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Oxygen and Carbon Dioxide Transport in the Blood
Dr Ziyad Sahe
Learning Objectives
Describe the structure of haemoglobin and its suitability for oxygen carriage.
Understand mutations in haemoglobin genes and their structural consequences.
Comment on the role of methaemoglobin in erythrocytes.
Draw and label the oxygen-haemoglobin dissociation curve.
Explain the influence of temperature and pH on the curve.
Define haematocrit and its regulation.
Discuss the transport mechanisms of carbon dioxide in blood.
Carriage of Oxygen
Oxygen is a powerful oxidizing agent. High concentrations can damage organic molecules.
Red blood cells (erythrocytes) are specially designed for oxygen transport.
Oxidation and Reduction
Oxidation
: loss of electrons; releases energy, often as heat.
Reduction
: gain of electrons; forms complex molecules from simpler ones and requires energy.
Example: Fe^{2+} \rightarrow Fe^{3+} + e^-
Reactions can be reversible if they involve small energy transfers.
Example reaction: NAD oxidation/reduction is crucial in bodily electron transfer processes.
Erythrocytes
Contain haemoglobin, enabling reversible oxygen binding without oxidation.
Structure
: Biconcave disks, approx. 7 μm in diameter and 2 μm thick.
Mature red cells lack nuclei and mitochondria, possibly due to oxygen damage.
Immature Erythrocytes: Reticulocytes
Comprise 1-2% of circulating red blood cells.
Characterized by a visible reticular network of ribosomal RNA when stained.
Cannot divide or repair themselves due to the absence of organelles.
Glucose Transporters in Erythrocytes: GLUT1
Mature red blood cells produce ATP via glycolysis (less efficient than aerobic metabolism).
High levels of lactate result in low pH in red cells.
GLUT1 facilitates unregulated glucose uptake, independent of insulin.
Erythrocytes and Oxidative Damage
Contain antioxidants like Vitamin C to mitigate oxidative damage from oxygen.
Over time, haemoglobin converts to methaemoglobin; aging RBCs are recognized and phagocytized by immune cells.
Haemoglobin Structure
Comprised of four subunits, each with a heme prosthetic group.
Stability arises from salt bridges, hydrogen bonds, and hydrophobic interactions among polypeptide chains.
Oxygen temporarily binds to the ferrous iron in the heme group but cannot fully oxidize due to steric hindrance.
Methemoglobinemia
In normal individuals, 1-2% of haemoglobin is methaemoglobin.
Levels above 2% indicate methaemoglobinemia, potentially genetic or due to chemicals.
Elevated methaemoglobin levels signal the removal of aging RBCs.
Congenital Hemoglobinemia
Caused by methaemoglobin reductase deficiency (rare).
Common in populations like Alaskan Inuit, where affected individuals develop polycythemia to compensate for increased methaemoglobin levels.
Sickle Cell Anemia
The most common hemoglobin disorder caused by hemoglobin S formation due to a valine substitution for glutamic acid.
Results in misshapen, inflexible RBCs that can obstruct blood vessels, impairing organ blood flow.
Variants of Haemoglobin
Variability in subunits affects oxygen affinity (e.g., adult hemoglobin A: (2\alpha, 2\beta) , fetal hemoglobin F: (2\alpha, 2\gamma) ).
Fetal hemoglobin possesses a higher oxygen affinity for optimal placental oxygen uptake.
Oxygen Binding and Unloading
2,3-DPG binds to deoxygenated β subunits of hemoglobin, enhancing oxygen release in hypoxic conditions.
Haemoglobin Saturation
Measured as the ratio of oxyhaemoglobin to total hemoglobin (SO2).
Typically assessed with a pulse oximeter (SpO2) or arterial oxygen saturation (SaO2).
Cooperative Binding of Haemoglobin
Oxygen dissociation curves illustrate the relationship between partial pressure of oxygen (pO2) and hemoglobin saturation.
Saturation is affected by each successive binding of O2, making the relationship non-linear.
The Oxygen-Haemoglobin Dissociation Curve
Characteristic 'S' shape; flat in high pO2 range, steep in medium/low pO2 range (20-40 mm Hg).
High pO2 in lungs maintains over 90% saturation.
Effects of Temperature and pH on the Curve
Temperature
: Elevated temperatures shift the curve right, facilitating oxygen unloading.
pH
: Increased CO2 in metabolically active tissues generates acidity, shifting the curve right (Bohr effect), enhancing oxygen release.
Myoglobin in Muscle
Myoglobin (Mb) is a single subunit hemoglobin variant with higher oxygen affinity than hemoglobin, storing oxygen in muscle tissues.
Rhabdomyolysis occurs when myoglobin is released from damaged muscle, potentially causing acute renal failure due to toxicity to renal epithelium.
Haematocrit
Percentage of blood volume occupied by red blood cells; typically about 45%.
Erythropoietin (EPO)
EPO is a hormone released by kidney interstitial cells during hypoxia.
Synthetic EPO can treat anemia from chronic kidney disease and cancer-related chemotherapy.
Carbon Dioxide Transport by Blood
RBCs carry CO2 back to the lungs, primarily converting it to bicarbonate (HCO3-) via carbonic anhydrase (CA).
CO2 transport: 70% as bicarbonate, 20% bound to hemoglobin (carbaminohemoglobin), 10% dissolved in plasma.
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