What does the cardiovascular and respiratory systems module cover?

It covers five linked areas: the structure of the heart and the double-circulation path of blood; the components of blood and the structure of arteries, veins and capillaries, including vascular shunting; the cardiac cycle with heart rate, stroke volume, cardiac output and target heart-rate zones; the structure of the respiratory system from nose to alveoli; and the mechanics of breathing and gas exchange, including how breathing rate and tidal volume change with exercise. Together they explain how oxygen is delivered to working muscles.

What is cardiac output and how is it calculated?

Cardiac output is the volume of blood pumped by the heart in one minute. It is calculated as heart rate multiplied by stroke volume, where heart rate is the number of beats per minute and stroke volume is the volume of blood pumped per beat. For example, a heart rate of 70 beats per minute and a stroke volume of 70 millilitres give a cardiac output of 4900 millilitres, or about 4.9 litres per minute. Cardiac output rises sharply during exercise to meet the muscles' demand for oxygen.

What is vascular shunting during exercise?

Vascular shunting is the redistribution of blood flow during exercise so that more blood is directed to the working skeletal muscles and less to organs such as the gut. It is achieved by vasodilation (widening) of the arterioles supplying active muscles and vasoconstriction (narrowing) of those supplying inactive areas. This ensures oxygen and nutrients reach the muscles that need them most while still maintaining essential supply to the brain.

How does the respiratory system respond to exercise?

During exercise, breathing rate (breaths per minute) and tidal volume (the volume of air per breath) both increase, raising minute ventilation, the total volume of air breathed per minute. This brings more oxygen into the alveoli for diffusion into the blood and removes more carbon dioxide. The rise is triggered mainly by increased carbon dioxide in the blood detected by the body, and it continues for a time after exercise to repay the oxygen debt.

SingaporeSports Science

Cardiovascular and Respiratory Systems: O-Level Exercise and Sports Science (SEAB 6081) module overview of the heart, blood, circulation, breathing and gas exchange

An O-Level Exercise and Sports Science overview of the cardiovascular and respiratory systems (SEAB 6081). How the heart pumps through a double circulation, what blood and vessels do, how breathing and gas exchange work, and how heart rate, stroke volume, cardiac output and breathing respond to exercise, with links to every dot point.

Generated by Claude Opus 4.87 min readSEAB-6081Updated 2026-06-13

Reviewed by: AI editorial process; not yet individually human-reviewed

Jump to a section

What this module is about
The heart: a double pump
Blood and the vessels that carry it
The cardiac cycle and the numbers that matter
The respiratory system and gas exchange
Check your knowledge

What this module is about

The cardiovascular and respiratory systems are the body's oxygen-delivery service, and the O-Level Exercise and Sports Science syllabus (SEAB 6081) treats them as one connected story: the lungs load oxygen into the blood, the heart pumps it, and the vessels deliver it to the working muscles. Every endurance and recovery question in the written theory paper rests on this module, and the heart-rate calculations here also feed the training module. This overview links the five dot points; work each in full for the worked answers and practice questions.

See the complete set for this subject at /sg-o-level/sports-science/syllabus.

The heart: a double pump

Start with the pump. The structure of the heart page sets out the four chambers (left and right atria and ventricles), the valves that keep blood flowing one way, and the major vessels. The key idea is the double circulation: the right side sends deoxygenated blood to the lungs, and the left side sends oxygenated blood to the body, which is why the left ventricle has the thickest muscular wall.

Blood and the vessels that carry it

The blood and circulation page covers what blood carries (red cells, white cells, platelets and plasma) and the structure of arteries (thick, elastic, high pressure), veins (thinner, with valves) and capillaries (one cell thick for exchange). It also explains vascular shunting, the redistribution of blood to working muscles during exercise.

The cardiac cycle and the numbers that matter

The cardiac cycle and heart rate page describes diastole (filling) and systole (pumping), then defines heart rate, stroke volume and cardiac output, and shows how to calculate target heart-rate training zones. These numbers connect directly to training thresholds and intensity in the training module.

The respiratory system and gas exchange

The respiratory system page traces the pathway of air from the nose and mouth, through the trachea and bronchi, to the alveoli, and explains the features of the alveoli that suit gas exchange (large surface area, thin walls, moist lining, rich blood supply). The gas exchange and breathing page then explains the mechanics of inspiration and expiration and how breathing rate, tidal volume and minute ventilation respond to exercise.

Calculating cardiac output and a target heart-rate zone

A typical question gives heart rate and stroke volume and asks for cardiac output, then asks for a training zone. Take an athlete aged 16 with a resting heart rate of 65 beats per minute and a stroke volume of 80 millilitres.

Step 1: Recall the cardiac output formula

Cardiac output is heart rate multiplied by stroke volume:

Q = HR \times SV

Step 2: Substitute the values

Q = 65 \times 80 = 5200 \text{ ml per minute}

So the resting cardiac output is about 5.2 litres per minute.

Step 3: Find maximum heart rate

Estimate maximum heart rate with the standard formula:

HR_{max} = 220 - \text{age} = 220 - 16 = 204 \text{ bpm}

Step 4: Calculate a 70 percent aerobic training zone

0.70 \times 204 = 142.8 \approx 143 \text{ bpm}

So to train aerobically at 70 percent of maximum, this athlete aims for a heart rate of about 143 beats per minute. Showing the formula, the substitution and the units secures every mark.

Check your knowledge

A mix of recall, calculation and application questions covering the module. Attempt them under timed conditions, then check against the solutions.

Why does the left ventricle have a thicker muscular wall than the right ventricle? (2 marks)
State the formula for cardiac output and calculate it for a heart rate of 75 bpm and a stroke volume of 70 ml. (2 marks)
Describe two features of an alveolus that make it efficient for gas exchange. (2 marks)
Explain what vascular shunting is and why it is useful during exercise. (3 marks)
State two ways the respiratory system increases oxygen intake during exercise. (2 marks)

Solutions

Q1: The left ventricle pumps oxygenated blood all the way around the body (the systemic circulation) at high pressure, whereas the right ventricle only pumps blood the short distance to the lungs at lower pressure. The thicker, more muscular wall of the left ventricle generates the greater force needed.
Q2: Cardiac output equals heart rate multiplied by stroke volume. So $Q = 75 \times 70 = 5250$ ml per minute (about 5.25 litres per minute).
Q3: Any two of: a large surface area for diffusion; very thin (one cell thick) walls for a short diffusion distance; a moist lining so gases dissolve; and a rich capillary blood supply that maintains a steep concentration gradient.
Q4: Vascular shunting is the redistribution of blood flow during exercise. Arterioles to the working muscles vasodilate (widen) while those to inactive areas such as the gut vasoconstrict (narrow). This directs more oxygenated blood to the muscles that need it, improving oxygen and nutrient delivery while still supplying essential organs.
Q5: Breathing rate (breaths per minute) increases and tidal volume (air per breath) increases; together these raise minute ventilation, bringing more oxygen to the alveoli and removing more carbon dioxide.

Sources & how we know this

Singapore-Cambridge GCE Ordinary Level Exercise and Sports Science (Syllabus 6081) — Singapore Examinations and Assessment Board (2026)