Red blood cell: Structure and function

Question 1

Rbc structure

Answer 1

RBC- unique amongst eukaryotic cells lacks nucleus, mitochondria and ER.

As they lack organelles, it is essential that they maintain their structure

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

Structural properties are linked to membrane

Question 2

Function of RBC

Answer 2

Primary function of RBC is transport of respiratory gases to and from the tissues.

To achieves this:

rbc traverse the microvascular system without mechanical damage, and retain a shape which facilitates gaseous exchange.

the red cell membrane should be extremely tough yet highly flexible

Cytoskeletal proteins interaction with the membrane lipid bilayer determines strength and flexibility.

Question 3

Structure of RBC membrane

Answer 3

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.

Question 4

RBC Membrane lipids

Answer 4

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

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 the formation of acanthocytes.
an increase in cholesterol and phospholipid is a cause of target cells

Question 5

Why is concentration of cholesterol in rbc important?

Answer 5

The concentration of cholesterol in the membrane is an important determinant of membrane surface area and fluidity:

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

Question 6

increased in lipids leads to

Answer 6

acanthocytes (just cholesterol)
increase in both cholesterol and phospholipid causes target cells.
Target cells occur in disproportional increase in SA:V; i.e abnormally high SA (increased SA),

Question 7

composition of lipids in rbc membrane

Answer 7

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

Question 8

RBC membrane proteins

Integral membrane proteins

Answer 8

Extend from outer surface and traverse entire membrane to inner surface
• There are 2 major types of integral proteins:

-> Glycophorins A, B, C
Glycophorins give RBCs their negative charge
When suspended in a test tube, blood will take time to settle due to repulsion
When there is an excess of inflammatory proteins, the cells settle quicker as they are
coated in these proteins
Glycophorin A is important for the parasite causing malaria, it uses this protein to enter
the RBC

-> Band 3: anion transporter
These help transfer ions in and out of cell to maintain cell shape

There are several other types of integral proteins as well
Na+/K+ ATPase, Aquaporin 1, surface receptors, e.g. TfR
TfR is transferase receptor found in the innnerpart of the RBC membrane

Question 9

Peripheral proteins

Answer 9

Limited to cytoplasmic surface of membrane and forms the RBC cytoskeleton
There are many major peripheral proteins

• Spectrin
• Ankyrin
  Primarily anchors lipid bilayer to membrane skeleton
  Via interaction with spectrin and Band 3
• Protein 4.1
  May link the cytoskeleton to the membrane by means of its associations with
  glycophorin
  Stabilises interaction of spectrin with actin
• Actin
  Responsible for contraction and relaxation of membrane

Question 10

Spectrin

Answer 10

The most abundant peripheral protein

Composed of alpha & beta chains

Very important in RBC membrane integrity because it binds with other peripheral proteins to form the cytoskeletal network of microfilaments on the inner surface of RBC membrane

Controls biconcave shape and deformability of cell

Question 11

Function of membrane

Answer 11

• Provides shape
Provides the optimum surface area to volume ratio for respiratory exchange AND is
essential to deformability

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

• Regulates intracellular cation concentration by use of Na/K ATPase and many other ion channels
Control of Ca2+ is very important
Accumulation of Ca2+ will affect RBC shape
May give a spherical shape
They are quite easily destroye
They may be removed from circulation by the reticuloendothelial system
Separates the contents of the cell from the plasma.

• Acts as the interface between the cell and its environment via membrane surface receptors.

Question 12

RBC cation pumps

Answer 12

Allows water and electrolytes to exchange via cation pumps.
RBC controls volume and H2O content primarily through control of intracellular concentrations of Na+ and K+ via these cationic pumps which require ATP.
ATP is also required in the Ca++ pump system that prevents excessive intracellular build-up of Ca++.
In ATP depleted cells there is an intracellular build-up of Na+ and Ca++ and a loss of K+ and water. This leads to dehydrated, rigid cells that are culled by the spleen.

Question 13

Defects in RBC Membrane

Answer 13

Hereditary Spherocytosis

Hereditary Elliptocytosis

Question 14

Defects in RBC Membrane

Hereditary Spherocytosis

Answer 14

Ankyrin def or abnormalities
A or B spectrin def or abnormalities
Band 3 protein abnormalities
Protein 4.2 abnormalities

Question 15

Defects in RBC Membrane

Hereditary Elliptocytosis

Answer 15

A or B spectrin mutation – defective spectrin dimer
A or B spectrin mutation – defective spectrin-ankyrin association
Protein 4.1 deficiency or abnormalities

Question 16

What features allow RBC to withstand life without structural deterioration?

