Introduction
Blood in vertebrates is a complex tissue comprising different cells that are suspended in a liquid matrix referred to as the plasma. Along with the transporting molecules, blood tends to stabilise the salt concentration and pH of the fluids in the body. It also serves as a conduit for the cells functioning in the immune system. Additionally, it is also involved in regulating the body temperature by transferring the heat between colder and warmer body regions, and between the body and the external surroundings.
The total volume of blood in an average-sized adult human is about 4 to 5 L. It accounts for about 8% of the weight of the body. A straw-coloured, clear fluid – plasma, accounts for about 55% of the blood volume in human males and 58% in females, on an average.
In the plasma are suspended three important types of blood cells – erythrocytes, leukocytes and the platelets. It makes up for the hematocrit – the remnants of the blood volume.
Development of Blood Cells
Blood cells in humans develop in the red bone marrow, mainly in the vertebrae, ribs, sternum and the pelvis. The blood cells take their origin from the cells referred to as pluripotent stem cells, which retain the embryonic capacity for division. These pluripotent stem cells differentiate into other two cell types – lymphoid and myeloid stem cells.
The myeloid stem cells produce the erythrocytes and platelets and 4 types of leukocytes – basophils, neutrophils, macrophages/monocytes and eosinophils. The lymphoid stem cells produce the two other types of leukocytes functioning in the immune system – T lymphocytes and B lymphocytes.
Take a look at the chart below for a better understanding-
Pluripotent stem cells
You may like: NEET MCQs on Blood
Components of Blood and their Functions
The table below depicts the different components of blood and their functions, along with their percentage or quantity.
- Hematocrit (cellular part): Males (45%), Females (42%) of the total mass
- Plasma constituents: Males (55%), Females (58%) of the total mass
Hematocrit
| Blood component | Function | Cells per microlitre |
| Red blood cells (erythrocytes) | Involved in the transportation of carbon dioxide and oxygen | 4.8X106 – 5.4X106 |
White blood cells (leukocytes)
|
Involved in immune response
Phagocytosis at the time of inflammation Defensive response against parasitic worms In all the defensive responses, phagocytosis Producing substances for inflammatory responses and eliminating fat from the body |
1000-2700
3000-6750 100-360 150-720 25-90 |
| Platelets | Participates in clotting | 150,000 to 450,000 |
Plasma Constituents
| Blood component | Function | Plasma volume percentage |
| Sugars, ions, amino acids, lipids, vitamins, hormones, dissolved gases | Participates in pH, extracellular fluid volume, etc. | 1-2 |
| Albumin, fibrinogen, globulin, etc – Plasma proteins | Transportation of lipids, clotting, defence, involved in extracellular fluid volume, etc. | 7-8 |
| Water | Serves as a solvent | 91-92 |
Red Blood Cells (Erythrocytes)
The red blood cells (RBCs), also referred to as the erythrocytes, deliver oxygen from the lungs to different tissues of the body. There are about 5 million of these cells per microlitre of human blood. They appear as disc-like, flattened cells with indentations on each of the flattened surfaces, hence the biconcave appearance – thicker at the edges than the middle.
These cells have a diameter of about 8 mm and are 2 mm wide. RBCs are extremely flexible cells that can squeeze through the capillaries that are narrow. They are derived from the precursors found in the bone marrow, produced from the myeloid stem cells. When it matures, the mammalian RBCs lose their nuclei, ribosomes and cytoplasmic organelles, hence restricting their metabolic lifespan and capabilities. The leftover cytoplasm has the enzymes required for glycolysis, abundantly available in haemoglobin.
Most of the ATP from glycolysis is utilised to fuel active transport mechanisms which pass ions in and out of the RBCs. The protein providing RBCs with its distinct red colour is haemoglobin. It comprises 4 polypeptides in turn, each of which is associated with a non-protein heme group containing an iron atom in their centre. This atom of iron is associated with the oxygen molecules as the blood circulates through the lungs, releasing oxygen as the blood continues to flow through the different tissues of the body.
In the event when the oxygen-carrying capacity of the blood declines below the usual level, it is identified by the kidneys, which secrete the EPO (erythropoietin) hormone into the bloodstream. The hormone triggers the stem cells in the bone marrow to increase the production of the RBC. The erythropoietin hormone is also secreted after the loss of blood, and when mammals are in higher altitudes.
The oxygen-carrying capacity increases as the new erythrocytes enter the bloodstream. If this capacity increases above normalcy, the production of the hormone drops in the kidneys and production of RBCs declines. The blood groups in humans are decided by the antigens present on the surface of the erythrocytes.
White Blood Cells (Leukocytes)
The white blood cells or leukocytes remove the dying and dead cells from the body, eliminating cellular debris and defending against the invading entities. WBCs are so called as they are colourless. It retains its nuclei, ribosomes and cytoplasmic organelles as it matures, unlike the RBCs. Consequently, these cells are fully functional. While some leukocytes are produced from the precursor cells found in the bone marrow that are derived from the myeloid stem cells, others are produced from the lymphoid stem cells in the bone marrow.
Platelets
The platelets are round or oval in shape having a diameter of about 2-4 mm, and have enzymes and other factors which participate in blood clotting. These are shed from the surface of large cells in the bone marrow and are obtained from the myeloid stem cells. When the blood vessels are destroyed, the collagen fibres contained in the extracellular matrix get disclosed to the blood that is flowing out.
Then, the blood adheres to the collagen fibres, releasing signalling molecules that induce surplus platelets to cling to it. Consequently, the process is pursued which forms a plug that aids in sealing the damaged areas. With the plug formation, the platelets release other factors which convert fibrinogen into fibrin. The cross-links from the fibrin (insoluble threads) form a mesh-like network trapping the cells and platelets, thereby sealing the wounded site.
The complete mass is a clot of blood. The diseases or mutations which interfere with factors and enzymes participating in the clotting mechanism have adverse effects. It results in uncontrolled bleeding.
Plasma
Plasma forms the liquid component of blood. It is a mix of sugar, water, proteins, fats and salts. The primary function of plasma is in the transportation of the blood cells all through the body with the nutrients, antibodies, wastes, proteins of clotting, proteins and hormones that aid in maintaining the fluid balance in the body.
You just read about components of blood and understood erythrocytes, leukocytes, platelets and plasma in detail.
Visit NEET BYJU’S to explore related articles for your NEET preparation.
More here:
- What Are the Parts of a Red Blood Cell?
- NEET Questions – Chapter 18 – Body Fluids and Circulation
- How Is Human Blood Produced?
- Is Blood Part Of The Circulatory System?
- How Does Blood Flow in the Heart?
Frequently Asked Questions on Components of Blood
What are the 4 components of blood?
Blood in the human body performs varied functions, such as delivering oxygen and nutrients to cells, and carrying metabolic wastes away from the cell. It has 4 main components, they are – white blood cells (WBCs), red blood cells (RBCs), platelets and plasma.
What is the lifespan of RBCs?
In an average human, about two to three million RBCs are produced every second. Its lifespan is about 120 days in the circulatory system. Towards the end of their lifespan, these RBCs are engulfed and destroyed by the macrophages – a type of leukocytes seen in the bone marrow, liver and spleen. The stability with regard to the number of RBCs is overlooked by the negative feedback mechanism linked with the oxygen-carrying capacity of the blood.
Comments