Lecture 13: RBCs & Hemostasis Flashcards
(38 cards)
Why do RBCs have a biconcave shape?
Flattened cross-section↓ diffusion distance
Easily deformed
- Cell membranes flexible, but NOT stretchable
- Biconcave can greatly deform w/o changing surface area
flatter = smaller radius = shorter diffusion distance
How is the RBC structure achieved? What is the major protein that plays a role?
There are proteins that sit just below the membrane’s surface and interact with the cytoskeleton and parts that embed within the lipid bilayer.
Spectrin
- Interacts with actin
- Long, rod-shaped protein
- Forms scaffolding under membrane
- Maintains cell shape & integrity
- Deficiencies cause spherocytosis
- Sphere-shaped RBCs (spherocytes)
- Fragile & short half-life (30-50 days)
What are the two ways that hemoglobin undergoes oxidative stress and what are the mechanisms by which RBCs deal with oxidative stress?
Oxidation by oxygen damages Hb.
One way it damages is by directly oxidizing Fe in hemoglobin to methemoglobin. Another way it damages is by oxidizing other parts of hemoglobin which causes cross-linking of cysteines (irreversible).
The two ways to reduce oxidative stress are:
The methemoglobin reductase pathway & the phosphogluconate pathway
What is methemoglobin?
hemoglobin that contains iron that has been oxidized to ferric iron (Fe3+)
non-functional
What is methemoglobin reductase?
RBC enzyme that restores iron to ferrous (Fe2+) form
- Hb function restored
Uses NADH as cofactor (produced in glycolysis)
- Levels maintained by glycolysis
What is glutathione’s role in preventing oxidative stress on hemoglobin? (the phosphogluconate pathway)
Oxidative species can damage RBC at locations other than Fe, cross-linking cysteines. In order to prevent this from happening, H2O2 (oxidizing species) can be cleared by reduced glutathione (GSH) which gets oxidized instead of Hb. Glutathione is sacrificed to protect Hb.
H2O2 + GSH –> (catalyzed by glutathione peroxidase) –> GSSG (oxidized glutathione) + water
Another set of enzymes called glutathione reductase takes the oxidized form and turns it back to the reduced form. The cell needs to maintain large amounts of reduced glutathione. It does this by using glutathione reductase GR which uses NADPH as to reduce.
NADPH levels are maintained by G6PD which dehydrogenases G6P to make 6-phosphogluconate
What does 2, 3 DPG (BPG) do?
it can interact with Hb and augment it’s affinity to bind to oxygen
Where does RBC formation happen at each stage of life?
Fetus
- Yolk sac (until 3 months)→ liver & spleen
At birth
- Exclusively from bone marrow
With age
- Distal ends of long bones fill with fat (first 25 years of life)
- Axial skeleton is site of hematopoiesis (after age 25)
What is the reticulocyte index?
The percentage of total RBCs that are reticulocytes (immature RBCs that still have reticular structures like ribosomes and ER)
Usually ~2% are reticulocytes
(~ 2 days as reticulocytes & ~ 100 days lifespan)
expected to be higher in someone with hemolytic anemia
O2 levels are detected in kidney by JG Apparatus
Low O2 → erythropoietin (EPO)
- Stimulates ↑ in # & maturation rate of erythrocyte precursors
Where does most of the iron used to make RBCs come from?
It is recycled from old dead RBCs
old cells are broken down, then transferrin (plasma protein) takes it to bone marrow. If not enough RBCs are dying, then ferritin (Fe storage in liver) is used. If no ferritin, get iron from GI tract/diet
How do RBCs die?
Senescent RBCs (near end of life)
- Loss of flexibility
- Denatured Hb forms rod-like lumps
Macrophages scan & assess RBCs
- Engulf old, low functioning RBCs
- Mainly in spleen, but can be anywhere that an RBC encounters a macrophage
- In spleen, there is a sinusoid (mesh sieve) that RBCs have to pass through. If they aren’t flexible they get stuck and macrophage digests
What is extravascular hemolysis?
Digestion of RBCs outside of the vasculature. RBCs do not release their contents to the blood stream. They are engulfed inside the macrophage extravascularly
Majority of RBCs die this way
Details:
RBCs broken down to Fe (goes to bone marrow via transferrin), globin (goes to amino acid pool), heme gets converted to bilirubin and CO is made as a byproduct
What is the process of intravascular hemolysis?
