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Respiration IIIRespiration IIIPartial pressure of gasesO2 and CO2 transport in the bloodVentilation and acid-base balance
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Partial pressure of gasesPartial pressure of gasesDalton’s law -
the total pressure of a gas mixture =the sum of the pressures that each gas would exert independently when the barometric pressure (PB) is 760 mm Hg
the composition of atmospheric air is -GAS % FRACTIONO2 20.93 .2093CO2 00.03 .0003N2 79.04 .7904TOTAL 100 1.0
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Calculation of partial pressuresCalculation of partial pressures
The partial pressure of any gas can be calculated by multiplying PB by the fraction of the gas
Examples - PO2 = 760 x .2093PO2 = 159 mm HgPN2 = 760 x .7904PN2 = 600 mm Hg
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OO22 transport by the blood transport by the blood
four O2 molecules combine with Hb forming oxy-haemoglobin in a reversible reactionHb + O2 HbO2
arterial blood is fully saturated with O2
the rest (98.5 %) is combined with iron (Fe2+) on haem units of haemoglobin (Hb) molecules in red blood cells
20 ml of O2 is carried by 100 ml of blood 0.3 ml (1.5%) is dissolved in solution in plasma
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Oxyhaemoglobin dissociation Oxyhaemoglobin dissociation curvecurve
This relates the % saturation of Hb to the PO2 in blood
A high PO2 = loading (curveright)A low PO2/reduced affinity = unloading (curveleft)This is described by the ‘S’ shaped
O2-Hb dissociation curveEach O2 molecule binding to Hb increases the rate of
binding of the next O2 molecule
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20
40
60
80
100
20 40 60 80 100
PCO2, pH, Temp
PCO2, pH Temp
PO2 (mm Hg)
% S
atur
atio
n
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Effects of pH & temperature on OEffects of pH & temperature on O22-Hb-Hb
Active tissues = PCO2 & lactic acid = [H+] [H+] weakens the bond between O2 & HbMore O2 is released for a given PO2 when pH is lowerO2-Hb dissociation curve shifts to the right
called the ‘Bohr effect’A similar shift to the right occurs with increased
blood temperature around exercising skeletal muscle
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COCO22 transport by the blood transport by the bloodCO2 diffuses into blood in systemic tissue capillaries7 % remains dissolved in blood plasmaThe rest (93%) diffuses into red blood cell (rbc)In rbc - 23% binds to Hb =carbaminohaemoglobin
- 70% converted into H2CO3
H2CO3 dissociates into HCO3- & H+
HCO3- diffuses into plasma in exchange with Cl-
H+ binds to buffers (Hb)This process is reversed in the lung capillaries
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Ventilation & acid-base balanceVentilation & acid-base balance
An increase in blood PCO2 leads to [H+] and this increases blood acidity (pH)
A decrease in blood PCO2 has the opposite effect and the blood becomes more alkaline
Excessive ventilation will exhale more CO2 and blood PCO2
Insufficient ventilation will result in a build-up of CO2 and blood PCO2