Gas transport in respiratory physiology by Dr. Isma S and moderator Dr Wan Fadz

1. Gas Transport
Dr Isma Syarina Ismail
Moderator : Dr Wan Fadzlina

2. OUTLINE
• Oxygen cascade
• Gas transport
• Oxygen
• Carbon Dioxide
• Oxygen flux

3. Ventilation of the lungs supplies O₂ to the alveolus
Diffusion of O₂ across the alveolus to the pulmonary
capillaries
O₂ carriage by blood ( Hb or dissolved in plasma)
Diffusion from capillary to miochondria

4. OXYGEN CASCADE
• Describe the sequential reduction in Po₂ from atmosphere to cellular
mitochondria (site of consumption)

5. 100mmHg

6. Alveolar gas equation
PAO2 = PIO2 – PACO2 / R
= 150 – (40/0.8)
= 100mmHg
• PACO2 = Alveolar partial pressure of oxygen
• PIO2 = Inspired partial pressure of oxygen
• PACO2 = Alveolar partial pressure of carbon dioxide
• RQ = Respiratory quotient = CO2 produced/ O2consumed = 0.8
• Respiratory exchange ratio

8. Arterial Blood PO₂
• Source of contribution
• Blood from bronchial and Thebesian vein drain directly into the
pulmonary vein, avoiding pulmonary capillaries
• V/Q mismatch – blood not fully oxygenated as it passes through poorly
ventilated areas of the lung eg, pulm pathology

9. • 3 factors my cause the Po₂ in the pulmonary vein < PAO2
1. V/Q mismatch
2. Shunt
3. Diffusion impairment
 Increase Alveolar-arterial (A-a) gradient

11. GAS TRANSPORT
OXYGEN TRANSPORT
Dissolved O₂ (2%) + Chemical O₂ (98%)

12. DISSOLVED O₂
• Obeys Henry’s law : the amount dissolved gas is proportional to the
partial pressure, Pa
• At T of 37°C, plasma contains 0.003ml O₂/mmHgPo2

13. O₂ CARRIAGE BY Hb
• Characteristics:
• Reversible
• Positive cooperative

15. Haemoglobin

16. • Hemoglobin is a complex molecule with a molecular weight of 64,500
• The protein globin has a tetrameric structure contains four
polypeptide chains
• Each of it is attached to a heme (iron porphyrin)
• Center ferrous ion
• Each hb molecule can bind with four oxygen molecules
• The chain are of two types, alpha and beta
• Hemoglobin A (normal adult Hb): 2α 2β

17. • Differences in their amino acid sequences give rise to various type of
human hemoglobin
• Hemoglobin F (fetal): 2α 2γ
• Higher affinity to oxygen

18. • Hemoglobin S (sickle): has valine instead of glutamic acid in the beta
chains
• Deoxygenated form is poorly soluble and crystallizes within the red
cell
• Cell shape changes from biconcave to crescent or sickle shaped
with increased fragility and a tendency to thrombus formation

19. • Methemoglobin
• Ferrous ion (Fe2+) oxidised to ferric (Fe3+) form by various drugs including
nitrites, sulfonamides and acetanilid or congenital cause in which the enzyme
methemoglobin reductase is deficient within the red blood cell.

20. Oxygen Dissociation Curve (ODC)
Sigmoid shape:
1.Allosteric modulation
2.Cooperative binding

21. P50
• The partial pressure of oxygen at which the oxygen carrying protein is 50%
saturated
• Normal p50 in adult hemoglobin is 26.6mmHg
• Lie at the steepest part of the curve, thus most sensitive point to detect the shift
of the curve
• Index of oxygen affinity
• P50 HbA = 26.6mmHg
• P50 HbF = 18mmHg
• P50 Myoglobin = 2.75mmHg
• The lower the P50 the higher the affinity towards O₂

22. Physiological significant
• Flat upper part acts as a buffer-
• PO2 can drop to 80mmHg, yet Hb remained highly saturated (96%) with
oxygen keeping the arterial [O2] high despite impairment in saturation in the
lungs
• Steep lower part allows large O2 unloading and maintain O2 diffusion
gradient (from capillary to cell) by only a small drop in PO2

