Blood Gas Transport

Question 1

How are blood gases transported around the body?

Answer 1

Gases carried in the blood, first dissolve in the plasma before mostly being transported in other forms

Question 2

Describe the route O₂ takes from lungs to tissues

Answer 2

O₂  exchange at the lung 
↓
O₂ dissolves in plasma 
↓
O₂ binds to Hb 
↓
Redissolves in plasma 
↓
O₂ diffuses into tissues

Question 3

Describe the transport of CO₂ from tissues to lungs

Answer 3

CO₂ produced by tissues
↓
CO₂ dissolves in plasma
↓
CO₂ transported as HCO₃⁻ or bound to Hb
↓
CO₂ redissolves in plasma 
↓
CO₂ exchanged at the lungs

Question 4

Describe how much % CO₂ is transported out the body in the different ways

Answer 4

HCO₃⁻ = 70%
HbCO₂ = 23%
Dissolved CO₂ = 7%

Question 5

Describe the % O₂ transported out the body bound to Hb and dissolved?

Answer 5

HbO₂ = 98%

Dissolved O₂ = 2%

Question 6

What is the significance of Hb?

Answer 6

• Hb is critical for O₂ transport
• Hb presence overcomes need for high alveolar PO₂
• Vast majority of O₂ transported by blood is bound to Hb
  >98%

Question 7

Why is Hb critical for O₂ transport?

Answer 7

O₂ has low solubility in plasma (0.225 ml/L/kPa)
in order to dissolve a sufficient amount of O₂ needed to supply tissues, an impossibly high alveolar PO₂ would be required
Binding to Hb enables O₂ to be concentrated in blood
(increases carrying capacity) at gas exchange surfaces and is released at respiring tissues

Question 8

How is the oxygen content of blood defined?

Answer 8

• O₂ partial pressure (PaO₂) is expressed as kPa
• Total O₂ content (CaO₂) is expressed as ml of O₂ per L of
  blood (ml/L)
• O₂ saturation (SaO₂) and SpO₂ (estimated by pulse
  oximetry) is expressed a s a%

Question 9

What does O₂ partial pressure mean?

Answer 9

This is the partial pressure of O₂ within a gas phase (at a gas-liquid interface) that would yield this much O₂ in plasma at equilibrium.

Question 10

Define what is meant by the total O₂ content

Answer 10

Volume of O₂ being carried in each litre of blood, including O₂ dissolved in plasma and bound to Hb

Question 11

What is the % O₂ saturation of blood?

Answer 11

% of total available Hb binding sites occupied by O₂

Question 12

What is the role of the oxygen-haemoglobin dissociation curve?

Answer 12

The relationship between [O₂], partial pressure, and saturation is shown by the Oxygen Dissociation Curve

The curve shifts to offload O₂ to demanding tissues

Question 13

Why is Hb so effective at transporting O₂ around the body?

Answer 13

The structure of Hb produces high O₂ affinity
∴ a high level of Hb-O₂ binding (and saturation) is
achieved at a relatively low PaO₂

[heme] group and Hb contained in RBCs enables a high carrying capacity

Question 14

What is the O₂ carrying capacity?

Answer 14

O₂ carrying capacity total = 200 ml/L

• 3 ml/L = Plasma
• 197 ml/L = Hb

Question 15

How does Hb-O₂ affinity change?

Answer 15

Hb O₂ affinity changes depending on the local environment, enabling O₂ delivery to be coupled to demand
Also enables decreased PvO₂ and more oxygen delivery per unit pf blood

Question 16

Describe the PO₂ and O₂ saturation in the lungs

Answer 16

In lungs:

• High PO₂
• Low PCO₂ = high pH
• High O₂ saturation

Question 17

Describe the PO₂ and O₂ saturation in resting tissue

Answer 17

In resting tissue:

• low PO₂
• low O₂ saturation
• O₂ moves from Hb to tissue

Question 18

What is the PO₂ and O₂ saturation in working tissue?

Answer 18

• v. low PO₂
• Anaerobic resp. and hypoxia produce H+, lactate, CO₂
  and 2,3-DPG, increasing O₂ demand
• high CO₂ = low pH and 2,3-DPG
• low Hb-O₂ affinity and binding
• low O₂ saturation
• Increased O₂ moving form Hb to tissues

Question 19

What is the role of Myoglobin?

