Aveolar gas and diffusion Lecture 4

Question 1

How is gas transported in the respiratory system?

Answer 1

Convection- large airways and blood vessels

Diffusion- from alveoli to pulmonary capillaries and from capillaries to tissues

Question 2

What is ficks law of diffusion?

Answer 2

Describes the relationship between flow of gas and the different things which infleunce the flow of that gas

Rate of diffusion= SA x 1/thickness x diffusibility x partial pressure gradient

V=A x 1/T x d x (P1-P2)

Question 3

Describe the diffusibility and partial pressures of oxygen and carbon dioxide

Answer 3

Dependant on molecular weight and solubulity.

CO2 is roughly 20x more diffusible than O2

Greater partial pressure gradient for oxygen

In alveoli, oxygen moves faster

CO2 is more souble in solution than O2

8kPa for oxygen 1kPa for carbon dioxide

Surface area and thickness of diffsuion barrier and diffusiblity of O2/CO2 are constant in health, the partial pressure gradient for O2 and CO2 is crucial to drive diffusion

Question 4

Lists the partial pressures of oxygen and carbon dioxide around the respiratory system

Answer 4

In air-

PO2- 21kPa PCO2- 0kPa

Mouth-

PO2- 20kPa PCO2- 0kPa

Partial pressure of O2 decreases due to humidifying it so less room for O2 as some H2O present

In conducting zone-

PO2- 20kPa PCO2- 0kPa (inspired)

No gas exchange so remains the same

In alveolar space. Alveolar gas is constant-

PO2- 13kPa PCO2- 5kPa

Less PAO2 and more PACO2

In mixed venous blood-

PO2- 5kPa PCO2- 6kPa

Enough partial pressure to drive oxygen into the blood from the alveolar space

Blood leaving pulmonary capillaries-

PO2- 13kPa PCO2- 5kPa

Enough partial pressure for CO2 to move into the lung, as CO2 is more diffusible, it is enough to move CO2

Expired air-

PO2- 13kPa PCO2- 5kPa

First bit is expired due to being in the airway and not the lngs, the pressure from the lungs is expired after.

Question 5

Roughly, what volume of air are we adding to our lungs when we inspire?

Answer 5

500ml inspiration

350ml added to about 2.5l already in the alveoli space

Question 6

What factors determine alveolar gas composition?

Answer 6

Metabolism - of glucose. Diet infleunces CO2 generated

Inspiration - noramlly in air, inspired gas is contant (PO2 = 21kPa PCO2 = 0kPa)

Alveolar ventilation- volume that takes part in gas exchange. VA = (VT - VD) x f (tidal voume - dead space) x respiratroy frequeceny

Question 7

Describe the changes in alveolar gas partial pressures

Answer 7

P(A)O2= k x 1/rate of oxygen diffusion

If alveolar ventilation is matched to rate of oxygen diffusion alveolar partial pressure of oxygen will remain constant

P(A)CO2= k x 1/Alveolar ventilation

If alveolar ventilation is matched to the rate of carbon dioxide diffusion the alveolar partial pressure of carbon dioxide will remain constant

The relationship between the alveolar partial pressures of O2 and CO2 are the inverse of each other

partial pressure gradient moves O2 into pulmonary capillary and CO2 out of pulmoonry capillary