gas exchange across alvoli

1. GAS EXCHANGE
Hari Rench
Moderator Dr. Himanshi
Department of Anaesthesiology
PIMS

2. CONTENTS
 Introduction
 Route of Air
 Alveoli
 Composition of Alveolar Air
 Mechanism of Exchange
 Steps in Gas Exchange
 Laws of Respiration
 Principles of Exchange
 O2 Cascade
 O2 Flux
 O2 Dissociation Curve
 ODC and Anaesthesia
 Effects of Anaesthesia on Exchange
 Clinical Aspects
 Points to Remember

3. INTRO
Gas Exchange is essential
because Energy Metabolism
requires O2 and produce CO2.
Food +O2--Tissue--ATP+CO2

4. ROUTE OF AIR

5. ALVEOLI

6. ALVEOLI

7. ALVEOLI

8. ALVEOLI
 Alveoli in the alvoelar ducts and air sacs are the basic
units of Gas Exchange.
 There are 300 million Alveoli. Sizes varies with the
degree of lung infiltration and position in the lung
 Largest in upper lungs
 Smallest in lower lungs
 Alveoli Epithelium is composed of
 Mainly Type I Squamous Epithelial cells
 And Type II Alveolar cells that produce
Surfactant(lipoprotein)

9. COMPOSITON OF ALVEOLAR AIR

10. COMPOSITION OF ALVEOLAR AIR
 Inspired air mixes with the alveolar gas
 Replacing the O2(Blood) and dilute the
CO2(Alveoli).
 A part of this mixture is Expired.
 O2 content in alveolar gas falls and its CO2 content
rises until the next Inspiration.
 Minimal increment of inspired and expired air has
relatively little effect on PO2 and PCO2.
 So the Composition of alveolar gas remains
constant remarkably.

11. MECHANISM OF GAS EXCHANGE
Gas exchange is the interchange of
O2 and CO2
between an organism and its
environment.
It is also called RESPIRATION

12. GAS EXCHANGE : A PASSIVE PROCESS
Gases Diffuse according to their Partial
Pressures.
External Respiration : Exchange b/w Air and
Blood
Internal Expiration : Exchange with Tissue
Fluids
O2 Transport : Bound to Hb in RBC or
dissolved in blood plasma
CO2 Transport : Dissolved in blood
plasma,bound to Hb or in the form of plasma
HCO3

13. FICK’S LAW
The rate of transfer of gas through a tissue
is proportional to its area and to the
partial pressure gradient of the gas and is
inversely proportional to the tissue
thickness.
The diffusion constant depends on the
 solubility of the gas and is inversely
proportional to the square root of its
molecular weight.
CO2 diffuses 20 times faster than O2 due
to its higher solubility.

15. Henry’s Law
The amount of 02 that dissolves
into the bloodstream is directly
proportional to the partial
pressure of O2 in Alveolar air.

16. PRINCIPLES OF GAS EXCHANGE
Ventilation
Diffusion
Perfusion

17. Ventilation means movement of air b/w
the environment and the lungs via
inhalation and exhalation.
Diffusion is the spontaneous movement
of gases without the use of any energy
or effort by the body.
Perfusion is the passage of fluid
through the circulatory system or
lymphatic system to an organ or a
tissue,usually delivery of blood to a
capillary bed in tissue.

18. DIFFUSION

20. OXYGEN CASCADE
The process of declining O2
tension from Atmosphere to
Mitochondria.

21. O2 CASCADE

22. STEPS IN O2 CASCADE
Uptake in the lungs
Carrying capacity of blood
Delivery from lungs to tissue capillaries
Delivery from capillary blood to
interstitium
Delivery from interstitium to individual
cells
Cellular use of O2

23. O2 FLUX

24. O2 DISSOCIATION CURVE

25. ODC AND ANAESTHESIA
ODC helps us to relate PO2 and Hb
saturation
A Left Shift gives a warning that tissue O2
delivery may be compromised even when
there is a minimal drop in PO2.
All Inhalational Agents including N2O
causes Shift to Right.
Intravenous Agents doesn’t have much
significant effect on ODC.

26. EFFECT OF ANAESTHESIA ON GAS
EXCHANGE
Increased Dead space(controlled
ventilation)
Hypoventilation
Increased intrapulmonary
shunting

27. DEAD SPACE
Volume of a breath that does not
participate in Gas Exchange.
It is Ventilation without
Perfusion.

28. INTRAPULMONARY SHUNTING
 A pulmonary shunt refers to passage of
deoxygenated blood from the right side of the
heart to the left without participation in gas
exchange in the pulmonary capillaries.
 ie , ventilation/perfusion ratio(the ratio of air
reaching the alveoli to blood perfusing them)is
zero.
 Main cause of Hypoxemia in pulmonary oedema
and also conditions like Pneumonia in which the
lungs become consolidated.

29. CLINICAL ASPECTS
Smoking
Emphysema
Pneumonia
Atelectasis
Asthma
Tuberculosis
Lung Cancer

30. SMOKING IS ONE OF THE DEADLIEST
ASSAULTS

31. Mucus and Cilia in the respiratory
passages protect the lungs.
Pollutants including Tobacco Smoke
can destroy these.
Over 50,00,000 deaths a year in the
world.

34. PNEUMONIA

35. ATELECTASIS

36. ASTHMA

37. TUBERCULOSIS

38. LUNG CANCER

39. POINTS TO REMEMBER
 What’s Gas Exchange and where it happens
 Principles of Exchange (Ventilation,Diffusion,Perfusion)
 O2 Cascade(Diagram is imp)
 O2 Flux=CO x SaO2 x Hb x 1.31(Hufner’s no.)
 Laws (Flick’s Law and Henry’s Law)
 ODC and the causes(Left and Right Shift)
 ODC and Anaesthesia
 Effects of Anaesthesia on Exchange
 Clinical Aspects

40. REFERENCE
Wylie and Churchill-Davidson’s A practice
of Anaesthesia 7th Edition
Miller’s Anaesthesia 9th Edition
Guyton and Hall Textbook Of Medical
Physiology 13th Edition
Ganong’s Review Of Medical Physiology
23rd Edition
Morgan & Mikhail’s Clinical
Anaesthesiology 6th Edition(international)

41. T