1 of 2 paths:
Hemoglobin –> Methemoglobin –> heme –> (uses hemopexin) –> bilirubin
Hemoglobin tetramer –> Hemoglobin dimer –> haptoglobin –> to liver and gets broken down
*Haptoglobin can be saturated, if so, the extra dimers will be eliminated in the urine
What are 2 tests to do if you suspect that someone has had a major intravascular hemolytic event?
Free haptoglobin & hemoglobinuria
Haptoglobin is what binds the hemoglobin dimers, so there would be very little free haptoglobin
Free Haptoglobin
- Low levels (up to 5 days) indicate intravascular hemolytic event
Hemoglobinuria
- Hemoglobin in urine, but only ~ 2 days
- Indicates intravascular hemolytic event
What are the blood groups? What is the antigen vs the antibody for each type? What is the Rh factor?
ABO Blood Groups
- Presence or absence of A and/or B antigens on RBC
Rh Group
- Presence (+) or absence (-) of D antigen on RBC
-Blood type A presents A antigens and has anti-B antibodies
-Blood type B presents B antigens and has anti-A antibodies
-Blood type AB presents A antigens & B antigens and has no antibodies
-Blood type O presents no antigens and has both anti-B and anti-A antibodies
What is Erythroblastosis Fetalis aka
Hemolytic Disease of the Newborn?
Problems with Rh-incompatibility can arise during pregnancy and threaten the life of the
fetus as a result of the following sequence of events:
1. An Rh-positive sperm donor and an Rh-negative egg donor conceive an Rhpositive fetus
2. Red cells from the baby introduce D antigen into the maternal circulation, generally
at birth when there may be mixing with the fetal blood
3. Antibodies against the D antigen form in the pregnant individual. This would have
no effect on the first baby since this baby is born before the maternal anti-D
antibodies develop
4. During ANY subsequent pregnancies with an Rh+ fetus, the maternal IgG
antibodies to the D antigen, generated in response to the first baby, can be taken
up by the placenta, attack the fetal red blood cells, and cause hemolysis
What is the standard of care for Erythroblastosis Fetalis?
Current standard of care:
Rh- pregnant individual treated near the time of birth of an Rh+ baby by injection of antibodies to D antigen
(Rhogam)
Injected antibodies rapidly remove any D antigen that may have entered the maternal circulation
- Prevent the pregnant individual’s immune surveillance system from detecting the antigen
- Prevent the maternal antibody production, which would otherwise follow
The injected antibodies subsequently degrade
and have no effect on later pregnancies
What happens after vessel injury?
Trigger is blood
entering tissues &
activation of Extrinsic
Pathway
Gives rise to Thrombin
Thrombin acts back &
activates intermediates (early factors of IP, turning it on) of Intrinsic Pathway
- w/o contact activation
- Explains why earlier factors before factors VIII and IX aren’t as vital
EP gets shut back off again by TFPI (tissue factor pathway inhibitor)
initial thrombin produced by EP is enough to kick off IP and keep it making more thrombin
Antithrombin III
Helps inactivate several of the facts and thus slow down the cascade
Thrombomodulin
Receptor on endothelial cells that thrombin binds to which activates protein C and protein S which themselves go back and inactivate various factors in the cascade
How are blood clots broken down?
The fibrinolytic system:
Plasminogen gets converted to plasmin which chops up fibrin
Prostacyclin and Nitric Oxide
Inhibits platelet aggregation/adhering to endothelial cells of blood vessel. It
is one of the mechanisms that limits the growth/spread of the platelet plug.
This is why we don’t turn into one big platelet plug, it stays at site of injury. Endothelial cells farther away are still making prostacyclin which prevents platelets from binding and sticking there
Vasodilators that balance endothelin when not clotting
What are factors that promote platelet aggregation?
ADP and Thromboxane A2
Von Willebrand factor
Glues/links the site of injury to the platelet.
Facilitates platelet adhesion by binding platelets while also binding sub-endothelial collagen. This can happen on it’s own, but VWF helps to speed up this process
