Gas transport

Question 1

• Oxygen is transported in blood in two ways:

Answer 1

– Physically dissolved in plasma ~2%

– Combined with haemoglobin ~98%

Question 2

Amount of O2 dissolved in plasma depends on

Answer 2

its solubility and partial pressure in blood (Recall Henry’s Law)

Question 3

what happens to o2 at body temp in regards to plasma

Answer 3

• At 37oC the solubility of O2 in plasma is poor - only 0.03ml/L/mmHg
• Partial pressure of O2 in arterial blood is ~100 mmHg
• Therefore only 3ml O2/L of blood can be transported in solution
• Equates to 15ml O2/min delivery to tissues
• BUT our bodies consume 250ml O2/min

Question 4

The Structure of Haemoglobin

Answer 4

• Normal Hb (HbA) is a tetramer
• Four O2-binding heme groups each attached
to a polypeptide (globin) chain
• HbA consists of 2α and 2β chains

Question 5

In Fetal haemoglobin (HbF)

Answer 5

the β- chains are replaced by γ-chains

Question 6

HbS causes sickle cell anaemia

Answer 6

– glutamate at position 6 in the β- globin is

replaced with a valine.

Question 7

Each haem group consists of

Answer 7

a porphyrin ring surrounding an Fe2+ molecule

Question 8

binding of O2

Answer 8

in Fe2+ (ferrous state)
• If iron oxidised to ferric (3+) state leads to
methaemoglobin (~1.5% Hb is in this state)
– methaemoglobin reductase uses the NADPH chain to reduce metHb back to Hb

Question 9

Deoxygenated Hb exists in

Answer 9

a tensed state (T) compared with oxygenated Hb in a relaxed state (R)
• In the tensed state strong ionic bounds form between the 4 polypeptide chains
– immobile and apart

Question 10

• β-globins also bind 2,3 DPG consequence

Answer 10

• The consequence of this is that the Fe lies deeper in the pocket and cannot bind O2
• As O2 binds the bonds break and the Fe moves to the plane of the prophyrin rings – relaxed state
• The colour of blood changes from dark red to bright red

Question 11

Haemoglobin Oxygen Dissociation Curve

Answer 11

• Binding of one O2 molecule makes it easier for the subsequent ones to attach
• Haem-haem interaction – cooperatively. This accounts for the shape of O2-Hb dissociation curve
• The colour change is utilised clinically to measure the O2 saturation of blood using the pulse oximeter

Question 12

O2 capacity and what it depends on

Answer 12

Amount of O2/L of blood attached to Hb, at full saturation and depends on the Hb concentration in blood

Question 13

Each g of Hb, when fully saturated carries

Answer 13

1.35ml of O2

Question 14

• MyHb and HbF shift the Haemoglobin Oxygen Dissociation Curve

Answer 14

to the left

Question 15

• HbF Consists of and its O2 properties

Answer 15

2 α-chains and 2 γ-chains HbF has higher O2 affinity than HbA due to special properties of γ-chains
May take up to 2 years to convert all HbF to HbA

Question 16

Factors that Affect Haemoglobin Affinity for O2

Answer 16

• CO2, H+ and 2,3 DPG affects the affinity of Hb for O2

Question 17

what does a left shift in the Haemoglobin Oxygen Dissociation Curve mean

Answer 17

high affinity

Question 18

what does a right shift in the Haemoglobin Oxygen Dissociation Curve mean

Answer 18

low affinity

Question 19

In systemic capillaries

Answer 19

increases in CO2, temperature and decrease in pH, move Hb to low affinity tensed state, so more O2 released (right shift)

Question 20

In pulmonary capillaries

Answer 20

temperature is lower, Pco2 is lower and pH is higher, moves Hb to higher affinity relaxed state , so more O2 taken up by Hb
(left shift)

Question 21

Bohr Effect (Shift)

Answer 21

• Bohr observed that respiratory acidosis shifted the Hb-O2 dissociation curve to right
• This respiratory acidosis has two components
– Decrease in pH (more acidic)
– Increase in Pco2

Question 22

Temperature

Answer 22

affects the O2 capacity of Hb, by affecting Hb structure

Question 23

Hb

Answer 23

• Hb good buffer for H+, as [H+] increases conformational change in Hb structure and O2 affinity reduces

Question 24

Effects of Hypercapnia on the O2-Hb Dissociation Curve

Answer 24

• Small portion of the Bohr effect
• Pco2 increase – CO2 combines with unprotonated amino group on Hb – carbamino groups
• Carbamino haemoglobin

Question 25

Effects of 2,3-diphosphoglycerate (DPG) on the O2-Hb Dissociation Curve

Answer 25

• RBC do not have mitochondria
– by-product of glycolysis
– decresing PO2 of rbc’s stimulates glycolysis resulting increased levels of 2,3-DPG
• 2,3-DPG interacts with β chains destabilising interaction of O2 with Hb

Question 26

Affect of Carbon Monoxide (CO) on Hb Affinity for O2

Answer 26

• CO, NO and H2S can also bind to Hb and snap it into relaxed state
• CO has a 200 fold greater affinity for Hb than O2
• maximal O2 capacity falls to extent that CO binds
• However, CO also increases O2 affinity of Hb and shifts dissociation curve to left
• Hb does not release O2 when it gets to tissue

Question 27

• CO2 is transported in blood in two main ways:

Answer 27

• In plasma – physically dissolved, combined with plasma proteins and as bicarbonate ions
• In red blood cells – in physical solution, combined with Hb and as bicarbonate ions
• HCO3- (majority – 70%)
• CO2 dissolved in plasma (10%)
• Carbaminohaemoglobin (20%)

Question 28

CO2 Release from Blood in Lungs

Answer 28

• Partial Pressure gradients for O2 and CO2 reverse
• High PO2 causes H+ to dissociate from Hb
• H+ and HCO3- combine to form CO2 and H2O
• HCO3- reenters RBCs and combines with H+ to form H2CO3 which dissociates to release CO2 and H2O

Question 29

CO2 dissociation curves demonstrate

Answer 29

how changes in PCO2 affect total CO2 blood content

Question 30

• Carriage of CO2 in blood depends on:

Answer 30

– PCO2

– plasma pH – PO2

Question 31

CO2 dissociation curves

Answer 31

• Near linear relationship between PCO2 and PO2 in physiological range
• Upshift of curve with decreasing PO2 – Haldane effect
• As blood enters systemic capillaries and release O2 , CO2 carrying capacity rises
• As blood enters pulmonary capillaries and binds O2, CO2 carrying capacity falls and blood dumps CO2