Answer 19

Myoglobin acts as an O₂ reservoir

only releases O₂ at low PO₂

Question 20

What is the difference between foetal and adult Hb-O₂ affinity?

Answer 20

Foetal Hb has a higher O₂ affinity and effectively steals O₂ from maternal Hb

Question 21

What colour does oxyhaemoglobin appear?

Answer 21

Hb-O₂ oxyhaemoglobin appears red

Question 22

What is the colour of deoxyhaemoglobin?

Answer 22

Hb deoxyhaemoglobin appears blue

Question 23

What is Cyanosis and why does it occur?

Answer 23

Purple discolouration of the skin and tissue, occurring when [deoxyhaemoglobin] becomes excessive

Question 24

Describe the features of Central cyanosis

Answer 24

• bluish discolouration of core, mucus membranes and
  extremities
• Inadequate oxygenation of blood
• e.g. hypoventilation, V/Q mismatch

Question 25

Describe the pathological features of peripheral cyanosis

Answer 25

- Bluish colouration confined to extremities (e.g. fingers) - Inadequate O₂ supply to extremities - e.g. small vessel circulation issues

Question 26

Why does insufficient Hb anaemia (hypoxia) occur?

Answer 26

Hypoxia can occur despite adequate ventilation and perfusion, if blood isn't able to carry sufficient O₂ to meet tissue demands

Question 27

What are the causes of insufficient RBCs/Hb anaemia?

Answer 27

``` Iron deficiency (decreased production) Haemorrhage (increased loss) ```

Question 28

Why is more CO₂ dissolved in plasma?

Answer 28

CO₂ has a higher H₂O solubility than O₂ ∴ a greater % of CO₂ is transported dissolved in plasma (7% compared to 1% of O₂)

Question 29

How can you calculate the [CO₂] dissolved in plasma?

Answer 29

concentration = partial pressure x solubility

Question 30

Why is less CO₂ transported bound to Hb?

Answer 30

CO₂ has different (lower affinity) Hb binding sites to O₂ CO₂ binds to R-NH₂ residues at the end of peptide chains, forming carbamino-Hb (R-NHCOOH) - lower % of CO₂ is transported like this (23%)

Question 31

How is the majority of CO₂ transported?

Answer 31

CO₂ reacts with water to form carbonic acid, which accounts for the majority (70%) of CO₂ transport: CO₂ + H₂O ⇋ H₂CO₃ ⇋ H⁺ + HCO₃⁻

Question 32

What is the haldane effect?

Answer 32

Principle that venous blood carries more CO₂ than arterial blood

Question 33

Explain how CO₂ is transported to tissues

Answer 33

1. CO₂ produced by respiring cells, dissolves in plasma to enter RBCs 2. CO₂ + H₂O ⇋ H₂CO₃ occurs within RBCs catalysed by carbonic anhydrase 3. Effective removal of CO₂ via H₂CO₃ production, enabling more CO₂ to diffuse into RBCs 4. H₂CO₃ ⇋ H⁺ + HCO₃⁻ occurs. RBC membrane is impermeable to H⁺, so H⁺ can't leave 5. Accumulation of H⁺ in cell so cessation of H₂CO₃ production is prevented by deoxy-Hb acting as a buffer and binding H⁺, causes movement of O₂ into tissues from RBCs Increased [deoxy-Hb] enables more CO₂ to be transported

Question 34

How is CO₂ transported to the lungs?

Answer 34

1. Low PaCO₂ creates gradient; CO₂ diffuses out of blood -> airspace 2. Increased PaO₂ leads to Hb-O₂ binding. Hb-O₂ binds less H⁺ than deoxy-Hb, increasing free [H⁺] 3. Increased free [H⁺], leads to increased H₂CO₃ and ultimately CO₂ which contributes to [CO₂] plasma 4. The changing equilibrium of the carbonic acid rxn -> decreased [HCO₃⁻] as it binds to free H⁺, creating a diffusion gradient, allowing HCO₃⁻ to enter RBCs in exchange fro Cl⁻ Net result: Haldane effect Deoxygenated blood carries more CO₂ Oxygenated of blood causes CO₂ removal