24. Bohr Effect
• The effect of CO2 and H+ (pH) toward the affinity of Hb for oxygen
• ↑CO2, H+ will cause ↓Hb affinity for O2, favour unloading, right
shifted ODC

31. Oxygen content
• 98% is carried bound to Hb in RBC, only 2% of O2 in arterial blood is
present as dissolved O2
• One gram of Hb can combine with 1.34 ml O2 when 100% saturated
• Functional Hb saturation= [HbO2] x ( [HbO2] + [DeoxyHb]
(1 gm pure Hb binds 1.39mls O2)
• Fractional saturation = ( [HbO2] x 100/total [Hb] )
where total [Hb] = [HbO2] + [DeoxyHb] + [metHb] + [COHb]
(Physiological value ~ 1.34 to 1.37 ml.O2/gmHb)

32. • Total O2 content of arterial blood
CaCO2 = [1.34x(Hb)xSaO2] + [PaCO2 x kO2]
• CaCO2=O2 content (mlO2/dl Blood)
• Hb= hemoglobin concentration (g/dl)
• SaO2= O2 saturation of Hb
• kO2= O2 solubility constant (0.003ml O2/mmHg/dl of blood)
• Normal blood contains 15 gm of Hb/dl of blood
• CaCO2= (1.34x15x1) + (0.003x100)
= 20.4 mls/dl blood
• Thus normal O2 content is about 20.4ml O2/ dl of blood
(if 100% saturated)

33. O₂ Consumption by cell ~250mlsO₂/min
• Arterial point PaO₂ 100mmHg, SaO₂97.5% = 19.9mls O₂/dL
• Venous point PaO₂ 40mmHg, SaO₂ 75%
CaCO2 = [1.34x(Hb)xSaO2] + [PaCO2 x kO2]
= 15.1 + 0.12
= 15.22 O₂/dL
O₂ Consumption 19.9.-15.2= 4.7mls O₂/dL
~ 5x50=250mls O₂/min

34. OXYGENT FLUX
• Amount of O₂ delivered to the peripheral tissues per minutes
• In healthy young adult, the tissues O₂ delivery is ~ 1000mls O₂/min
• Tissues extract 250mls O₂/min (body O₂ consumption)
• 750mls O₂ return to right heart

35. • Oxygen Flux equation
oxygen flux = chemical O2 delivery + Dissolved O2 delivery
= [CO x (Hb) x SaO2 x k] + [CO x PaO2 x 0.003]
= [50 x 15 x 0.99 x 1.34] + [50 x 100 x 0.003]
= 995 + 15
= approx 1000 mls O2/ min
k – Huffner’s number (1.34mlO2/gm.Hb)
CO in dl/min; Hb in gm/dl

36. CₐO₂ (O₂ content) X CO (cardiac output) = O₂ delivery ,DO₂ (Oxygen flux)
How to increase CₐO₂ and DO₂
• Increase circulating Hb
• Maintain high oxygen saturation
• Increase dissolved oxygen by increase partial pressure of oxygen
• Optimise HR and rhytm (sinus )
• Optimise SV (preload/contractality)
• Maintain perfusion pressure to ensure organ oxygen delivery (afterload)

37. • The amount of O₂ in the blood is determined by :
• Amount of dissolved O₂
• Amount of Hb in the blood
• Affinity of Hb to the O₂

38. GAS TRANSPORT
CARBON DIOXIDE TRANSPORT
Dissolved CO2 (5%)
Bicarbonate (90%)
Carbamino compounds (5%)

39. Carbamino compound
• Formed from CO₂ reaction with
• Terminal amino group of protein
• Amino groups in the side chains of arginine and lysine

42. Chloride shift
• Excess Hco3 leaves the RBC in exchange of Chloride

43. Haldane Effect
• Refer to the effect of O2 on affinity of Hb to CO2
• Removal of O2 from Hb increases the affinity of Hb for CO2.
• Favour the loading of CO2 in the tissue level
• Arterial blood contains 48mls of CO2 at PCO2 of 40mmHg
• Mixed venous blood contains 52 mls of CO2 at PCO2 of 46mmHg

45. THANK YOU