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Respiratory Physiology > O2 Transport > Flashcards

O2 Transport Flashcards

1
Q

How is O₂ transported in the blood?

A

In one of 2 forms:
Bound to Hb:
◆ Accounts for 98% of the O₂ carried in blood.
◆ Each gram of fully satured Hb can bind 1.34 ml of O₂ (Hufner's constant).

Dissolved in plasma:
◆ Accounts for 2% of the O₂ carried in blood.
◆ The volume of O₂ dissolved in blood, is proportional to the PO₂ (Henry's law).

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2
Q

Describe what is Hufner's constant.

A

Each gram of fully satured Hb, can bind to 1.34 ml of O₂.

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3
Q

Describe what is Henry's law.

A

The volume of dissolved O₂ in blood, is proportional to the partial pressure of O₂ (PO₂).

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4
Q

How to calculate the total volume of O₂ carried by the blood?

A

◉ It is the sum of Hufners constant with Henry’s law.

➜ O₂ content equation.

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5
Q

Describe the O₂ content equation.

A

O₂ content per 100 ml of blood = [(1,34 x Hb x ❨SPO₂/100%❩) + (0.023 x PO₂)]

◆ 0.023 solubility coefficient for O₂.

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6
Q

Define Fick’s law of diffusion.

A

Diffusion occurs following a pressure gradient.

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7
Q

Explain Fick's law and the importance of the O₂ dissolved in plasma.

A

◆ The partial pressure of O₂ in the blood (PO₂) is measured from the dissolved amount in plasma.
◆ The PO₂ within the RBC is low because all the O₂ is bound to Hb.

➔ Relation to Fick’s law:
◆ O₂ diffuses to the tissues from the dissolved amount in the plasma (not from Hb).
◆ Plasma O₂ [.] ⬇︎
◆ O₂ dissociates from Hb to replenish the plasma.

Hb is a much more efficient means of O₂ carriage than O₂ dissolved in plasma though.

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8
Q

How is O₂ stored in the blood?

A

➜ Very little O₂ is stored in the blood (which means apnoea is quickly followed by hypoxia).

➔ Storage:
◆ In the lungs as FRC.
◆ In the blood (bound and dissolved).
◆ In the muscles bound to myoglobin.

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9
Q

What is the consumption of O₂ at rest in an adult?

A

250 ml/min.

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10
Q

Describe the structure of RBC’s.

A

◆ Small
◆ Biconcave discs
◆ Able to change form to fit through small capillaries.
◆ No nucleus
◆ Cytopalsm without mitochondria (aerobic metabolism not possible).

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11
Q

What is Haemoglobin?

A

◆ It is a large protein that contains iron (Fe).
◆ It is contained within the RBC’s.

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12
Q

Describe the adult HbA.

A

➜ HbA:
◆ Accounts for 98% of adult Hb.

➜ It has a quaternary structure compromising of 4 (polypeptide) globin subunits:
◆ 2 𝝰 chains
◆ 2 β chains

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13
Q

Describe the globin subunits chains

Adult HbA: 2 𝝰 chains & 2 β chains.

A

◆ The 4 globin chains are held together by weak electrostatic forces.
◆ Each globin chain cointains iron in the ferrous state (Fe²⁺).

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14
Q

How is O₂ bound to the haem group?

A

◆ O₂ is reversibly bound to the Fe²⁺ ion in the haem group.
◆ 4 O₂ molecules can bind to Hb (one for each haem group).

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15
Q

Describe cooperative binding of Hb.

A

It’s the increase in O₂ affinity to Hb, with each successive binding.

First step
* The first O₂ molecules bind with relative difficulty.
* Hb is on a tense conformation due to the β-chains being far apart.
* Strong electrostatic charges must be overcome to achieve the required conformational change.

2nd step
* After the 1st Hb is bound, the conformation of the β-chains and Hb come close together.
* This allow a 2nd O₂ molecule to have a higher binding affinity to Hb.

3rd step
* After the 2nd O₂ molecule has bound the 3rd is easier to bind, and subsequently the 4th.

4th step
* Once the 4th O₂ binds, the Hb protein achieves its relaxed conformation.

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16
Q

Define what is oxyhaemoglobin and deoxhymoglobin.

A

Oxyhaemoglobin
• Fully saturated Hb.

Deoxhymoglobin
• Fully desaturated Hb.

17
Q

Define what is the Oxyhaemoglobin dissociation curve.

A
  • Describes the relationship between SaO₂ (arterial O₂ saturation by arterial blood gas) and PaO₂.
  • The sigmoid shape of the graph is due to cooperative binding of the 4 O₂ molecules and Hb.
18
Q

Draw and explain the Oxyhaemoglobin dissociation curve

A

X axis
* PaO₂

Y axis
* Percentage of oxyhaemoglobin (measured by SaO₂)

There 3 main points in the graph:
* Arterial point
* Venous point
* P50

Arterial point
* 97 to 100% of Hb is saturated at PO₂ of 13.3 kPa.

Venous point
* Hb is 75% at 5.3 kPa

P50
* The partial pressure of O₂ (PO₂) at which Hb is 50% saturated (bound to O₂).
* Usually at PO₂ of 3.5 kPa.
* It is the reference point to determine left or right shift.

19
Q

What is myoglobin?

A
  • It is a large protein molecule, containing iron and capable of binding to O₂ (like Hg).
  • It stores O₂ in the skeletal muscles (O₂ demand is high).

Unlike Hb, it contain only:
* 1 haem group
* 1 globin chain
* There is no cooperative binding

20
Q

Describe the oxymyoglobin dissociation curve.

