respiratory: gaseous diffusion and transport

Question 1

Henry’s law

Answer 1

C=kP
C is concentration of dissolved gas
K is Henry’s constant (different for different gases)
P is partial pressure of gas

Question 2

What is alveolar water vapour pressure? Why is it constant

Answer 2

6.3kPa
Water vapour pressure depends on temperature and saturation
Temperature is constant at 37C
Saturation is 100% after passing over moist surfaces

Question 3

Equation for PIO2

Answer 3

(Pb-6.3)x0.209

Question 4

How to measure pAO2? Why cant we measure this directly

Answer 4

Co2 diffuses into alveolus to replace o2 diffusing into capillary
Usually more o2 is used than co2, co2 production:o2 consumption = 4:5 (R=0.8)
PAo2= pIo2-pAco2/R

Question 5

PO2, PCO2, PH2O of atm, mixed expired air, trachea and alveolar gas

Answer 5

Atm: po2=21kpa; pco2=0kpa; ph2o=variable
Expired air: po2=16kpa; pco2=3.5kpa; ph2o=variable
Trachea (during inspiration): po2=20kpa; pco2=0kpa; ph2o=6.3kpa
Alveolar gas: po2=13.5kpa; pco2=5.3kpa; ph2o=6.3kpa

Question 6

What is po2 at venous end of capillary? When does capillarypo2=alveolarpo2

Answer 6

5.3kpa;

About 1/3 along capillary

Question 7

Solubility co-efficient of co2 and o2;

Which gas will diffuse faster? How many times faster

Answer 7

O2=0.03ml/L/mmhg
Co2=0.7ml/L/mmhg
Co2 diffuses faster. It is 85% of the molecular weight of o2; it is 0.7/0.03=23x more soluble
Diffuses 23x0.85=20x faster

Question 8

What factors are diffusion across a membrane affected by? How is the rate of transfer of gas calculated in the lungs?

Answer 8

Proportional to area; proportional to pressure gradient; inversely proportional to thickness
DLG is the transfer factor (area and thickness considered)
Rate of transfer=DLG (P2-P1)

Question 9

How is oxygen diffusing capacity calculated (DLO2)

Answer 9

DLO2= oxygen uptake from lungs/(pAo2-pco2)

Question 10

How is carbon monoxide diffusing capacity measured (DLCO) measured?

Answer 10

Inhale Low amount of CO + tracer gas -> composition of exhaled gas examined
DLCO=CO uptake from lungs/(pAco-pcCO)
PcCO assumed to be zero as haemoglobin has great affinity for CO-> most CO binds to haemoglobin once in blood

Question 11

What is DLCO reduced and increased by

Answer 11

Reduced by: anaemia/ increased thickness (eg fibrosis/oedema)/ reduced area (eg emphysema/emboli)
Increased by: polycythaemia (too many blood cells)/increased pulmonary blood volume (increases area)

Question 12

Haemoglobin content in blood
Haemoglobin carrying capacity 
Oxygen capacity in blood
Oxygen saturation of pulmonary venous blood 
Oxygen saturation in Vena cava

Answer 12

Haemoglobin content=150g/L
Haemoglobin carrying capacity=1.34ml/g
Oxygen capacity=150 x 1.34= 200ml/L
Oxygen saturation in pulmonary venous=100%
Oxygen saturation in VC=75%

Question 13

How does haemoglobin dissociation curve change from arterial-> venous blood? Why is there a change like that?

Answer 13

Moves right wards;

Bohr shift: ph decreases/pco2 increases/temperature increases/2,3DPG increases

Question 14

How much o2 do tissues extract

How much co2 do tissues produce

Answer 14

5ml of o2/dL

4ml of co2/dL

Question 15

How does anaemia affect oxygen dissociation curve? Why are anaemics fatigued?

Answer 15

Causes a downward shift
Hb capacity is halved, venous capacity Low at 3.6kpa
Anaemia limits o2 extraction during exercise-> fatigue

Question 16

Negative effects of CO binding to Hb
How does CO shift oxygen dissociation curve
Problems caused

Answer 16

Reduces o2 bound to Hb 
Increases o2 affinity of remaining binding sites on Hb -> decreases unloading of o2
Downward shift, now a plateauing shape 
Venous po2=2kpa 
Headache coma death

Question 17

Cyanosis types; what is it?

Answer 17

Central and peripheral

O2 supply to tissues is deficient-> content of de-oxy hb in tissue capillaries is high-> blueish tinge

Question 18

Peripheral cyanosis

Causes

Answer 18

Reduced blood flow to a region-> hypoxic tissue

Causes of reduced blood flow: cardiovascular shock/Low temp/reduced CO/poor arterial supply

Question 19

Central cyanosis

Causes

Answer 19

Arterial hypoxaemia (reduction in o2 content)
Blood contains 1.5-2 g/dL of deoxyhb; o2 saturation is 85%
COPD/right to left heart shunts

Question 20

Forms and percentages in which co2 is carried

Answer 20

60% HCO3
30% HbCO2
10% dissolved

Question 21

What happens to H and HCO3 formed by CA in RBC

Answer 21

H is buffered by deoxyhaemoglobin

HCO3 enters plasma via HCO3/CL antiporter

Question 22

Formation of carbamino compounds

Answer 22

Co2 reacts with nh2 on plasma proteins + deoxy-hb

Oxygenation of hb-> co2 unloaded

Question 23

Haldane effect and causes

Answer 23

At any pco2, venous blood carries more co2 than arterial blood
This is bc hb forms carbamino compounds more readily when deoxygenated
And hb buffers H+ more readily when deoxygenated-> formation of HCO3 favoured

Question 24

Whats respiratory quotient

Answer 24

Co2 produced: o2 utilised

Question 25

Consequences of hyperventilation

Answer 25

Low paco2-> cerebral vasoconstriction-> cerebral hypoxia-> dizziness
Alkalosis-> decreased free ca (proteins bind more strongly to ca at higher pH)-> increased excitability of VGCC

Question 26

Hypoventilation consequences

Answer 26

Hypercapnia
Acidosis
Vasodilation/flushed skin/full pulse
Very high pco2-> CNS depressed-> confusion/drowsiness/coma

Question 27

Values for PAO2
PaO2 
Arterial o2 content
Mixed venous po2
Mixed venous o2 content 
Mixed venous hb saturation

Answer 27

PAO2=13.5 kpa 
PaO2=12.5 kpa 
Arterial o2 content= 200ml/L
Mixed venous po2= 5.3kpa
Mixed venous o2 content=150ml/L
Mixed venous hb saturation= 75%

Question 28

PACO2
PaCO2
Arterial co2 content 
Mixed venous paco2
Mixed venous co2 content 
Arterial HCO3

Answer 28

PACO2= 5.3kpa
PaCO2=5.3kpa 
Arterial co2 content= 480 ml/L
Mixed venous pco2= 6.1kpa 
Mixed venous co2 content= 520ml/L
Arterial HCO3= 21-27mmol/L