Human Respiratory & Circulatory Systems

Air composition. Parts and functions of the respiratory system (nose, windpipe, lungs) and circulatory system (heart, blood, blood vessels). How they work together.

gas exchangeheart & bloodlungs
The two systems and how they work together
Respiratory system
NoseFilters, warms, moistens air
WindpipeCarries air to lungs
LungsGas exchange: O₂ in, CO₂ out
Air in: more O₂, less CO₂
Air out: less O₂, more CO₂, more water vapour
Circulatory system
HeartPumps blood around body
Blood vesselsCarry blood throughout body
BloodTransports O₂, CO₂, food, water
Heart → body → heart → lungs → heart (repeats)
How they link: Lungs absorb O₂ into blood → blood carries O₂ to all body cells → cells use O₂ (respiration) → produce CO₂ → blood carries CO₂ back to lungs → breathed out.
Part A — Multiple choice (Booklet A style · 2 marks each)
Question 1 — MCQ (2 marks) Booklet A Respiratory system parts

Which of the following correctly describes the function of the windpipe?

AIt exchanges oxygen and carbon dioxide between air and blood.
BIt filters, warms and moistens the air before it reaches the lungs.
CIt is a tube that carries air from the nose and mouth to the lungs.
DIt pumps blood containing oxygen to all parts of the body.

Each option describes a different part: A = lungs, B = nose, C = windpipe ✓, D = heart. The windpipe is purely a passageway — it does not filter air (nose) or exchange gases (lungs).

Question 2 — MCQ (2 marks) Booklet A Inhaled vs exhaled air

Compared to inhaled air, exhaled air contains —

AMore oxygen and less carbon dioxide.
BMore oxygen and more carbon dioxide.
CLess oxygen and less carbon dioxide.
DLess oxygen, more carbon dioxide and more water vapour.

During respiration, body cells use O₂ and produce CO₂ and water. So exhaled air has less O₂, more CO₂, and more water vapour than inhaled air. Note: exhaled air still contains oxygen — we do not use it all up.

Part B — Structured questions (Booklet B style)
Question 3 — Structured (4 marks) Booklet B Circulatory system · blood's role

The diagram shows a simplified view of how the heart, lungs and body cells are connected by blood vessels.

Heart pumps blood Lungs gas exchange Body cells O₂-rich CO₂-rich CO₂-rich O₂-rich

(a) What happens to blood as it passes through the lungs? Describe the gas exchange that takes place. (2 marks)

(b) A student says: "The heart is part of the respiratory system because it helps transport oxygen." Is the student correct? Explain your answer. (2 marks)

(a)As blood passes through the lungs, oxygen from the inhaled air passes into the blood (blood becomes oxygen-rich). At the same time, carbon dioxide from the blood passes into the air in the lungs and is breathed out. The blood changes from carbon dioxide-rich to oxygen-rich.
(b)The student is not correct. The heart is part of the circulatory system, not the respiratory system. The heart's function is to pump blood around the body. It transports oxygen carried in the blood, but it does not carry out any gas exchange. Gas exchange happens in the lungs, which are part of the respiratory system.
Lungs: O₂ into blood, CO₂ out · Heart: pumps blood (circulatory system)
Common confusion: The two systems work closely together but are separate. The respiratory system (nose, windpipe, lungs) handles breathing and gas exchange. The circulatory system (heart, blood vessels, blood) handles transport. The heart is never part of the respiratory system.
Question 4 — Structured (4 marks) Booklet B Exercise · linked systems · data Distinction level

A student measures his breathing rate (breaths per minute) and heart rate (beats per minute) at rest and after running for 5 minutes. The results are shown below.

Condition Breathing rate Heart rate At rest 15 breaths/min 72 beats/min After running 38 breaths/min 148 beats/min

(a) Explain why the breathing rate increases after running. (2 marks)

(b) Explain why the heart rate increases at the same time as the breathing rate. (2 marks)

(a)During running, the muscles need more energy. To release this energy through respiration, the body cells use more oxygen and produce more carbon dioxide. The breathing rate increases to take in more oxygen and to remove the extra carbon dioxide more quickly.
(b)The heart rate increases because blood needs to be pumped faster to deliver the extra oxygen to the working muscles and to carry away the extra carbon dioxide to the lungs more quickly. The circulatory and respiratory systems work together — faster breathing alone is not enough if blood circulation does not also speed up.
More energy needed → more O₂ used, more CO₂ made → both systems speed up

The key phrase for (a) is "muscles need more energy for respiration." Always link the cause (exercise) → effect (more energy needed) → why breathing increases (more O₂ in, more CO₂ out). Two steps = 2 marks.

Question 5 — Structured (5 marks) Booklet B Systems working together · hypothesis · application Distinction level

A doctor tells a patient that a blockage in one of his blood vessels is preventing enough blood from reaching his heart muscle. The patient notices he feels very tired even when resting, and gets breathless easily.

(a) The heart is a muscle. Explain why a reduced blood supply to the heart muscle would cause the heart to pump less effectively. (2 marks)

(b) Explain why a less effective heart would cause the patient to feel breathless easily. (2 marks)

(c) A doctor suggests the patient exercises gently to strengthen the heart. A student says: "Exercising will make breathlessness worse because it increases breathing rate." Do you agree? Explain your answer. (1 mark)

(a)Blood carries oxygen and food (nutrients) to the heart muscle. With reduced blood supply, the heart muscle receives less oxygen and food, so it cannot release enough energy through respiration. Without enough energy, the heart muscle cannot contract strongly or pump blood effectively.
(b)A less effective heart pumps less blood per beat to the body and lungs. Muscles and cells receive less oxygen and accumulate more carbon dioxide. The body responds by increasing the breathing rate to try to get more oxygen — causing the feeling of breathlessness, even with little activity.
(c)Disagree (partially). While exercise temporarily increases breathing rate, gentle regular exercise strengthens the heart muscle over time, making it pump more efficiently. A stronger heart can deliver more oxygen per beat, so the patient eventually becomes less breathless for the same level of activity. Short-term breathlessness during exercise is expected, but the long-term benefit outweighs it.
Blood → O₂ + food to heart → energy → pumping · Less pump → less O₂ → breathless
For (a) — don't just say "less oxygen." Full marks require: (1) blood supplies oxygen AND food to heart muscle, (2) less blood = less oxygen/food, (3) heart muscle cannot get enough energy, (4) therefore pumps less effectively. Shorter chains score 1/2.