Structure and function of red blood cells (RBC)

Physiology Unit

AIMST University

Learning outcome-

1.Describe the morphologic characteristics of mature RBC.

2.Describe the functions of the cytoskeleton with special reference to RBC.

3.Describe the structure of hemoglobin and enumerate its properties (including its reactions with oxygen).

4.List the factors that affect the oxygen carrying capacity of hemoglobin.

5.What is the oxygen carrying capacity of adult hemoglobin?

6.Hb F has greater affinity for oxygen compared to Hb A. What is the underlying mechanism?

RBC are flexible , bi-concave and non-nucleated disc.

Composition:

i. Water : 65%

ii. Solid : 35%

a. Hemoglobin : 33%

b. Stromal mash : 02%

(Protein , phospholipid, cholesterol ester and neutral fat)

Fig: Red blood cell in electron microscope

Morphology of RBCShape & size: Bi concaveThickness: i. At the thickness point- 2.5 µm

ii. At the centre- 1µmDiameter : 7.2 -7.8 µmSurface area: 120-140 square micronVolume: 90-95 cubic micronLife span : 120 daysHb : each RBC contains approx. 29 pg hemoglobin or 34 gm/dl of cells.

Normal count of RBC:In adult male: 4.5 to 5.5 million/cu.mm of bloodIn adult female: 4.0 to 5.0 million/cu.mm of bloodAdvantage of absence of nucleus in RBCIt gives the bi- concavity of RBCIt also gives more room for Hb.

Importance of bi-concave shape are:1.It can easily squeeze when it pass through capillary pores.2.It allows considerable alteration of the cell volume without increasing the tension of the cell wall.3.It can accommodate much fluid in hypotonic environment.4.Hb remains distributed in a very thin layer. This facilitates quick saturation or desaturation with the gases.

Functions of RBC:1.Respiratory function: RBC contain Hb which carry O2 from lungs to tissue and CO2 from tissue to lungs.

2.Acid- base balance: RBC helps to maintain acid base balance. It carried out by the buffering action of hemoglobin.

3.Ion balance: RBC maintain intercellular ion balance by special permeability of the cell membrane.

4.Maintenance of viscosity- RBC helps to maintain the viscosity of blood. If RBC is increased in plasma, viscosity of blood will increased.

5.Various pigments are derived from Hb after the disintegration of red cells e.g. bilirubin, biliverdin etc.

6.It contains antigens.

Functions of cytoskeleton of RBC lipid bilayer, trans membrane proteins, and a filamentous meshwork of proteins that forms a membrane skeleton . Most abundant protein is spectrin.

Throughout the course of their 120 day lifespan, the cells are exposed to high amounts of shear force as they navigate the narrow capillaries of the microvasculature and hence need to undergo rapid, reversible deformations. To cope with this stress, the red blood cell is equipped with a specialized cytoskeleton that provides the mechanical stability and flexibility necessary to withstand forces experienced during circulation.

Mature red cells do not have a nucleus, mitochondria, or endoplasmic reticulum,

They have cytoplasmic enzymes that are capable of metabolizing glucose and forming small amounts of adenosine triphosphate.

These enzymes also (1)maintain pliability of the cell membrane, (2)maintain membrane transport of ions,(3)keep the iron of the cell’s hemoglobin in the ferrous form rather than ferric form(4) prevent oxidation of the proteins in the red cells.

Hemoglobin is a conjugate protein or metaloporphyrin. It is the red pigment inside the red blood cell.i.Globin: 96%ii.Heam: 4%Normal countIn male : 14-18 gm/100 ml of bloodIn female : 12-15.5gm/100ml of bloodAt birth : 23gm/100ml of bloodAt first year: 10gm/100ml of blood

Functions of hemoglobin1. Transport of respiratory gases (Oxygen from the lungs to the tissues and carbon dioxide, from the tissues to the lungs.)2. Regulation of blood pH3. Various pigment of bile, stool, urine etc. are derived from it.4. It reserves iron and protein.

Synthesis of hemoglobin Begins in Proerythroblasts and continues slightly even into reticulocyte stages.

Heam portion synthesized mainly from acetic acid and glycine, in the cell mitochondria

Globin portion in the ribosome of endoplasmic reticulum.

1. Heam portion: Acetic acid is changed in the Krebs cycle in to succinyl Co-A and then two molecule of glycine to form a pyrrole compound. In turn four pyrrole compounds combine to form a protoporphyrin compound. One of the protoporphyrin compounds known as protoporphyrin IX, then combines with iron to form the heam molecule.

2. Globin portion: It is composed of four large polypeptide chain, synthesized by the ribosomes.

Finally each heam molecules combine with a very long polypeptide chain forming a subunit of Hb called hemoglobin chain.

Steps of Formation of hemoglobin:

Fig: Structure of Hb molecule

Fig: Structure of Hb

Type of hemoglobinApart from species differences, several varieties of hemoglobin occur in man; in all the heam moiety is the same, physical and chemical differences being due to variation in the composition of peptides of the globin fraction. Physiological hemoglobinThese can be separated by electrophoresis. Physiological Hb is two types, Adult & fetal hemoglobin. Adult hemoglobinAdult Hb is of two typesHemoglobin A (Hb A1): 98% in normal adult. It contains 2α & 2β chain of globin.Hemoglobin A2 (Hb A2): 2% in normal adult. It contains 2α chain identical with HB A, but β chains are replaced by delta chain.

