Carbon dioxide transport

“CO₂ in blood behaves like sugar in tea: most dissolves as bicarbonate, while a small spoon stays as gas.”

≈70 % of CO₂ is transported as bicarbonate ions formed by carbonic anhydrase inside RBCs; ~23 % binds to haemoglobin as carbamino compounds and ~7 % remains dissolved. The chloride shift exchanges HCO₃⁻ for Cl⁻ to maintain electroneutrality. Forgetting the bicarbonate route loses points in transport-type questions.

Memory trick: CO₂ → HCO₃⁻ (≈70%); remember chloride shift.

Carbon dioxide transport — common mistake

“When you pop a soda, most CO2 escapes as bubbles, but in blood it’s hidden as bicarbonate.”

About 70% of CO₂ is carried in blood as HCO₃⁻ formed by carbonic anhydrase inside RBCs; ~23% as carbamino-Hb and ~7% dissolved. The trap is to assume haemoglobin-bound CO₂ is the majority – remember the bicarbonate route dominates.

Memory trick: 70% HCO₃⁻, 23% Hb, 7% dissolved – 7-23-70 rule

Bohr effect

“Like a sports car revving hotter, low pH in muscles forces hemoglobin to dump oxygen.”

Higher CO₂ and H⁺ lower blood pH, shifting the O₂-Hb dissociation curve right (higher P₅₀). This releases more O₂ where it’s needed. Exam trap: forgetting that both CO₂ and H⁺ contribute; a pH drop alone is enough to cause the shift.

Memory trick: Low pH = high P₅₀ = O₂ out

Bohr effect — common mistake

“Adding lemon to water makes it sour; the acidity in active tissue pushes oxygen out of hemoglobin.”

Acidic (low pH) conditions shift the O₂-Hb curve to the right, decreasing affinity and increasing O₂ release. A common mistake is to think low pH shifts the curve left, raising affinity – it does the opposite.

Memory trick: Acid = right shift

Pepsin works best at pH ~2

“Pepsin is a chef that only works in a super-sour kitchen of pH ≈ 2, the stomach’s acid bath.”

Pepsin’s activity peaks at pH 1.5–2; above pH 4 its catalytic efficiency falls sharply. Students often forget that pepsinogen activation also requires the same acidic environment.

Pepsin works best at pH ~2

Memory trick: Pepsin = pH 2 chef

~97% O2 carried by haemoglobin

“A train (haemoglobin) carries 97% of the passengers (oxygen), while the tracks (plasma) hold the rest.”

≈97% of O₂ in arterial blood is bound to haemoglobin; only ≈3% is dissolved in plasma and determines PaO₂. The trap: ignoring dissolved O₂, which drives diffusion to tissues.

~97% O2 carried by haemoglobin

Memory trick: 97% Hb, 3% plasma

~70% CO2 carried as bicarbonate

“Your soda fizz turns CO₂ into a hidden salt – like bicarbonate swirling in blood plasma.”

≈70 % of CO₂ from tissues is converted to HCO₃⁻ inside red cells by carbonic anhydrase, then exported to plasma via the chloride shift. The remainder is carried bound to haemoglobin (carbamino) or dissolved. The 70 % value applies to total blood transport, not just plasma, and the conversion happens in RBCs, not in plasma.

~70% CO2 carried as bicarbonate

Memory trick: CO₂ → HCO₃⁻ (70%) via RBC enzyme, then plasma

Tidal volume ~500 mL

“Like a soda bottle releasing a quick puff of 500 mL when you open it.”

Tidal volume is the air moved in a normal quiet breath of a resting adult, about 500 mL. It is a component of minute ventilation and stays roughly constant in healthy adults. Trap: do not mix it with vital capacity or inspiratory reserve volume, which are much larger.

Tidal volume ~500 mL

Memory trick: TV = 5×10² mL, think “TV” as 5-zero-zero.

inspiration vs expiration

“When you inhale, your diaphragm pulls down like a drum-skin, pulling air into the lungs.”

Inspiration is always active: diaphragm contracts, external intercostals lift ribs, thoracic volume rises, intrapulmonary pressure falls, and air flows in. Quiet expiration is passive – diaphragm relaxes, ribs fall, elastic recoil expels air. During forced breathing both phases become active, a frequent exam trap.

Memory trick: Inspire = Pull; Expire = Push (passive unless forced)

oxygen transport vs carbon dioxide transport

“Your blood carries O2 like passengers on a train (Hb), while CO2 rides mostly as dissolved bicarbonate in the carriage.”

≈98% of O2 binds to haemoglobin; the rest dissolves. CO2 is transported mainly as bicarbonate ions (≈70%), with smaller amounts dissolved and as carbamino-Hb. Trap: never assume CO2 is only dissolved; the bicarbonate route requires carbonic anhydrase in RBCs.

Memory trick: O2 = Hb train; CO2 = BICar (bicarbonate) car.

mechanical digestion vs chemical digestion

“Chewing an apple is like shredding paper before the printer (enzymes) can read the words.”

Mechanical digestion physically breaks food, increasing surface area for enzymes. Chemical digestion uses specific enzymes to hydrolyze macromolecules into absorbable units. Trap: remember that chemical digestion starts in the mouth (salivary amylase) and the stomach also does some mechanical mixing.

Memory trick: MECHE = M(ach) + CHEMical; first M = mash.

Memory trick: SI = Super Absorber; Stomach = S for Small (only limited).

Double circulation

“A two-stage elevator first lifts passengers to a sky deck (lungs) then sends them down to the lobby (body).”

