Components of Blood and Blood Cells

Formed Elements

The formed elements are found freely suspended in the liquid plasma.

There are three types of formed elements:

• Red blood corpuscles (RBCs)

• White blood corpuscles (WBCs)

• Platelets

Red Blood Corpuscles (RBCs)

Erythrocytes

They are small biconcave circular cells also called erythrocytes.

Blood, a Liquid Tissue

They are thicker at the edges than in the centre.

The erythrocytes are flexible so that they can pass through the narrow capillaries easily.

They number 5 million per cubic mm in adult males and 4.5 million per cubic mm in adult females. Their number is higher in early infancy. The cells are composed of a network of fats and proteins between which are enmeshed numerous pigments called the haemoglobin which give the blood its colour.

Haemoglobin is composed of an iron containing pigment called haeme and a protein called globin. The haemoglobin pigments combine with oxygen to form oxyhaemoglobin in the lungs. Around the body cells, the oxyhaemoglobin dissociates due to low oxygen concentration. Here, some of them combine with the carbon dioxide.

Haemoglobin has a strong affinity for carbon monoxide (CO) with which it forms a stable compound. When carbon monoxide is present in the inhaled air the pigments prefer it to oxygen thereby greatly reducing the oxygen supplied to the body. This is called carbon monoxide poisoning and can even result in death. This is one of the reasons why the vehicle exhaust gases that contain carbon monoxide are harmful.

The red blood cells do not have nucleus, mitochondria or endoplasmic reticulum. The lack of nucleus creates more space inside the cell for the haemoglobin. Lack of mitochondria means that all the oxygen carried by the cell is transported and none of it is used by the cells.

The red blood cells are synthesised in the bone marrows of ribs, sternum and vertebrae at the rate of 1.2 million cells per second. The life span of the cells is only about 120 days. They are destroyed in the liver. The iron part is retained and the pigment is excreted in the bile juice as bilirubin.

Functions of Erythrocytes

• They are carriers of oxygen and carbon dioxide

• They maintain the viscosity of blood

• They maintain acid-base balance

• They maintain ionic balance

• The disintegration of haemoglobin leads to formation of many other pigments like the bilirubin, biliverdin, etc. in the liver

White Blood Corpuscles (WBCs)

They are also called leucocytes. They lack haemoglobin and are therefore colourless. They are nucleated and amoeboid.The amoeboid nature of the leucocytes helps them to squeeze through the walls of the blood vessels in order to engulf bacteria. They do not contain haemoglobin. They number 6000 to 8000 per cubic mm. The ratio of WBC to RBC is 1:7.

During leukamia, a cancer of blood, the number of WBCs increases abnormally at the cost of the number of RBCs usually resulting in death.

They are of five distinct types whose life span varies from 12 hours to 300 days.

The leucocytes are basically of two types:

• Granular leucocytes

• Agranular leucocytes

Granular Leucocytes

The leucocytes have granular cytoplasm. They are of three types based on the stains they take:

• Neutrophils

• Eosinophils

• Basophils

Neutrophils

Neutrophils contain neutral-staining granules. The nucleus is 2 to 7 lobed. They number 3000 to 6000 per cubic mm. Their life span is 12 hours to 3 days. Their function is phagocytosis of bacteria.

Eosinophils

Eosinophils contain granules that take acidic stains like eosin. The nucleus is 2 to 3 lobed. They number 100 to 400 per cubic mm. Their life span is 3 to 5 days. Their function is thought to be anti-allergic properties.

Basophils

Basophils contain granules that take basic stain. The nucleus is variously lobed. They number 25 to 200 per cubic mm. Their life span is 9 to 18 months. They develop into mast cells of the connective tissue.

Agranular Leucocytes

The cytoplasm is devoid of granules. They are of two types based on their origin:

• Lymphocytes

• Monocytes

Lymphocytes

They are produced in the lymph system. Their nucleus is large and occupies most of the cell. The cytoplasm is basophilic. They are more in number in children than in adults. They have a life span of 100 - 200 days. Their functions are phagocytosis and antibody-production.

Monocytes

They are produced in the bone marrow. They have a large kidney-shaped nucleus. The cytoplasm is more than in lymphocytes. They number 100 to 700 per cubic mm. They function as tissue macrophages feeding on damaged tissues.

Functions of Leucocytes

Phagocytosis

- Engulfing solid particles is called phagocytosis.

The leucocytes can also squeeze out between the cells of the blood vessels to engulf the bacteria. This is called diapedesis.

A White Blood Cell coming out of the Blood Vessel (Diapedesis)

The WBCs, mainly the neutrophils and lymphocytes, engulf the bacteria by forming an invagination in their plasmalemma.

WBC Engulfing Bacteria

This protects the body against bacterial attacks. A count of 50,000 or more WBCs per cubic mm in blood is considered abnormal and indicates infection.

Production of Antibodies

The WBCs are involved in the production of antibodies that either neutralise, kill or poison the germs. The WBCs can be induced to produce antibodies with the help of vaccinations thus preparing the body for an attack.

Inflammation at a spot is due to localised infection. The spot becomes red and swollen. There may be pus formation and oozing. The pus is nothing but dead WBCs that have died fighting the infection and dead tissue attacked by the bacteria.

Anti-allergic

Eosinophils are thought to protect the body against allergies. The count of eosinophils in the body goes up during some allergies.