O2 Transport

Question 1

How is O₂ transported in the blood?

Answer 1

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).

Question 2

Describe what is Hufner's constant.

Answer 2

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

Question 3

Describe what is Henry's law.

Answer 3

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

Question 4

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

Answer 4

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

➜ O₂ content equation.

Question 5

Describe the O₂ content equation.

Answer 5

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

◆ 0.023 solubility coefficient for O₂.

Question 6

Define Fick’s law of diffusion.

Answer 6

Diffusion occurs following a pressure gradient.

Question 7

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

Answer 7

◆ The partial pressure of O₂ in the blood (PO₂) is measured from the dissolved amount in plasma.

➔ 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.

Question 8

How is O₂ stored in the blood?

Answer 8

➜ 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.

Question 9

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

Answer 9

250 ml/min.

Question 10

Describe the structure of RBC’s.

Answer 10

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

Question 11

What is Haemoglobin?

Answer 11

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

Question 12

Describe the adult HbA.

Answer 12

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

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

Question 13

Describe the globin subunits chains

Adult HbA: 2 𝝰 chains & 2 β chains.

Answer 13

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

Question 14

How is O₂ bound to the haem group?

Answer 14

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

Question 15

Describe cooperative binding of Hb.

Answer 15

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.

Question 16

Define what is oxyhaemoglobin and deoxhymoglobin.

Answer 16

Oxyhaemoglobin
• Fully saturated Hb.

Deoxhymoglobin
• Fully desaturated Hb.

Question 17

Define what is the Oxyhaemoglobin dissociation curve.

Answer 17

• 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.

Question 18

Draw and explain the Oxyhaemoglobin dissociation curve

Answer 18

X axis
◆ PaO₂

Y axis
◆ Percentage of oxyHb (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 (99.7 mmHg).

Venous point
◆ Hb is 75% at 5.3 kPa (39.7 mmHg)

P50
◆ The partial pressure of O₂ (PO₂) at which Hb is 50% saturated (bound to O₂).
◆ Usually at PO₂ of 3.5 kPa (26 mmHg).
◆ It is the reference point to determine left or right shift.

Question 19

What is myoglobin?

Answer 19

► 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

Question 20

Describe the oxyMYoglobin dissociation curve.

Answer 20

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

Question 21

Describe the right shift of the oxyhaemoglobin curve.

Answer 21

➔ It causes Hb to have 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)

Question 22

Give examples of the physiological importance of the right shift of the OxyHb curve?

Answer 22

HDue to the following physiological mechanisms:

Bohr effect:
◆ When blood arrives at the capillaries, the oxyHb dissociation curve is shifted to the right, offloading O₂ where it is most needed.

Anaerobic metabolism:
◆ When cellular PO₂ ↓ below a threshold value, anaerobic metabolism predominates.
◆ Energy is produced through the breakdown of glucose to pyruvate (in a process called glycolysis);
◆ Which is then converted to lactate.
◆ 2,3-DPG is also produced.
◆ ↑ anaerobic metabolism ➝⬆︎ 2,3-DPG concentration.
◆ 2,3-DPG binds specifically to the β-chains of deoxyHb, stabilising
this configuration ➝ thus reducing the O₂-binding affinity of Hb.
◆ This mechanism means that additional O₂ is offloaded to cells undergoing anaerobic metabolism.

O₂ loading in the lungs:
◆ When blood reaches the lungs, CO₂ is exhaled and the pH normalises.
◆ The P50 of the oxyHb dissociation curve then returns to its central position.
◆ The binding affinity of O₂ therefore increases;
◆ Dissolved O₂ binds to Hb, which in turn lowers the blood O₂ tension, facilitating O₂ diffusion across the alveolar–capillary barrier.

Question 23

Explain what is the Bohr effect.

Right shift of OxyHb curve

Answer 23

◉ 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.
◆ CO₂ binding to Hb reduces its affinity for O₂ and causes a right shift of the O₂-Hb dissociation curve.

Question 24

Explain the Anaerobic metabolism.

Right shift of OxyHb curve

Answer 24

◉ 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.

Question 25

Describe the **`left shift`** of the **OxyHb curve**.

