S2 Science: Respiration, Circulation And Excretion (Paper 2 (intermediate))

S2 Science: Respiration, Circulation and Excretion (Paper 2 (intermediate))

2026-11-14

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Tutor Prep Guide · Respiration and gas exchange, features of alveoli, blood components and the three vessel types, the four heart chambers and double circulation, kidney filtration and reabsorption, dialysis and sweat

S2 Science: Respiration, Circulation and Excretion (Paper 2 (intermediate))

This paper moves on to mechanisms and comparisons: lactic acid and muscle soreness, the origin of the pulse, what each side of the heart does, valves and gas exchange in the lung capillaries, kidney filtration and reabsorption, and the need for double circulation plus the role of sweat in excretion.

Direct answers　Lactic acid comes from anaerobic respiration, the pulse comes from the left ventricle, the right heart serves the lungs and the left the body, valves prevent backflow, lung exchange is two way diffusion, the kidney filters then reabsorbs, double circulation keeps the lungs at low pressure and the body at high pressure, and sweat carries a little urea but mainly cools the body.

01　Paper details (check answers afterwards)

Item	Detail
Questions	10
Suggested time	20
Difficulty mix	Foundation 3 · Intermediate 3 · Challenge 3 · Error redo 1
Chapter	Chapter: S2 Science: Respiration, Circulation and Excretion
Level	S2

02　Questions (10 items)

FoundationWhich substance builds up to cause muscle soreness after hard exercise?
FoundationWhat causes the pulse?
FoundationWhere do the right and left sides of the heart pump blood to?
IntermediateWhat is the job of the valves in the heart?
IntermediateWhat exchange happens as blood passes through the lung capillaries?
IntermediateApart from removing water, what two main jobs do the kidneys do?
ChallengeWhich kind of respiration do human muscles use when oxygen is short? Give the word equation.
ChallengeWhy does the human body need a double circulation?
ChallengeWhich two main steps form urine?
Error redoSweating is said to be only about cooling and not excretion at all. What is wrong?

03　Answers and one-line notes

#	Answer	One-line note
1	Lactic acid	Lactic acid is made by anaerobic respiration when oxygen is short and is slowly removed after exercise.
2	The surge of blood when the left ventricle contracts into the aorta makes the artery wall expand and recoil	That is why the pulse rate matches the heart rate and can be felt at the wrist or the neck.
3	The right side pumps to the lungs (pulmonary circulation) and the left side to the whole body (systemic circulation)	The left ventricle has the thickest wall because it must send blood all round the body at the highest pressure.
4	They keep blood flowing one way only and stop it flowing backwards	The atrioventricular valves stop blood returning from ventricle to atrium, and the semilunar valves stop it returning from artery to ventricle.
5	Carbon dioxide diffuses from blood into the alveoli and oxygen diffuses from the alveoli into the blood	Both gases move down their concentration gradients, so no extra energy is needed.
6	They remove nitrogenous waste such as urea and regulate the salt content and acidity of the blood	By selective reabsorption the kidney returns useful glucose, water and salts to the blood.
7	Anaerobic respiration: glucose -> lactic acid + a small amount of energy	With too little oxygen glucose is only partly broken down, far less energy is released and lactic acid builds up in the muscle.
8	Blood passes through the heart twice, once to the lungs for gas exchange and once to send oxygenated blood round the body at enough pressure	A single circuit could not keep the lungs at low pressure while supplying the body at high pressure.
9	Blood is first filtered under pressure, then useful substances are reabsorbed back into the blood	What stays in the capsule is filtrate, and only what is left after reabsorption becomes urine.
10	Sweat carries water, salt and a trace of urea, so the skin is one excretion route, though cooling is its main job	Excretion means removing metabolic waste, and sweat does carry a little urea, though far less than urine.

04　What to do after marking

After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
Repeat the same item two days later; move to the next paper only when both attempts are correct.
Parents only need the answer and note columns; there is no need to explain every item.

For respiration and circulation questions ask three things: whether the question is about structure, process or numbers, which way the air or blood is moving, and whether the units have been converted correctly.Tutor Registration →

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