Four chambers: atria fill, ventricles pump
The heart is a double pump. The atria (upper) receive returning blood; the ventricles (lower) pump it out, with thicker walls. The right side carries deoxygenated blood (body to lungs), the left oxygenated blood (lungs to body). Valves keep blood flowing one way -- for 0413 you need their location and function, not names. The left ventricle has the thickest wall, pumping at high pressure round the body; the right only reaches the lungs.
The one-way pathway of blood
Blood runs a fixed one-way route, and examiners want the vessels in order. Deoxygenated (to the lungs): vena cava to right atrium to right ventricle to pulmonary artery to lungs. Oxygenated (to the body): pulmonary vein to left atrium to left ventricle to aorta to body. Two memory hooks stop the classic slips: the pulmonary artery is the one artery carrying deoxygenated blood, the pulmonary vein the one vein carrying oxygenated blood.
Cardiac output: Q = SV x HR
Three linked measures. Heart rate (HR) = beats per minute (bpm). Stroke volume (SV) = blood pumped in one beat (ml). Cardiac output (Q) = blood pumped per minute (ml/min). They combine as Q = SV x HR: at SV 70 ml and HR 72 bpm, Q = 70 x 72 = 5040 ml/min. Two marks are lost here: always write the unit (ml/min), and remember "per minute" belongs to cardiac output -- stroke volume is per single beat, so "per unit time" is not the heart rate.
Drawn from real examiner reports.
Stroke volume is not cardiac output
Stroke volume is blood pumped in one beat; cardiac output is blood pumped per minute (ml/min). Candidates swap them, or define cardiac output as "the blood the heart pumps" without the crucial "per minute" -- and "per unit time" gets confused with heart rate. Lock phrase to word: per beat = stroke volume, per minute = cardiac output.
s22 P11 Q5a(ii)
Cardiac output always needs its unit
Cardiac-output calculations are usually numerically correct, yet the unit is regularly missing or wrong -- an easy mark lost. The answer to Q = SV x HR is a volume per minute, so it must end in ml/min (or cm3/min; 1 ml = 1 cm3). Never leave a bare number, and never write "bpm" -- that is the heart rate, not the cardiac output.
s22 P11 Q5a(ii)
Right and left chambers get transposed
The right and left sides are frequently swapped -- naming the right atrium when the left ventricle is meant -- and the right ventricle is a particular weak point. Anchor to the blood: the right side handles deoxygenated blood heading to the lungs, the left side oxygenated blood heading to the body. Get the blood right and the chamber follows.
w22 P11 Q5c; s23 P12 Q5a; w22 P12 Q11a; w23 P12 Q11a
Explain why, do not re-describe the graph
Asked to explain WHY heart rate differs between a fit and unfit person, candidates re-describe the graph ("the fit line is lower") instead of giving a reason. Credit needs the physiology: a fitter person has a bigger, stronger heart and larger stroke volume, so fewer beats give the same output; age and genetics count too. Describing data is not explaining it.
s23 P13 Q12b; s23 P11 Q6a(i)
In exercise, stroke volume rises too
During exercise, some candidates raise only the heart rate when calculating or explaining cardiac output, forgetting that stroke volume also rises. Both increase, so cardiac output multiplies up sharply -- e.g. 120 ml x 160 bpm = 19,200 ml/min, far above a resting ~5000 ml/min. Change both values, not just heart rate.
Atria receive, ventricles pump
The atria are the upper chambers that receive blood coming back to the heart; the ventricles are the lower chambers that pump it out, which is why ventricle walls are thicker and more muscular. Candidates sometimes reverse these roles. Tie "atrium" to receiving blood, "ventricle" to pumping it out under pressure.
A fitter heart beats slower at rest
Students assume a "fitter" heart must beat faster at rest -- the opposite is true. Training makes the heart bigger and stronger, so it pumps more blood per beat (a larger stroke volume). Because resting cardiac output is fixed, fewer beats now move the same blood, so the resting heart rate falls. A low resting rate signals a fitter heart, not a weaker one.
s23 P13 Q12b; s23 P11 Q6a(i)
Describe wants what, explain wants why
Read the command word. "Describe" the effect of exercise = say what changes (heart rate, stroke volume and cardiac output all rise). "Explain" = say why (working muscles need more oxygen, so the heart pumps more per minute). An "explain" answer that only describes is capped.
Route an explain answer through Q = SV x HR
For a top-band "explain" answer: name the change in both heart rate and stroke volume, link them through Q = SV x HR so cardiac output rises, then say why -- more oxygen delivered, more carbon dioxide removed. The equation turns a list into an explanation.
Apply it to a named activity
Failing to apply an answer to a named activity is the most repeated Paper 1 criticism. Tie the change to a named endurance activity -- distance running, swimming -- not a generic statement, and for a trained-versus-untrained comparison give both sides.
The heart is a double pump with a right side and a left side.
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