The mechanics of breathing Flashcards
Equation for airflow
Airflow (V) = (sum)Pressure (P) / Resistance (R)
Describe resistance and how it affects airflow
Air generates resistance as it comes into contact with airway surface. The mores resistance present, the lower the rate of flow.
What are the factors that affect the level of resistance in airways
Cross sectional area of the airway lumen, airflow pattern
Describe the Hagen-Poiseuille equation
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)
Name some examples of respiratory diseases that impact the cross-sectional area of the lumen
Excessive mucus secretion, oedema, damage to airway structure
Describe how pattern of airflow can affect airflow
Airflow linear -> turbulent, increased airway resistance. Decrease in airflow
What is a ‘loss of patency’
Patency refers to the state of being open/unobstructed. Loss of patency refers to a closing or obstruction of the airway
What two things maintain the open structure of the airways
Elastic fibres within the wall of the airway and radial traction
What is transpulmonary pressure
Difference between the pressure within the alveoli and intrapleural space
This determines the level of force acting to expand or compress the lungs
What is lung compliance
Relationship between change in lung volume produced by a particular change in transpulmonary pressure, describes how easily the lungs can be distended
Equation for lung compliance
Compliance (CL) = (sum)Volume / (sum)Pressure
How do diseases such as fibrosis and emphysema affect static lung compliance
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
Describe the internal surfaces of alveoli
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
Describe Law of Laplace
P = 2T / r
therefore if T remains constant, P is inversely proportional to bubble radius. The bubble generates pressure, (smaller generates more)
What is pulmonary surfactant
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
How does pulmonary surfactant equalise pressure between alveoli of varying size
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
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?
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
What is NRDS
Neonatal respiratory distress syndrome - Condition in infants born prematurely, who develop and produce insufficient levels of pulmonary surfactant.
What are the adverse effectsw of NRDS
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
How is NRDS treated
Supplementation of affected infants with artificial surfactant, and/or by administering glucocorticoids to mothers deemed high risk (poor diabetic control)
What do glucocorticoids do
Increase surfactant production via maturation of type 2 pneumocytes
