Gas exchange

Question 1

Learning outcomes

Answer 1

• Describe the pulmonary and bronchial circulations
• Describe the diffusion of gases across the alveolar-capillary membrane and factors which affect rate
• Describe the reaction of gases with blood
• Explain the effect of ventilation/perfusion mismatch
• Explain the effects of a shunt
• Explain the effects of increased dead space
• Recognise whether hypoxic hypoxia is due to impaired ventilation or impaired gas exchange
• Describe gas exchange in the tissues

Question 2

Hypoxic hypoxia with and without hypercapnia

Answer 2

Hypoxic hypoxia with hypercapnia will reduce ventilation, impairing CO2 removal and oxygen inhalation- O2 therapy is unsafe (inc. PaCO2)
Hypoxic hypoxia without hypercapnia will reduce transer, where o2 therapy is safe (decreased PaCO2)
(Hypercapnia- too much CO2 in bloodstream)

Question 3

Pulmonary circulation

Answer 3

• The flow of blood through alveolar walls, from right ventricle to pulm artery > arterioles> capillaries (wedge catheter in artery to find pressure) > pulmonary veins, drain flow back to left atria -
• Low pressure circulation, can be measured by catheterisation through arm into P. artery and 25/10 sys/dia with a mean of 15mmHg
  Much lower than systemic circulation (120/80mmHg)

Pulm. vascular resistance= perfusion pressure/ cardiac pressure (units mmHg.min.L-1)

Question 4

Bronchial circulation

Answer 4

• Supplies nutritive needs of bronchial walls –muscle and glands
• Systemic arterial supply –oxygenated blood (aortic branches)
• Small rate of blood flow, not much tissue to be supplied
• High vascular resistance –sympathetic tone (noradrenergic nerve fibres) adrenergic fibres control tone of bronchioles , metabolite dilatation
• Capillaries –oxygen extracted
• Venules–deoxygenated admixture with pulmonary capillaries
• Physiological shunt
• Slightly lower PO2 in systemic arteries than equilibrated with alveoli

Question 5

Gas transfer

Answer 5

1. Diffusion through alveolo-capillary membrane (between alveolus and the capillary (blood))
   Fick’s law: Rate of diffusion ∞ PP gradient x surface area x solubility in water / membrane thickness x √molecular weight
2. Reaction with blood
   Oxygen- 4O2 + Hb4 <> Hb4O8
   Carbon dioxide- CO2+H2O (carbonic anhydrase)<> H2CO3 <> H+ + HCO3─
   [CO2 + Hb(NH2) <> Hb(NHCOOH)]
3. Matching ventilation to perfusion
   Ventilation-perfusion ration= rate of vent/ rate of perfusion
   Mismatching occurs in transfer pathologies, where ratios are mismatched (blood in poor vent alveoli will not fully oxgyenate blood, when mixed with fully oxygenated blood no extra oxygen, cannot compensate: HYPOXIC HYPOXIA)
   Compensation for this: reducing perfusion to poorly ventilated areas, hypoxic pulmonary vasoconstriction in arterioles, VQ ratio back to normal and better chance of full oxygenation

Question 6

Types of hypoxia

Answer 6

Hypoxic- dec. oxygen tension- high altitude, hypoventilation, VQ MISMATCH
Anaemic- dec. carrying capacity, anaemia, blood loss, CO poisoning
Stagnant- dec. perfusion, heart failure, shock, ischaemia
Histotoxic- cellular hypoxia, cellular poisons eg cyanide, shifting of O2-Hb curve

Question 7

Shunts

Answer 7

• Deoxygenated blood that does not flow through alveolar walls for oxygenation- hypoxia and hypocapnia
  To differentiate between VQ mismatch and shunt, ask patient to breathe in 100% oxygen:
  In shunting no oxygen will not correct hypoxic hypoxia
  With mismatch more PO2 in inhalation normal PP can be returned, full saturation of blood can occur (compensation for mismatch)
• Dead space- space where there is no blood flow back to left atria OR gas exchange, usually confined to conducting airways
• Air does not get to blood and cannot affect blood gases
• In some conditions some alveoli cannot exchange gases, contributes to dead space that still must be ventilated (muscle fatigue, fail to maintain required ventilation)

Question 8

How to distinguish between ventilatory and tranfer hypoxic hypoxia

Answer 8

PaCO2 is how you can determine whether or not hypoxic hypoxia is due to ventilation or transfer ( higher in ventilation as CO2 cannot be exhaled properly)

Question 9

Gas exchange in tissues

Answer 9

-Systemic arteries > capillaries > veins
-Vasomotion in pulmonary circulation- Variable proportion of capillaries closed and open
Oxygen extracted as o.blood passes along capillaries, PaO2 drops

No equilibration with systemic tissues and alveolar wall
PaO2 in tissues depends on tissue location (how close cells are to capillary, if capillary open/closed)

Question 10

Measurement of transfer factor

Answer 10

Single breath carbon monoxide transfer test