Hemoglobin F (α2γ2):Normally present in the fetal red blood cell and has usually disappeared by 2-3 months after birth. It differs from Hb A in having γ chain in place of β chain and in having a greater affinity for oxygen.

Pathological hemoglobin

Hemoglobin S

Hemoglobin C

Difference between HbA & HbF

Adult hemoglobin

1. It contains 2α & 2β chains

2. β chain contains 146 amino acids

3. It is normally 98% in adult.

4. It saturates at a relatively high O2 tension.

5. It binds with 2-3 di- phos phoglycerate

6. It is not resistant to the action of alkali

Foetal hemoglobin

1. It contains 2α & 2γ chains

2. γ chain contains 146 amino acids but 37 of which differ from chain of Hb A

3. It is normally absent in adult but present in foetus.

4. It saturates at a low O2 tension.

5. It binds less avidly with 2-3 di- phos phoglycerate

6. It is resistant to the action of alkali

Oxygen carrying capacity of adult hemoglobin-Hemoglobin is a protein made up of four subunits, each of which contains a heam moiety attached to a polypeptide chain. In normal adults, most of the hemoglobin molecules contain two ∞ and two ß chains. Heme is a porphyrin ring complex that includes one atom of ferrous iron. Each of four iron atoms in hemoglobin can reversibly bind one O2 molecule. Since it contains 4Hb unit, the hemoglobin molecule can also be represented as Hb4 and it actually reacts with 4 molecule of O2 to form Hb4O8.

I gm. Hb contains 1.34 ml of O2100 ml of arterial blood contains 19.8 ml of O2100 ml of venous blood contains 15.2ml of O2

Hb4 + O2 ↔ Hb4O2

Hb4O2 + O2 ↔ Hb4O4

Hb4O4 + O2 ↔ Hb4O6

Hb4O6 + O2 ↔ Hb4O8

Factor affecting the oxygen carrying capacity of hemoglobin:

1. pH

2. Blood temperature

3. 2,3- diphosphoglycerate

4. CO2 & Hydrogen ion

5. Fetal hemoglobin

Hb F has greater affinity for oxygen compared to Hb A. What is the underlying mechanism?Fetal hemoglobin's affinity for oxygen is substantially greater than that of adult hemoglobin. Notably, the P50 value for fetal hemoglobin (i.e., the partial pressure of oxygen at which the protein is 50% saturated; lower values indicate greater affinity) is roughly 19 mmHg, whereas adult hemoglobin has a value of approximately 26.8 mmHg. As a result, the so-called "oxygen saturation curve", which plots percent saturation vs. pO2, is left-shifted for fetal hemoglobin in comparison to the same curve in adult hemoglobin.

This greater affinity for oxygen is explained by the lack of fetal hemoglobin's interaction with 2,3-bisphosphoglycerate (2,3-BPG or 2,3-DPG). In adult red blood cells, this substance decreases the affinity of hemoglobin for oxygen. 2,3-BPG is also present in fetal red blood cells, but interacts less efficiently with fetal hemoglobin than adult hemoglobin., as a result, oxygen will bind to it with higher affinity than adult hemoglobin.

For mothers to deliver oxygen to a fetus, it is necessary for the fetal hemoglobin to extract oxygen from the maternal oxygenated hemoglobin across the placenta.

This requires the fetal hemoglobin to have a higher oxygen affinity than that of the maternal carrier. This is achieved by a fetal hemoglobin subunit γ (gamma), instead of the ß (beta) subunit.

The γ subunit has fewer positive charges than the adult hemoglobin ß subunit.

This means that 2,3-BPG is less electrostatically bound to fetal hemoglobin as compared to adult hemoglobin and therefore less effective in lowering the oxygen affinity of the fetal hemoglobin.

This lowered affinity allows for adult hemoglobin (the maternal hemoglobin) to readily transfer its oxygen to the fetal hemoglobin.

Fate of RBCThe life span of red blood cell is 120 days, than the red blood cells are destroyed by Macrophages. Hemoglobin is released and both iron & globin are split off and bilirubin is formed. The released iron is stored in the body binding with tissue protein called Apo ferritin to form ferritin.

Fig: Fate of RBC

Hemoglobin split into globin and heme, and the heme ring is opened to give-1.Free iron that is transported in the blood by transferrin.2.A straight chain of four pyrrole nuclei that is the substrate from which bilirubin will eventually be formed.The first substance formed is biliverdin, but this is rapidly reduced to free bilirubin, which is gradually released from the macrophages in to the plasma. Within hours, the free bilirubin is absorbed through the hepatic cell membrane. thereafter conjugated about 80% with the glucuronic acid to form bilirubin glucuronide, about 10% with sulfate and the final 10% with other substances.

In these form bilirubin is excreted from the hepatocytes by an active transport process in to the bile canaliculi and then into the intestines.

Formation of fate of

Urobilinogen:Once in the intestine, about one half of the “ conjugated” bilirubin is converted by bacterial action in to the substance Urobilinogen, Some of the Urobilinogen is reabsorbed through the intestinal mucosa back into the blood. Most of this is once again re-excreted by the liver back into the gut, but about 5% is excreted by the kidneys into the urine. After exposure to air in the urine, the Urobilinogen becomes oxidized to urobilin, or in the feces, it becomes altered and oxidized form stercobilin.