RP - Gas Transport: Oxygen Transport by Blood

Question 1

Describe the partial pressures of oxygen and carbon dioxide:

Answer 1

Oxygen:

• PO2 in alveoli 100 mmHg
• arterial blood = 100 mmHg
• venous blood = 40 mmHg
• oxygen moves from higher to lower partial pressure:
  Alveoli → arterial blood → tissues

Carbon dioxide:

• PCO2 in alveoli = 40 mmHg
• arterial blood = 40 mmHg
• venous blood = 45 mmHg
• CO2 follows reverse gradient = Tissues → venous blood → alveoli

Question 2

Describe the general characteristics of the oxygen dissociation curve

Answer 2

Sigmoid shape due to co-operative binding (O₂ binding increases Hb affinity for more O₂)

Curve represents relationship between PO2 and haemoglobin saturation (SaO2)

Flat plateau region:
- high affinity for O₂ → haemoglobin is nearly fully saturated in arterial blood
- Acts as a buffer → oxygen remains bound even if alveolar PO2 slightly decreases

Steep Middle Portion:
- rapid oxygen unloading in tissues with decreasing PO2
- ensures efficient oxygen delivery to metabolically active tissues

Low PO2 Region:
- tissue oxygen extraction
- occurs in exercising muscles or hypoxic conditions where maximal oxygen release is required

Question 3

Describe oxygen transport:

Answer 3

Oxygen is inhaled into the alveoli, where it dissolves into the pulmonary capillaries

98% of oxygen binds to haemoglobin (Hb) in red blood cells

The oxygen-rich blood is pumped to tissues, where oxygen is released to cells

Cells use oxygen for aerobic respiration, producing carbon dioxide as a byproduct

Question 4

Describe haemoglobin and myoglobin:

Answer 4

Haemoglobin:

• tetrameric protein in RBCs, consisting of 2 alpha & 2 beta chains
• each molecule carries 4 oxygen molecules via reversible binding to iron in heme groups
• porphyrin ring
• HbA = Adult haemoglobin
• HbF = Foetal haemoglobin, higher O₂ affinity for oxygen transfer across the placenta

Myoglobin:

• Monomeric protein in muscle cells that stores oxygen
• Higher O₂ affinity than haemoglobin, allowing it to release oxygen only during hypoxia

Question 5

What are the factors causing a right shift + left shift in the oxygen dissociation curve ?

Answer 5

Right shift:

• decreased affinity + more O2 unloading
• increase CO2 (Bohr Effect), CO2 binds to Hb reducing affinity for O2
• decrease pH (Bohr effect), H⁺ ions stabilise the T-state of Hb, reducing O₂ affinity
• increased temperature increases metabolism promoting oxygen unloading
• ↑ 2,3-DPG (BPG), binds to Hb reducing O2 affinity

Left shift:

• occurs when O2 needs to be retained by Hb reducing tissue delivery
• decreased CO2 (Bohr effect), Less CO₂ binding increases Hb affinity for O₂
• increase pH (Bohr effect), H⁺ ions decrease, stabilising the R-state of Hb
• decrease temperature
• ↓ 2,3-DPG (BPG), less competition for Hb binding sites

Question 6

Describe the carriage of oxygen

Answer 6

Bound to haemoglobin:

• Each haemoglobin molecule can bind 4 molecules of oxygen → oxyhaemoglobin (HbO₂)
• O₂ binding is reversible and governed by Partial pressure of O₂ (PaO₂) and Affinity of Hb for O₂

Dissolved in plasma:

• Follows Henry’s Law: oxygen conc = alpha x PaO2

Question 7

Describe the Fick principle and Oxygen Extraction Ratio:

Answer 7

Fick principle:

• oxygen consumption (VO2) = blood flow x oxygen extraction

V02 = Q x (CaO2 - CvO2)

• CaO2 - CvO2 = Arterial-venous oxygen difference (AVO₂ difference)

Oxygen Extraction:

OER = VO2/ DO2
- tells us how efficiently tissues use oxygen
- DO2 = Oxygen delivery, Q x CaO2

Question 8

Define hypoxia and the different types

Answer 8

Hypoxia is insufficient oxygen supply to tissues despite adequate perfusion

Hypoxic hypoxia - low PaO2

Anaemic hypoxia - low haemoglobin levels

Stagnant hypoxia - low blood flow

Historic hypoxia - deficiency of tissue utilisation

Question 9

Describe the Alveolar Gas & Oxygen/Carbon Dioxide Cascade:

Answer 9

Oxygen cascade:

• The stepwise drop in O₂ partial pressure from ambient air → mitochondria

CO₂ Transport & Cascade:

• CO₂ diffuses in opposite direction:

Tissue → venous blood → alveoli → expired air.

Question 10

Describe the anaemia dissociation curve:

Answer 10

Anaemia does not shift the curve itself because Hb affinity for O₂ remains unchanged

since fewer Hb molecules are available, the total O₂ carrying capacity is reduced

Patients compensate with increased cardiac output (CO) and 2,3-BPG production, facilitating O₂ unloading in tissues

Severe cases = right shift as metabolic acidosis, increased 2,3-BPG, and higher temperature promote O₂ unloading