Blood is the body’s transport highway — a connective tissue with a liquid matrix. It carries gases, nutrients, wastes, hormones and heat, fights infection and clots to seal leaks. CBSE Class 11 dedicates a full chapter to it and NEET pulls at least one question every year. We will cover composition, formed elements, blood groups, clotting and the numbers examiners love.
Core Concepts
Composition of blood
Blood is roughly 55% plasma and 45% formed elements. Plasma is 90% water plus proteins (albumin, globulin, fibrinogen), ions, nutrients, hormones, gases and wastes. Formed elements are RBCs, WBCs and platelets. Total blood volume in an adult is 5–6 litres.
The ratio of formed elements to total blood is called the haematocrit (or PCV — packed cell volume). A normal haematocrit for adult males is about 40–45%. Anaemia drops this number, while polycythemia raises it.
Plasma minus clotting factors (especially fibrinogen) is called serum. This is what we get when blood clots in a test tube and the liquid is squeezed out. NEET loves testing this distinction.
Plasma proteins and their jobs
Three major plasma proteins matter for exams:
| Protein | % of plasma protein | Function |
|---|---|---|
| Albumin | ~60% | Maintains osmotic pressure, transport carrier |
| Globulin | ~35% | Alpha and beta: transport; gamma: antibodies (immunoglobulins) |
| Fibrinogen | ~4% | Clotting cascade precursor to fibrin |
Albumin keeps water inside blood vessels. When albumin drops (liver disease, malnutrition), fluid leaks into tissues causing oedema. This is a favourite NEET mechanism question.
Red blood cells (erythrocytes)
Biconcave, without a nucleus in mammals, 5–5.5 million per mm in men and 4.5–5 million in women. They live about 120 days, are made in red bone marrow and destroyed in spleen and liver. Their whole job is oxygen transport via haemoglobin. Camel RBCs are oval but still without a nucleus.
Each RBC contains roughly 270 million molecules of haemoglobin. Each haemoglobin has four haem groups, each with one iron atom (Fe). That iron atom binds one O molecule. So one haemoglobin carries up to 4 O molecules.
The production of RBCs is regulated by erythropoietin (EPO), a hormone released by the kidneys when blood oxygen drops. Athletes who train at high altitude naturally boost EPO levels — this is why altitude training improves endurance.
The spleen is called the “graveyard of RBCs” because old RBCs are broken down there. The iron is recycled, the globin is broken into amino acids, and the haem ring is degraded to bilirubin — which the liver excretes in bile. High bilirubin causes jaundice.
White blood cells (leukocytes)
6000 to 11000 per mm. Two families — granulocytes (neutrophil, eosinophil, basophil) and agranulocytes (monocyte, lymphocyte). Neutrophils are the most abundant at 60 to 65% and are the first responders to bacterial infection. Lymphocytes run adaptive immunity.
| WBC type | % of total | Nucleus shape | Key function |
|---|---|---|---|
| Neutrophil | 60–65% | Multi-lobed (3–5 lobes) | Phagocytosis of bacteria |
| Eosinophil | 2–3% | Bilobed | Parasites, allergy modulation |
| Basophil | 0.5–1% | Bilobed (S-shaped) | Histamine, heparin release |
| Monocyte | 6–8% | Kidney-shaped | Becomes macrophage in tissues |
| Lymphocyte | 20–25% | Round, large | B-cells (antibodies), T-cells (cell-mediated) |
A differential WBC count is a diagnostic goldmine. Bacterial infections raise neutrophils (neutrophilia). Viral infections raise lymphocytes (lymphocytosis). Parasitic infections and allergies raise eosinophils (eosinophilia). NEET loves assertion-reason questions on this.
Platelets (thrombocytes)
Cell fragments from megakaryocytes, 1.5 to 4 lakh per mm. They plug leaks and trigger the clotting cascade. Low count causes easy bruising and bleeding. Platelet count below 50,000 per mm is called thrombocytopenia and carries a bleeding risk. Dengue fever is notorious for dropping platelet counts.
Platelets survive only 8–10 days. They contain no nucleus (like mature RBCs) but do carry mitochondria, ER and granules loaded with clotting factors.
