Red blood cells (RBCs) are the non-nucleated formed elements in the blood. Red blood cells are also known as erythrocytes (erythros = red). Red color of the red blood cell is due to the presence of the coloring pigment called hemoglobin. RBCs play a vital role in transport of respiratory gases. RBCs are larger in number compared to the other two blood cells, namely white blood cells and platelets.
RBC count ranges between 4 and 5.5 million/cu mm of blood. In adult males, it is 5 million/cu mm and in adult females, it is 4.5 million/cu mm.
MORPHOLOGY OF RED BLOOD CELLS
Normally, the RBCs are disk-shaped and biconcave (dumbbell-shaped). Central portion is thinner and periphery is thicker. The biconcave contour of RBCs has some mechanical and functional advantages.
Advantages of Biconcave Shape of RBCs
1. Biconcave shape helps in equal and rapid diffusion of oxygen and other substances into the interior of the cell.
2. Large surface area is provided for absorption or removal of different substances.
3. Minimal tension is offered on the membrane when the volume of cell alters.
4. Because of biconcave shape, while passing through minute capillaries, RBCs squeeze through the capillaries very easily without getting damaged.
|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. This difference in thickness is because of the biconcave shape.|
|Surface area||120 sq µ.|
|Volume||85 to 90 cu µ.|
Red blood cells are nonnucleated. Only mammal, which has nucleated RBC is camel. Because of the absence of nucleus in human RBC, the DNA is also absent. Other organelles such as mitochondria and Golgi apparatus also are absent in RBC. Because of absence of mitochondria, 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.
RBC has a special type of cytoskeleton, which is made up of actin and spectrin. Both the proteins are anchored to transmembrane proteins by means of another protein called ankyrin. Absence of spectrin results in hereditary spherocytosis. In this condition, the cell is deformed, losses its biconcave shape and becomes globular (spherocytic). The spherocyte is very fragile and easily ruptured (hemolyzed) in hypotonic solutions.
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 (pleural = rouleau) formation. It is accelerated by plasma proteins globulin and fibrinogen.
Specific gravity of RBC is 1.092 to 1.101.
PACKED CELL VOLUME
Packed cell volume (PCV) is the proportion of blood occupied by RBCs expressed in percentage. It is also called hematocrit value. 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.
LIFESPAN OF RED BLOOD CELLS
Average lifespan of RBC is about 120 days. After the lifetime the senile (old) RBCs are destroyed in the reticuloendothelial system.
Determination of Lifespan of Red Blood Cells
Lifespan of the RBC is determined by radioisotope method. RBCs are tagged with radioactive substances like radioactive iron or radioactive chromium. Life of RBC is determined by studying the rate of loss of radioactive cells from circulation.
FATE OF RED BLOOD CELLS
When the cells become older (120 days), the cell membrane becomes more fragile. Diameter of the capillaries is less or equal to that of RBC. Younger RBCs can pass through the capillaries easily. However, because of the fragile nature, the older cells are destroyed while trying to squeeze through the capillaries. The destruction occurs mainly in the capillaries of red pulp of spleen because the diameter of splenic capillaries is very small. So, the spleen is called ‘graveyard of RBCs’.
Destroyed RBCs are fragmented and hemoglobin is released from the fragmented parts. Hemoglobin is immediately phagocytized by macrophages of the body, particularly the macrophages present in liver (Kupffer cells), spleen and bone marrow.
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. Daily 10% RBCs, which are senile, are destroyed in 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.
Daily 10% RBCs, which are senile, are destroyed in normal young healthy adults. It causes release of about 0.6 g/dL of hemoglobin into the plasma. From this 0.9 to 1.5 mg/dL bilirubin is formed.normal young healthy adults. It causes release of about 0.6 g/dL of hemoglobin into the plasma. From this 0.9 to 1.5 mg/dL bilirubin is formed.
FUNCTIONS OF RED BLOOD CELLS
Major function of RBCs is the transport of respiratory gases. Following are the functions of RBCs:
1. Transport of Oxygen from the Lungs to the Tissues
Hemoglobin in RBC combines with oxygen to form oxyhemoglobin. About 97% of oxygen is transported in blood in the form of oxyhemoglobin.
2. Transport of Carbon Dioxide from the Tissues to the Lungs.
Hemoglobin combines with carbon dioxide and form carbhemoglobin. About 30% of carbon dioxide is transported in this form.
RBCs contain a large amount of the carbonic anhydrase. This enzyme is necessary for the formation of bicarbonate from water and carbon dioxide. Thus, it helps to transport carbon dioxide in the form of bicarbonate from tissues to lungs. About 63% of carbon dioxide is transported in this form.
3. Buffering Action in Blood
Hemoglobin functions as a good buffer. By this action, it regulates the hydrogen ion concentration and thereby plays a role in the maintenance of acidbase balance.
