Physiology of red blood cells

2021-06-14 04:39 PM

Red blood cells have neither nucleus nor organelles, the main component of red blood cells is haemoglobin, which makes up 34 percent of its weight.

Shape - structure

Red blood cells make up more than 99% of the visible components of blood. These are biconcave disc-shaped cells, 7-8 mm in diameter, 2-2.5 mm thick at the periphery and 1 mm in the centre, with an average volume of 90-95 mm3. This shape has two advantages as follows:

Increasing the contact surface area increases the gas diffusion capacity by 30% compared with erythrocytes of the same volume that are spherical in shape.

Makes red blood cells extremely flexible, able to pass through narrow capillaries without damaging the capillaries or the red blood cells themselves.

Red blood cells have neither nucleus nor organelles. The main component of red blood cells is haemoglobin (Hb), which accounts for 34% of the weight (concentration 34 g/dl in cytoplasmic fluid). The structure of red blood cells is particularly adapted to the function of transporting oxygen.

Amount

In a normal person, the number of red blood cells in the peripheral blood is:

Nam: 5.400.000 ± 300.000 /mm3.

Female: 4,700,000 ± 300,000/mm3.

According to the initial results of research on some biological indicators of Vietnamese people in 1996, the number of red blood cells in the blood of normal Vietnamese people varies depending on the author.

The red blood cell count can be variable in some physiological circumstances. In neonates, the red blood cell count is high within the first week or two, followed by red blood cell rupture causing physiological jaundice. In addition, the number of red blood cells may increase in people who do heavy work, live in highlands.

The process of differentiation of blood cell lines

The process of differentiation of blood cell lines.

Function

The main function of red blood cells is to carry oxygen to the tissues. In addition, red blood cells also have the following functions: transport a part of CO2 (through haemoglobin), help plasma transport CO2 (through the enzyme carbonic anhydrase), regulate acid-base balance thanks to the buffering effect of haemoglobin.

Structure of haemoglobin

Haemoglobin, also known as haemoglobin, is a chromoprotein composed of two components, the heme nucleus and the globin. (Figure).

Heme is a red pigment. Each heme consists of a porphyrin ring and a central Fe2+. A haemoglobin molecule has four heme nuclei, accounting for 5%.

Globin is a protein composed of four identical polypeptide chains, one by one. Normal human haemoglobin is HbA consisting of two a and two b chains. Foetal haemoglobin is HbF consisting of two a and two g chains.

Structure of the haemoglobin molecule

Structure of the haemoglobin molecule.

Abnormalities of the globin chains will change the physiological characteristics of the Hb molecule. For example, in sickle cell anaemia, the amino acid valine substitutes for glutamic at one position in each b-chain causing HbA to become HbS.

Normal human haemoglobin levels are:

Nam: 13.5-18 g / 100 ml (g%).

Female: 12-16 g/100 ml (g%).

Children: 14-20 g/100 ml (g%).

The haemoglobin concentration of normal Vietnamese people studied in 1996 has different values ​​depending on the author.

Gas transport function

Transporting O2 gas:

Red blood cells transport O2 from the lungs to the tissues by the following reaction:

Hb + O2 ---- HbO2 (oxyhaemoglobin)

where O2 is loosely bound to Fe2+. This is a reversible reaction, the direction of which is determined by the O2 pressure gradient. In the Hb molecule, O2 is not ionized but is transported as an O2 molecule.

When air is high in CO (carbon monoxide), haemoglobin combines with CO to produce carboxyhaemoglobin according to the following reaction:

Hb + CO   δ   HbCO

The affinity of Hb for CO is more than 200 times that for O2, so once combined with CO, Hb is no longer able to transport O2. The first sign is bright red skin, the patient falls into a state of excitement, then drowsiness, coma and death. CO is usually produced when fuel is burned incompletely. Treatment is by taking the patient out of the CO2 environment and giving O2 at the same time. The amount of CO in the air is an indicator of the level of environmental pollution.

When the blood is exposed to oxidizing drugs or chemicals, the Fe2+ in the heme nucleus converts to Fe3+ and the haemoglobin becomes methaemoglobin which is no longer able to carry O2. Methaemoglobin when present in the blood will cause cyanosis symptoms. This situation occurs when poisoning some derivatives of aniline, sulphonamide, phenacetin, nitro-glycerine, nitrate in food...

