Lecture 18: O2 and CO2 EXCHANGE Flashcards
What is the formula for respiratory minute volume?
Tidal volume x respiratory rate
What are the units of respiratory minute volume?
L/min
What are the units of tidal volume?
L/breath
What are the units of respiratory rate?
Breaths/min
What is tidal volume also known as?
Breathing strength
What is respiratory rate also known as?
Breathing speed
Can all the respiratory minute volume be used?
No, some of the air never gets to the alveoli and so cannot gas exchange
What is the air which can’t gas exchange called?
Dead space (Vd)
How much is dead space?
About 150mL
What is dead space interpreted as?
About 150mL of each breath just fills upper airways (conducting airways) and is of no use for gas exchange
What is really important?
The amount of air that gets into the alveoli (alveolar ventilation)
What is the formula for alveolar ventilation?
(tidal volume - dead space) x respiratory rate
What are the units of dead space?
L/breath
What is dead space also known as?
Air that can’t be used. It is constant
What does each gas do in a gas mixture (air?
Exert its own own individual pressure, called a partial pressure
What is the pressure of a mixture of gases equal to?
The sum total of the pressures of each individual gas
What is inhaled air?
Dry and as it becomes filled with water vapour and moistened, the partial pressure of nitrogen and oxygen decrease
What is the goal?
To move gas back and forth between the alveoli and the capillaries
Where is inhaled oxygen transported?
Into the capillaries
Where is carbon dioxide transported?
Into the alveoli to be exhaled
Where do gases move?
Across the membranes between the alveoli and the capillaries by diffusion
What determines rate of diffusion?
Surface area of the membranes, thickness of the membranes and pressure difference between the two sides
What is the formula for flux (amount flowing)?
F=(A/T)*D(P1-P2)
What is A?
Surface area
What is T?
Thickness
What is diffusion constant?
D
What os P1-P2?
Pressure difference
What does the diffusion constant depend on?
Gas solubility (S) and its molecular weigth
How do carbon dioxide and oxygen differ in diffusion speed?
On a per molecule basis, carbon dioxide diffuses about 20 times faster than oxygen due to higher solubility of carbon dioxide
Does diffusion constant affect flux?
Not usually
What is the surface area?
Bulbous structure of alveoli and the high density of capillaries creates lots of surface area for gas exchange
What is emphysema?
A disease characterised by dilation of the alveolar spaces and destruction of the alveolar walls
What does reduction in surface area mean?
Less contact between the air and capillaries so oxygen exchange is greatly reduced
What is the relationship between flux and thickness?
Flux is inversely proportional to thickness
What is the blood air barrier mostly comprised of?
The alveolar and capillary walls
What is the distance between the alveolar air and the blood?
Very small
What happens in pulmonary fibrosis?
Thicker walls so diffusion decreases
What does gas transfer by diffusion depend on?
Alveolar area for diffusion and thickness of alveolar membrane
What is the main factor of gas transfer by diffusion?
Pressure difference is driven by diffusion (alveolar - arterial)
What does alveolar partial pressure of oxygen depend on?
Partial pressure of oxygen in inspired air, alveolar ventilation and oxygen consumption - blood oxygen
What is most important in alveolar partial pressure of oxygen?
Balance between oxygen consumption and alveolar ventilation because the atmospheric partial pressure of oxygen is usually constant
What does low activity do to blood oxygen?
Reduced oxygen consumption by cells so higher levels of oxygen in the venous blood
What does high activity do to blood oxygen?
Increased oxygen consumption by cells so lower levels of oxygen in the venous blood
What does alveolar partial pressure of carbon dioxide depend on?
Partial pressure of carbon dioxide in inspired air, alveolar ventilation and carbon dioxide production
What is alveolar partial pressure of carbon dioxide usually determined by?
The balance between carbon dioxide production and alveolar ventilation because atmospheric partial pressure of carbon dioxide is negligible
What is the partial pressure of oxygen in the capillaries and alveolus?
It is high in the alveolus and low in the capillary
Where does oxygen diffuse?
Down the pressure gradient from alveolus to capillaries until a balance of the partial pressure of oxygen is reached
What is the partial pressure of carbon dioxide in the capillaries and alveolus?
High in the capillaries and low in the alveolus
Where does carbon dioxide diffuse?
Down the pressure gradient from capillaries to alveolus until a balance of partial pressure of carbon dioxide is reached
What are the gradients in external respiration?
Gradient for oxygen to move into capillaries and carbon dioxide to diffuse into the alveolus
What are the gradients in internal respiration?
Gradient for oxygen to move into the cells and carbon dioxide to diffuse into the capillaries