Blood groups
ABO system is based on antigens on the RBC surface. Group A has antigen A and anti-B antibody, group B has antigen B and anti-A, AB has both antigens and no antibodies, O has no antigens and both antibodies. Rh system is based on the D antigen — Rh positive has it, Rh negative does not.
| Blood group | Antigen on RBC | Antibody in plasma | Can donate to | Can receive from |
|---|---|---|---|---|
| A | A | Anti-B | A, AB | A, O |
| B | B | Anti-A | B, AB | B, O |
| AB | A and B | None | AB | A, B, AB, O |
| O | None | Anti-A, Anti-B | A, B, AB, O | O |
Group O is the universal donor (RBCs have no antigens to trigger a reaction) and group AB is the universal recipient (plasma has no antibodies to attack incoming RBCs). However, modern blood banks always cross-match before transfusion because minor antigens can still cause reactions.
Erythroblastosis foetalis (Rh incompatibility)
When an Rh-negative mother carries an Rh-positive foetus, foetal RBCs can leak into the maternal circulation at delivery. The mother develops anti-Rh antibodies. In the next Rh-positive pregnancy, these IgG antibodies cross the placenta and attack foetal RBCs, causing haemolytic disease of the newborn.
Prevention: an injection of anti-D immunoglobulin within 72 hours of the first delivery destroys foetal RBCs before the mother mounts a lasting immune response.
Blood clotting
A cascade triggered by tissue damage. Platelets adhere to the break, release factors, prothrombin is converted to thrombin, thrombin converts soluble fibrinogen into insoluble fibrin, and the fibrin mesh traps cells to form a clot. Calcium ions and vitamin K are essential cofactors.
Stage 1 — Thromboplastin formation. Damaged tissues release tissue factor (extrinsic pathway) or collagen exposure activates factor XII (intrinsic pathway). Both converge on factor X.
Stage 2 — Thrombin formation. Activated factor X (with factor V, Ca and phospholipids) converts prothrombin to thrombin.
Stage 3 — Fibrin formation. Thrombin cleaves fibrinogen into fibrin monomers, which polymerise into an insoluble mesh. Factor XIII cross-links the fibrin for strength.
Vitamin K is needed by the liver to synthesise prothrombin and factors VII, IX and X. A patient on the blood thinner warfarin has reduced vitamin K activity — that is how warfarin works. Newborns are routinely given a vitamin K injection because their gut flora (which produces vitamin K) is not yet established.
Key Formulas
A person with 15 g/dL haemoglobin carries about 20 mL of oxygen per 100 mL of blood when fully saturated.
Oxygen delivery equals cardiac output times arterial oxygen content — basic ICU physiology.
Normal: males 40–45%, females 36–40%. Lower in anaemia, higher in dehydration or polycythemia.
Worked Examples
Group O has no A or B antigens, so its RBCs do not trigger anti-A or anti-B in any recipient — universal donor. Group AB has no anti-A or anti-B in plasma, so it can receive any ABO blood — universal recipient. Rh must also match.
The biconcave shape gives a higher surface area to volume ratio than a sphere, speeding oxygen diffusion. It also lets RBCs deform to squeeze through capillaries narrower than their own diameter.
A bacterial infection typically raises neutrophils. A viral infection raises lymphocytes. A parasite or allergy raises eosinophils. Examiners love turning this into an assertion-reason question.
A patient has Hb = 12 g/dL. Oxygen carrying capacity per gram of Hb = 1.34 mL. So oxygen content = mL O per 100 mL blood. With a cardiac output of 5 L/min, total oxygen delivery = mL O/min. At rest, the body uses roughly 250 mL O/min — so there is a comfortable reserve.
During delivery, foetal blood mixes with maternal blood. If the mother is Rh and the baby is Rh, the mother’s immune system sees the D antigen as foreign and starts making anti-D IgG antibodies (this takes weeks). In the second pregnancy with an Rh foetus, these antibodies cross the placenta and lyse foetal RBCs — causing erythroblastosis foetalis. The anti-D injection given within 72 hours of the first delivery mops up foetal RBCs before the mother’s B-cells form memory cells.
Common Mistakes
Writing that mature mammalian RBCs have nuclei. They do not. Exception awareness: RBCs of camel and llama are oval and non-nucleated; frog and other amphibian RBCs are nucleated and oval.
Saying plasma and serum are the same. Serum has no fibrinogen because clotting has already removed it.
Thinking an Rh-negative mother with her first Rh-positive baby always has a problem. Usually the first baby is fine; the sensitisation happens at delivery and threatens the next Rh-positive pregnancy.
Confusing thrombin and fibrin. Thrombin is the enzyme; fibrin is the insoluble mesh product.
