Outline the physiological production of blood and its constituents
- Are 7.5μm in diameter
- Are 2um thick
- Have a lifespan of 120 days
- Have a biconcave disc shape
This maximises surface area (optimising gas transfer) and makes the cells flexible enough to pass through capillary beds (which are narrower than the cell).
- Are important in:
- Delivering O2 to the tissues and delivering CO2 to the lungs
- Acid-Base balance
- Metabolism of some drugs
- Carry ~29pg of haemoglobin
- Comprise 40-50% of blood volume
- Proerythroblasts begin synthesis of Hb, with ongoing production occurring until the cell is mature
- Further differentiation results in successive loss of organelles, increasing Hb content
- The loss of ribosomes and nucleus of the reticulocyte are the final stage of erythropoiesis
- The entire process takes ~7-10 days
- Gas Carriage
- Acid-Base Buffering
- Production of HCO3-
- Binding of H+ to Hb
Esterases (and other -ases) in erythrocytes metabolise many drugs, including:
Old red cells are removed from circulation via:
- Phagocytosis by macrophages in:
- Bone marrow
~10% of red cell breakdown occurs in circulation, where the Hb dimers are then bound to haptoglobin by haemopexin.
- This is important to prevent glomerular filtration of haeme, and loss of iron
Haemoglobin is broken down into:
Broken down into constituent amino acids.
Re-enters haemoglobin synthetic pathway.
Complex metabolic pathway, notable as it is the only metabolic process that produces carbon monoxide:
- Metabolised to biliverdin by splenic macrophages in the reticuloendothelial system of the spleen
- Biliverdin is reduced to unconjugated bilirubin
This is fat soluble, and binds to albumin.
- Unconjugated bilirubin is conjugated in the liver to conjugated bilirubin
- Conjugated bilirubin is secreted in bile by active transport
This is impaired during hepatic disease, leading to increased bilirubin levels in plasma.
- Secreted conjugated bilirubin is metabolised to urobilinogen by gut bacteria
- Urobilinogen may have a number of fates:
- Enterohepatic recirculation and elimination in bile (again)
- Further metabolism by gut bacteria to stercobilinogen and then to stercobilin
- Enterohepatic recirculation and urinary excretion, where it is oxidised to urobilin
|Prehepatic disease||↑ Unconjugated bilirubin||↑ Urobilinogen, bilirubin not present||Normal|
|Intrahepatic disease||↑ Conjugated bilirubin, ↑ Unconjugated bilirubin||Bilirubin present||May be pale due to decreased urobilinogen excreted in bile|
|Posthepatic disease||↑ Conjugated bilirubin||↓ Urobilinogen, bilirubin present||Pale|
- Barrett KE, Barman SM, Boitano S, Brooks HL. Ganong's Review of Medical Physiology. 24th Ed. McGraw Hill. 2012.