Gas Transport Flashcards

1
Q

Learning outcomes

A
  • List the ways by which oxygen is carried in the blood
  • Recognise what proportion of oxygen is carried in each form
  • Describe the oxygen-haemoglobin dissociation curve
  • Explain the physiological significance of the shape of the curve
  • Describe the factors that cause the curve to shift to the right or to the left
  • Calculate how much oxygen is carried in the blood
  • List the ways by which carbon dioxide is carried in the blood
  • Recognise what proportion of carbon dioxide is carried in each form
  • Describe the role of the red blood cells in carbon dioxide carriage
  • Describe how carbon dioxide is converted to bicarbonate
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2
Q

What are the primary functions of the resp and cardiovascular systems

A
  • One of the primary functions of the cardiovascular systems is to transport O2 from the lungs to all tissues in the body
  • And CO2 from the tissues to the lungs
  • The lungs expire this CO2 to the atmosphere
  • Both gases move by diffusion down their concentration gradients
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3
Q

Discuss oxygen transport

A

• Oxygen is transported in blood in two ways:
– Physically dissolved in plasma ~2%
– Combined with haemoglobin ~98%

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4
Q

Discuss O2 as it is found in plasma

A
  • Amount of O2 dissolved in plasma depends on its solubility and partial pressure in blood (Recall Henry’s Law)
  • Henry’s law states that at equilibrium for a given temperature

[O2]Dis = solubility O2 x PO2 (O2 sanscript)

  • 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
  • So this mechanism of O2 transport is completely inadequate
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5
Q

Discuss the structure of haemoglobin

A

• 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
• In Fetal haemoglobin (HbF) the β- chains are replaced by γ-chains

• HbS causes sickle cell anaemia
– glutamate at position 6 in the β- globin is
replaced with a valine.

• Each haem group consists of a porphyrin ring surrounding an Fe2+ molecule

• O2 can only be bound 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

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6
Q

Discuss haemoglobin structural changes on oxygenation

A
  • Deoxygenated Hb exists in 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

• β-globins also bind 2,3 DPG

  • 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 porphyrin rings – relaxed state
  • The colour of blood changes from dark blue to bright red
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7
Q

Discuss the haemoglobin oxygen dissociation curve

A
  • 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
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8
Q

Discuss maximal load of O2 in haemoglobin

A
  • Amount of O2/L of blood attached to Hb, at full saturation, is called O2 capacity and depends on the Hb concentration in blood
  • Each g of Hb, when fully saturated carries 1.35ml of O2
  • Maximal O2 bound to Hb can be calculated as:

max 𝑂2 𝑏𝑜𝑢𝑛𝑑 𝐻𝑏 = 𝑂2 𝑐𝑎𝑝𝑎𝑐𝑖𝑡𝑦 ∗ [𝐻𝑏]
max 𝑂2 𝑏𝑜𝑢𝑛𝑑 𝐻𝑏 = 1.35𝑚𝑙/𝑔 ∗ 150𝑔/𝐿
=203ml O2/L blood

• Equates to 235ml O2/min delivery to tissues
• % 𝑠𝑎𝑡𝑟𝑛 𝐻𝑏
=
((𝑂2 𝑎𝑐𝑡𝑢𝑎𝑙𝑙𝑦 𝑏𝑜𝑢𝑛𝑑 𝑡𝑜 𝐻𝑏) OVER 𝑂2 𝑐𝑎𝑝𝑎𝑐𝑖𝑡𝑦 𝑜𝑓 𝐻𝑏) ∗ 100

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9
Q

Discuss myoglobin and foetal haemoglobin in comparison to adult haemoglobin interactions

A
  • MyHb and HbF shift the curve to the left
  • HbF Consists of 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
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10
Q

Discuss factors that affect haemoglobin affinity for O2

A
  • CO2, H+ and 2,3 DPG affects the affinity of Hb for O2
  • Left shift – high affinity
  • Right shift – low affinity
  • CO2, H+ and 2,3 DPG affect the globins
  • In systemic capillaries increases in CO2, temperature and decrease in pH, move Hb to low affinity tensed state, so more O2 released (right shift)

• In pulmonary capillaries 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)

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11
Q

Discuss the Bohr effect (or shift)

A

• 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

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12
Q

Effects of temperature and pH on the 02-Hb dissociation curve

A

Temperature affects the O2 capacity of Hb, by affecting Hb structure
• Changes in pH account for most of Bohr effect
• Metabolic acidosis

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

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13
Q

Discuss the effects of hypercapnia on the 02-Hb dissociation curve

A
  • Small portion of the Bohr effect
  • Pco2 increase – CO2 combines with unprotonated amino group on Hb – carbamino groups
  • Carbamino haemoglobin
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14
Q

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

A

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

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15
Q

Affect of carbon monoxide (CO) on Hb affinity for 02

A
  • 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
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16
Q

Discuss CO2 blood transport

A
  • Metabolism generates 200 ml CO2/min at rest
  • Solubility of CO2 in plasma is 20 times that of O2
  • CO2 is transported in blood in two main ways:
  • 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
17
Q

Discuss CO2 transport in blood from tissues

A
  1. CO2 enters RBC and is converted to HCO3-
  2. HCO3- is carried in plasma after being exchanged for Cl-
  3. H+ binds to haemoglobin and enhances 02 release (reduced affinity)

4 Protonated haemoglobin becomes substrate for carbamino formation

Carbon in the plasma is thus transported in 3 forms:

  • HCO3- (majority – 70%)
  • CO2 dissolved in plasma (10%)
  • Carbaminohaemoglobin (20%)
18
Q

discuss CO2 release from blood in lungs

A
  • 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
19
Q

Discuss CO2 dissociation curves

A

• CO2 dissociation curves demonstrate how changes in PCO2 affect total CO2 blood content

• Carriage of CO2 in blood depends on:
– PCO2
– plasma pH
– PO2

• Near linear relationship between PCO2 and PO2 in physiological range

• Upshift of curve with decreasing PO2
– Haldane effect (deoxygenated haemoglobin increasing capacity for CO2 transport)

  • 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
20
Q

Summary

A

• O2 consumption in adults 250 ml/min, rising to 4L/min in heavy exercise
• High PO2 in lungs facilitates O2 binding to Hb (left shift in dissociation
curve due to ↓PCO2, ↓ Temp, ↑ pH)
• Low PO2 in the tissues encourages O2 release (right shift in dissociation curve - Bohr shift - due to ↑PCO2, ↑ Temp, ↓ pH)
• CO carried predominantly as HCO - in red blood cells