Fundamentals
The core facts every aspirant should own — each a titled nugget with a real-world story, the concept in plain words, and a memory trick. Works even when the internet doesn't.
300 fundamentals
“When you wash greasy dishes, soap breaks the oil into tiny droplets—bile does the same inside your duodenum.”
Bile is a watery secretion of the liver stored in the gall-bladder. Bile salts emulsify fats into fine droplets, raising surface area for pancreatic lipase. The exam trap: bile contains no enzymes, so it never directly hydrolyses triglycerides.
Memory trick: Bile = Breaks, not Burns (no enzymes)
“Like laundry detergent lifts grease without cutting it, bile lifts fat droplets for enzymes.”
Bile is a liver-derived fluid stored in the gall-bladder; its bile salts act as a detergent, emulsifying fats into small droplets. It does not digest fat; lipase does that after emulsification. Beware of calling bile a digestive enzyme in answers.
Memory trick: Bile = detergent, not digestive
“Swallowing a gummy bear, you feel a smooth wave of squeezes that pushes it down the throat – that’s peristalsis.”
Peristalsis is a coordinated wave of circular muscle contraction followed by longitudinal muscle shortening that propels the bolus aborally. It works independent of gravity, so it continues even if you’re upside-down. Remember: segmentation mixes contents but does not give net forward movement; reverse peristalsis is a separate reflex for vomiting.
Memory trick: Peri = around, stals = squeeze → squeeze around → push forward
“Even if you hang upside-down, food still slides down your esophagus because peristalsis drives it.”
The gut’s muscular wall generates peristaltic waves that move chyme independent of body position. Students often lose marks by writing that gravity moves food; the correct mechanism is muscle-driven peristalsis.
Memory trick: Peri beats gravity
“The intestine’s inner wall is a forest of trees (villi) whose leaves (microvilli) multiply the absorbing area.”
Villi are finger-like folds of the mucosa that increase the gut’s surface area. Each enterocyte on a villus bears microvilli (brush border) that further amplify the area for nutrient uptake. Do not mix up the two levels.
Memory trick: Villi = villages, microvilli = mini-leaves
“Pepsin works in the stomach’s acidic kitchen; trypsin cooks proteins in the alkaline kitchen of the small intestine.”
Pepsin, secreted as pepsinogen, is activated by HCl (pH≈2) in the stomach and hydrolyses proteins there. Trypsin, from pancreatic trypsinogen, is activated by enteropeptidase and works at pH≈8 in the duodenum. Remember the pH difference.
Memory trick: Pepsin = pH 2, Trypsin = pH 8
“Chewing an apple is like a hand-grater shredding it, while salivary amylase works like a key unlocking the starch molecules.”
Mechanical digestion physically reduces food size by chewing and stomach churning, raising surface area. Chemical digestion uses enzymes (amylase, pepsin, lipase) to cleave molecular bonds. Exam trap: enzymes start in the mouth and act throughout, not only after chewing stops.
Memory trick: M = Motion, C = Catalyst (enzymes)
“Bile in the gallbladder acts like dish-washing liquid, spreading fat droplets so lipase can act, but it never cuts any chemical bonds itself.”
Bile is a watery mix of bile salts, bilirubin and cholesterol that emulsifies lipids, forming tiny droplets that enlarge surface area for pancreatic lipase. It contains no enzymes, so it does not hydrolyze fats. Trap: never write “bile digests fats” – it only emulsifies.
Memory trick: Bile = B for Break-up, not Break-down
“Just as perfume spreads across a room, O₂ spreads from alveoli into blood by diffusion.”
Gas exchange relies on a steep PO₂ gradient (high in alveoli, low in capillary blood) and a large surface area with a thin moist barrier. CO₂ follows the opposite gradient. Ignoring surface-area loss (e.g., emphysema) is a frequent mistake.
Memory trick: Short distance + large area = rapid gas swap.
“Your lungs don’t have a pump for gases; they rely on the same passive diffusion that lets a drop of ink spread in water.”
O₂ and CO₂ move across the alveolar membrane by passive diffusion down their partial-pressure differences; no cellular energy is spent. The common error is to picture an active ‘gas pump’ in the lung. Remember, only ventilation moves air, diffusion moves gases.
Memory trick: Diffusion = no pump, just gradient.
“Imagine trying to dissolve a spoon of sugar in a glass of water; similarly, only a tiny O₂ amount dissolves in plasma.”
Only about 0.3 mL O₂ dissolves per 100 mL plasma, while haemoglobin carries ~20 mL per 100 mL blood. The misconception that plasma is the main carrier leads to under-estimating blood O₂ content. Always count Hb-bound O₂ first.
Memory trick: Plasma O₂ = tiny sip; Hb = big gulp.
“CO₂ in blood is like a chameleon, turning into bicarbonate (HCO₃⁻) for the long journey.”
Roughly 70 % of CO₂ is converted to bicarbonate ions in RBCs via carbonic anhydrase, about 23 % forms carbamino compounds on haemoglobin and ~7 % stays dissolved. Ignoring the bicarbonate route gives wrong acid-base balance calculations.
Memory trick: CO₂ → HCO₃⁻ (≈70%) = main route.
“Like soda fizzing when opened, most CO₂ in blood is hidden as bicarbonate, not riding on hemoglobin.”
≈70 % of CO₂ is carried as HCO₃⁻ in plasma (via carbonic anhydrase). ≈20–23 % binds to hemoglobin as carbamino compounds, and ≈7 % stays dissolved. Exams often list only the bicarbonate part; remember the two smaller parts together make about 30 % of transport.