4. In Blood Group Determination
RBCs carry the blood group antigens like A antigen, B antigen and Rh factor. This helps in determination of blood group and enables to prevent reactions due to incompatible blood transfusion.
VARIATIONS IN NUMBER OF RED BLOOD CELLS
1. PHYSIOLOGICAL VARIATIONS
A. Increase in RBC Count
Increase in the 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:
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.
Before puberty and after menopause in females the RBC count is similar to that in males. During reproductive period of females, the count is less than that of males (4.5 million/cu mm).
3. High altitude
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. The erythropoietin in turn stimulates the bone marrow to produce more RBC.
4. Muscular exercise
There is a temporary increase in RBC count after exercise. It is because of mild hypoxia and contraction of spleen. Spleen stores RBCs. Hypoxia increases the sympathetic activity resulting in secretion of adrenaline from adrenal medulla. Adrenaline contracts spleen and RBCs are released into blood.
5. Emotional conditions
RBC count increases during the emotional conditions such as anxiety. It is because of increase in the sym pathetic activity as in the case of muscular exercise.
6. Increased environmental temperature
Increase in atmospheric temperature increases RBC count. Generally increased temperature increases all the activities in the body including production of RBCs.
7. After meals
There is a slight increase in the RBC count after taking meals. It is because of need for more oxygen for metabolic activities.
B. Decrease in RBC Count
Decrease in RBC count occurs in the following physiological conditions:
1. High barometric pressures
At high barometric pressures as in deep sea, when the oxygen tension of blood is higher, the RBC count decreases.
2. During sleep
RBC count decreases slightly during sleep and immediately after getting up from sleep. Generally all the activities of the body are decreased during sleep including production of RBCs.
In pregnancy, the RBC count decreases. It is because of increase in ECF volume. Increase in ECF volume, increases the plasma volume also resulting in hemodilution. So, there is a relative reduction in the RBC count.
1. Pathological Polycythemia
Pathological polycythemia is the abnormal increase in the RBC count. Red cell count increases above 7 million/cu mm of the blood. Polycythemia is of two types, the primary polycythemia and secondary polycythemia.
2. Primary Polycythemia – Polycythemia Vera
Primary polycythemia is otherwise known as polycythemia vera. It is a disease characterized by persistent increase in RBC count above 14 million/cu mm of blood. This is always associated with increased white blood cell count above 24,000/cu mm of blood. Polycythemia vera occurs in myeloproliferative disorders like malignancy of red bone marrow.
3. Secondary Polycythemia
This is secondary to some of the pathological conditions (diseases) such as:
1. Respiratory disorders like emphysema.
2. Congenital heart disease.
3. Ayerza’s disease (condition associated with hypertrophy of right ventricle and obstruction of blood flow to lungs).
4. Chronic carbon monoxide poisoning.
5. Poisoning by chemicals like phosphorus and arsenic.
6. Repeated mild hemorrhages.
All these conditions lead to hypoxia which stimulates the release of erythropoietin. Erythropoietin stimulates the bone marrow resulting in increased RBC count.
Abnormal decrease in RBC count is called anemia.
VARIATIONS IN SIZE OF RED BLOOD CELLS
Under physiological conditions, 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).
Microcytes are present in:
i. Iron-deficiency anemia
ii. Prolonged forced breathing.
iii. Increased osmotic pressure in blood.
Macrocytes are present in:
i. Megaloblastic anemia.
ii. Decreased osmotic pressure in blood.
Anisocytes occurs in pernicious anemia.
VARIATIONS IN SHAPE OF RED BLOOD CELLS
Shape of RBCs is altered in many conditions including different types of anemia.
1. Crenation: Shrinkage as in hypertonic conditions.
2. Spherocytosis: Globular form as in hypotonic conditions.
3. Elliptocytosis: Elliptical shape as in certain types of anemia.
4. Sickle cell: Crescentic shape as in sickle cell anemia.
5. Poikilocytosis: Unusual shapes due to deformed cell membrane. The shape will be of flask, hammer or any other unusual shape.
VARIATIONS IN STRUCTURE OF RED BLOOD CELLS
1. PUNCTATE BASOPHILISM
Striated appearance of RBCs by the presence of dots of basophilic materials (porphyrin) is called punctate basophilism. It occurs in conditions like lead poisoning.
2. RING IN RED BLOOD CELLS
Ring or twisted strands of basophilic material appear in the periphery of the RBCs. This is also called the Goblet ring. This appears in the RBCs in certain types of anemia.
3. HOWELL-JOLLY BODIES
In certain types of anemia, some nuclear fragments are present in the ectoplasm of the RBCs. These nuclear fragments are called HowellJolly bodies.