Human circulation is double: the pulmonary circuit carries deoxygenated blood to lungs and returns oxygenated blood, while the systemic circuit delivers it to tissues and returns CO₂-rich blood. The septum keeps the loops separate; fetal shunts (foramen ovale, ductus arteriosus) bypass them, but any septal defect can cause mixing—a classic exam pitfall.

Memory trick: Pulmonary up, systemic down – two loops, no mix.

Double circulation — common mistake

“Like a two-lane highway, blood takes separate routes: lungs then body, ensuring oxygen picks up and drops off efficiently.”

Humans have double circulation: a low-pressure pulmonary circuit carries deoxygenated blood to the lungs and returns oxygen-rich blood to the left atrium; a high-pressure systemic circuit distributes it to all tissues. Mixing the two pressures gives wrong MAP calculations.

Memory trick: Pulmonary = lungs, Systemic = body; two loops.

Cardiac cycle

“A heart works like a bicycle pump: it pushes blood out (systole) then draws fresh blood in (diastole).”

Each cardiac cycle begins with atrial systole, adding ~20 % to ventricular filling (the atrial kick), followed by ventricular systole that ejects blood, then diastole when all chambers relax and refill. Ignoring the atrial kick loses marks in pressure-volume questions.

Memory trick: Push-draw-kick: systole, diastole, atrial contribution.

Cardiac cycle — common mistake

“Even when you hold your breath, the heart pauses between beats, like a metronome’s silent beat.”

After ventricular systole the heart enters diastole, a relaxation phase where valves open and ventricles fill; it is not idle. A common trap is treating diastole as ‘no activity’—it is vital for coronary perfusion and stroke volume.

Memory trick: Beat = push-pause-push.

Conducting system

“The SA node is the heart’s metronome, ticking ~70 times per minute to set the rhythm.”

Impulse starts at the SA node, spreads through atria, then pauses ~0.1 s at the AV node (allowing ventricular filling) before traveling via the Bundle of His and Purkinje fibres to contract ventricles. Missing the AV-node delay loses points in timing diagrams.

Memory trick: SA → AV (delay) → Purkinje = rhythm chain.

Conducting system — common mistake

“When you faint, your heart still beats—proof the brain isn’t the beat-generator.”

The SA node autonomously generates each impulse; the brain modulates rate via sympathetic and parasympathetic nerves but does not fire each beat. Mistaking the CNS as the primary pacemaker leads to wrong answers on autonomic regulation questions.

Memory trick: Heart’s own pacemaker; brain just adjusts tempo.

Blood pressure

“The cuff’s snap feels the left ventricle’s surge, like a wave crashing on a beach.”

Blood pressure peaks (systolic) when the left ventricle ejects blood and falls (diastolic) during ventricular relaxation. For calculations, remember mean arterial pressure ≈ DBP + 1/3 × pulse pressure; omitting the 1/3 factor is a frequent error.

Memory trick: MAP ≈ DBP + 1/3 (SBP-DBP).

Blood pressure — common mistake

“Checking a car’s tire gauge, the larger reading shows needed pressure—just as systolic is the higher number in BP.”

Blood pressure is recorded as systolic over diastolic (e.g., 120/80 mmHg). Systolic is the peak pressure when the heart contracts; diastolic is the minimum during relaxation. A common mistake is swapping them—always write the larger value first.

Memory trick: S > D, larger first.

Nephron and filtration

“Like a pressure cooker, the glomerular capillaries push at ~55 mmHg, but opposing forces leave a net ~10 mmHg to filter plasma.”

Net filtration pressure (NFP) ≈10 mmHg, calculated as P_gc (≈55 mmHg) – P_bc (≈15 mmHg) – π_gc (≈30 mmHg). Only this resultant drives plasma into Bowman’s space. A common trap is to quote the 55 mmHg capillary pressure as the filtration pressure, which overestimates the driving force.

Memory trick: 55-15-30 = 10

Nephron and filtration — common mistake

“Imagine a raw batter poured into a pancake pan; the batter (filtrate) is later flipped and cooked (reabsorption) before serving as a pancake (urine).”

Glomerular filtration produces a filtrate that contains water, ions, glucose, and waste, but no proteins. The tubular system then reabsorbs needed solutes and water and secretes additional wastes. The trap is assuming the filtrate exiting the glomerulus is already urine; it becomes urine only after tubular processing.

Memory trick: Filtrate ≠ urine; tubules transform it.

Counter-current mechanism

“The loop of Henle is a two-track ski lift: water drains down the permeable descending limb, while the thick ascending limb actively pumps salt upward, creating the medullary gradient.”

In the descending limb water exits (high permeability) while solutes stay, concentrating the tubular fluid. The thick ascending limb is water-impermeable and actively transports Na⁺/K⁺/2Cl⁻ into the interstitium, raising medullary osmolarity. The trap: assuming the descending limb moves salt; only the ascending limb pumps salt out.

Memory trick: Down = water out; Up = salt in.

Counter-current mechanism — common mistake

“When you steep tea, the pot’s layers create a stronger brew at the bottom—similarly, the kidney’s medullary gradient concentrates solutes deeper down.”

The medullary osmotic gradient, built by the counter-current multiplier, is essential for concentrating urine; without it, the collecting duct cannot reabsorb water even under ADH. Students often ignore this gradient, assuming ADH alone concentrates urine. Remember, the gradient is the foundation.

Memory trick: Gradient first, ADH then.