Red blood cells (erythrocytes) are the most abundant blood cells: 5 to 5.5 million per mm3 in a healthy adult man. They are biconcave, have no nucleus in most mammals, and carry haemoglobin, a red, iron-containing protein (12-16 g per 100 ml of blood) that transports respiratory gases. Memory hook: "5 million red discs, no nucleus, packed with iron, live 120 days, die in the spleen."
Biconcave, enucleate red blood cell packed with haemoglobin, with the key NCERT numbers NEET repeatedly tests: count, haemoglobin level, origin, life span and site of destruction.
Your doubts, answered
How many RBCs are there in blood and where are they made?
A healthy adult man has, on average, 5 to 5.5 million RBCs per mm3 of blood. RBCs are the most abundant of all blood cells. In adults they are formed in the red bone marrow. This exact NCERT count is a common one-mark fact in NEET.
Why do mammalian RBCs have no nucleus?
NCERT states RBCs are devoid of a nucleus in most mammals. The NEET-approved reason is that losing the nucleus frees all the internal space for oxygen transport, so more haemoglobin can be packed inside. It is NOT because 'they do not need to reproduce' or 'they do not metabolize' - those are wrong options in the 2017 PYQ.
What exactly is haemoglobin and how much is normal?
Haemoglobin is a red-coloured, iron-containing complex protein inside RBCs. It gives blood its red colour and its name (haemo = iron/blood, globin = protein). A healthy individual has 12-16 g of haemoglobin in every 100 ml of blood. It plays a major role in transporting respiratory gases (oxygen and carbon dioxide).
How long do RBCs live and where are they destroyed?
RBCs have an average life span of 120 days. After that they are destroyed in the spleen, which NCERT calls the 'graveyard of RBCs'. This spleen fact is directly tested in the NEET 2022 statement question.
What does biconcave shape mean and why does it matter?
Biconcave means the disc is pushed inward on both flat sides (like a doughnut without a hole). This shape increases surface area for gas exchange and lets the cell bend to squeeze through narrow capillaries. Shape and enucleate nature together are the deeper topic covered in the next concept.
⚠️ The NEET trap ✗ A healthy adult has 6000-8000 RBCs per mm3 of blood. ✓ 6000-8000 per mm3 is the WBC count. RBCs are 5 to 5.5 MILLION per mm3. Never mix the two - RBCs are in millions, WBCs are in thousands. 🧠 NTA loves swapping the RBC count with the WBC count.
Real NEET questions
NEET 2017
Adult human RBCs are enucleate. Which of the following statement(s) is/are most appropriate explanation for this feature? (a) They do not need to reproduce (b) They are somatic cells (c) They do not metabolize (d) All their internal space is available for oxygen transport. Select the correct option:
A · Only (d) ✓
B · Only (a)
C · (a), (c) and (d)
D · (b) and (c)
Solution: Only statement (d) is correct. Losing the nucleus in mature mammalian RBCs frees all the internal space so it can be filled with haemoglobin for maximum oxygen transport. RBCs still metabolise (anaerobically, via glycolysis), so (c) is false; (a) and (b) are not valid explanations for being enucleate.
NEET 2022
Given below are two statements: Statement I: The coagulum is formed of a network of threads called thrombins. Statement II: Spleen is the graveyard of erythrocytes. In the light of the above statements, choose the most appropriate answer:
A · Both Statement I and Statement II are correct
B · Both Statement I and Statement II are incorrect
C · Statement I is correct but Statement II is incorrect
D · Statement I is incorrect but Statement II is correct ✓
Solution: Statement II is correct: NCERT calls the spleen the graveyard of RBCs, where erythrocytes are destroyed after their 120-day life span. Statement I is wrong because the coagulum (clot) is a network of threads called FIBRINS, not thrombins. So only Statement II is correct.
Solved Body Fluids and Circulation NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
What is the normal RBC count in a healthy adult man?
5 to 5.5 million RBCs per mm3 of blood, as per NCERT.
How much haemoglobin is present in normal blood?
12 to 16 grams of haemoglobin in every 100 ml of blood.
What is the life span of a red blood cell?
About 120 days, after which the RBC is destroyed in the spleen (graveyard of RBCs).
Do all mammals have enucleate RBCs?
NCERT says RBCs are devoid of a nucleus in MOST mammals, not all. Camel and llama are known exceptions with nucleated RBCs, but for NEET follow the NCERT wording 'most mammals'.
Why is haemoglobin red in colour?
Because it is an iron-containing complex protein; the iron gives it the red colour, which in turn gives red blood cells their name and colour.