Answer 25

➔ **Hb has a higher affinity to O₂ ⟶ easier binding of O₂.** *`Causes`*: ◆ ⬇︎ PCO₂ ◆ Alkalosis ◆ Hypothermia (⬇︎ 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)*.

Question 26

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

Answer 26

◆ 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.

Question 27

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

Answer 27

◆ 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.

Question 28

Describe the **classification** of different types of `Hb`.

Answer 28

◉ **Physiological** ◆ HbA ◆ HbA₂ ◆ HbF ◉ **Pathological** ◆ Thalassemia ◆ HbS ◆ MetHb ◆ COHb

Question 29

Describe the `HbA₂`.

Answer 29

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

Question 30

Describe the `HbF`.

Answer 30

◆ 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

Question 31

Describe `Thalassemia Hb`.

Answer 31

◆ 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.

Question 32

Describe the `HbS` (sickle cell anaemia).

Answer 32

◆ 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.

Question 33

Describe the `MetHb`.

Answer 33

◆ 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.

Question 34

Describe the `COHb`.

Answer 34

◆ Formed when Hb binds to inhaled carbon monoxide.

Question 35

Describe the **Hb-binding affinity** to `carbon monoxide`.

Answer 35

➔ The Hb-binding affinity to carbon monoxide is **`250 times greater`** than that of O2. ➔ In the presence of carbon monoxide, `Hb preferentially forms COHb rather than oxyhaemoglobin`, **resulting in a reduced O2-carrying capacity**.

Question 36

Describe what happens to the **hemoglobin dissociation curve** in the presence of `COHb`.

Answer 36

**There's a:** ◆ a leftward shift of the oxyhaemoglobin dissociation curve. **That causes a:** ◆ reduction in the offloading of O2 to the tissues. **Resulting in:** ◆ exacerbation of tissue hypoxia.

Question 37

What are the **clinical features / symptoms** of `COHb`?

Answer 37

* 15–20% causes mild symptoms – headache and confusion. * At higher concentrations – weakness, dizziness, nausea and vomiting. * At COHb >60% – convulsions, coma and death.

Question 38

Describe the treatment for carbon monoxide intoxication.

Answer 38

◆ High FiO2 OR ◆ Hyperbaric chamber

Question 39

Describe the MOA of cyanide intoxication.

Answer 39

► **Reduced O2 carrying capacity:** ◆ Cyanide binds irreversibly to the O₂ binding site of the Hb Fe₂⁺ ion. ◆ Resulting in a functioning anaemia that can’t be treated with O₂. ► **Inhibition of the electron transport chain:** ◆ The main toxic effect of cyanide is inhibition of cytochrome c oxidase (Complex IV) of the mitochondrial electron transport chain. ◆ The mitochondria are unable to make use of the O₂ that reaches them.

Question 40

What are the **clinical signs** of *cyanide intoxication*?

Answer 40

◆ `Bright red colour of venous blood`, where the blood has passed through the tissue capillary network without offloading O₂. ***In other words:*** ◆ `Mixed venous Hb-O₂ saturation` is ⬆︎, with a lactic acidosis resulting from anaerobic metabolism.

Question 41

Causes of cyanide poisoning?

Answer 41

* **Following inhalation of smoke from burning nylon materials in house fires**. * Following administration of sodium nitroprusside.

Question 42

Management of **cyanide intoxication**?

Answer 42

◆ Supportive measures; ◆ Hydroxocobalamin (vitamin B12) in high doses; ◆ The cobalt cation of hydroxocobalamin binds cyanide ions; ◆ Forming cyanocobalamin (vitamin B12), which is non-toxic and renally excreted; ◆ **Hydroxocobalamin can enter the mitochondria**, where `it irreversibly binds cyanide ions`, thus restoring oxidative metabolism. ## Footnote **Hydroxocobalamin can enter the mitochondria**, where `it irreversibly binds cyanide ions` forming a non-toxic and renally excreted compound, thus restoring oxidative metabolism / aerobic metabolism.

Question 43

Describe the **O₂-Hb dissociation curve** in `CO intoxication`.

Answer 43

◆ It is shifted to the left; ◆ P50 will be reduced causing increased affinity for the few HgB left; ◆ Difficult to unload O₂ to the tissues.