oxygen and carbon dioxide transfer between alveoli, blood and tissues Flashcards
what is gas movement
gas movement through the conducting airways occurs by convection
gas exchange across the blood-gas barrier in the alveolus occurs by diffusion (Fick’s law)
how is oxygen transported by blood
oxygen is transported in two forms in blood
physical
chemical
what is physical transport of oxygen
plasma soluble O2 (2%)
less soluble than CO2
function of partial pressure of O2 in alveolus
0.3ml O2/100ml blood > at PO2 of 100mmHg
0.3ml O2 = normoxic alveolar PO2
what is chemical transport of oxygen
O2 bound to haemoglobin (98%)
rapid and reversible interaction to enable oxygen off-loading to tissues
Hb + O2 <> HbO2
how much oxygen is carried by haemoglobin
CaO2 = 20.3mlO2/100ml blood
CvO2 = 15.28mlO2/100ml blood
oxygen carrying capacity of haemoglobin is 1.34 x 15 = 20mls O2/100ml blood
what is oxygen content
CaO2 or CvO2
determined by amount of haemoglobin and oxygen in blood
what is oxygen saturation
SaO2
proportion (%)
SaO2 = oxyhaemoglobin / oxygen carrying capacity of haemoglobin
saturation can remain the same even if oxygen content of blood differs
what is the oxyhaemoglobin dissociation curve
P50 value gives PO2 required for half maximal Hb saturation
venous blood enters alveolus at 40mmHg, 75% saturation
equilibriates to alveolar PO2 of 100mmHg, 97% saturation
arterial plateau phase ensures maximal HbO2 saturation even if alveolar PO2 is below the normal (normoxic) oxygen tension
steep phase of curve favours off-load of arterial oxygen to tissues
greater HbO2 dissociation for small changes in tissue PO2
what is the normal P50 of the oxyhaemoglobin dissociation curve
27mmHg at pH7.4 and PCO2 of 40mmHg
what is the left-shift P50 of the oxyhaemoglobin dissociation curve
increased Hb-O2 affinity
reduced oxygen offloading to tissues
e.g. foetal HB
what is the right-shift P50 of the oxyhaemoglobin dissociation curve
decreased Hb-O2 affinity
raised O2 offloading to tissues
e.g. high altitude
may be induced by stressors such as acidosis, fever and hypoxia
what is the bohr effect
pCO2 and pH alter haemoglobin-oxygen binding affinity
what is haemoglobin
Hb
heterotetramer composed of 2⍺ and 2β subunits
contains 4 iron-binding haem domains
oxygen reversibly binds to Fe3+ ions in the centre of the haem ring
CO2, pH and 2,3-BPG alter this affinity by interacting with charged amino acids between the ⍺ and β subunits
what is the affect of pH on oxy-haemoglobin binding affinity
alters the ability of oxygen to bind to haemoglobin
amino terminus of the haemoglobin ⍺ subunit binds to the carboxy-terminal histidine in the β subunit > this stabilises the structure
this interaction is pH and O2 sensitive
in acidosis, decreased pH (increased [H+]) favours the ⍺-β subunit interaction and reduces the binding of oxygen to haem
what is the affect of CO2 on oxy-haemoglobin binding affinity
CO2 alters blood pH and reduces haemoglobin affinity for oxygen
increased blood pCO2 releases oxygen from haemoglobin in two ways;
1. production of carbonic acid in red blood cell
2. carbamate reaction at N-terminal amino groups on Hb ⍺-subunit
H+ = acid stabilisation of ⍺-β subunit interaction = low Hb affinity for oxygen
what is physical transport of carbon dioxide by the blood
plasma soluble CO2 - soluble CO2 gas (5%), bicarbonate ion (90%)
CO2 is 20x more soluble in plasma than oxygen
what is the CO2 content of arterial blood
CaCO2
48mlCO2/100ml blood
what is the CO2 content of venous blood
CvCO2
52mlCO2/100ml blood
how much CO2 is exhaled
4mlCO2/100ml blood
what is the chemical transport of carbon dioxide by blood
carbamino haemoglobin
carbamate reaction at N-terminal amino groups of Hb ⍺ subunit (5%)
how is CO2 carried in a red blood cell
carbonic amhydrase in red blood cell enables CO2 carriage as bicarbonate anion
exchange of bicarbonate out of the cell for chlorine into the cell maintains the gradient for CO2 uptake into red cell and buffers pH of blood plasma to approx 7.4
what is the carbon dioxide dissociation curve
blood carriage of CO2 in all 3 forms
relationship near straight line in physiological range
altered by tissue oxygenation
what is the bohr and haldane effect
enable reciprocal oxygen and carbon dioxide gas exchange
describe the process of carbon dioxide release from tissue/ oxygen release from red blood cell
- carbon dioxide dissolves into plasma and red cell along partial pressure gradient
- low tissue oxygen favours carbon dioxide carriage by blood (haldane effect)
- carbamate reaction reduces HbO2 affinity (bohr effect)
- carbonic anhydrase reaction generate carbonic acid which protonates. bicarbonate leaves the cell and maintains inward carbon dioxide gradient
- increased red blood cell [H+] reduces HbO2 affinity by promoting ⍺,β Hb subunit interaction (bohr effect)
describe the process of carbon dioxide release from red cell/ oxygen uptake from alveolus
- oxygen dissolves into plasma and red cell along partial pressure gradient
- high affinity of Hb for oxygen reverses Hb carbamation and protonation raising availability of high affinity Hb (bohr effect)
- carbon dioxide diffuses into alveolus along pressure gradient. high pO2 decreases CO2 affinity for Hb (haldane effect)
- movement of carbon dioxide out of red cell increases bicarbonate uptake and chlorine moves out of the cell
- proton release from Hb and increased bicarbonate drive carbonic anhydrase reaction in reverse, maintaining outward CO2 gradient and lowering plasma bicarbonate
