Gas transport Lecture 5

Question 1

How is the majority of oxygen transported in the blood, and why?

Answer 1

Gas in solution - amount of gas dissolved in solution is proportional to its partial pressure

Amount dissolved = Partial pressure x Solubility coefficeint

Haemoglobin- 4 haem moieties - 4 binding sites per molecule

2 alpha and 2 beta globulin chains

Alpha and beta globin chains will also bind CO2 and H on histadines residues

Beta chains also bind 2,3-DPG (diphosphoglycerate)

Carried discretely in RBCs

Haemoglobin carries both O2 and CO2 and buffers pH

Isolated from plasma:

Plasma would have a much higher viscosity (decreased viscosity in RBC due to less work by heart to pump around)

If haemogloblin was free in plasma, it Would be more open to degradation by enzymes and excretion by the kidneys

If there wass higher PO2 in alvelolar phase, there would be higher PO2 in RBC which will shift equilbrium to make more O2 bind to Hb.

Question 2

Describe oxygen transport

Answer 2

Saturation (%)= O2 bound x 100/ O2 capacity

Graph shows that as you increase PO2, % saturation increases

Sigmoid all shape of graph represents the cooperative binding of oxygen to haemoglobin due to allosteric changes. When oxygen binds it changes the shape of the molecule.

1st and 4th oxygen is difficult to bind

2nd and 3rd bind with ease

P50 is the partial pressure needed to achieve saturation ‘Plateau’ indicates that the lungs could almost half their partial pressure and still saturate the blood

Supplying oxygen to tissues ‘deoxygenates’ the blood but it is still ~75% saturated

Question 3

What can affect O2 transport?

Answer 3

The Bohr effect

Decreased affinity (right shift) results from an increase in: PCO2, [H+], 2,3-DPG, Temperature

All of these are found in metabolising tissues- help off-load oxygen from blood in to the tissue

Acidosis decreases the affinity of Hb for O2

Question 4

What does an increase in lung PO2 do to oxygen content in blood?

Answer 4

Haemoglobin remains saturated- 100% The amount dissolved in the plasma increases do there is a slight incline to the ‘plateau’ as PO2 increases

Question 5

What is the O2 capacity of the blood?

Answer 5

O2 capacity is the capacity of Hb to bind O2

Capacity= [Hb]x 1.39ml O2/g of Hb

=145g/L x 1.39= 202mlO2/L of blood In equilibrium with the 2.4ml dissolved in the plasma

Content= Hb(A-V)+P(A-V)O2= 242+8= 250mlO2/min

O2 content = Hb bound + dissolved

Oxygen consumption = the rate at which oxygen is utilised by the periphery - the difference between the oxyge ncontent in arterial blood and in mixed venous blood in the pulmonary artery.

Question 6

How is CO2 carried in the blood?

Answer 6

4% dissolved on the plasma

1% reacts with amino groups in proteins to form R-NHCOO- +H

5% dissolved in the cytoplasm of the RBCs

5% bound as deoxy-Hb-NHCOO-

Carbonic anhydrase in the RBCs forms H2CO2, from CO2 and H2O, which dissociates to form HCO3- and H- most of this is transported into the plasma coupled with the chloride shift to maintain neutrality

This reaction also happens spontaneously and slowly in the plasma 20% in RBCs 65% in the plasma

Question 7

What is that Haldane effect?

Answer 7

Initial rapid increase in CO2 as PCO2 increases

Still large changes seen in CO2 from arterioles to veins

CO2 carriage is non-saturable

Hypoxia increases Hb affinity for CO2 moving the curve up

When H+ is gone to the deoxygenated Hb, we can pump out the HCO3- using the chloride sift and the reaction can continue to go on .

In the lungs you get rid of O2, you will liberate H+, and Co2 will be generated. O2 will bind to dexoygengated Hb which has a low affininty for CO2