Mixing up the intrinsic and extrinsic pathways of clotting. The extrinsic pathway starts with tissue factor from damaged cells (fast, takes seconds). The intrinsic pathway starts with factor XII activation by exposed collagen (slower, takes minutes). Both converge on factor X.
Practice Questions
Q1. What is the normal RBC count in adult males?
5 to 5.5 million per mm. In females, it is 4.5 to 5 million per mm.
Q2. Name the protein in RBCs that carries oxygen and state how many O molecules one unit can carry.
Haemoglobin. Each molecule has four haem groups, each binding one O. So one haemoglobin can carry up to four O molecules.
Q3. A patient has blood group B. Which antigens and antibodies are present?
Antigen B on the RBC surface. Anti-A antibodies in the plasma. This person can donate to B and AB, and receive from B and O.
Q4. Why is the spleen called the graveyard of RBCs?
Old and damaged RBCs (older than about 120 days) are phagocytosed by macrophages in the spleen. Haemoglobin is broken down: iron is recycled, globin is hydrolysed, and the porphyrin ring becomes bilirubin.
Q5. What is the role of calcium ions in blood clotting?
Calcium ions (Ca, also called factor IV) are needed at several steps in the coagulation cascade, including the activation of prothrombin to thrombin. Chelating calcium with EDTA or citrate prevents clotting — this is how blood banks store blood.
Q6. Differentiate between serum and plasma.
Plasma is the liquid part of blood with all clotting factors intact. Serum is what remains after blood has clotted and the clot is removed — it lacks fibrinogen and most clotting factors. Plasma is obtained using an anticoagulant; serum is obtained without one.
Q7. What causes erythroblastosis foetalis?
It occurs when an Rh mother carries a second Rh foetus. Anti-Rh IgG antibodies from the first pregnancy cross the placenta and destroy foetal RBCs, causing severe anaemia and jaundice in the newborn.
Q8. A WBC differential count shows 80% neutrophils. What does this suggest?
Neutrophilia (elevated neutrophils) indicates a bacterial infection. Neutrophils are the first responders to bacterial invasion. A viral infection would raise lymphocytes instead.
Exam Weightage and Revision
| Exam | Typical weight | What they ask |
|---|---|---|
| CBSE Class 9 | 2–3 marks | Basic blood components, functions |
| CBSE Class 11 | 4–6 marks | Formed elements, blood groups, clotting |
| NEET | 1–2 questions | RBC lifespan, differential count, Rh incompatibility |
| State boards | 3–6 marks | Diagram-based long answer on composition |
NEET PYQs on blood cluster around six facts: RBC count (5 million), WBC count (6000–8000), platelet count (2.5 lakh), lifespan of RBC (120 days), universal donor (O), and the role of vitamin K in clotting. Lock those six and you clear most questions on this topic.
Memorise six numbers — RBC 5 million, WBC 7000, platelets 2.5 lakh, Hb 15 g/dL, plasma 55%, clotting time 2 to 8 minutes. They unlock most single-line questions.
FAQs
Why is blood considered a connective tissue?
Blood meets all three criteria of connective tissue: cells (RBCs, WBCs, platelets) scattered in an abundant matrix (plasma), derived from mesoderm. The matrix is liquid, which makes it unique among connective tissues, but the structural classification holds.
What happens if platelet count drops very low?
Below 50,000 per mm (thrombocytopenia), the risk of spontaneous bleeding rises. Below 20,000, even minor bumps can cause dangerous internal bleeding. Dengue is a common cause in India. Treatment includes platelet transfusions and treating the underlying cause.
Why do AB blood group individuals have no antibodies in plasma?
Antibodies are made against foreign antigens. Since AB individuals have both A and B antigens on their own RBCs, their immune system recognises both as “self” and does not produce anti-A or anti-B. This is immunological tolerance.
What is the difference between haemoglobin and myoglobin?
Haemoglobin is in RBCs and carries oxygen in the blood. Myoglobin is in muscle cells and stores oxygen locally. Myoglobin has a higher oxygen affinity than haemoglobin, so it grabs oxygen from haemoglobin and holds it until the muscle needs it during exercise.
Can blood clot inside a blood vessel?
Yes — this is called a thrombus. If a piece breaks off and travels through the bloodstream, it becomes an embolus. If it lodges in a coronary artery, it causes a heart attack; in a cerebral artery, a stroke. Anticoagulants like heparin and warfarin are used to prevent this in high-risk patients.
Treat blood as a river with freight. Cells are the cargo, plasma is the water, and every function is transport, defence or repair. Keep that trio in mind and the chapter writes itself.