Memory trick: 70-20-7 ≈ B-C-D (Bicarb, Carbamino, Dissolved)
“Think of a bellows: the diaphragm pulls down like the bottom plate, expanding the chest to draw air.”
Inhalation occurs when the diaphragm contracts and moves downward while external intercostals lift the ribs, increasing thoracic volume and lowering intrapulmonary pressure, so air flows in. During quiet breathing exhalation is passive, driven by elastic recoil; forced exhalation uses internal intercostals and abdominal muscles. Mistake: writing that lungs actively pump air.
Memory trick: Diaphragm down = draw in; lungs just inflate
“Your lungs are like balloons; they expand only because the chest wall pushes them, not because they have their own muscles.”
Lungs are elastic sacs without contractile fibers; they expand only because the surrounding chest wall changes volume. The diaphragm and intercostal muscles create the pressure gradient that moves air. Confusing lungs as active pumps leads to wrong statements in answers.
Memory trick: Lungs = passive balloon
“Breathing is like opening a window for fresh air; cellular respiration is the furnace inside using that air to burn fuel.”
Breathing (ventilation) is the mechanical movement of air in and out of the respiratory tract, caused by pressure changes in the thorax. Cellular respiration is the set of biochemical reactions in mitochondria that oxidize glucose, using O2 and producing CO2 and ATP. Exams often mix the two, so keep the processes separate.
Memory trick: Ventilation ≠ metabolism
“During inspiration your diaphragm flattens like a drum skin, enlarging the chest; during expiration it relaxes, letting the drum snap back.”
Inspiration: diaphragm contracts (flattens) and external intercostals raise ribs, expanding the thoracic cavity and lowering pressure, drawing air in. Expiration (quiet): diaphragm and intercostals relax, elastic recoil reduces volume, raising pressure, pushing air out. Forced expiration recruits internal intercostals and abdominal muscles; the trap is treating expiration as always active.
Memory trick: Insp = diaphragm down; Exp = diaphragm up (relax)
“Like soda fizz, ~70% CO2 rides in blood as bicarbonate, while O2 hops onto hemoglobin like train passengers.”
O2 is carried mainly bound to Fe²⁺ in hemoglobin (~98% of total O₂). CO₂ is transported chiefly as HCO₃⁻ in plasma (~70%) formed by carbonic anhydrase in RBCs; the rest is as carbamino-Hb and dissolved gas. Exams ask for the dominant form: O₂ → Hb, CO₂ → bicarbonate.
Memory trick: O2 on Hb; CO2 → HCO3-
“While jogging, your lungs draw in O₂, but the muscles’ mitochondria use that O₂ to release energy.”
Breathing (ventilation) is the mechanical movement of air in and out of the lungs. Cellular respiration is the set of biochemical reactions inside cells that use O₂ to oxidize substrates and produce ATP. Exams require you to state this two-step distinction, not treat them as identical.
Memory trick: Breathe = air flow; Respire = energy flow – B moves, R burns.
“Your heart works like a two-stop subway: one stop sends blood to the lungs, the second sends it to the rest of the city.”
Humans have double circulation: the pulmonary circuit moves deoxygenated blood from the right heart to the lungs and back, while the systemic circuit carries oxygenated blood from the left heart to the body and returns it. Mistaking fish for double circulation is a classic error.
Memory trick: Pulmonary = P-to-Lungs, Systemic = S-to-Body.
“Imagine a two-door house: the left doors let fresh air (oxygenated blood) in, the right doors let stale air (deoxygenated) out.”
The heart’s four chambers keep oxygenated and deoxygenated blood separate: right atrium and ventricle receive and pump deoxygenated blood to the lungs; left atrium and ventricle receive oxygen-rich blood from the lungs and pump it to the body. Mixing them is a frequent slip-up in diagrams.
Memory trick: Left = Lungs-return, Right = Rest-deoxygenated.
“Think of a two-lane highway: the left lane carries cars (oxygenated blood) toward the city, the right lane carries cars (deoxygenated) away.”
A common mistake is swapping the sides: the left side of the heart actually carries oxygen-rich blood, the right side carries oxygen-poor blood. Remember left receives lungs-returned blood; right receives body-used blood. Exams often test this reversal.
Memory trick: L = Lungs, R = Return-depleted.
“Blood is like a mixed fruit salad: plasma is the juice, red cells are the red berries carrying oxygen, white cells are the protective nuts, platelets are the tiny seeds that clot.”
Blood comprises plasma (water, proteins, nutrients), erythrocytes (contain hemoglobin to bind O₂), leukocytes (defend against pathogens), and thrombocytes (initiate clot formation). A trap is to think plasma contains cells; cells are suspended in plasma, not part of it.
Memory trick: Plasma = liquid, RBC = red, WBC = white guard, PLT = plug.
“When you sip a smoothie, the liquid you see is like plasma, the red beads are RBCs, the tiny specks are WBCs, and the clumps are platelets.”
Blood is a mixture of plasma (≈55% water with proteins), red blood cells that bind O₂ via hemoglobin, white blood cells that provide immunity, and platelets that initiate clotting. RBCs lack nuclei, giving space for hemoglobin. A frequent trap is mixing up WBC function with oxygen transport.
Memory trick: Plasma-Liquid, RBC-Red-O₂, WBC-White-Defense, PLT-Plate-Clot