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Composition of Blood: Plasma, RBC, WBC and Platelets

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Blood is the red liquid that flows through your arteries and veins, carrying food, oxygen and waste to every part of the body. An adult human has about 5 litres of blood. It looks like one liquid, but it is actually a mixture of a watery part called plasma and tiny solid parts called formed elements.

This topic matters because every organ depends on blood for supply and cleaning. When doctors test a drop of blood, they can learn about infection, anaemia, sugar level and many other health problems, because each component of blood does a different and important job.

Key Concepts

  • Plasma is the pale yellow liquid part, about 55% of blood. It is 90-92% water, with dissolved proteins, salts, glucose, amino acids, hormones and waste like urea.
  • Plasma proteins include albumin (keeps water in blood), globulins (fight infection as antibodies) and fibrinogen (helps clotting).
  • Red blood cells (RBCs) are biconcave discs with no nucleus in mammals. They contain haemoglobin, the red protein that carries oxygen. Normal count is about 5 to 5.5 million per cubic millimetre of blood.
  • White blood cells (WBCs) are larger, colourless cells with a nucleus, about 6,000-8,000 per cubic millimetre. They defend the body against germs.
  • Platelets are tiny cell fragments, about 1.5 to 3.5 lakh per cubic millimetre, needed for blood clotting.

How It Works

  1. The heart pumps blood to the lungs, where RBCs pick up oxygen and become bright red.
  2. Oxygen-rich blood returns to the heart and is pumped to all body tissues.
  3. Plasma carries digested food, hormones and salts to the cells and brings back carbon dioxide and urea as waste.
  4. If germs enter, WBCs move towards them, swallow them or make antibodies to destroy them.
  5. When a vessel is cut, platelets gather at the injury and start clot formation to stop bleeding.

Worked Example

Suppose a blood sample has a total volume of 200 mL. Plasma forms about 55% of blood. So plasma volume = 55/100 x 200 = 110 mL. The formed elements form the remaining 45%, which is 45/100 x 200 = 90 mL. Inside those 90 mL, almost all the volume is RBCs, because WBCs and platelets form only a very thin layer. This is why after spinning blood in a centrifuge, you see a large red layer at the bottom, a thin white middle layer, and yellow plasma on top.

Common Misconceptions

  • "Blood is blue inside veins." Wrong. Blood is always red; oxygen-rich blood is bright red and oxygen-poor blood is dark red. Veins only look blue because of how skin absorbs light.
  • "Plasma and serum are the same." Wrong. Serum is plasma without fibrinogen and clotting factors.
  • "RBCs have a nucleus like other cells." Wrong in mammals. Mature human RBCs lose their nucleus, which gives more space for haemoglobin.

Real-World Applications

Blood tests use these components daily: low RBC count shows anaemia, high WBC count suggests infection, and low platelets explain easy bleeding in diseases like dengue. Plasma is separated and given to burn and shock patients, while packed RBCs are transfused to people who have lost blood. Understanding composition helps doctors choose exactly which part a patient needs instead of giving whole blood every time.

In short, blood is plasma plus RBCs, WBCs and platelets. Each part has a clear job: transport, oxygen carriage, defence and clotting. Together they keep every cell alive and protected.

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