They are the most common type of blood cell (5.1 to 5.8 million/per cubic mm)
They give a red colour to the blood. The red colour is due to a pigment haemoglobin (a protein). It is oxygen-carrying red coloured pigment. Haemoglobin maintains pH of blood and acts as a buffer. Less amount of haemoglobin results in anaemia.
Individual RBC appears yellowish in colour but in bulk, they give a red colour to the blood.
They are non-nucleated, small in size, round, disc-like, and biconvex in shape (thin in the middle and thick around the edges).
They contain oxygen-carrying red coloured pigment haemoglobin (a protein).
They do not contain a nucleus, mitochondria, endoplasmic reticulum and Golgi complex. Hence more space is available for oxygen-carrying pigment haemoglobin.
The normal RBC range for men is 4.7 to 6.1 million cells per mL, For women who aren’t pregnant, the range is 4.2 to 5.4 per mL. In the pregnancy, the RBC count decreases. For children, the range is 4.0 to 5.5 per mL. For the persons living at high altitude and persons engaged in hard physical work, the RBC count is high.
The number of erythrocytes is counted by the instrument called haemocytometer.
RBCs are formed in the bone marrow, under the influence of hormone erythropoietin (formed in the kidneys). The formation of RBCs is called erythropoiesis. In the foetus, they are formed in liver and spleen whereas in the adult they are formed in the red bone marrow.
RBCs are formed in the bone marrow, under the influence of hormone erythropoietin (formed in the kidneys),
The breakdown of old and worn out RBCs takes place in spleen, liver and bone marrow. The process of the breakdown of old and worn out RBCs is called haemolysis.
The iron released during the breakdown of the RBCs is retained and reused by the bone marrow to form haemoglobin. The breakdown of protein part of RBCs forms bilverdin., which is carried to the liver. In the liver, it is converted into bilirubin and is excreted out with bile.
The yellow colour of the plasma is due to the presence of the pigment bilirubin.
Due to the absence of the nucleus, the lifespan of RBCs is about 120 days.
Structural Advantages of RBCs:
The biconcave nature of red blood cells is due to the loss of nucleus. It increases their surface area and assists in better oxygen diffusion.
Due to the absence of nucleus RBCs remain flexible and can squeeze through small blood capillaries.
Loss of Mitochondria:
The lack of nucleus and mitochondria means that red blood cells must get their energy differently using a process called glycolysis to produce ATP (anaerobic respiration) followed by lactic acid production.
Due to the absence of mitochondria, the oxygen absorbed by the haemoglobin is not used in the cell for producing energy and almost all the oxygen absorbed, can be delivered to the cell
Due to the absence of mitochondria RBCs remain flexible and can squeeze through small blood capillaries.
Small, Elastic Disc-like Structure:
The diameter of the capillary is smaller than the red blood cell. Due to the special shape, the cell can pass through the capillary after getting folded, squeezed, twisted,
Structural Disadvantages of RBCs:
Due to the lack of nuclei and organelles, the mature red blood cells do not contain DNA and cannot synthesize any RNA, and consequently, they cannot divide or repair themselves, limiting their lifespan.
It is iron containing conjugated protein and oxygen-carrying pigment in RBCs. Its empirical formula is C2952H4664N812O832S8Fe4. It is a macromolecule and has the molecular mass greater than 66000.
The haemoglobin count in blood is measured by an instrument known as haemometer or haemoglobinometer. In human adult male has haemoglobin count of 15.0 g per 100 mL, for females 10.0 to 14.0 g per 100 m and for infants 16.5 – 19.5 g per 100 mL
Heme is iron porphyrin compound. Porphyrin is a tetrapyrrole structure (Pyrroles are a five-atom ring with four carbon and one nitrogen atoms).
Ferrous iron occupies the centre of the porphyrin ring and establishes linkages with all the four nitrogens of all the pyrrole rings. It is also linked to the nitrogen of imidazole ring of histidine present in globin part. Globin part is made of four polypeptide chains, to identical α-chains and two identical β-chains in normal adult haemoglobin. Each chain contains a “heme” in the so-called ‘heme pocket’. So one haemoglobin molecule possesses four heme units.
Note that in chloroplast the central atom of porphyrin ring is magnesium.
Haemoglobin forms an unstable, reversible bond with oxygen. In its oxygenated state, it is called oxyhemoglobin and is bright red.
Haemoglobin + Oxygen ⇌ Oxyhaemoglobin
RBCs carrying oxygen travel to tissues in different parts of the body. In the tissues oxyhaemoglobin readily liberates oxygen.
Oxyhaemoglobin ⇌ Haemoglobin + Oxygen
A small amount of carbon dioxide from tissues combine with Globin protein of haemoglobin and brings it to the lungs.
Functions of RBCs:
transporting oxygen and nutrients from the lungs to the tissues.
Carrying carbon dioxide from the tissues to lungs. 23% carbon dioxide formed in tissues is carried to lungs by this method.
forming blood clots to prevent excess blood loss
carrying cells and antibodies to the site of action that fights infection
bringing waste products to the kidneys and liver, which filter and clean the blood
regulating body temperature
Haemoglobin is an excellent acid-base buffer and hence maintains the pH of the blood.