7. Alveolar ventilation & pulmonary gas exchange Flashcards
Which alveoli are suspended?
Apical alveoli - top ones
Which alveoli are smaller? Effect on Pleural Pressure?
Lower alveoli less –ve Ppl
Which alveoli are bigger? Effect on Pleural Pressure?
Top alveoli more –ve Ppl
Where is ventilation greatest in the lung?
bottom of lung
How is regional ventilation measured?
Upright individual Inhales radioactive Xenon gas Radiation recorded from ventilated areas
Gas transfer between alveoli and blood: the barrier
3 layers: Alveolar epithelium Capillary endothelium Interstitium (sometimes absent)
Thin portions of barrier
Gas exchange takes place
Thick portions of barrier
Fluid exchange takes place
Dalton’s law of partial pressures
Total pressure x Fractional concn in mixture
Henry’s law
The concentration of gas in a liquid is proportional to its partial pressure
Fick’s law: diffusion is proportional to
1) Area 2)Partial pressure gradient 3)Diffusion constant
Fick’s law: diffusion is Inversely proportional to
1) Barrier thickness
How long does blood take to traverse a pulmonary capillary at rest?
0.75 seconds
Effects of reduced transit time?
Strenuous exercise increases cardiac output May not have time for complete equilibrium to occur
Effects of increased barrier?
Pulmonary oedema Fibrosis
What limits the amount of gas we can get into the blood?
1) CO 2) Nitrous Oxide 3) O2 uptake
CO bound avidly by Hb
Little in solution
not much rise in PCO
Still diffusing at end of capillary
DIFFUSION LIMITED
N2O just stays in solution
No binding
rapid rise in PN2O in blood
partial pressure gradient lost
Net diffusion stopped before end of capillary
FLOW LIMITED
O2 intermediate: lots bound
Normally FLOW LIMITED
Can be diffusion limited in disease
Diffusion in the lung is decreased by:
1) Decreased SA (emphysema) 2) Increased membrane thickness (pulmonary fibrosis) 3) Pulmonary hypertension 4) Anaemia
Diffusion in the lung is increased by:
1) Exercise just before the test 2) Supine position 3) Asthma 4) Pulmonary haemorrhage 5) Polycythaemia (too many rbcs) 6) Mild left heart failure
