diffusion gas exchange and solubility

Question 1

partial pressures of gases (daltons law)
- what is daltons law
- what is atmospheric pressure
- composition of air in atmosphere
- partial pressures of atmospheric gases
- partial pressures of atmospheric gases in fully saturated alveoli (what is water vapour pp)
- impact of high altitude on these

Answer 1

• The pressure of a gas mixture is equal to the sum of the partial pressures of the individual gases.
• 760 mmHg
• 78.1% Nitrogen (N2)
  20.9% Oxygen (O2)
  0.033% Carbon dioxide (CO2)
• N2 = 760 mmHg x 78.1% = 594 mmHg
  O2 = 760 mmHg x 20.9% = 159 mmHg
  CO2 = 760 mmHg x 0.033% = 0.25 mmHg
• N2 = (760 mmHg – 46 mmHg) x 78.1% = 558 mmHg
  O2 = (760 mmHg – 46 mmHg) x 20.9% = 149 mmHg
  CO2 = (760 mmHg – 46 mmHg) x 0.033% = 0.24 mmHg
• atmospheric pressure lower, contribution of water vapour is more relevant as reduce pp of gases further

Question 2

ficks law of diffusion
- calc, what each part means and implication for rate of diffusion

Answer 2

• Rate of diffusion directly proportional to- A/T x (P1 − P2) x D
  A= surface area (large in lungs due to many alveoli),
  T= thickness of barrier (alveoli thin, contact close with blood)
  P1-P2= partial pressure difference either side of barrier. Greater pp diff, greater diffusion of gas.
  Constant D for gases= the solubility of a certain gas- the ease with which the gas dissolves in solution
• From Fick’s law, greater solubility of a gas = greater the rate of diffusion (CO2 more soluble than O2 and it diffuses across membrane a lot easier)

Question 3

the diffusion barrier
- how thick is it
- cells of the alveolar wall in detail
- what holds the capillar wall
- where are hb, how does gas attach,

Answer 3

• Around 0.1-1.5 micrometers
• aleveolar epithetlium (type 1 cells allow gas to diffuse across with surfactant on surface which allows gases to dissolve)
• vascular endothelium (capillaries) that are held together by fused basement membranes.
• attached to red blood cells (o2 has to diffuse through cell wall of RBC before binding to HB) in the plasma transporting o2 and some co2

Question 4

pulmonary gas exchange
- p02 and co2 in alveoli and deoxygenated blood
- describe the transport of gas

Answer 4

• lungs, po2= 100mmHg, pco2= 40mmHg
• deoxygenated blood, po2= 40mmHg, pco2= 46mmHg
• Gases diffuse down partial pressure gradients until equilibrium

Question 5

systemic gas exchange
- po2 and co2 in cells
- what happens to the diff gases and what are we left with
- what happens during exercise

Answer 5

• po2 < 40mmHg, pco2 > 46mmHg
• cells use o2 and produce co2 so gases diffuse down pressure gradients and we are left with deoxygenated blood
• the diffusion gradients are steeper so more o2 used by tissues and more co2 produced

Question 6

partial pressure in solution
- what do o2 and co2 do in solution
- difference between partial pressures and concentrations in air and water for o2 and co2 at equilibrium
- what do movements of o2 and co2 depend on when making equilibrium (3)

Answer 6

• dissolve
• partial pressure are the same for both gases in and out of water. concentration is lower in the water for both, but more co2 dissolves as its more soluble
• temp (hotter faster)
• pressure difference (steeper faster)
• solubility (more the more soluble)

Question 7

importance of fast diffusion
- why does it need to be fast
- impact of exercise on rate
- what can cause sub normal diffusion and what it leads to

Answer 7

• fast enough so gases can equilibrate in gas exchange while in contact
• faster during exercise as pressure gradients steeper
• exchange surface thickening, altitude, disease, leading to lower oxygen content limiting activity and exercise

Question 8

Pathological limitation of diffusion and its impacts
- normal lung
- emphysema
- fibrotic lung disease
- pulmonary oedema
- asthma

Answer 8

• Large surface area, thin barrier, good ventilation of alveoli, elastic recoil - Po2 normal in blood and lungs
• Normal thickness of barrier, but limited surface area and poor elastic recoil as alveolar sacs damaged, struggle to breathe out - Po2 in blood low as less capillaries contact
• Thick barrier and decrease of lung compliance (stiff lungs, hard to inflate) - Po2 in blood low due to diffusion distance
• Caused by pressure increase in capillaries, creating fluid that separates epi/endothelium - Increased diffusion distance so po2 in blood low
• Po2 in alveoli low due to constricted bronchioles limiting airflow, and p02 in alveoli - so reduced gradient and po2 in blood low