Answer 16

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 (mean cell haem conc)
as MCHC rises, viscosity rises exponentially.

Question 17

Haemoglobin structure

Answer 17

Haemoglobin (Hb) is a globular haemoprotein

haemoproteins are 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.

Question 18

Haemoglobin structure

Globin

Answer 18

Globin:
4 polypeptide subunits
2 α-globin chains
2 β-globin chains

Question 19

adult hameoglobin

globin %

Answer 19

Hb A
structure a2b2
96-98%

Hba2
structure a2d2
1.5-3.2%

HbF
a2Y2
0.5-0.8%

Question 20

Functions of haemoglobin

Answer 20

Oxygen delivery to the tissues
• One Hb can bind to four O2 molecules
• Less than .01 sec required for oxygenation.
• When oxygenated 2,3-DPG is pushed out; the Β-chains move closer.
• Β-chains are pulled apart when O2 is unloaded, permitting entry of 2,3-DPG resulting in
lower affinity of O2
• Increased amount of 2,3-diphosphoglycerate allows RBCs to release oxygen easily and vice
versa

Question 21

2,3-diphosphoglycerate (2,3-DPG)

Answer 21

2,3-diphosphoglycerate (2,3-DPG) is a substance made in the red blood cells. It controls the movement of oxygen from red blood cells to body tissues.

The more 2,3-DPG in the cell, the more oxygen is delivered to body tissues. Conversely, the less 2,3-DPG in the cell, the less oxygen is delivered.

Increasing the amount of 2,3-DPG is the body’s primary way of responding to a lack of oxygen.

As Hb molecule loads and unloads O2, the individual globin chains move on each other.

The Beta chains slide on the a1b2 and a2b1 contacts during oxygenation and deoxygenation.

When O2 is unloaded the beta chains are pulled apart, permitting entry of the metabolite 2,3-diphosphoglycerate (2,3-DPG), resulting in lower affinity for O2.

This move is responsible for the sigmoid shape of the O2-dissociation curve

Question 22

Hb-oxygen dissociation curve

Answer 22

The normal position of curve depends on

Concentration of 2,3-DPG
H+ ion concentration (pH)
CO2 in red blood cells
Structure of Hb (eg fetal hemoglobin does not release oxygen as easily as adult hemoglobin)

Standard conditions:
Temp = 37OC
pH = 7.40
BE = 0

Question 23

BE

Answer 23

BE stands for Base Excess, and it is a measure of the metabolic acid level. Zero means blood base level is normal which 48mmol/L, or blood pH of 7.4.

For instance, if the patient is acidotic, the base excess will be a negative value; this means acid would have to be taken away to bring the pH to normal. For a base excess of -6 would mean that 6 mEq per litre of base would have to be added to bring the patient’s blood to normal pH (7.4)

Question 24

What shifts the curve to the right?

Answer 24

Changes in blood pH shift the oxyhemoglobin dissociation curve. An increase in CO2 production by tissue and release into blood results in the generation of hydrogen ions (H+) and a decrease in pH. This shifts the dissociation curve to the right, which has a beneficial effect by aiding in the release of O2 from Hgb for diffusion into tissues. The shift to the right in the dissociation curve is due to the decrease in pH and to a direct effect of CO2 on Hgb

hence increased temp, lower pH, increase 2-3DPG and increase co2 levels

Question 25

Changes in structure of Hb

Haemoglobinopathies

Answer 25

Inherited disorders of Hb; 2 categories:

Thalassaemias
Sickle Cell Disease

Question 26

RBC metabolic pathways why?

Answer 26

Metabolism provide energy required for:

Maintenance of cation pumps
Maintenance of RBC integrity and deformability
Maintenance of Hb in reduced state
Maintenance of reduced sulfhydryl groups in Hb and other proteins

Question 27

Key Metabolic Pathways for the RBC

Answer 27

Glycolytic or Embden-Meyerhof Pathway
Hexose Monophosphate Shunt
Methaemoglobin reductase Pathway
Luebering-Rapoport shunt

Question 28

Glycolytic Pathway

Answer 28

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

Generates 90- 95% of energy needed by RBC’s

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

Question 29

Pentose Phosphate Pathway/Hexose monophosphate shunt

Answer 29

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;

Question 30

Methemoglobin Reductase pathway

Answer 30

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

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

Question 31

Luebering-Rapoport shunt

Answer 31

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