoil) -> Increase in compliance

Fibrosis - Scarring and deposition of structural fibres such as collagen, making the lung stuff and less compliant

Question 13

Describe the internal surfaces of alveoli

Answer 13

Lined with fluid, water-air interface between lining fluid and pseudo-spherical alveolar airspace creates a bubble. Surface tension arises due to the relative strength of hydrogen bonds between water molecules, exerts collapsing force towards centre of the bubble

Question 14

Describe Law of Laplace

Answer 14

P = 2T / r

therefore if T remains constant, P is inversely proportional to bubble radius. The bubble generates pressure, (smaller generates more)

Question 15

What is pulmonary surfactant

Answer 15

Phospholipoprotein secreted by type II pneumocytes. Amphipathic molecules, with hydrophilic head and tail regions.

The molecules act to disrupt attractive forces between water molecules, reducing surface tension and the collapsing pressure generated

Question 16

How does pulmonary surfactant equalise pressure between alveoli of varying size

Answer 16

As alveoli size increases during inflation, conc of surfactant molecules at the interface decreases. Wherever pulmonary surfactant is present, surface tension increases with increasing alveolar surface area. Air will naturally flow from larger alveoli to smaller ones, helping to distribute air across the lung during inspiration

Question 17

Surface tension produced at the air-liquid interface reduces hydrostatic pressure in the alveolar tisue. What effect does this have and how is pulmonary surfactant important?

Answer 17

Pulls fluid out of the surrounding pulmonary capillaries into the alveoli and interstitial tissue. Pulmonary surfactant reduces surface tension, helping to prevent alveolar oedema due to excessive fluid being plled from capillaries

Question 18

What is NRDS

Answer 18

Neonatal respiratory distress syndrome - Condition in infants born prematurely, who develop and produce insufficient levels of pulmonary surfactant.

Question 19

What are the adverse effectsw of NRDS

Answer 19

Respiratory failure due to alveoli collapsing
Decreased lung compliance
Increased likelihood of alveolar oedema (reduced rate of gas exchange)
Increased force in the lungs can damage alveoli and capillaries

Question 20

How is NRDS treated

Answer 20

Supplementation of affected infants with artificial surfactant, and/or by administering glucocorticoids to mothers deemed high risk (poor diabetic control)

Question 21

What do glucocorticoids do

Answer 21

Increase surfactant production via maturation of type 2 pneumocytes