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
“Your small intestine, unfolded, would cover a badminton court — all to absorb your lunch.”
Mechanical digestion physically breaks food (chewing, churning); chemical digestion uses enzymes to break molecular bonds.
“Your small intestine, unfolded, would cover a badminton court — all to absorb your lunch.”
Bile is an emulsifier that breaks fat into droplets; it contains no digestive enzymes.
“You breathe about 20,000 times a day without ever thinking about it.”
Oxygen and carbon dioxide swap by diffusion across the thin, moist walls of the alveoli.
Memory trick: gas exchange is passive diffusion, driven by concentration differences.
“You breathe about 20,000 times a day without ever thinking about it.”
A frequent error is thinking gases are actively pumped across the alveoli. In reality, oxygen and carbon dioxide swap by diffusion across the thin, moist walls of the alveoli.
Memory trick: gas exchange is passive diffusion, driven by concentration differences.
“You breathe about 20,000 times a day without ever thinking about it.”
A frequent error is thinking oxygen mostly dissolves in the plasma. In reality, most oxygen is carried by haemoglobin in red blood cells as oxyhaemoglobin.
Memory trick: only a little O₂ dissolves in plasma; haemoglobin carries the rest.
“You breathe about 20,000 times a day without ever thinking about it.”
Most CO₂ travels in the blood as bicarbonate ions.
Memory trick: CO₂ is mostly bicarbonate; only a fraction rides on haemoglobin.
“You breathe about 20,000 times a day without ever thinking about it.”
A frequent error is thinking CO₂ is carried mainly by haemoglobin like oxygen. In reality, most CO₂ travels in the blood as bicarbonate ions.
Memory trick: CO₂ is mostly bicarbonate; only a fraction rides on haemoglobin.
“You breathe about 20,000 times a day without ever thinking about it.”
The diaphragm and intercostal muscles change chest volume to draw air in and push it out.
Memory trick: lungs are passive; the diaphragm does the pumping.
“You breathe about 20,000 times a day without ever thinking about it.”
A frequent error is thinking the lungs themselves have muscles that pump air. In reality, the diaphragm and intercostal muscles change chest volume to draw air in and push it out.
Memory trick: lungs are passive; the diaphragm does the pumping.
“You breathe about 20,000 times a day without ever thinking about it.”
Breathing is the physical movement of air in and out; cellular respiration is the chemical release of energy from glucose inside cells.
“You breathe about 20,000 times a day without ever thinking about it.”
In inspiration the diaphragm contracts and chest volume rises to pull air in; in expiration it relaxes and air is pushed out.
“You breathe about 20,000 times a day without ever thinking about it.”
Oxygen is carried mostly by haemoglobin; carbon dioxide is carried mostly as bicarbonate in the plasma.
“You breathe about 20,000 times a day without ever thinking about it.”
Breathing is the physical exchange of air; cellular respiration is the biochemical release of energy in cells — they are different.
“Your heart beats ~100,000 times a day, pushing blood through 100,000 km of vessels.”
A frequent error is thinking humans have single circulation like fish. In reality, blood passes through the heart twice in one complete circuit — once to the lungs, once to the body.
Memory trick: pulmonary (to lungs) + systemic (to body) = double circulation.
“Your heart beats ~100,000 times a day, pushing blood through 100,000 km of vessels.”
Four chambers — two atria and two ventricles — keep oxygenated (left) and deoxygenated (right) blood separate.
Memory trick: Left side = oxygenated ('L' for lungs-returned); right side = deoxygenated.
“Your heart beats ~100,000 times a day, pushing blood through 100,000 km of vessels.”
A frequent error is mixing up which side carries oxygenated blood. In reality, four chambers — two atria and two ventricles — keep oxygenated (left) and deoxygenated (right) blood separate.
Memory trick: Left side = oxygenated ('L' for lungs-returned); right side = deoxygenated.
“Your heart beats ~100,000 times a day, pushing blood through 100,000 km of vessels.”
Plasma (fluid), red cells (carry O₂), white cells (defence) and platelets (clotting).
Memory trick: red cells carry oxygen; white cells fight infection.
“Your heart beats ~100,000 times a day, pushing blood through 100,000 km of vessels.”
A frequent error is thinking white cells carry oxygen. In reality, plasma (fluid), red cells (carry O₂), white cells (defence) and platelets (clotting).
Memory trick: red cells carry oxygen; white cells fight infection.
“Your heart beats ~100,000 times a day, pushing blood through 100,000 km of vessels.”
Red blood cells carry oxygen (no nucleus in humans); white blood cells defend against infection (have a nucleus).
“Your heart beats ~100,000 times a day, pushing blood through 100,000 km of vessels.”
Systole is heart-muscle contraction (pumping blood out); diastole is relaxation (chambers refill).
“Your heart beats ~100,000 times a day, pushing blood through 100,000 km of vessels.”
The pulmonary artery is the exception — it carries deoxygenated blood from the heart to the lungs.
“Your kidneys filter your entire blood volume about 60 times every day.”
A frequent error is thinking the whole kidney filters as one unit. In reality, the microscopic filtering unit of the kidney where urine formation begins.
Memory trick: each kidney has about a million nephrons doing the actual filtering.
“Your kidneys filter your entire blood volume about 60 times every day.”
Ultrafiltration in the glomerulus, then selective reabsorption and secretion in the tubules.
Memory trick: about 99% of the filtrate is reabsorbed; only ~1% becomes urine.
“Your kidneys filter your entire blood volume about 60 times every day.”
A frequent error is thinking all the filtered fluid leaves as urine. In reality, ultrafiltration in the glomerulus, then selective reabsorption and secretion in the tubules.
Memory trick: about 99% of the filtrate is reabsorbed; only ~1% becomes urine.