1. Faisal I. Mohammed, MD, PhD
Respiratory system L3
Faisal I. Mohammed, MD, PhD
University of Jordan

2. Terms Used to Describe Lung Volumes and Capacities

3. Exchange of Oxygen and Carbon Dioxide
Dalton’s Law
Each gas in a mixture of gases exerts its own pressure as if no other gases were present
Pressure of a specific gas is partial pressure Px
Total pressure is the sum of all the partial pressures
Atmospheric pressure (760 mmHg) = PN2 + PO2 + PH2O + PCO2 + Pother gases
Each gas diffuses across a permeable membrane from the are where its partial pressure is greater to the area where its partial pressure is less
The greater the difference, the faster the rate of diffusion
University of Jordan

4. Partial pressure of the gas
- is determined by its concentration in the mixture and by the overall pressure of the gas mixture
PatmO2 = Patm * FO2
PatmO2 = 760 mm Hg * 0.21
- in the liquid - partial pressure of the gas component, which is balanced with the liquid

5. Partial Pressures of Gases in Inhaled Air
PN2
=0.786
x 760mm Hg
= mmHg
PO2
=0.209
= mmHg
PH2O
=0.004
= 3.0 mmHg
PCO2
=0.0004
= 0.3 mmHg
Pother gases
=0.0006
= 0.5 mmHg
TOTAL
= mmHg
University of Jordan

6. Henry’s law
Quantity of a gas that will dissolve in a liquid is proportional to the partial pressures of the gas and its solubility
Higher partial pressure of a gas over a liquid and higher solubility, more of the gas will stay in solution
Much more CO2 is dissolved in blood than O2 because CO2 is 24 times more soluble
Even though the air we breathe is mostly N2, very little dissolves in blood due to low solubility
Decompression sickness (bends)
University of Jordan

8. External Respiration in Lungs
Oxygen
Oxygen diffuses from alveolar air (PO2 105 mmHg) into blood of pulmonary capillaries (PO2 40 mmHg)
Diffusion continues until PO2 of pulmonary capillary blood matches PO2 of alveolar air
Small amount of mixing with blood from conducting portion of respiratory system drops PO2 of blood in pulmonary veins to 100 mmHg
Carbon dioxide
Carbon dioxide diffuses from deoxygenated blood in pulmonary capillaries (PCO2 45 mmHg) into alveolar air (PCO2 40 mmHg)
Continues until of PCO2 blood reaches 40 mmHg
University of Jordan

9. Ventilation Mechanical process that moves air in and out of the lungs.
[O2] of air is higher in the lungs than in the blood, O2 diffuses from air to the blood.
C02 moves from the blood to the air by diffusing down its concentration gradient.
Gas exchange occurs entirely by diffusion:
Diffusion is rapid because of the large surface area and the small diffusion distance.
Insert 16.1

10. Surface Tension (continued)
Law of Laplace:
Pressure in alveoli is directly proportional to surface tension; and inversely proportional to radius of alveoli.
Pressure in smaller alveolus would be greater than in larger alveolus, if surface tension were the same in both.
Insert fig

11. Surfactant Phospholipid produced by alveolar type II cells.
Lowers surface tension.
Reduces attractive forces of hydrogen bonding by becoming interspersed between H20 molecules.
Surface tension in alveoli is reduced.
As alveoli radius decreases, surfactant’s ability to lower surface tension increases.
Disorders:
RDS.
ARDS.
Insert fig

12. Internal Respiration Internal respiration – in tissues throughout body
Oxygen
Oxygen diffuses from systemic capillary blood (PO2 100 mmHg) into tissue cells (PO2 40 mmHg) – cells constantly use oxygen to make ATP
Blood O2 drops to 40 mmHg by the time blood exits the systemic capillaries
Carbon dioxide
Carbon dioxide diffuses from tissue cells (PCO2 45 mmHg) into systemic capillaries (PCO2 40 mmHg) – cells constantly make carbon dioxide
PCO2 blood reaches 45 mmHg
At rest, only about 25% of the available oxygen is used
Deoxygenated blood would retain 75% of its oxygen capacity
University of Jordan

14. Rate of Pulmonary and Systemic Gas Exchange
Depends on (D  p*S *A /  M.Wt )
Partial pressures of gases
Alveolar PO2 must be higher than blood PO2 for diffusion to occur – problem with increasing altitude
Surface area available for gas exchange
Diffusion distance
Molecular weight and solubility of gases
O2 has a lower molecular weight and should diffuse faster than CO2 except for its low solubility - when diffusion is slow, hypoxia occurs before hypercapnia
University of Jordan

15. Internal Respiration Factors affecting the internal respiration:
Partial pressure of gases (diffusion)
Distance between the cells and the capillary
Rate of metabolic rate: cellular oxidation that leads to formation of CO2
Speed of the blood flow in capillary

16. Oxygen Diffusion from the Alveoli to the Pulmonary circulation
O2 diffuses into the pulmonary capillaries because the Po2 in the alveoli is high.
Po2 in the pulmonary capillaries increased very fast (1/3 distance).

17. Transport in arterial blood & Pulmonary shunt flow
Due to the bronchial circulation the arterial Po2 fall to 95 mm Hg

18. Po2 in systemic circulation (Diffusion from peripheral capillaries)
Oxygen is always being used by the cells. Therefore, the intracellular Po2 in the peripheral tissue cells remains lower than the Po2 in the peripheral capillaries.

19. Diffusion of CO2 from the Tissue Cells into the Capillaries

20. Diffusion of CO2 from the Pulmonary Capillaries into the Alveoli

21. Thank You
University of Jordan