Plasma and the formed elements

What comes out of a cut

A small cut on a finger bleeds. The red fluid that comes out is blood. It is a fluid connective tissue. Like every connective tissue, it has cells lying in a matrix. The matrix is a liquid called plasma, and the cells are the formed elements.

Blood carries oxygen, carbon dioxide, food, wastes, hormones and heat. It also defends the body. If you spin a sample of blood in a tube, it separates into two layers.

Plasma

Plasma is a straw coloured, slightly sticky fluid. It makes up about 55 per cent of the blood. These are the facts to remember.

• Water is 90 to 92 per cent of the plasma.
• Proteins are 6 to 8 per cent. There are three main kinds.
• Minerals such as Na$^+$, Ca$^{2+}$, Mg$^{2+}$, HCO$_3^-$ and Cl$^-$ are present in small amounts. So are glucose, amino acids and lipids.
• Clotting factors are present too, but they stay inactive until there is an injury.

Serum is plasma from which the clotting factors, including fibrinogen, have been removed. It is what is left when blood has clotted.

Red blood cells

The red blood cells, also called erythrocytes, are the most abundant cells of the blood. A healthy adult has 4.5 to 5.5 million of them in every cubic millimetre of blood.

• They are made in the red bone marrow of an adult. The hormone erythropoietin, made by the kidney, speeds up their making.
• In most mammals, a red cell has no nucleus when it is mature. It is biconcave, like a disc with a dent on each side.
• The red colour comes from haemoglobin, a protein that contains iron. A healthy woman has about 12 to 16 g of haemoglobin in every 100 mL of blood, and a healthy man about 13 to 17 g. It carries oxygen.
• A red cell lives for about 120 days. The spleen destroys the old cells, so it is called the graveyard of the red cells.

White blood cells

The white blood cells, also called leucocytes, are colourless, because they have no haemoglobin. They have a nucleus. They are far fewer than the red cells: 4000 to 11000 in every cubic millimetre. They live only a short time.

There are two groups. The granulocytes have granules in the cytoplasm, and the agranulocytes do not.

A way to remember the order of the numbers, from the most to the least: Never Let Monkeys Eat Bananas. It gives neutrophil, lymphocyte, monocyte, eosinophil, basophil.

Platelets

Platelets, or thrombocytes, are not whole cells. They are small pieces that break off from large bone marrow cells called megakaryocytes. There are 150,000 to 450,000 platelets in every cubic millimetre of blood. They release substances that start the clotting of blood. If their number falls, the blood may not clot well, and too much blood may be lost from a wound.

A note on the numbers. Counts differ from person to person, and laboratories give slightly different ranges. This course uses the ranges that haematology books and the World Health Organization accept as normal. A man usually has more red cells and more haemoglobin than a woman. NCERT prints narrower ranges, for example 5 to 5.5 million red cells, 6000 to 8000 white cells and 150,000 to 350,000 platelets in each mm³. If a test follows the textbook, use the textbook number.

You should find that the plasma is always the larger part when the formed elements are below 50 per cent. A larger sample holds more of everything, but the shares stay the same.

Try it in Python

The program separates a sample of blood into plasma and formed elements and counts the red cells.

Practise with new numbers

Common mistakes

• Saying that plasma and serum are the same. Serum has no clotting factors. Plasma has them.
• Saying that every mature red cell has a nucleus. In most mammals it has none.
• Counting the lymphocytes as granulocytes. The lymphocytes and the monocytes are agranulocytes.
• Saying that platelets are cells. They are small pieces of cells, made from megakaryocytes.
• Mixing up the defenders. Neutrophils and monocytes eat germs. Lymphocytes make the immune response. Basophils release histamine.

Remember these

• Blood is a fluid connective tissue. Plasma is 55 per cent, and the formed elements are 45 per cent.
• Plasma: 90 to 92 per cent water, 6 to 8 per cent proteins (fibrinogen, globulins, albumins).
• Red cells: 4.5 to 5.5 million per mm³. Haemoglobin: 12 to 16 g per 100 mL in a woman, 13 to 17 g in a man. They live 120 days and are destroyed in the spleen.
• White cells: 4000 to 11000 per mm³. Order of number: neutrophil, lymphocyte, monocyte, eosinophil, basophil.
• Platelets: 150,000 to 450,000 per mm³, from megakaryocytes, needed for clotting.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. What per cent of the blood is plasma? What is most of the plasma made of?

2. Name the three main plasma proteins, and give one work of each.

3. Which cells of the blood are the most numerous? Where are they made, and where are they destroyed?

4. List the five kinds of white cells from the most numerous to the least numerous.

5. Where do platelets come from, and why are they needed?

References

• Betts J. G. et al. (2022). Anatomy and Physiology 2e, OpenStax, chapter 18, The Cardiovascular System: Blood.
• Bain B. J., Bates I. and Laffan M. A. (2017). Dacie and Lewis Practical Haematology, 12th edition, Elsevier, chapter Reference Ranges and Normal Values.
• World Health Organization (2024). Guideline on haemoglobin cutoffs to define anaemia in individuals and populations, Geneva.
• NCERT (2023). Biology, Textbook for Class XI, chapter 15.