Transporting CO2 gas:

Red blood cells transport CO2 from the tissues to the lungs according to the following reaction:

Hb + CO2 --- HbCO2 (carbaminohaemoglobin)

CO2 is attached to the NH2 group of globin. This is also a reversible reaction, the direction of which is determined by the CO2 pressure. Only about 20% of CO2 is transported in this form, the rest is carried by the alkaline salts of the plasma.

Red blood cell reproduction

The process of erythrocyte differentiation

 

 

The process of red blood cell line differentiation

The process of red blood cell line differentiation.

Protoplasts are the first cells of the red blood cell line that we recognize. The process of differentiation from pro-erythrocytes takes place according to scheme 1.

The stage from stem cells to reticulocytes takes place in the bone marrow, after which reticulocytes are released into the peripheral blood 24-48 hours, the reticulum disappears and becomes mature red blood cells. Peripheral reticulocyte count does not exceed 1%. This ratio allows evaluating the rate of erythropoiesis of the bone marrow after the treatment of anaemia or after acute blood loss.

Haemoglobin synthesis occurs from the pro-erythrocytic stage and is progressive. By the stage of eosinophilic erythropoiesis, saturation is reached.

Regulation of red blood cell reproduction

The number of red blood cells in the circulatory system is tightly regulated so that it changes only to a narrow range. The number of red blood cells must meet the following two requirements:

Sufficient oxygen supply to the organization.

Not too much to avoid impeding blood circulation.

The tissue oxygen concentration is the main factor controlling the rate of erythropoiesis. The rate of erythropoiesis will increase in cases where the amount of oxygen transported to the organization does not meet the needs of the organization and vice versa. The rate of erythropoiesis will increase in the following cases:

When anaemia is caused by blood loss, the bone marrow increases the production of red blood cells. In addition, in people with partial bone marrow damage from x-ray therapy, for example, the remaining bone marrow will increase red blood cell production to meet the body's needs.

People who live in the highlands.

Cases of persistent heart failure or chronic lung diseases.

The hormone that stimulates red blood cell production is the hormone erythropoietin. In a normal person, 90% of erythropoietin is secreted by the kidneys, the rest is mainly produced by the liver. When there is a lack of oxygen in the tissues, erythropoietin is secreted in the blood and promotes the production of pro-erythroblasts from hematopoietic stem cells in the bone marrow. Once erythrocytosis has been formed, erythropoietin promotes its rapid transition to the erythroblasts stages to form mature erythrocytes. In addition, erythropoietin increases Hb synthesis in erythrocytes and increases reticulocyte transport to the peripheral blood.

Nutritional components involved in red blood cell formation

To make red blood cells, it is necessary to provide adequate protein, iron, and vitamins B12, B9 (folic acid).

Protein is required for the synthesis of globin chains and structural components of red blood cells.

Iron required for heme nucleation: the daily iron requirement is 1 mg in men and 2 mg in women. For pregnant women, the need for iron increases, so it is necessary to provide more iron tablets every day.

Vitamin B12 and folic acid are needed for DNA synthesis for cell division. The daily requirement of B12 is 1-3 mg.

Red blood cell life

The average life of red blood cells in peripheral blood is 120 days. Over time, the red blood cell membrane loses its flexibility and eventually the red blood cells burst as they pass through the small capillaries of the spleen. Haemoglobin released from ruptured erythrocytes is phagocytosed by fixed macrophages of the liver, spleen, and bone marrow.

Macrophages release iron into the blood; This iron, along with iron from food, is absorbed by the small intestine and is transported as transferrin to the bone marrow to form new red blood cells, or to the liver and other tissues for storage as ferritin and hemosiderin.

The porphyrin part of heme will be metabolized through several stages in macrophages to form the pigment bilirubin, which is released into the blood, to the liver, and then excreted in the bile. The metabolism of bilirubin will be studied in detail in the chapter on digestion.

In addition, the globin part of haemoglobin is broken down into amino acids that will be used to synthesize proteins for the body.

Some clinical disorders of erythrocyte flow

Anaemia

According to the World Health Organization, anaemia is a decrease in haemoglobin levels:

Male: < 13 g/100 ml of blood.

Female: < 12 g/100 ml of blood.

Neonates: < 14 g/100 ml of blood.

Anaemia can be caused by hookworm, haemorrhage, haemolysis, marrow failure...

Polycythaemia vera

Also known as Vaquez disease, it is caused by a genetic defect in the hematopoietic lineage. These fibroblasts do not stop making red blood cells, even when the number is sufficient. The red blood cell count is usually 7-8 million/mm3.