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fundamentals of pathology > W10 RBC structure + function > Flashcards

W10 RBC structure + function Flashcards

1
Q

RBC Structure

A

lacks nucleus, mitochondria and ER.

It is essentially a “bag of haemoglobin

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2
Q

RBC link to membrane?

A

Structural properties are linked to membrane

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3
Q

RBC ability to move

A

Biconcave, 8 micron cells, but able to deform & pass through 3 micron capillaries or reticuloendothelial system without fragmentation

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4
Q

RBC membrane

A

Have flexible membrane with a high surface-to-volume ratio

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5
Q

RBC primary function

A

Transport of respiratory gases to and from the tissues

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6
Q

RBC structure-function

A

RBC should be capable of traversing the microvascular system without mechanical damage, and retain a shape.

The red cell membrane should be extremely tough yet highly flexible

Interactions between the membrane & cytoskeletal proteins determines strength and flexibility

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7
Q

RBC biconcave shape

A

Maximises the surface area to increase efficiency of oxygen absorption
Functions of the red cell membrane

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8
Q

RBC membrane function

A 
A.  Maintenance of cell volume
            1.  Na+ and K+content 
            2.  Osmotic fragility
B.  Ca+2 Homeostasis
C.  Anion exchange
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9
Q

Factors affecting red cell deformability

A

Cytoplasmic viscosity
Intracellular rubbish
Membrane rigidity
Surface to volume ratio

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10
Q

What is red cell membrane structure?

A

A semi-permeable lipid bilayer; with proteins scattered throughout:

an outer hydrophilic portion composed of glycolipids, glycoproteins, and proteins

a central hydrophobic layer containing proteins, cholesterol, and phospholipids

an inner hydrophilic layer of mesh-like cytoskeletal proteins to support lipid bilayer

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11
Q

The red cell membrane consists of

A

Proteins~50%, Lipids~40%, Carbohydrates~10%

Lipds consist of 60% phospholipid, 30% natural lipids (mainly cholesterol) and 10% glycolipids

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12
Q

Cholestrol in RBC membrane

A

Cholesterol will not form a membrane by itself, but inserts into a bilayer of phospholipids with its polar hydroxyl group close to the phospholipid head

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13
Q

RBC elasticity

A

RBC is highly elastic (100-fold softer than a latex membrane of comparable thickness), rapidly responds to applied fluid stresses (time constants in the range of 100 milliseconds), and is stronger than steel in terms of structural resistance

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14
Q

RBC membrane lipids

A

Asymmetric phospholipid distribution.

Unesterified free cholesterol between.

Uncharged phospholipids of outer layer

Phosphatidyl choline and Sphingomyelin

Charged phospholipids of inner layer:
Phosphatidyl ethanolamine
Phosphatidyl serine

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15
Q

Glycolipids

A

Lipids with a carbohydrate attached by a glycosidic (covalent) bond

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16
Q

Glycolipids function

A

Tto maintain the stability of the cell membrane and to facilitate cellular recognition, which is crucial to the immune response and in the connections that allow cells to connect to one another to form tissues

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17
Q

Concentration of cholesterol in the membrane

A

Important determinant of membrane surface area and fluidity

Increase in membrane cholesterol leads to an increased surface area and decreased deformability

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18
Q

Membrane cholestrol in equilbrium with…

A

Membrane cholesterol exists in free equilibrium with plasma cholesterol:

an increase in free plasma cholesterol results in an accumulation of cholesterol in the RBC membrane

RBCs with increased cholesterol appear distorted resulting in acanthocytosis

an increase in cholesterol and phospholipid is a cause of target cells

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19
Q

Excess plasma CL w/in outer leaflet of RBC membranes

A

This makes the red blood cells less deformable and they are remodeled as they passage through the spleen, forming acanthocytes (spiked CM)

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20
Q

RBC membrane proteins

A

Integral membrane proteins: Extend from outer surface and traverse entire membrane to inner surface

2 major integral membrane proteins:
Glycophorins: types of glycophorins identified:A, B, and C

Band 3: anion transporter

Other integral proteins:-
Na+/K+ ATPase, Aquaporin 1, surface receptors, e.g. TfR

21
Q

Glycophorins

A

are the major integral membrane proteins, accounting for location of RBC antigens. They impart –ve charge to cell, reducing interaction with other cells/endothelium.

They are all glycophorins are receptors or transport proteins 3 types: Glycophorins A, B, and C.

GlycophorinC/ Protein 4.1/ p55 complex, and
GlycophorinA, appear important for P falciparum invasion of and development in RBC

22
Q

Band 3

A

(acts as anion transport channel). links lipid bilayer to underlying membrane cytoskeleton (ankyrin).

    - Na+/K+ ATPase.
    - glucose transport protein.
    - surface receptors. (the most important is the transferrin receptors)
23
Q

Peripheral proteins

A

Limited to cytoplasmic surface of membrane and forms the RBC cytoskeleton

Major peripheral proteins include:
Spectrin, Ankyrin, Protein 4.1 and Actin

24
Q

RBC cytoskeleton

A

The Cytoskeleton provides rigid support and stability to lipid bilayer.

