Understanding Human Blood And Its Components

Blood is an essential component of our body and performs some critical functions. Blood is a type of fluid connective tissue, which is required in the body for transport of nutrients and gases. It also helps in the removal of waste products such as carbon dioxide, ammonia and other metabolites. It has a vital role to play in immunity as well as in maintaining homeostasis. Hormones are transported to the target organs through blood only. The main constituents of the human blood include fluid matrix, i.e. plasma and formed elements, which include all types of blood cells, viz., RBCs, WBCs and platelets. On average an adult healthy individual has 4.5 to 5.5 litres of blood.

Letโ€™s make ourselves familiar with these important constituents of the blood.


It is the fluid part of the blood and constitutes 55% of the blood. Plasma is mainly water (90-92%), proteins (6-8%) and some amounts of ions, other biomolecules such as amino acid, glucose, lipids, etc. The proteins and ions help in maintaining the pH of blood. Hormones are also transported through plasma. The main plasma proteins include:

  • Albumin – helps in osmotic balance and pH of blood.
  • Fibrinogen – involved in blood coagulation. When there is an injury blood clotting helps in preventing excessive blood loss. Plasma also contains various clotting factors.
  • Globulin – Immunoglobulins are required for immunity. Antibodies neutralise foreign antigens.

If we remove clotting factors from the plasma, it is known as serum.

Formed Elements

Formed elements or blood cells account for 45% of the blood. Formed elements include erythrocytes (red blood cells or RBC), leucocytes (white blood cells or WBC) and platelets.


Erythrocytes or RBCs are an important constituent of the blood and are required for oxygen transport to various tissues. Human erythrocytes are biconcave in shape and devoid of the nucleus. They are formed in the bone marrow and have a lifespan of 120 days. They are destroyed in the spleen, which is known as the graveyard of RBCs.


We all know about haemoglobin present in the blood and its diagnostic importance. Red blood cells contain iron-containing respiratory pigment, which is known as haemoglobin. The main haemoglobin function is to carry oxygen from the lungs to various tissues in our body through blood. It carries oxygen as oxyhaemoglobin. Who doesnโ€™t know about pulse oximeters these days, which measures the blood oxygen saturation level. It estimates the amount of oxygen bound to haemoglobin.

Haemoglobin also transports 20-25% of carbon dioxide from tissues back to the lungs as carbaminohemoglobin. The normal Hb level of blood in a healthy individual is 12-16 g/dl. An interesting fact about erythrocytes is that they lack mitochondria and due to absence of mitochondria, RBCs can not utilise the oxygen itself.


Leucocytes or WBCs are nucleated and less in number compared to RBCs. They have a distinct role to play in the defence mechanism of the body. They can identify and kill foreign pathogens. White blood cells can be categorised into two categories:

  • Granulocytes – Neutrophils, eosinophils and basophils
  • Agranulocytes – Lymphocytes and monocytes

All the different types of WBCs play specific roles. Eosinophils are involved in allergic reactions. Neutrophils and monocytes destroy foreign antigen by phagocytosis. Basophils are associated with inflammation. They secrete histamine, heparin, serotonin, etc. involved in inflammatory reactions. Lymphocytes, i.e. B and T lymphocytes are involved in the immune response and responsible for humoral and cell-mediated immunity. They are produced in the bone marrow but mature in different organs such as thymus, lymph nodes, etc.


They are also known as thrombocytes. Platelets are involved in blood clotting. They get attracted to the site of injury to the blood vessel and form a plug. It then releases certain factors and stimulates the mechanism of blood clotting. It is a cascade of enzymatic reactions, which ultimately result in the formation of fibrin thread from inactive fibrinogen and forms the blood clot.

All types of blood cells are formed from the stem cells present in the bone marrow known as hematopoietic cells. The constituents of blood are an important diagnostic tool. When you are unwell, the first thing your doctor tells you to do is the blood test. When next time you go for the complete blood count, do check all the constituents of the blood closely.


This was in brief about the blood. For more information related to the human blood and the circulatory system, subscribe to BYJU’S YouTube Channel for more information.














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