Respiration IIIRespiration IIIPartial pressure of gasesO2 and CO2 transport in the bloodVentilation and acid-base balance

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

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

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

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

20

40

60

80

100

20 40 60 80 100

PCO2, pH, Temp

PCO2, pH Temp

PO2 (mm Hg)

% S

atur

atio

n

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

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

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