COMPOSITION AND FUNCTIONS OF BLOOD: A TEACHER’S COMPREHENSIVE GUIDE
Welcome, future healthcare professionals!
The haemopoietic system is the system of organs and tissues that are responsible for the production of blood cells. It includes the bone marrow, spleen, thymus, and lymph nodes. Blood is a special type of connective tissue that performs many vital functions — it carries out several life-sustaining processes and protects the body from diseases. The study of blood, its components, blood-forming organs, and blood-related diseases is known as Haematology.
Dpharmguru’s exam insights:
In my years of teaching haematology, I have observed that students often get confused between plasma and serum. Remember: Plasma contains clotting factors, while serum is plasma without clotting factors. This is a very common exam question!
COMPOSITION OF BLOOD
Blood is a connective tissue made up of cells and cell fragments suspended in a liquid medium called plasma. It forms about 8% of the total body weight.
- Adult male: 5–6 litres of blood
- Adult female: 4–5 litres of blood
Blood is made up of two main components:
- Blood Plasma – the liquid portion of blood
- Formed Elements – the solid portion, made up of red blood cells (RBCs), white blood cells (WBCs), and platelets.
1. Blood Plasma
Plasma makes up about 55% of total blood volume (around 2.7–3.0 litres in an average adult). It is a golden-yellow coloured fluid that contains:
- 92% water
- 8% plasma proteins
- Trace amounts of other substances
Components of Plasma
Over 70% of the solids in plasma are proteins, such as:
- Immunoglobulins (antibodies) – for immunity
- Serum albumin – for maintaining osmotic pressure
- Apolipoproteins – for transport of fats
- Transferrin – for transport of iron
- Fibrinogen and prothrombin – for blood clotting
Main Plasma Proteins
- Albumin: Made in the liver. Most abundant plasma protein. Maintains osmotic pressure and keeps fluid inside blood vessels.
- Globulin: Main immune proteins (alpha, beta, and gamma globulins). Gamma globulins are also called immunoglobulins (antibodies). Help in defense and maintaining osmotic balance.
- Fibrinogen: Key protein involved in blood clotting. Converts into fibrin fibres to stop bleeding.
Dpharmguru’s exam insights:
Albumin is the most abundant plasma protein and is produced in the liver. Fibrinogen is essential for blood clotting. A common exam question is: “Which plasma protein is responsible for osmotic pressure?” The answer is albumin.
Functions of Plasma Proteins
- Maintain Oncotic Pressure: Helps keep fluid within blood vessels (mainly by albumin).
- Transport Function: Carry hormones, vitamins, lipids, metals, and drugs throughout the body.
- Immunity: Immunoglobulins (antibodies) fight against bacteria and viruses.
- Clotting Function: Fibrinogen and other factors help in blood clotting and wound healing.
- Buffering Action: Maintain normal pH balance of blood.
- Acute Phase Response: During infection or inflammation, the liver produces special proteins like C-reactive protein (CRP) that help healing.
- Enzyme Activity: Some plasma proteins act as enzymes in body metabolism.
2. Formed Elements
The formed elements are the cellular components of blood. They are suspended in plasma and include:
- Red Blood Cells (RBCs) – Erythrocytes
- White Blood Cells (WBCs) – Leukocytes
- Platelets – Thrombocytes
Red Blood Cells (RBCs or Erythrocytes)
RBCs are disc-shaped cells and make up about 99% of total blood cells. Each RBC contains a red pigment called haemoglobin, which binds with oxygen. RBCs have no nucleus, and their cytoplasm is made up of 65% water and 35% solid material (mostly haemoglobin and some proteins, lipids, and cholesterol).
Functions of RBCs:
- Oxygen Transport: RBCs carry oxygen from lungs to body tissues.
- ATP Release and Vessel Dilation: When blood vessels narrow, RBCs release ATP, which relaxes and widens vessels.
- S-nitrosothiol Release: During low oxygen, RBCs release this compound to improve blood flow.
- Immunity Role: When bacteria damage RBCs, the released free radicals kill bacteria and help protect the body.
Dpharmguru’s exam insights:
RBCs have no nucleus, which allows more space for haemoglobin. This is why they can carry more oxygen. Also, the average lifespan of an RBC is about 120 days — this is a common exam question!
White Blood Cells (WBCs or Leukocytes)
WBCs are the defence cells of the body. They fight infection and foreign substances. WBCs are divided into two main types:
| Type | Presence of Granules | Example |
|---|---|---|
| Granulocytes | Have granules | Neutrophils, Eosinophils, Basophils |
| Agranulocytes | No granules | Monocytes, Lymphocytes |
Granular Leukocytes (Granulocytes)
All granulocytes are formed from myeloblasts in red bone marrow.
