Gas Exchange Flashcards
Purpose of respiration
> O2 from air to tissues
> CO2 from tissues to air
Daltons law
> Partial pressure = pressure exerted by single gas within a mixture (Kpa x 7.5 = mmHg)
Normal partial pressures for air
- O2 (~21%) = 159 mmHg
- CO2 (0.01%) = 0.3 mmHg
Diffusion
> Air in respiratory zone moves via diffusion (conducting zone = bulk flow)
Ficks law = high partial pressure to low
Rate is controlled by:
- area + thickness of barrier
- diffusing ability of gas
- partial pressure difference
(area x difference / distance = proportional to rate)
Influenced by:
- temperature
- movement of molecules
External respiration
> O2/Co2 exchange at alveoli
PaO2: 100(alveoli) to 40 (venous blood)
PaCo2: 40(alveoli) to 45 (venous blood)
*PaCo2 is higher in lungs than air as not all air breathed out so conc. is higher
At lungs
> O2 dissolves in surfactant
diffuses through surfactant, alveolar wall, capillary wall, plasma to RBC
Combines with haemoglobin to release H+ (deoxyhaemoglobin - HHb + O2 to oxyhaemoglobin HbO2 + H)
*rest then dissolves in plasma (~1.5%)
H then combines with bicarbonate to form water and Co2
Co2 then diffuses into alveoli (more soluble + small partial pressure difference so takes about same time as oxygen to reach equilibrium ~0.25 seconds)
Haemoglobin
> transports 98.5% of oxygen - doesn’t count towards PaO2 of blood
consists of two alpha and two beta polypeptide chains which each associate with a heme molecule (to form a protein subunti)
O2 binds to heme - so one haemoglobin molecule can carry four oxygen molecules
Amount of oxygen bound to haemoglobin is calculated as % saturation
Henry’s Law
> amount of gas in solution depends on: - solubility of gas - partial pressure of gas > O2 - low solubility - high partial pressure > Co2 - high solubility (20x of O2) - low partial pressure
Oxyhaemoglobin dissociation
> happens in sequence; 1st + 4th oxygen molecules = hardest to get on/off haemoglobin
factors that affect dissociation
- temp (increase = less O2 affinity)
- partial pressure of Co2 (higher = less O2 affinity)
- pH (lower = less O2 affinity)
- 2-3 diphosphoglycerate (protein that aids release of O2 from haemoglobin - higher levels = less O2 affinity)
- anaemia (less Hb to carry O2)
- Carbon monoxide - competes with O2 to bind
- Nitric oxide - (higher - less O2 affinity)
> levels of PaO2
- curve is steep between 20-40mmHg so if oxygen level drops then substantial unloading occurs
- 40mmHg = 75% sats
- 70 mmHg = 94.1%
- 100mmHg (alveolar PaO2) = 97.4%
Bohr effect
> more hydrogen ions = more deoxyhaemoglobin
> occurs at high Co2 levels
Internal respiration
> O2/Co2 exchange at tissue
PaO2: 95 (blood) to 40 (tissue)
PaCo2: 40(blood) to 45(tissue)
At tissue
> Oxygen diffuses through RBC, plasma, capillary well, tissue membrane to mitochondria
*Co2 goes reverse way
Co2 enters RBC - some combines with Hb, the rest combines with H2O to form H+ and bicarbonate
Bicarbonate moves into plasma to act as a buffer
H+ combines with oxyhaemoglobin to form deoxyhaemoglobin and oxygen
O2 diffuses into tissue
How do partial pressures change in exercise/disease
> Exercise
- don’t change
- no limit on diffusion/perfusion (more capillaries are recruited + blood moves more quickly)
> Disease
- abnormal alveolar/capillary interface (thicker interface so diffusion rate decreases - lower oxygen levels = less exercise tolerance)
Transport of Carbon dioxide + O2
> Co2
- 5-10% dissolved in plasma
- 5-10% bound to amino acids
- 80-90% as bicarbonate ions
> O2
- 98.5% is bound to haemoglobin
- 1.5% is dissolved in plasma
Henderson-Hasselback equation
H20+Co2 > Carbonic acid (H2Co3) > H+ and HC03- (bicarbonate ion)
> requires enzyme = carbonic anhydrase
Haldane effect
> more deoxyhaemoglobin = more Co2 carried by blood (ie at tissues)
more oxyhaemoglobin = less Co2 carried by blood (ie at lungs)
