Gas Transport in the Blood

Question 1

explain the effect of partial pressure in gas solubility with Henry’s Law

Answer 1

Henry’s law - the amount of a given gas dissolve in a given type and volume of liquid (e.g. blood) at a constant temperature is;
proportional to he partial pressure of the gas in equilibrium with the liquid

this means if partial pressure in the gas phase is increased, the concentration of the gas in the liquid phase would increase proportionally

Question 2

describe and explain the means of oxygen carriage in the blood

Answer 2

oxygen picked up by blood a the lungs
oxygen is present in blood in 2 forms;
bound to Hb
physically dissolved

3ml oxygen per litre of blood at Po2 of 13.3 kPa (another mechanism must be involved in oxygen transportation)
most oxygen in blood is transported bound to haemoglobin in red blood cells –> 200 ml oxygen per litre of blood (Hb concentration 150g per litre)
==> 98.5% of oxygen bound to Hb

Question 3

draw and explain the O2-Hb dissociation curve and role of partial pressure; and the significance of the sigmoid shape of the curve

Answer 3

refer to PP
curve is sigmoid due to the co-operativity of Hb
flat upper portions means that moderate fall in alveolar Po2 will not much affect oxygen loading
steep lower part means that the peripheral tissues get a lot of oxygen for a small drop in capillary Po2

Question 4

explain oxygen delivery index (DO2I) to the tissues

Answer 4

function of cardiac output and the oxygen content of arterial blood  
DO2I = CaO2 x Cl
CaO2 = oxygen content of arterial blood
CI = cardiac index 

cardiac index relates the cardiac output to the body surface area (i.e. size of individual)
normal range for CI: 2.4-4.2 L/min/metre^2

Question 5

describe oxygen content of arterial blood (CaO2)

Answer 5

determined by the concentration of haemoglobin in the blood and the percentage saturation of haemoglobin with oxygen
CaO2 = 1.34 x [Hb] x SaO2
SaO2 = %Hb saturated with oxygen (determined by PO2)

Question 6

describe how oxygen delivery can be impaired

Answer 6

decreased partial pressure of inspired oxygen
respiratory disease - decrease arterial PO2 ad hence decrease Hb saturation with oxygen and oxygen content of the blood
heart failure - decreases cardiac output
anaemia - decreases Hb concentration and hence decreases oxygen content of the blood

Question 7

describe the Bohr effect and its significance in the liberation of oxygen from Hb at tissue level

Answer 7

Bohr Effect - a shift of the curve t the right

e.g. increased release of oxygen by conditions at the tissues (competitive inhibitor)

Question 8

know that foetal haemoglobin (HbF) differs from adult haemoglobin (HbA) in structure and affinity for oxygen binding

Answer 8

HbF has 2 alpha and 2 gamma subunits
HbF interacts less with 2,3 - Biphosphoglycerate in red blood cells
HbF has a higher affinity for oxygen compared to HbA (O2-Hb dissociation curve for HbF is shifted to the left compared to HbA)
this allows oxygen to transfer from mother to foetus even if the PO2 is low
HbF is usually replaced with HbA within a few months after birth

Question 9

explain the difference between oxygen-Hb and oxygen-myoglobin dissociation curves and the significance of the hyperbolic shape of the oxygen-myoglobin dissociation curve

Answer 9

Mb is present in skeletal and cardiac muscle (not blood, if present in blood this indicates muscle damage)
Mb - one haem group per molecule, no cooperative binding of oxygen
the dissociation curve of Mb is hyperbolic
Mb releases oxygen at a very low PO2
this provides a short term storage of oxygen for anaerobic conditions