Red blood cells are the cellular component of blood, millions of which in the circulation of vertebrates give the blood its characteristics colour and carry oxygen from the lungs to the body tissues. RBCs take up oxygen from the lungs or gills of fish and release it into tissues while squeezing through the body’s capillaries.

Red blood cells are derived from stem cells in red bone marrow. The production of red blood cells is called ‘erythropoiesis’.

Red blood cells is also known as erythrocytes (erythros = red and cyte translated as ‘cell’ in modern usage). The red colour of RBC is due to the presence of the colouring pigment called hemoglobin. 95% of the proteins in RBC is hemoglobin and 5% of protein are enzymes.

Red blood cells are non-nucleated formed elements in the blood. It lacks cytoplasmic organelles such as nucleolus, mitochondria and ribosomes. Among the mammals, camel has nucleated RBC.

RBCs are larger in number as compared to the other two blood cells, namely white blood cells and platelets but, they are the smallest cells in the mammalian body.

Normal range of red blood cells

RBC count ranges between 4 and 6 million/cu mm of blood in adult person.

  • In adult males        :   4.5 to 6 million/cu mm
  • In adult females     :   4 to 5.5 million/cu mm
  • At birth                  :   6.7 million/cu mm

Morphology of red blood cells

Shape:

In humans, mature red blood cells are disk shaped and biconcave (dumbbell shape). Central portion is thinner and periphery is thicker. The biconcave shape of RBCs has some mechanical and functional advantages-

  • It helps in equal and rapid diffusion of oxygen and other substances into the interior of the cells. Oxygen does not have to travel a great distance for the diffusion.
  • Biconcave shape increases the surface area of the RBC. Large surface area is provided for absorption or removal of different substances.
  • While passing through minute capillaries, RBCs can easily squeeze itself without getting damaged. 
  • Minimal tension is offered on the membrane when the volume of cell alters.

Size:

  • Diameter : 7.2 μ (6.9 to 7.4 μ).
  • Thickness : At the periphery it is thicker with 2.2 μ and at the center it is thinner with 1 μ because of the biconcave shape.
  • Surface area : 120 sq μ.
  • Volume : 85 to 90 cu Î¼.

Structure:

Red blood cells are anucleated (lack of cell nucleus). Unlike other cells of body, mature red blood cells do not contain a nucleus, mitochondria and ribosomes. Due to the absence of nucleus in human RBC, the DNA is also absent in RBC. The energy is produced from glycolytic process. Red cell does not have insulin receptor and so the glucose uptake by this cell is not controlled by insulin. The absence of these cell structures leaves space for the hundreds of millions of hemoglobin molecules. RBC contain some structural proteins that helps the blood cells maintain their unique structure and enable them to change their shape to squeeze while passing through minute capillaries. This includes the protein actin and spectrin, a cytoskeletal protein element.

Properties of red blood cells

  • When blood is taken out of the blood vessel, the RBCs pile up one above another like the pile of coins. This property of the RBCs is called rouleaux formation. It is accelerated by plasma proteins globulin and fibrinogen.
  • Specific gravity of RBC is 1.092 to 1.101.
  • Packed cell volume (PCV) or hematocrit value is the proportion of blood occupied by RBCs expressed in percentage. It is 45% of the blood and the plasma volume is 55%.
  • During circulation, the RBCs remain suspended uniformly in the blood. This property of the RBCs is called the suspension stability.
  • Average lifespan of RBC is about 120 days. After the lifetime the senile (old) RBCs are destroyed in reticuloendothelial system.

Fate of red blood cells

When the cells become older than 120 days, the cell membrane becomes fragile. The cell ruptures during passage through minute capillaries. Young cells are flexible. They can squeeze themselves while passing through minute capillaries. The destruction occurs mainly in the capillaries of red pulp of spleen because the diameter of capillaries is very small in spleen. So, the spleen is called ‘graveyard of RBCs’.