A
  • It’s a hyperbolic graph positioned to the left of the oxyhemoglobin curve.
  • The P50 of myoglobin is much lower than Hb.
21
Q

Describe the right shift of the oxyhaemoglobin curve.

A

➔ Hb has a lower affinity to O₂ ⟶ easier offload of O₂.

Causes:
◆ ⬆︎ PCO₂
◆ Acidosis
◆ Hyperthermia (↑Tº)
◆ ⬆︎ 2,3-diphosphoglycerate (2,3-DPG)
◆ Exercise
◆ Pregnancy
◆ Altitude
◆ Sickled cell haemoglobin (HbS)

22
Q

What is the clinical importance of the right shift of the OxyHb curve?

A

Due to the following physiological mechanisms:

◆ Bohr effect
◆ Anaerobic metabolism
◆ O₂ loading in the lungs

23
Q

Explain what is the Bohr effect.

Right shift of OxyHb curve

A

◉ Bohr effect
◆ Tissues that are metabolically active produce: CO₂, H⁺ and heat.
◆ When blood arrives to these capillaries, the oxyhaemoglobin curve shifts to the right ⟶ unloading O₂ where is needed the most.

24
Q

Explain the Anaerobic metabolism.

Right shift of OxyHb curve

A

◉ Anaerobic metabolism
◆ When PO₂ falls below a certain value anaerobic metabolism ensues.
◆ Glucose breaksdown into pyruvate and produces in the process 2ATP (glycolysis).
◆ The pyruvate further breaksdown producing lactate.
◆ One of the intermediates of the glycolytic pathway is converted to 2,3-DPG in a side pathway.
◆ Hence ⬆︎ anaerobic metabolism ⟶ ⬆︎ 2,3-DPG.
◆ 2,3-DPG binds specifically to β-chains of deoxyhaemoglobin ⬇︎ the Hb affinity to O₂.
◆ This means that additional O₂ is offloaded to cells undergoing anaerobic metabolism.

25
Q

Describe the left shift of the OxyHb curve.

A

➔ Hb has a higher affinity to O₂ ⟶ easier binding of O₂.

Causes:
◆ ⬇︎ PCO₂
◆ Alkalosis
◆ Hyporthermia (↑Tº)
◆ ⬇︎ 2,3-diphosphoglycerate (2,3-DPG)
◆ Foetal Hb (P50 is 2.5) (HbF)
◆ Carbon monoxide (carboxyhaemoglobin COHb)
◆ Methaemoglobin (iron in haem group is in Fe³⁺ ferric state and cannot bind to O₂).

26
Q

Explain the clininal relevance of the left shift of the OxyHb curve in foetal life.

A

◆ HbF has to be able to extract O₂ from maternal OxyHb.
◆ HbF must then have a higher affinity to O₂ than maternal Hb.

This is caused by:
➔ HbF causes a left shift in the OxyHb curve ⟶ ⬆︎ affinity to O₂.
➔ HbF is contains 2𝝰 and 2ɣ globin subunits.
➔ 2,3-DPG although present in foetal RBC’s It cannot bind to HbF due to lack of β-chains ⟶ This further ↑ the binding affinity of HbF for O₂.

27
Q

What is the clinical relevance of 2,3-DPG in blood transfusion?

A

◆ In stored blood, the erytrocytes 2,3-DPG [.] rapidly ⬇︎ to zero after 1-2 weeks.
◆ OxyHb curve will shift to the left ⬆︎ the affinity to O₂.
◆ When stored blood is transfused, it takes up to 24h for erytrocyte 2,3-DPG [.] to ⬆︎.
◆ Transfused blood is not as effective at offloading O₂ to the tissues as native blood cells are.
◆ Cell-salvaged blood maintains most of its 2,3-DPG which makes it a better option.

28
Q

Describe the classification of different types of Hb.

A

◉ Physiological
◆ HbA
◆ HbA₂
◆ HbF

◉ Pathological
◆ Thalassemia
◆ HbS
◆ MetHb
◆ COHb

29
Q

Describe the HbA₂.

A

◆ It accounts for 2% of the adult Hb.
◆ It has 2𝝰 and 2δ globin subunits.

Normal variant

30
Q

Describe the HbF.

A

◆ Normal variant during foetal life.
◆ It has 2𝝰 and 2ɣ globin subunits.
◆ It has a higher affinity to O₂ than HbA.
◆ HbF is produced up to 3mths.
◆ By 6 mths the all HbF is replaced by HbA.

Normal variant

31
Q

Describe Thalassemia Hb.

A

◆ Inherited autosomal recessive blood disorder.
◆ ⬇︎ synthesis of one of the globin chains (𝝰 or β).
◆ The ensuing anaemia can be 𝝰 or β depending on which globin chain is being underproduced.

32
Q

Describe the HbS (sickle cell anaemia).

A

◆ Inherited autosomal recessive blood disorder.
◆ There is an abnormal β globin subunit (due to a genetic mutation in the amino acid sequence where the amino acid valine is replaced
by glutamic acid)
◆ In low PaO₂, the homozygous state forms sickle cells causing the erythrocytes to obstruct the microcirculation, leading to painful crises and infarcts.

33
Q

Describe the MetHb.

A

◆ Methaemoglobinaemia is where the ferrous iron (Fe²⁺) within the Hb molecule is oxidised to ferric iron (Fe³⁺).
◆ Fe3+ cannot bind O₂, so MetHb cannot participate in O₂ transport.

34
Q

Describe the COHb.

A

◆ Formed when Hb binds to inhaled carbon monoxide.

Respiratory Physiology (3 decks)

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