Respiration III  Partial pressure of gases  O 2 and CO 2 transport in the blood  Ventilation and acid-base balance

Partial pressure of gases  Dalton’s law - the total pressure of a gas mixture = the sum of the pressures that each gas would exert independently when the barometric pressure (P B ) is 760 mm Hg the composition of atmospheric air is - GAS%FRACTION O CO N TOTAL

Calculation of partial pressures  The partial pressure of any gas can be calculated by multiplying P B by the fraction of the gas Examples -PO 2 = 760 x.2093 PO 2 = 159 mm Hg PN 2 = 760 x.7904 PN 2 = 600 mm Hg

O 2 transport by the blood  four O 2 molecules combine with Hb forming oxy-haemoglobin in a reversible reaction Hb + O 2  HbO 2  arterial blood is fully saturated with O 2  the rest (98.5 %) is combined with iron (Fe 2+ ) on haem units of haemoglobin (Hb) molecules in red blood cells  20 ml of O 2 is carried by 100 ml of blood  0.3 ml (1.5%) is dissolved in solution in plasma

Oxyhaemoglobin dissociation curve  This relates the % saturation of Hb to the PO 2 in blood  A high PO 2 = loading (curve  right)  A low PO 2 /reduced affinity = unloading (curve  left)  This is described by the ‘S’ shaped O 2 -Hb dissociation curve  Each O 2 molecule binding to Hb increases the rate of binding of the next O 2 molecule

 PCO 2,  pH,  Temp  PCO 2,  pH  Temp PO 2 (mm Hg) % Saturation

Effects of pH & temperature on O 2 -Hb  Active tissues =  PCO 2 & lactic acid =  [H + ]   [H + ] weakens the bond between O 2 & Hb  More O 2 is released for a given PO 2 when pH is lower  O 2 -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

CO 2 transport by the blood  CO 2 diffuses into blood in systemic tissue capillaries  7 % remains dissolved in blood plasma  The rest (93%) diffuses into red blood cell (rbc)  In rbc -23% binds to Hb =carbaminohaemoglobin - 70% converted into H 2 CO 3  H 2 CO 3 dissociates into HCO 3 - & H +  HCO 3 - diffuses into plasma in exchange with Cl -  H + binds to buffers (Hb)  This process is reversed in the lung capillaries

Ventilation & acid-base balance  An increase in blood PCO 2 leads to  [H + ] and this increases blood acidity (  pH)  A decrease in blood PCO 2 has the opposite effect and the blood becomes more alkaline  Excessive ventilation will exhale more CO 2 and  blood PCO 2  Insufficient ventilation will result in a build-up of CO 2 and  blood PCO 2