Biology
🔒 Log in to trackHuman body — organs, blood, digestion, glands
🔒 Log in to trackRecord-holders of the human body
| Fact | Answer |
|---|---|
| Largest organ | Skin |
| Largest gland / largest internal organ | Liver |
| Longest bone | Femur (thigh bone) |
| Smallest bone | Stapes (middle ear) |
| Bones in an adult | 206 (about 300 at birth; many fuse) |
| Strongest/hardest substance | Tooth enamel |
| Master gland | Pituitary |
| Largest part of the brain | Cerebrum |
| Functional unit of the kidney | Nephron |
| Structural and functional unit of the nervous system | Neuron |
| Normal body temperature | 37 °C (98.6 °F) |
| Normal blood pressure | About 120/80 mm Hg |
| Normal resting heart rate | About 72 beats per minute |
Blood and circulation
- William Harvey described the circulation of blood.
- RBCs carry oxygen with haemoglobin (iron-containing), live about 120 days, are made in red bone marrow; the spleen is the 'graveyard of RBCs'. WBCs fight infection (immunity). Platelets help clotting.
- Blood groups — Karl Landsteiner (ABO). O is the universal donor (O negative, strictly) and AB the universal recipient (AB positive, strictly). The Rh factor was also found by Landsteiner and Wiener (named after the rhesus monkey).
- The human heart has 4 chambers; the SA node is the natural pacemaker. Arteries carry blood away from the heart (all oxygenated except the pulmonary artery); veins carry blood to the heart (all deoxygenated except the pulmonary vein).
Digestion
| Where | Secretion / enzyme | Action |
|---|---|---|
| Mouth | Salivary amylase (ptyalin) | Starch → maltose |
| Stomach | HCl + pepsin | Kills germs; proteins → peptides |
| Liver | Bile (stored in the gall bladder) | Emulsifies fats — contains no enzyme |
| Pancreas | Trypsin, lipase, amylase | Proteins, fats, starch |
| Small intestine | Villi | Most absorption of digested food |
| Large intestine | — | Absorbs water |
- The small intestine is the longest part of the alimentary canal.
Nervous and endocrine systems
- Cerebrum — intelligence, memory, voluntary actions. Cerebellum — balance and coordination. Medulla oblongata — involuntary actions (heartbeat, breathing, swallowing).
| Gland | Hormone | Note |
|---|---|---|
| Pituitary | Growth hormone and others | 'Master gland' |
| Thyroid | Thyroxine (needs iodine) | Deficiency → goitre |
| Pancreas (islets of Langerhans) | Insulin (β-cells), glucagon (α-cells) | Insulin lowers blood sugar; lack → diabetes mellitus |
| Adrenal | Adrenaline | 'Fight or flight' hormone |
| Testes / ovaries | Testosterone / oestrogen, progesterone | Sex hormones |
- The pancreas is both an endocrine and an exocrine gland. Insulin was isolated by Banting and Best (1921).
- In the eye, the retina holds the image; the cornea is what is donated in eye donation.
- The lungs' air sacs (alveoli) are the site of gas exchange.
Detailed notes
Body records — the standard list
| Record | Answer |
|---|---|
| Largest organ | Skin |
| Largest gland / largest internal organ | Liver |
| Largest endocrine gland | Thyroid |
| Longest bone | Femur (thigh) |
| Smallest bone | Stapes (middle ear) |
| Smallest muscle | Stapedius (ear) |
| Strongest muscle (by force) | Jaw (masseter) |
| Bones in an adult | 206 |
| Bones in a newborn | about 300 (many fuse later) |
| Ribs | 12 pairs = 24 |
| Normal body temperature | 37 °C (98.6 °F) |
Blood and circulation
Blood ≈ 5 litres. Plasma (55%, the liquid), RBC (oxygen on haemoglobin), WBC (defence), platelets (clotting). Blood groups: ABO + Rh; O− is the universal donor, AB+ the universal recipient. The heart has 4 chambers (2 atria + 2 ventricles); its natural pace-maker is the SA node; humans have double circulation (blood passes through the heart twice per loop). Blood pressure is measured with a sphygmomanometer; normal ≈ 120/80 mm Hg. A resting heart beats ≈ 72 times a minute.
Digestive system — organ by organ
Mouth: salivary amylase starts starch digestion. Stomach: HCl + pepsin (protein); a mucous lining saves the stomach from itself. Liver makes bile (stored in the gall bladder — no enzymes, it only emulsifies fat). Pancreas sends trypsin (protein) and lipase (fat). The small intestine is the real digestion + absorption site (villi); the large intestine absorbs only water.
Endocrine glands — hormones
Pituitary = 'master gland' (growth hormone; under- or over-secretion gives dwarfism/gigantism). Thyroid = thyroxine (contains iodine) — deficiency causes goitre; pancreas = insulin (from the islets of Langerhans) controls blood sugar — its failure is diabetes; adrenal = adrenaline (the 'fight-or-flight' hormone); testes/ovaries = testosterone/estrogen.
Excretion and the brain
Kidneys filter the blood in about a million nephrons each; urine (urea + water + salts) reaches the bladder. The brain: cerebrum (thinking, largest part), cerebellum (balance, posture), medulla oblongata (involuntary heartbeat/breathing — a spine injury there is fatal); reflex actions are handled by the spinal cord without waiting for the brain.
Quick revision
- 206 adult bones; stapes smallest, femur longest; skin largest organ; liver largest gland.
- O− universal donor, AB+ universal recipient; SA node = pace-maker; BP 120/80 (sphygmomanometer).
- Bile has no enzymes (emulsifies fat); HCl + pepsin in stomach; villi absorb in the small intestine.
- Pituitary master gland; thyroxine needs iodine; insulin from pancreas fights diabetes; adrenaline = fight-or-flight.
