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.
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.
Question types you will see
Each type: how to recognise it, the method step by step, and one question to try.
Body records (largest/smallest/number)
'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.
pure recall — keep one list, test the three largest-X pairs against each other.
The smallest bone in the human body is —
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Stapes in the middle ear (about 3 mm). The smallest muscle, stapedius, sits right beside it.
Blood, groups and circulation
'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.
Which blood group is the universal recipient?
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AB+ — it carries no anti-A or anti-B antibodies. O− is the universal donor counterpart.
Organ ↔ secretion/function
'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.
Bile, which helps digest fats, is produced by the —
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Liver (largest gland) and stored in the gall bladder; it emulsifies fat but contains no enzymes.
Hormone ↔ gland and deficiency
'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.
gland–hormone–disease is a fixed triangle; any corner can be asked.
Which gland is called the 'master gland'?
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Pituitary — its hormones steer the other endocrine glands, including the thyroid and adrenals.
Digestion steps and enzymes
'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.
Digested food is absorbed mainly in the —
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Small intestine, through millions of villi that multiply the absorbing surface. The large intestine takes up only water and salts.
Body numbers (computed)
'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.
At a resting rate of 72 beats per minute, the heart beats about how many times a day?
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72 × 60 × 24 = 103,680 — roughly 1 lakh beats a day, pumping thousands of litres.
Formula sheet
18.5–24.9 is the normal range (WHO)
Shortcuts that save time
O has no A/B antigens → can give to all. AB has no anti-A/anti-B antibodies → can take from all.
The universal recipient blood group is?
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AB.
Thinking/memory — cerebrum; balance/posture — cerebellum; heartbeat/breathing — medulla oblongata.
Mistakes to avoid
Where most students lose marks on this subtopic.
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.
Quick revision
Read this the night before the exam.
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.
Practice: 22 questions
Sets of 10, mixed across the question types above. Every answer has a step-by-step explanation.
Topic test · 10 questions
Suggested time 3 min · wrong answers go to your mistake notebook automatically.