10. Carriage of Oxygen in the Blood Flashcards
Describe a typical RBC
What is microcytic and macrocytic anaemia?
What are reticulocytes?
Biconcave disc about 7um diameter, contains about 270 mill Hb molecules, no nuclei/mitochondria, live about 120 days
Microcytic anaemia: RBC smaller than usual, Macrocytic anaemia: RBC larger than usual
Just before and after RBC leave bone marrow, 1-2% circulating blood, reticular rRNA visible with stains. mature after 1 day.
What happens after a RBC has lived for around 120 days?
Why do RBC require small amounts of ATP?
How do RBC produce this ATP?
Increased level of methaemoglobin -> Changes plasma membrane markers, recognised by phagocytes -> phagocytosis in spleen/liver/bone marrow
To maintain Na+ pumps in cell membranes/other ion pumping operations
Glycolysis (glucose -> pyruvate -> lactic acid) so RBC have low pH. NB. their glucose uptake IS NOT regulated by insulin.
What kinds of cells are indicated by the arrows?
Reticulocytes
How do RBC protect against oxidative damage?
How are RBC progressively damaged?
Contain antioxidants e.g. vit C
Hb gradually converted to methaemoglobin (oxidised Hb), and cells damaged by having to bend through small capillaries.
NB: some repair can be done by methaemoglobin reductase
Describe how structure allows RBC to transport O2 without being oxidised.
Fe2+ atom is hexavalent and has 6 unpaired d orbital electrons - 4 in one plane, 1 above and 1 below. The 4 in 1 plane are held by 4 covalent bonds to N atoms in a porphyrin ring.
PORPHYRIN RING + FE CENTRE = HAEM GROUP
A histidine group underneath the porphyrin ring binds 5th Fe electron leaving ONE to react with other molecules. In Hb O2 forms weak reversible bond with the electron but can’t get close enough to Fe to remove it due to steric hindrance from other parts of the Hb molecule.
Explain steric hindrance
Hb is made of 4 subunits, each with a haem prosthetic group, and bound together by salt bridges, H bonds, and hydrophobic interactions. The 3D folding of the subunits creates steric hindrance so O2 cannot get close enough to Fe to remove the electron.
How does pO2 affect O2 and Hb bonding?
What is methaemoglobinemia?
Why do Alaskan Inuits have polycthemia (and what is it?)
When pO2 high (lungs) = O2Hb, when pO2 low (tissues) = deoxyHb
A higher percentage of methaemoglobin than 1-2% of normal Hb - can be genetic or caused by exposure to certain chemicals
Polycythemia = making more RBC than normal so O2 carrying capacity increased. Alaskan inuits have congenital deficiency of methaemoglobin reductase - may have as much as half of their total Hb in form of metHb.
What does steric hindrance amount depend on?
How do adult and foetal Hb differ, and how does it look on a O2/Hb dissociation curve?
Depends on particualr mix of Hb subunit (diff forms exist) in a molecule.
Adult: 2 alpha and 2 beta subunits, Foetal: 2 alpha and 2 gamma subunits and HIGHER AFFINITY FOR O2 THAN ADULT so can remove it from placental blood. Foetal curve to left of maternal.
What is 2,3 diphosphoglycerate (2,3 DPG)?
Small seperate molecule bound loosely in centre of Hb. When beta subunits start to deoxygenate, it binds to them and increases rate of O2 release, thus enhances ability of RBCs to release O2 in hypoxic tissues.
How can you measure the proportion of O2Hb in blood?
What are normal oxygen saturation values?
In terms of this, how is hypoxaemia described?
Pulse oximeter
96-99%
SaO2 (arterial O2 sat) below 90%
Describe the O2/Hb saturation curve, and its shape at high and low pO2.
What effect does heat have on the curve?
What effect does pH have on the curve?
S-shaped curve showing progressive binding to the 4 subunits of Hb. Flat at high pO2 (e.g. in lungs) and steep at medium and low pO2 (e.g. in tissues).
Flat upper part means that Hb more than 90% saturated by O2 over wide range of pO2 in lungs (from 70->100mmHg). Steep middle means that Hb releases large amounts of O2 for small decrease in pO2 (over 20-40mmHg)
Heat: heavily metabolising tissue heats up (and vice versa), and moves Hb curve to right thus unloading more O2 at any given partial presure. Cold moves it left - a cold limb may become hypoxic even if well perfused.
pH: heavily metabolising tissue generates more CO2 -> more acidic -> curve moves right. This pH-driven shift = BOHR SHIFT.
What is myoglobin and how is it useful?
What is rhabdomyolysis?
What does Mb look like on the O2/Hb saturation curve?
Hb in muscle, single subunit, greater O2 affinity than Hb so O2 is transferred to MbO2 as blood passes through muscle capillaries - Mb forms buffer store of O2 in muscles.
Mb released from damaged muscle tissue -> filtered by kidneys but toxic to renal tubular epithelium (may cause acute renal failure)
To left.
Describe the negative feedback loop that occurs if haematocrit falls below normal levels (45%).
Haematocrit falls -> renal hypoxia -> EPO increase (erythropoetin - continually released from interstitial cells in kidney) -> RBC production in bone marrow -> haematocrit returns to 45%
How can synthetic EPO be used?
(erythropoetin) to treat anaemia from chronic kidney disease, from cancer treatment and other illnesses.
What are the 3 ways CO2 is carried from muscles to lungs? Which is the main way?
- RBC convert CO2 -> carbonic anhydrase (H2CO3). Most HCO3- expelled to plasma and carried to lungs. As HCO3- leaves, Cl- diffuses into RBC = maintain electrical neutrality = CHLORIDE SHIFT. In lungs -> HCO3- reenters RBC -> converted to CO2 and leaves. Cl- leaves RBC to balance charge. Main way.
- CO2 binds to O2Hb and displaces O2 in acid conditions -> HbCO2. In lungs high pO2 and low pH displaces CO2 fro Hb. BOHR EFFECT enhances O2 unloading to muscles.
- CO2 can be dissolved in plasma
