Lecture 3: Gas exchange and transport

Question 1

What is the O2 partial pressure of air?

Answer 1

21pKa

Question 2

Why does the partial pressure of oxygen decrease in the airways compared to the air?

Answer 2

Air is humidified in the upper airways and water pressure is present in inspired air and alveolar air.

Water vapour pressure, in air 100% saturated with water, has a pressure of 6.3kPa.

The total pressure of air stays the same (Patm at sea level pressure is 101 kPa), but now water vapour dilutes the amount and partial pressure of O2.

Question 3

Why does the partial pressure of oxygen decrease in the alveoli compared to the airways?

Answer 3

The alveolus is not a closed space for gases. Oxygen leaves (and CO2 enters)

Question 4

What is the equation for PAO2 (the alveolar gas equation)?

Answer 4

PAO2 = [(Patm – PH2O) x FiO2] – (PaCO2/R)

Question 5

Why does the partial pressure of o2 decrease in blood compared to alveolus?

Answer 5

A few small veins that dump deoxy blood directly into the arteries (shunts) so is diluted

This is called anatomical shunt as this blood doesn’t pass through the lungs

Venous admixture

Question 6

What are the two examples of an anatomical shunt?

Answer 6

The thesbian and bronchial veins carrying “deoxygenated” blood empty into the arterial circulation

Question 7

What is a venous admixture?

Answer 7

Term to describe the mixing of highly oxygenated arterial blood with the small amount of oxygen-poor venous blood

This mixing accounts for the small drop in PAO2 to PaO2 between alveoli and blood

Question 8

Why does the partial pressure of oxygen decrease in the interstitial fluid compared to the blood?

Answer 8

The large difference in the PaO2 (12.7 pKa) to interstitial fluid PO2 (5.3 pKa) drives transportof oxygen

The drop in intravascular PO2 favours Hb unloading - abundant transfer of O2 out of blood and into tissues

Question 9

Why does the partial pressure of oxygen decrease in the cells/mitochondria compared to the interstitial fluid?

Answer 9

Oxygen diffuses from interstitial fluid to inside tissue cells

Diffusion across a thin membrane is FAST but diffusion through water is SLOW

Mitochondria consume oxygen - in steady state times, the intracellular PO2 can equilibrate with tissues

Question 10

What is the maximum distance of a cell from its nearest capillary?

Answer 10

Usually less than 1/10th of a mm

Question 11

What is formed when CO binds to Hb?

Answer 11

Carboxyhaemoglobin (CO-Hb)

Question 12

How much greater is CO affinity for Hb compared to O2?

Answer 12

> 200 times affinity for Hb compared to O2

Question 13

How does CO poisoning occur?

Answer 13

CO impairs both loading and unloading of O2:

• O2 content is reduced due to CO outcompeting O2 for Hb binding
• CO stabilizing the R state (ie left shift) for the remaining sites - this impedes release of O2

Question 14

What are the symptoms of CO poisoning?

Answer 14

Headache, nausea/vomiting, dizziness/lethargy/weakness

Confusion

Coma and death (when CO-Hg > 40%)

Question 15

What is MET-Hb?

Answer 15

Normal state of iron in Hb = Fe2+ (ferrous)

Can get oxidized to Fe3+ form (called ferric)

Ineffective oxygen binding at Fe3+ subunits reduces oxygen content and delivery

Question 16

What are the three forms of CO2 transport in the blood?

Answer 16

Bicarbonate(90%)

Carbamino compounds (~5%), most of which is Hb-CO2

Soluble gas (5%)

Question 17

What are the two forms of O2 transport in the blood?

Answer 17

Dissolved and bound to Hb

Question 18

How can you measure PACO2?

Answer 18

Deep breath out – last air will have come from v inside of lung – end tidal – can measure conc of CO2