Destroyed RBCs are fragmented and hemoglobin is released from the fragmented parts. Hemoglobin is degraded into iron, globin and porphyrin. Iron combines with the protein called apoferritin to form ferritin, which is stored in the body and reused later. Globin enters the protein depot for later use. Porphyrin is degraded into bilirubin, which is excreted by liver through bile.

Function of red blood cells

The most important function of RBCs is the transfer of respiratory gases.
  • Transport oxygen from the lungs to the tissues (oxyhemoglobin).
  • Transport carbon dioxide from tissues to the lungs (carboxyhemoglobin).
  • Hemoglobin acts as a buffer and regulates the hydrogen ion concentration (acid base balance).
  • RBCs carry the blood group antigens like A antigen, B antigen and Rh factor. This helps in determination of blood group.

Variations in number of red blood cells

1. Physiological variation

Increase in RBC count:

Increase in RBC count is known as polycythemia. It occurs in both physiological and pathological conditions. When it occurs in physiological conditions it is called physiological polycythemia. The increase in number during this condition is marginal and temporary. It occurs in the following conditions:

Sex:- The red blood cell count in male is similar to female before puberty and after menopause. The RBCs count in female during reproductive period is less than that of males (4.5 million/cu mm).

Age:- At birth, the RBC count is 8 to 10 million/cu mm of blood. The count decreases within 10 days after birth due to destruction of RBCs causing physiological jaundice in some newborn babies. However, in infants and growing children, the cell count is more than the value in adults.

Emotional conditions:- RBC count increases during the emotional conditions such as excitement and stress. It is because at this condition, adrenaline is released from adrenal gland, which stimulates the spleen.

After meals:-  There is a slight increase in the RBC count after taking meals. It is because of need for more oxygen for metabolic activity.

High altitude:-The inhabitants of mountains (above 10,000 feet from mean sea level) have an increased RBC count of more than 7 million/cu mm. It is due to hypoxia (decreased oxygen supply to tissues) in high altitude. Hypoxia stimulates kidney to secrete a hormone called erythropoietin which stimulates the bone marrow to produce RBCs.

Exercise:- There is a temporary increase in RBC count after exercise. It is because of mild hypoxia and contraction of spleen.

Increased environmental temperature:- Increase in atmospheric temperature increases RBC count.

Decrease in RBC count:

It occurs in the following conditions:

High barometric pressures:-  At high barometric pressures as in deep sea, the RBC count decreases when the oxygen tension of blood is higher.

During sleep:- RBC count decreases slightly during sleep.

Pregnancy:-  In pregnancy, the RBC count decreases. It is because of increase in extracellular volume. Increase in extracellular volume, increases the plasma volume also resulting in hemodilution. So, there is a relative reduction in the RBC count.

2. Pathological variation

Pathological polycythemia:- It is the abnormal increase in the RBC count. Red cell count increases above 7 million/cu mm of the blood.

Anemia:- Abnormal decrease in RBC count is called anemia.

Variations in size of red blood cells

The size of RBCs in venous blood is slightly larger than those in arterial blood. In pathological conditions, the variations in size of RBCs are:

  1. Microcytes (smaller cells)
  2. Macrocytes (larger cells)
  3. Anisocytes (cells with different sizes).

Variations in shape of red blood cells

The shape of RBCs is changes in many conditions including different types of anemia.

Crenation:- Shrinkage as in hypertonic conditions.

Spherocytosis:- Globular form as in hypotonic conditions.

Elliptocytosis: Elliptical shape as in certain types of anemia.

Sickle cell:- Crescentic shape as in sickle cellanemia.

Poikilocytosis:- Unusual shapes due to deformed cell membrane. The shape will be of flask, hammeror any other unusual shape.

Variations in structure of red blood cells

Punctate basophilism:- Striated appearance of RBCs by the presence of dots of basophilic materials (porphyrin).

Ring:-  Ring or twisted strands of basophilic material appear in the periphery of the RBCs.

Howell-jolly bodies:- In certain types of anemia, some nuclear fragments are present in the ectoplasm of the RBCs.