PHS 3342 Lecture Notes - Lecture 2: Nitric Oxide, Red Blood Cell, Cell Membrane
28 Apr 2018
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January 12, 2018
Erythrocytes
Erythrocytes
Dimensions: ~8 Ī¼m diameter, 1-2 Ī¼m thick
-Smaller than WBCs, bigger than platelets
-Not very thick
Biconcave (no nucleus): collapses in the middle once the nucleus is ejected
-Spectrin: maintains biconcave shape and allows for deformity
ā¢Mainly located under the plasma membrane
ā¢Allows RBCs to change their shape without rupturing when squeezing through especially skinny capillaries
Hemoglobin: protein globin bound to red heme pigment
-Globin = 4 polypeptide chains (2 Ī± + 2 Ī²) with each chain wrapped around a central heme group
-Iron is in the centre of each heme group - reversible binding site of oxygen
ā¢Reversible binding is especially important in order for blood to be passed to the tissues - canāt act as a delivery
system otherwise
Hemoglobin binds to:
-*Oxygen
ā¢*NB: binds to the heme portion
-Carbon dioxide: most is dissolved in plasma as bicarbonate, but some is bound to hemoglobin
ā¢*NB: binds with the globin (protein) section
-H+: replaces oxygen when it comes off
-*Carbon monoxide: 200x higher afļ¬nity for hemoglobin than oxygen
ā¢Binds to the same location and replaces O2
ā¢Causes people to suffocate
-Nitric oxide: can help signal blood vessels to open - makes it easier to deliver oxygen
Hb in erythrocytes: advantageous to package Hb in RBCs instead of as a loose protein in the blood
-Not lost through capillary membranes - keeps Hb in a protected membrane
-Doesnāt contribute directly to osmotic pressure and blood viscosity - packaging in RBCs prevents the blood from
becoming over-viscous
Specialized RBC characteristics that optimize function:
-Small size and biconcave shape
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January 12, 2018
ā¢Two advantages:
-Provides a larger surface area for diffusion of oxygen across the membrane
-Thinness enables oxygen to diffuse rapidly between the exterior and innermost regions of the cell
ā¢More efļ¬cient - less distance required for oxygen to travel in order to ļ¬nd an available hemoglobin binding spot
-~280 million Hb molecules/RBC
-Generate ATP anaerobically
ā¢Donāt use up the oxygen that they are transporting
Erythropoiesis
Type of hematopoiesis (formation of new formed elements)
Erythropoiesis: production of new erythrocytes - necessary because erythrocytes canāt divide to replenish their own
numbers
-Occurs in red bone marrow (network of reticular CT + wide capillaries)
-Reticulocytes migrate through thin capillary walls to blood
-Haematopoietic stem cell (hemocytoblast): stem cell for all formed elements
ā¢Cells will become committed to a particular pathway
-RBC formation:
ā¢Stem cell: hemocytoblast
ā¢Committed cell: proerythroblast
-Point of no return - can only become erythrocytes after this point
-Still has nucleus
-Developmental pathway: will now undergo ribosome synthesis to allow for hemoglobin synthesis, will then
eject the nucleus and will eventually degenerate the ribosomes
ā¢Mature cell: erythrocyte
-Basically just hemoglobin and cytoplasm contained by a plasma membrane
-Reticulocyte to blood/begins to transport O2, becomes a mature erythrocyte after ~2 days
Regulation of erythropoiesis: mainly occurs in the kidneys
-Kidneys monitor the oxygen supply being delivered to them in order to estimate the number of RBCs available
-Balance between production and destruction:
ā¢Too few: anemia
ā¢Too many: polycythemia
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