Gas Exchange Flashcards
What is atmospheric pressure
Pressur exerted by the weight of the air above the earth in the atmosphere
Higher up, less air -> less weight pushing down -> lower pressure
What is saturated vapour pressure
Pressure exerted by water in the air when the air is fully saturated with water
Pressure exerted by water displaces some of the pressure exerted by gases in air -> decreases their partial pressure
How do you calculate the amount of gas dissolved in a liquid
Amount dissolved = partial pressure x solubility coefficient of the gas
Describe how the oxygen content of blood is high despite it not being very soluble in water
Oxygen content is high due to oxygen binding to Hb
Oxygen binds to Hb until Hb becomes saturated and then oxygen dissolves in plasma until equilibrium is reached where pO2 of plasma = pO2 of alveolar air
This allows a greater oxygen content of blood as oxygen is stored in Hb and then released when the pO2 of plasma decreases as O2 is removed from the blood
Alveolar air’s pO2 is lower than inhaled while pCO2 is greater than inhaled. Why is this
O2 diffuses across the alveolar wall
Inhaled air mixes with residual volume - has higher pCO2
CO2 enters the alveoli from blood
What factors affect the rate of diffusion in the lungs
Partial pressure difference across the membrane
Surface area available for diffusion
Thickness of the surface
Solubility of the gas in the liquid
Molecular weight of the molecule
Which molecule diffuses faster, CO2 or O2 and why
CO2 is more soluble than O2 -> diffuses fast
Molecular weight of CO2 > O2 -> O2 diffuses faster
As solubility has a greater effect on diffusion speed, CO2 diffuses faster than O2 -> in diseased lungs, O2 gas exchange is impaired more than CO2 gas exchange due to slower diffusion rate
Name the diffusion barriers
Fluid film lining alveoli
Epithelial cells of alveolus
Interstitial space
Endothelial cell of capillary
Plasma
Red cell membrane
What factors affect rate of gas diffusion
Membrane thickness
- Increased thickness due to oedema fluid in interstitium and alveoli
- Lung fibrosis
Surface area of membrane
- Removal of entire lung
- Emphysema
Diffusion coefficient of the gas
What factors can help tissues remove more O2 from the blood
Low partial pressure of the tissue - tissue pO2 must still be high enough to drive diffusion of O2 to cell though
High capillary density - decreases diffusion distance -> can lower partial pressure further
The Bohr Shift - acidic pH drives dissociation curve right
Temperature - high temp drives dissociation curve right
2,3-BPG - high concentration drives dissociation curve right
How can hypoxia occur
If Hb levels are low
If pO2 levels are low
In conditions where there is reduced blood flow - shock
When tissues use O2 faster than it can be delivered:
- Peripheral arterial disease
- Raynaud’s syndrome
How does Pulse Oximetry work
Works by detecting the amount of red light absorbed by the blood
Oxgenated Hb absorbs less red light than deoxygenated Hb -> can see how much red light is absorbed and then determined Hb saturation