- Cerebrum thinks, cerebellum balances, medulla keeps the heart beating.
Types of questions asked
Every way this subtopic shows up in exams — how to recognise it, the formula or logic to use, and a solved example.
Type 1: Body records (largest/smallest/number)very common4 practice Q
'Largest gland / largest organ / longest bone / smallest bone', 'how many bones does an adult have', 'which muscle is the smallest', number-of-ribs questions.
- Records: skin = largest organ; liver = largest gland (and largest internal organ); thyroid = largest ENDOCRINE gland; femur = longest bone; stapes = smallest bone; stapedius = smallest muscle.
- Numbers: adult 206 bones, newborn ≈ 300, ribs 12 pairs.
- Trap: 'largest gland' = liver; 'largest endocrine gland' = thyroid; both are often mixed. Why: pure recall — keep one list, test the three largest-X pairs against each other.
Example: The smallest bone in the human body is —
Stapes in the middle ear (about 3 mm). The smallest muscle, stapedius, sits right beside it.
Type 2: Blood, groups and circulationvery common4 practice Q
'Universal donor / recipient', 'which blood cells fight infection / help clotting', 'pace-maker of the heart', 'how many chambers does the heart have', instrument for BP.
- Cells: RBC carry oxygen; WBC defend; platelets clot.
- Groups: O− universal donor; AB+ universal recipient; Rh factor decides +/−.
- Heart: 4 chambers; SA node = pace-maker; double circulation in humans.
- BP instrument = sphygmomanometer; normal 120/80 mm Hg; pulse ≈ 72/min resting.
Example: Which blood group is the universal recipient?
AB+ — it carries no anti-A or anti-B antibodies. O− is the universal donor counterpart.
Type 3: Organ ↔ secretion/functionvery common4 practice Q
'Insulin is secreted by…', 'bile is produced in… / stored in…', 'which organ filters the blood', 'HCl is secreted in…', 'nephron belongs to…'.
- Digestive secretions: mouth–salivary amylase; stomach–HCl + pepsin; liver–bile (stored in gall bladder, no enzymes); pancreas–trypsin + lipase (+ insulin from islets of Langerhans).
- Kidney: filtration unit = nephron; brain parts: cerebrum–thinking, cerebellum–balance, medulla–involuntary control.
- 'Bile stored where' = gall bladder; 'bile made where' = liver — split them.
Example: Bile, which helps digest fats, is produced by the —
Liver (largest gland) and stored in the gall bladder; it emulsifies fat but contains no enzymes.
Type 4: Hormone ↔ gland and deficiencyvery common3 practice Q
'Insulin / thyroxine / adrenaline is secreted by', 'master gland', 'which hormone controls blood sugar', 'deficiency of growth hormone causes…'.
- Master gland–pituitary (growth hormone); thyroid–thyroxine (iodine; low → goitre); pancreas–insulin (diabetes); adrenal–adrenaline (fight-or-flight).
- Sex hormones: testes–testosterone, ovaries–estrogen.
- Link each hormone to its imbalance: dwarfism/gigantism–growth hormone, goitre–thyroxine, diabetes–insulin. Why: gland–hormone–disease is a fixed triangle; any corner can be asked.
Example: Which gland is called the 'master gland'?
Pituitary — its hormones steer the other endocrine glands, including the thyroid and adrenals.
Type 5: Digestion steps and enzymescommon2 practice Q
'Which enzyme digests protein / starch / fat', 'where is food fully digested and absorbed', 'function of villi', 'which juice has no enzyme', 'peristalsis is…'.
- Enzyme-to-food: amylase–starch, pepsin/trypsin–protein, lipase–fat.
- Small intestine = final digestion + absorption (villi increase area); large intestine = water only.
- Bile = no enzymes, emulsifies fat; peristalsis = wave-like muscle movement pushing food.
Example: Digested food is absorbed mainly in the —
Small intestine, through millions of villi that multiply the absorbing surface. The large intestine takes up only water and salts.
Type 6: Body numbers (computed)common2 practice Q
'How many times does the heart beat in a day', 'normal body temperature in °F', 'how much blood does an adult have', questions needing 72 × 60 or 37 → 98.6 conversion.
- Heart: 72 beats/min → 72 × 60 × 24 = 103,680 beats/day.
- Temperature: 37 °C = 98.6 °F (F = 9C/5 + 32).
- Blood volume ≈ 5 litres; RBC count ≈ 5 million/mm³; WBC ≈ 6,000–8,000/mm³.
- Compute the arithmetic in your head by splitting: 72 × 60 = 4,320; × 24 ≈ 1.04 lakh.
Example: At a resting rate of 72 beats per minute, the heart beats about how many times a day?
72 × 60 × 24 = 103,680 — roughly 1 lakh beats a day, pumping thousands of litres.
Formulas
18.5–24.9 is the normal range (WHO)
Shortcut tricks
⚡ Blood group giving: 'O gives, AB takes'
O has no A/B antigens → can give to all. AB has no anti-A/anti-B antibodies → can take from all.
Example: The universal recipient blood group is?
AB.
⚡ Brain parts: 'Cerebrum thinks, Cerebellum balances, Medulla keeps you alive'
Thinking/memory — cerebrum; balance/posture — cerebellum; heartbeat/breathing — medulla oblongata.
Where students lose marks
Saying bile contains digestive enzymes — it only emulsifies fats.
Answering thyroid for the master gland — it is the pituitary.
Naming the heart or liver as the largest organ — skin is the largest organ; liver is the largest gland/internal organ.
Practice sets — 22 questions
Sets of 10, mixed across the question types above. Each answer comes with a step-by-step explanation.
Topic test · 10 questions
Suggested time 3 min · wrong answers go to your mistake notebook automatically.