It is also responsible for deformability properties of the RBC membrane, leading to shape change

Strong cohesion between bilayer and membrane skeleton maintains surface area

25
Q

Outer leaflet

A

leaflet consists predominantly of phosphatidylcholine, sphingomyelin, and glycolipids

26
Q

Inner leaflet

A

inner leaflet contains phosphatidylethanolamine, phosphatidylserine, and phosphatidylinositol

27
Q

Maintenance of asymmetric distribution of phospholipids

A

The maintenance of asymmetric distribution of phospholipids, in particular exclusive localization of PS and phosphoinositides to the inner monolayer, has several functional implications:

  • macrophages recognize + phagocytose red cells that expose PS at their outer surface
  • confinement of this lipid in the inner monolayer is essential if the cell is to survive its frequent encounter w/macrophages of the reticuloendothelial system, especially the spleen
28
Q

Spectrin

A

The most abundant peripheral protein

composed of alpha & beta chains

very important in RBC membrane integrity;

binds with other peripheral proteins to form the cytoskeletal network of microfilaments.

controls biconcave shape and deformability of cell

29
Q

Microfilaments

A

Microfilaments strengthen membrane, protecting cell from being broken

For spectrin to participate in interaction with other proteins, it must be phosphorylated by a protein kinase that requires ATP.

So decrease in ATP = decreased phosphorylation of spectrin

30
Q

Unphosphorylated spectrin

A

Can no longer bind to actin to give the membrane its elastic properties, leading to loss in membrane deformability and decreased RBC survival time

31
Q

Mutations in cytoskeletal proteins

A

(spectrin, protein 4.1) weaken the horizontal linkage, decrease the membrane mechanical stability and are responsible for hereditary
elliptocytosis (HE)

32
Q

Ankyrin

A

Primarily anchors lipid bilayer to membrane skeleton

via interaction with spectrin and Band 3;

33
Q

Protein 4.1

A

may link the cytoskeleton to the membrane by means of its associations with glycophorin;

stabilises interaction of spectrin with actin

34
Q

Actin

A

responsible for contraction and relaxation of membrane

35
Q

What does red cell membrane do?

A

Provides shape:
Provides the optimum surface area to volume ratio for respiratory exchange

Provides deformability, elasticity
allowing for passage through micro vessels (capillaries)

Regulates intracellular cation concentration

36
Q

Defects of band 3, spectrin, ankyrin, or protein 4.2

A

Defects of band 3, spectrin, ankyrin, or protein 4.2 lead to destabilisation of the overlying lipid bilayer and release of lipid in microvesicles

37
Q

What features allow RBC to withstand life without structural deterioration?

A

Geometry of cell; surface area to volume ratio.

facilitates deformation whilst maintaining constant surface area.

Membrane deformability

spectrin molecules undergo reversible change in conformation: some uncoiled and extended, others compressed and folded.

Cytoplasmic viscosity determined by MCHC

as MCHC rises, viscosity rises exponentially.

38
Q

Haemoglobin structure

A

globular haemoprotein

group of specialized proteins that contain haem as a tightly bound prosthetic group

Haem is a complex of protoporphyrin IX and ferrous iron (Fe2+)

Iron held in the centre of haem molecule by bonds to the 4 nitrogen of a porphyrin ring

39
Q

Prosthetic gp

A

The nonprotein component of a conjugated protein, as the heme group in haemoglobin

40
Q

Metabolism provide energy required for

A

Maintenance of cation pumps

Maintenance of Hb in reduced state

Maintenance of reduced sulfhydryl groups in Hb and other proteins

Maintenance of RBC integrity and deformability

41
Q

Key Metabolic Pathways for the RBC

A

Glycolytic or Embden-Meyerhof Pathway

Hexose Monophosphate Shunt

Methaemoglobin reductase Pathway

Luebering-Rapoport shunt

42
Q

Glycolytic Pathway

A

Glucose is metabolized and generates two molecules of ATP (energy).

Generates 90- 95% of energy needed by RBCs

Functions in the maintenance of RBC shape, flexibility and the cation pumps

This pathway also produces NADPH needed by the enzyme methaemoglobin reductase

43
Q

hexose monophosphate pathway

A

used for production of NADPH from NADP

44
Q

ATP energy for…

A

ATP provides energy to maintain red cell volume, shape and flexibility

45
Q

A membrane ATPase sodium pump

A

A membrane ATPase sodium pump requiring ATP is used to control movement of Na+ and K+; pumps 3 Na+ out and 2 K + into cells

46
Q

Pentose Phosphate Pathway

A

Pentose Phosphate Pathway

The pentose phosphate shunt provides the reducing power, NADPH

NADPH maintains glutathione in the reduced form (GSH)

RBC uses GSH to protect it from oxidative damage

An alternative to glycolysis and generates NADPH (oxidative phase) and pentoses (5-carbon sugars, nonoxidative phase).

47
Q

Methemoglobin Reductase pathway

A

Maintains iron in the ferrous (Fe++) state.

In the absence of this enzyme, methaemoglobin accumulates and it cannot carry oxygen

48
Q

Luebering-Rapoport shunt

A

A biochemical pathway in mature erythrocytes involving the formation of 2,3-bisphosphoglycerate and which regulates oxygen release from haemoglobin and delivery to tissues.

Permits the accumulation of 2,3-DPG which is essential for maintaining normal oxygen tension, regulating haemoglobin affinity

fundamentals of pathology (23 decks)

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