Lecture 17 Respiration 3 Flashcards
Gas transport in blood
Most O2 in blood bound to haemoglobin
Hb+O2 <-> HbO2
Hb proteins in erythrocytes - red blood cells
4 polypeptide chains
- each chain contains a haem group or single iron atom
- each haem binds an atom of O2 so one Hb carries 4 O2
Method of transport in blood
O2 physically dissolved 1.5%
bound to haemoglobin 98.5%
CO2 physically dissolved 10%
Bound to haemoglobin 30%
As bicarbonate (HCO3-) 60%
Oxygen transport
1g (100% saturated) Hb carries 1.34 ml of O2
Blood contains 150gL-¹ Hb (1.34x150) = 200ml O2 carried by Hb
Normal PO2 100mm Hg Hb 98% saturated.
CO= 5 L.min-¹ (5x200) = 1000ml. min-¹ to tissue
At rest need 250 ml min-¹
25% O2 diffuses into tissue
75% remains bound to Hb
(PO2 40mmHg)
Venous blood still saturated with O2
Anaemia occurs when 100% saturated but content reduced due to reduced quantity of haemoglobin
Hemorrhage reduces internal pressure
Oxygen haemoglobin dissociation curve
steep part is systemic capillaries where Hb unloads O2 to cells
Plateau where pp O2 high (lungs)
% sat high where PO2 high - lungs
% sat low where PO2 low - tissue cells
At tissue O2 dissociates from Hb opposite of saturation
Hb O2 dissociation curve
Sigmoidal curve
O2 binding induces confirmational change
Binding 1 O2 increases affinity of Hb for next O2
O2 binding induces confirmational changes - cooperatively
Below 15mmHg deoxygenated form low O2 affinity so low % saturation
Increase PO2 Hb oxygenation increase O2 affinity
PO2 above 60mm Hg few binding sites available as saturation nears 100%
Systemic veins PO2 40mm Hg
Hb 75% saturated 25% used by respiring tissue
Huge capacity to unload more O2 if tissue metabolism increases e.g. exercise
Shape of curve reflects affinity of Hb for O2 (%sat of Hb / PO2 mm Hg)
Various factors affect affinity of Hb for O2
Changes in affinity shift curve left or right
Reduced affinity shifts curve right - higher PO2 required to achieve given level of saturation (aid unloading)
Increased affinity shifts curve left - lower PO2 required to achieve given level saturation O2 loaded more efficiently onto Hb (aids binding)
Factors affecting affinity of Hb for O2
Temperature - increase, right shift
pH ( Bohr effect)
Increased acidity decreased affinity of Hb for O2
Respiring tissue produces H+ facilitating unloading
In lungs increased pH and reduced acidity increases affinity of Hb for O2 facilitates loading (left shift)
2.3 - DPG produced during glycolysis binds reversibly to Hb lowering affinity for O2 - right shift - promotes unloading
Special haemoglobin
Myoglobin
- O2 binding protein in skeletal muscle
- 1 haem 1 polypeptide chain
- higher affinity for O2 than Hb
- at low PO2 50% saturated
- liberates O2 when PO2 down 10mm Hg
Foetal Hb
-PO2 blood to foetus via umbilical veins (placenta) 35-40mm Hg
- PaO2 foetus 20mmHg expect low Hb sat infavt 60% sat.
- Hb diff structure+ greater affinity for O2 than adult (left shift)
- exchange steep part Hb-O2 curve
- small diff between arterial and venous blood PO2
Most CO2 transported as bicarbonate ion
1) CO2 diffuses out of cells into systemic capillaries
2) only 7% (sometimes written ~10%) of CO2 remains dissolved in plasma
3) nearly 25% (sometimes written ~30%) of CO2 binds to hemoglobin forming carbaminohemoglobin
4) 70% of CO2 load combines with H20 forming carbonic acid. Enzyme carbonic anhydrase facilitates this in RBC. Carbonic acid dissociates to bicarbonate and H+ ions. Hb buffers H+
5) HCO3- enters plasma in exchange for Cl- entering RBC (chloride shift)
6) at lungs dissolved CO2 diffuses out of the plasma
7) by law of mass action CO2 unbinds from hemoglobin and diffuses out of RBC
8) carbonic acid reaction reversed pulling HCO3- back into the RBC and converting it back to CO2 ( in lungs)
Haldane effect and Bohr effect
By the Haldane effect the removal of O2 from hemoglobin at the tissue cells increases the ability of Hb to bind CO2
Haldane effect promotes CO2 loading
Bohr effect promotes O2 unloading
Acid base balance
Carriage of CO2 generates H+ ions
CO2 production has a profound effect on the acid/base balance in blood
Blood pH calculated by Henderson Hasselbach equation
H+ ions bind Hb so buffered
Some remain free in venous blood so slightly more acidic pH 7.36 than lung blood 7.4
Hb crucial role in maintenance of pH balance
Acid- base balance
Hypoventilation raises PCO2 and H+ ions increasing blood acidity - respiratory acidosis - increase of PCO2 and HCO3- decrease in pH
If respiratory acidosis persists kidneys start to conserve HCO3-
Hyperventilation lowers PCO2 and H+ ions alkalising the blood - respiratory alkalosis - decreased PCO2 and HCO3- increased pH
If respiratory alkalosis persists renal compensation excretes HCO3-
Relationship of pH, PCO2 and HCO3- can be seen on a Davenport plot
