1. The document discusses gas exchange in the lungs, explaining factors like respiratory quotient (RQ), alveolar and blood gas partial pressures, and diffusion of gases. 2. It describes how oxygen and carbon dioxide diffuse passively across the alveolar-capillary membrane, moving from areas of high to low partial pressure down gradients established by pulmonary ventilation and perfusion. 3. Membrane characteristics like thinness and large surface area, along with high perfusion and ventilation, enable efficient gas exchange in the lungs.

1. EXCHANGE OF
GASES
1
Dr Chandana Hewawasam
Department of physiology

2. OBJECTIVES
 Explain RQ (Respiratory Quotient)
 Discuss the factors that determine alveolar gas
pressures
 Describe the process of gas exchange between the
blood and alveoli
 Describe the responses with regard to ventilation
- perfusion inequality 2

3. RESPIRATORY QUOTIENT (RQ)
 RQ = VCO2 produced / VO2 consumed
 The ratio between the volume of carbon dioxide
produced to the volume of oxygen consumed by
the body per unit time (at the steady state).
 RQ varies according to the source that is used
for energy production
o Carbohydrate – 1
o Protein – 0.8
o Fat – 0.7
3

4. Respiratory Exchange Ratio (R)
 R = VCO2 exhaled / VO2 inhaled
 The ratio between the volume of CO2
expelled (across the alveolar membrane) at
the lungs to the volume of O2 intake during
one minute.
Under normal conditions R = RQ
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5. ALVEOLAR AND BLOOD
GAS PARTIAL
PRESSURES
5

6. PHYSIOLOGICAL
SHUNT 6

7. 7

8. EXCHANGE OF GASES IN THE
LUNGS
Takes place across the alveolar-
capillary or RESPIRATORY membrane
By diffusion
The movement of materials from a
higher to a lower concentration
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9. 9

10. Due to partial pressure gradients
of oxygen and carbon dioxide
between blood and alveoli;
i. Carbon dioxide diffuses from blood to
alveoli and is then expired out
ii. Inspired oxygen diffuses into blood
from alveoli 10

11. DIFFUSION OF GASES
 Oxygen and carbon dioxide move
between air and blood by simple
diffusion: from an area of high to low
partial pressure.
 It is a passive process which requires
no energy.
 Diffusion obeys the gas laws.
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12. LAWS OF GAS DIFFUSION
 Dalton's law: in a mixture of non-reacting
gases, the total pressure exerted is equal to
the sum of the partial pressures of the
individual gases.
e.g. PAlv = PH2O + PO2 + PCO2 + PN2
 Boyle's Law: P1V1 = P2V2 (at constant temp)
 volume of a gas is inversely proportional to its
pressure.
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13. 13
Graham's Law: rate of diffusion of a gas
is inversely proportional to the square
root of its molecular weight (MW).

14. FICK'S LAW
the rate of transfer of a gas through a sheet of
tissue is,
 proportional to the tissue area (A)
 proportional to the gas tension
difference(P1-P2)
 proportional to the diffusion coefficient (D)
 inversely proportional to the tissue
thickness(T) 14

15. 15

16. DETERMINANTS OF GAS DIFFUSION
1. Characteristics of the Gas
(solubility)
2. Pressure Gradient
3. Membrane Characteristics
(thickness, surface area)
16

17. CHARACTERISTICS OF THE GAS
(SOLUBILITY)
 Solubility  D X MW
 where, D = diffusion constant
 CO2 diffuses about 20 times more rapidly
through tissues since it has a much
higher solubility but not very different
molecular weight. 17

18. MEMBRANE CHARACTERISTICS
 The blood-gas barrier in the lung is
extremely thin (0.6m) and
 has a very large surface area (70 m2).
 Therefore, it is well suited to its
function.
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19.  Gas exchange also depends on
 properties of the circulation,
 haemoglobin level,
 and whether the gas is perfusion or
diffusion limited
 Substances that are diffusion-limited have
higher diffusing capacities
19

20. 20
Diffusing capacity (DL)
 Diffusing capacity is defined as “the volume
of a gas that diffuses thorough the
membrane per one minute when the partial
pressure difference (between alveoli and
blood) of that gas is 1 mm Hg”.
 Units- mL/min/mmHg
DL = V / (PA – PB)

21. o Because transfer of CO is entirely
diffusion limited it is an ideal gas to
use for diffusion capacity
measurements.
DL = VCO / (PA CO – PB CO)
 Since PCO in the capillary blood is
negligible
DL = VCO / (PACO)
21

22.  Diffusing capacity for O2= 25ml/min/mmHg
DO2 = VO2 / (PA O2 – PC O2)
DO2 = diffusing capacity
VO2 = oxygen consumption
PA O2 = alveolar PO2
PC O2 = mean pulmonary capillary PO2
v the diffusing capacity for CO2 is at least 20
times that of oxygen
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23. FACTORS THAT AFFECT DIFFUSING CAPACITY
23
•Ventilation
•Perfusion
•Haemoglobin level
•Surface area
•Thickness of the membrane

24. REDUCED DIFFUSING CAPACITY IS SEEN IN;
•Extra-pulmonary diseases
•Airways diseases
•Interstitial lung diseases
•Diseases of pulmonary vasculature
•Anaemia
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25. INCREASED DIFFUSING CAPACITY IS SEEN IN;
•Exercise
•Supine position
•Obesity
•Polycythaemia
•Left-to-right shunt
•Pulmonary haemorrhage
•Asthma 25

26. VENTILATION - PERFUSION
RATIO (VA / Q)
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27. VENTILATION - PERFUSION RATIO (VA / Q)
27

28. Partial pressures of alveolar gases
O2 100 mm Hg CO2 40 mm Hg
N2 573 mm Hg H2O 47 mm Hg
Partial pressures of alveolar gases is
determined by: * the rate of alveolar
ventilation and
* the rate of diffusion of gases between the
alveoli and the pulmonary capillary blood
All alveoli are not ventilated equally and
blood flow to all alveoli is not equal

29. Alveolar ventilation
Pulmonary ventilation =
Tidal volume x respiratory rate = 500ml x 12=
= 6L.min-1
Dead space = 150 ml
Therefore, per breath only 350 ml reaches the
alveoli for gas exchange
Alveolar ventilation (VA) = 350 x 12 = 4.2 L.min-1

30.  Perfusion of both lungs = cardiac output
is about 5.25 L/min-1 ( 70 x 75 = 5250)
 Therefore, ventilation/ perfusion ratio
(VA/Q) at rest for both lungs:
4.2/ 5.25 = 0.8
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31. REGIONAL DIFFERENCES IN PULMONARY
BLOOD FLOW
31

32. V-Q MISMATCH AND PHYSIOLOGICAL
ADJUSTMENTS
 During exercise
 Cardiac output (CO) increase
 pulmonary blood flow
 capillaries dilate
 More capillaries are recruited for gas
exchange(in apex of the lungs)
  O2 in the circulation.
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33. THE END
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