- Neutrophils: 60–70% of total WBCs. Nucleus has 5–7 lobes. Contain digestive enzymes that destroy bacteria. Main function: phagocytosis (engulfing microbes).
- Eosinophils: 1–5% of total WBCs. Nucleus has 2–3 lobes. Stained with acid dyes. Active in allergic reactions and parasite infections.
- Basophils: Least numerous. Nucleus is S-shaped. Contain heparin (anticoagulant) and histamine (allergy mediator).
Agranular Leukocytes (Agranulocytes)
- Monocytes: Largest WBCs (12–20 µm). Kidney-shaped nucleus. Formed in bone marrow from monoblasts. When they move into tissues, they become macrophages, which eat dead cells and bacteria.
- Lymphocytes: 20–40% of total WBCs. Small and round with large nucleus. Formed from lymphoblasts in bone marrow. Types:
- B-Lymphocytes: Produce antibodiesT-Lymphocytes: Control immune responsesNatural Killer (NK) Cells: Destroy virus-infected and cancer cells
Dpharmguru’s exam insights:
Neutrophils are the most abundant WBCs and are the first responders to bacterial infections. Eosinophils increase in allergic conditions and parasitic infections. Lymphocytes are responsible for specific immunity — B-cells make antibodies, and T-cells destroy infected cells.
Functions of WBCs
- Phagocytosis: Neutrophils and monocytes eat and destroy germs.
- Antibody Formation: B-lymphocytes form antibodies.
- Wound Repair: WBCs form fibroblasts that help tissue healing.
- Prevent Clotting: Basophils release heparin.
- Fight Allergies: Granulocytes neutralize allergic reactions using histamine.
Platelets (Thrombocytes)
Platelets are tiny, non-nucleated cell fragments made in bone marrow from megakaryocytes. Normal count: 2,00,000 to 3,50,000 per mm³ of blood. Their main role is in haemostasis — the process that stops bleeding.
Functions of Platelets:
- Vasoconstriction: Platelets release serotonin, which narrows damaged blood vessels.
- Platelet Plug Formation: Platelets stick together at the injury site to form a temporary plug.
- Coagulation (Blood Clotting): Platelets help convert prothrombin → thrombin → fibrin, forming a solid clot.
- Fibrinolysis: Once healing starts, platelets help break down the clot through plasmin enzyme activity.
Dpharmguru’s exam insights:
Platelets are essential for blood clotting. A low platelet count (thrombocytopenia) can cause excessive bleeding. This is a common exam question in haematology!
FUNCTIONS OF BLOOD
- Transportation: Carries oxygen and carbon dioxide, removes waste products, transports nutrients and hormones.
- Thermoregulation: Helps maintain body temperature; increases skin blood flow to lose heat.
- Hydraulic Function: Helps in tissue engorgement during physiological responses.
- Carrier Function: Transports hormones, vitamins, and essential chemicals.
- Ion Balance: Maintains proper ion concentration in tissues.
- Water Balance: Regulates water level between blood and tissue fluids.
- Acid-Base Balance: Maintains normal pH with help of haemoglobin and plasma proteins.
- Homeostasis: Keeps the internal environment stable and prevents large heat fluctuations.
- Storage Function: Acts as a reservoir for proteins, glucose, sodium, and potassium.
- Defensive Function: WBCs destroy bacteria, foreign particles, and produce immunity.
Dpharmguru’s exam insights:
Blood is the lifeline of the body. Understanding its composition and functions is essential for understanding how the body maintains health. In exams, pay special attention to the functions of different WBCs and the role of platelets in clotting. Remember: “Blood is a connective tissue with a liquid matrix (plasma) and cellular elements (RBCs, WBCs, platelets).”
REFERENCES AND FURTHER READING
- Tortora, G. J., & Derrickson, B. H. (2017). Principles of Anatomy and Physiology (15th ed.). John Wiley & Sons.
- Marieb, E. N., & Hoehn, K. (2019). Human Anatomy & Physiology (11th ed.). Pearson Education.
- Hoffman, R., Benz, E. J., Silberstein, L. E., et al. (2018). Hematology: Basic Principles and Practice (7th ed.). Elsevier.
- Bain, B. J., Bates, I., & Laffan, M. A. (2017). Practical Haematology (12th ed.). Wiley-Blackwell.
- National Institutes of Health (NIH). (2022). Blood and Haematology Resources. Retrieved from https://www.nih.gov.
Disclaimer: This article is for educational purposes only and does not constitute medical advice. Always consult qualified healthcare professionals for medical concerns.
written by:
Dr. N. Sujith Kumar
