1. Lecture – 5 Dr. Zahoor Ali Shaikh
GAS EXCHANGE
Lecture – 5
Dr. Zahoor Ali Shaikh

2. Gas Exchange in Alveoli
Gas Exchange takes place in alveoli and then at tissue level.
Why we are breathing?
To provide a continuous supply of oxygen to tissues.
Gas Exchange in Alveoli
It is diffusion of O2 and CO2 through a permeable membrane of alveoli.
It is down the partial pressure gradients of O2 and CO2.

3. Partial Pressures
First we will see Composition of Atmospheric Air ( Inspired Air )
- Nitrogen (N2) – 79%
- Oxygen (O2) – 21%
- Carbon dioxide (CO2) – 0.04%
[0% for practical purpose]
- Water Vapor
- Other gases and pollutants
These gases exert an atmospheric pressure of 760mmHg at sea level.

4. Partial Pressures
The pressure exerted by a particular gas is directly proportional to the percentage of that gas in a total air mixture.
Example: N2 is 79%,
- Partial Pressure of N2 will be 79% of 760
= (79 * 760)/100 = 600mmHg
- O2 is 21%
= (21*760)/100 = 160mmHg

6. Partial Pressure What is Partial Pressure [Pgas] ?
It is the pressure exerted by each gas in a mixture of gases, is known as ‘Partial Pressure’ of gas.
NOTE – Gases dissolved in a liquid such as blood also exert a partial pressure. Greater the partial pressure of the gas in the liquid, more of that gas is dissolved.

7. Partial Pressure Gradient
A difference in partial pressure between capillary wall and surrounding structure is known as ‘Partial Pressure Gradient’.
Partial Pressure Gradient exist between alveolar air and pulmonary capillary blood.
NOTE – A gas always diffuses down its partial pressure gradient i.e. from area of high partial pressure to area of low partial pressure.

8. Partial Pressure
We will see the Alveolar Air Composition and Partial Pressure of O2 and CO2 in the alveolar air.
Alveolar Air Partial Pressure
O2 – 14% Partial pressure 100mmHg
CO2 – 6% Partial pressure 40mmHg
N2 – 80% Partial pressure 573mmHg
H2O – 47mmHg
( Total Pressure = 760mmHg )

9. PO2 and PCO2 Gradients Across Pulmonary Capillaries
As blood passes through the lungs, it picks up O2 and gives CO2 by diffusion down the partial pressure gradient between blood and alveoli.
Pulmonary Capillary has Venous blood [Deoxygenated].
Pulmonary Capillary blood
PO2 – 40mmHg
PCO mmHg

10. PO2 and PCO2 Gradients Across Pulmonary Capillaries
As blood flows through Pulmonary capillaries, it is exposed to alveolar air.
As alveolar air has PO2 at 100mmHg, which is higher than PO2 of 40mmHg in the blood of Pulmonary capillaries [Venous blood].
Therefore O2 diffuses from alveoli to capillary blood till no further gradient exist i.e. PO2 in blood capillary becomes 100mmHg [oxygenated blood].

12. PO2 and PCO2 Gradients Across Pulmonary Capillaries
Partial Pressure of CO2 in Pulmonary capillary [Venous blood] is 46mmHg, whereas alveolar PCO2 is only 40mmHg, therefore, CO2 diffuses from the blood into the alveoli.
Blood leaving the Pulmonary capillary has PCO2 of 40mmHg.

13. PO2 and PCO2 Gradients Across Pulmonary Capillaries
SUMMARY
Blood leaving the lungs [oxygenated]
PO mmHg
PCO mmHg
Blood coming to the lungs from tissues [Venous Blood]
PO mmHg
PCO mmHg

14. Factors Affecting the GAS TRANSFER across the Alveolar Membrane
Diffusion constant means ‘solubility of gas’.

15. Gas Diffusion Applied Aspect
In Emphysema – there is loss of alveolar walls, resulting in large but less alveoli , therefore
lung surface area decreases, therefore, decreased gas exchange.
Pulmonary fibrosis --Increased thickness of alveolar membrane due to pulmonary fibrosis leads to decrease exchange of gases.
NOTE – Diffusion of CO2 is 20 times more than O2 in alveolar membrane as CO2 is more soluble, therefore, in diseased lung O2 transfer is more seriously affected than CO2 transfer.

17. Gas Exchange across Systemic Capillaries
Gas exchange across systemic capillaries also occurs down partial pressure gradient.
In Systemic capillaries O2 and CO2 move to tissues by diffusion down their partial pressure gradients.

18. Gas Exchange across Systemic Capillaries
Arterial Capillary PO2 – 100mmHg
Arterial Capillary PCO2 – 40mmHg
Oxygen is given to tissues and CO2 diffuses from tissues to blood.
Tissue
- PO2 – 40
- PCO2 – 46
Venous Capillary Blood
This blood [Venous] goes to right side of the heart and then to lungs to get oxygen and give out CO2 and cycle repeats.

20. Composition OF INSPIRED, Alveolar, EXPIRED AIR

21. What do you know from this lecture
Composition of Inspired Air, Alveolar Air, and Expired Air
Partial Pressure of O2 and CO2 in Alveolar Air
Partial Pressure of O2 and CO2 in Oxygenated [Arterial] and in Deoxygenated [Venous] blood
Partial Pressure of O2 and CO2 in tissues
Factors Affecting the Gas transfer (Diffusion) across the Alveolar Membrane
Gas Exchange across the Systemic Capillaries
Solubility of O2 and CO2