Blood Gas Transport Flashcards
How are blood gases transported around the body?
Gases carried in the blood, first dissolve in the plasma before mostly being transported in other forms
Describe the route O₂ takes from lungs to tissues
O₂ exchange at the lung ↓ O₂ dissolves in plasma ↓ O₂ binds to Hb ↓ Redissolves in plasma ↓ O₂ diffuses into tissues
Describe the transport of CO₂ from tissues to lungs
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
Describe how much % CO₂ is transported out the body in the different ways
HCO₃⁻ = 70%
HbCO₂ = 23%
Dissolved CO₂ = 7%
Describe the % O₂ transported out the body bound to Hb and dissolved?
HbO₂ = 98%
Dissolved O₂ = 2%
What is the significance of Hb?
- 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%
Why is Hb critical for O₂ transport?
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
How is the oxygen content of blood defined?
- 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%
What does O₂ partial pressure mean?
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.
Define what is meant by the total O₂ content
Volume of O₂ being carried in each litre of blood, including O₂ dissolved in plasma and bound to Hb
What is the % O₂ saturation of blood?
% of total available Hb binding sites occupied by O₂
What is the role of the oxygen-haemoglobin dissociation curve?
The relationship between [O₂], partial pressure, and saturation is shown by the Oxygen Dissociation Curve
The curve shifts to offload O₂ to demanding tissues
Why is Hb so effective at transporting O₂ around the body?
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
What is the O₂ carrying capacity?
O₂ carrying capacity total = 200 ml/L
- 3 ml/L = Plasma
- 197 ml/L = Hb
How does Hb-O₂ affinity change?
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
Describe the PO₂ and O₂ saturation in the lungs
In lungs:
- High PO₂
- Low PCO₂ = high pH
- High O₂ saturation
Describe the PO₂ and O₂ saturation in resting tissue
In resting tissue:
- low PO₂
- low O₂ saturation
- O₂ moves from Hb to tissue
What is the PO₂ and O₂ saturation in working tissue?
- 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
What is the role of Myoglobin?
Myoglobin acts as an O₂ reservoir
only releases O₂ at low PO₂
What is the difference between foetal and adult Hb-O₂ affinity?
Foetal Hb has a higher O₂ affinity and effectively steals O₂ from maternal Hb
What colour does oxyhaemoglobin appear?
Hb-O₂ oxyhaemoglobin appears red
What is the colour of deoxyhaemoglobin?
Hb deoxyhaemoglobin appears blue
What is Cyanosis and why does it occur?
Purple discolouration of the skin and tissue, occurring when [deoxyhaemoglobin] becomes excessive
Describe the features of Central cyanosis
- bluish discolouration of core, mucus membranes and
extremities - Inadequate oxygenation of blood
- e.g. hypoventilation, V/Q mismatch
Describe the pathological features of peripheral cyanosis
- Bluish colouration confined to extremities (e.g. fingers)
- Inadequate O₂ supply to extremities
- e.g. small vessel circulation issues
Why does insufficient Hb anaemia (hypoxia) occur?
Hypoxia can occur despite adequate ventilation and perfusion, if blood isn’t able to carry sufficient O₂ to meet tissue demands
What are the causes of insufficient RBCs/Hb anaemia?
Iron deficiency (decreased production) Haemorrhage (increased loss)
Why is more CO₂ dissolved in plasma?
CO₂ has a higher H₂O solubility than O₂ ∴ a greater % of CO₂ is transported dissolved in plasma
(7% compared to 1% of O₂)
How can you calculate the [CO₂] dissolved in plasma?
concentration = partial pressure x solubility
Why is less CO₂ transported bound to Hb?
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%)
How is the majority of CO₂ transported?
CO₂ reacts with water to form carbonic acid, which accounts for the majority (70%) of CO₂ transport:
CO₂ + H₂O ⇋ H₂CO₃ ⇋ H⁺ + HCO₃⁻
What is the haldane effect?
Principle that venous blood carries more CO₂ than arterial blood
Explain how CO₂ is transported to tissues
- CO₂ produced by respiring cells, dissolves in plasma to
enter RBCs - CO₂ + H₂O ⇋ H₂CO₃ occurs within RBCs catalysed by
carbonic anhydrase - Effective removal of CO₂ via H₂CO₃ production,
enabling more CO₂ to diffuse into RBCs - H₂CO₃ ⇋ H⁺ + HCO₃⁻ occurs. RBC membrane is
impermeable to H⁺, so H⁺ can’t leave - 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
How is CO₂ transported to the lungs?
- Low PaCO₂ creates gradient;
CO₂ diffuses out of blood -> airspace - Increased PaO₂ leads to Hb-O₂ binding. Hb-O₂ binds
less H⁺ than deoxy-Hb, increasing free [H⁺] - Increased free [H⁺], leads to increased H₂CO₃ and
ultimately CO₂ which contributes to [CO₂] plasma - 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
