Blood is the river of life that flows through every part of the human body, and although it looks like a simple red liquid, it is actually a living fluid connective tissue made of cells floating in a watery fluid. The sections below move from the broad picture inward — what blood is and how much we carry, then its liquid plasma and its solid cells, the red cells, white cells and platelets one by one, how blood clots and how it is grouped into types, and finally the disorders and the quick high-yield facts examiners ask most.
Blood is a special connective tissue in which the living cells are suspended in a flowing liquid rather than held by solid fibres, which is why it is called a fluid connective tissue.
| Feature | Value / Fact |
|---|---|
| Volume in adult body | 5–6 litres (≈ 5040 mL, 7–8% of body weight) |
| Nature | fluid connective tissue |
| pH (slightly alkaline) | 7.35–7.45 |
| Site of formation (baby, 2–3 yrs) | liver |
| Site of formation (adult) | red bone marrow |
| Temperature | about 37°C |
Blood is classified as which type of tissue?
The normal pH range of human blood is:
In an adult, blood cells are formed in the (haematopoiesis).
Blood minus the clotting protein fibrinogen leaves a pale yellow fluid called .
Blood breaks into two broad parts — a liquid part and a solid part — and a spun (centrifuged) sample separates into clear layers.
| Layer | Approx. share | What it contains |
|---|---|---|
| Plasma (top) | 55% | salts/minerals, water, proteins |
| Buffy coat (middle) | 1% | white blood cells, platelets |
| Red blood cells (bottom) | 45% | red blood cells (erythrocytes) |
The red-cell fraction (~45%) of centrifuged blood is called the:
The thin middle 'buffy coat' layer of centrifuged blood contains mainly:
Blood = Plasma (liquid) + (the solid formed elements).
Red blood cells make up about percent of whole blood.
The liquid in which all blood cells float is the plasma, the transport medium of the body.
| Component | Share of plasma |
|---|---|
| Water | 90% |
| Plasma proteins | 6–8% |
| Electrolytes (Na⁺ & Cl⁻) | 1% |
Plasma makes up approximately what fraction of whole blood?
Water forms roughly what share of blood plasma?
The plasma protein acts as an anticlotting agent, keeping blood flowing inside vessels.
Electrolytes such as Na⁺ and Cl⁻ form about percent of plasma.
Floating in their millions through the plasma are the red blood cells (erythrocytes), the most numerous cells in the body and the ones giving blood its red colour.
| Feature | Value / Fact |
|---|---|
| Normal count | 4.5–5.5 million per mm³ |
| Lifespan | 120 days |
| Shape | biconcave disc, no nucleus (in humans) |
| Pigment | haemoglobin (contains iron) |
| Haemoglobin | 12–16 g per 100 mL |
| Made in | red bone marrow |
| Destroyed in | spleen and liver |
What is the lifespan of a mature human red blood cell?
A mature human red blood cell notably lacks which structure?
Old red blood cells are mainly destroyed in the , called the 'graveyard of RBCs'.
The red colour of blood comes from present in haemoglobin.
Far fewer in number but vital for survival are the white blood cells (leukocytes), the body's defenders.
| Specific WBC | Function | Differential % |
|---|---|---|
| Neutrophil | general phagocytosis; effective in acute bacterial infections | 54–62% |
| Lymphocyte | produces antibodies; gives long-term immunity | 25–33% |
| Monocyte | phagocytosis of large particles in typhoid, malaria etc. | 3–9% |
| Eosinophil | kills parasites; helpful in allergic conditions | 1–3% |
| Basophil | releases heparin and histamine; involved in allergic reactions | 1% |
Which is the most abundant type of white blood cell?
Which white blood cell produces antibodies and provides long-term immunity?
White blood cells move towards infection sites by movement.
The white blood cell that releases heparin and histamine is the .
The third of the formed elements are the platelets (thrombocytes), tiny cell fragments whose one great task is to stop bleeding.
The normal platelet count in blood is about:
Which disease is well known for sharply lowering the platelet count?
During clotting, thrombin converts soluble fibrinogen into threads that seal the wound.
Blood clotting requires calcium and vitamin .
One of the most heavily tested topics is the ABO blood group system, discovered by Karl Landsteiner, which sorts blood by the antigens on red cells and the antibodies in plasma.
| 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 only | A, B, AB, O |
| O | none | anti-A and anti-B | A, B, AB, O | O only |
The ABO blood group system was discovered by:
Which blood group is the universal recipient?
Group has no antigens on its red cells and is the universal donor.
Rh incompatibility in pregnancy can cause a condition called .
A change in any cell count or substance shows up as a recognisable disease, and examiners often ask which cell is at fault.
| Disease | Cause |
|---|---|
| Anaemia | low RBC (or low haemoglobin) |
| Polycythemia | high RBC |
| Leukopenia | low WBC |
| Leukaemia | high WBC ("blood cancer") |
Leukaemia, often called 'blood cancer', is characterised by:
Anaemia is caused by:
An abnormally low platelet count is called .
The inherited disorder in which blood fails to clot is called .
Pulling the parts together, blood as a whole keeps every other organ alive — a transport system, defence force, repair crew and thermostat in one.
| Function | How it is carried out |
|---|---|
| Transport of oxygen | haemoglobin in RBCs carries O₂ from lungs to tissues |
| Transport of nutrients | plasma carries glucose, amino acids to cells |
| Removal of wastes | plasma carries CO₂ and urea to lungs and kidneys |
| Defence (immunity) | WBCs and antibodies destroy germs |
| Clotting | platelets and fibrin seal wounds, stop bleeding |
| Temperature & pH control | distributes heat; buffers hold pH at 7.35–7.45 |
Which blood component carries oxygen from the lungs to the tissues?
Blood helps maintain the body's pH within which range?
By distributing heat around the body, blood helps regulate body .
Immune defence in blood is carried out mainly by .
A last sweep of the highest-yield facts gathers the numbers and names that come up again and again.
Take 5 questions at a time — tap an option to check. After each round, revise the notes above and take the retest for 5 fresh questions, until you've mastered the whole chapter.