School › Heart and lungs › Ages 15–16
Circulation, gas exchange and cardiac output
Double circulation, the cardiac cycle, gas exchange at the alveoli and calculating cardiac output. With an exercise and recovery investigation.
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Mammals have a double circulation: the pulmonary circuit carries blood from the right ventricle to the lungs and back, and the systemic circuit carries blood from the left ventricle to the rest of the body.[4][1]
Each heartbeat has two phases: systole, when the muscle contracts and pushes blood out, and diastole, when it relaxes and the chambers fill. Valves keep the flow one-way.[4][1]
At the alveoli, blood arriving from the pulmonary artery is low in oxygen and high in carbon dioxide, so oxygen moves from the air into the blood and carbon dioxide moves out, down their concentration gradients.[2]
Exchange with the body's cells happens in the capillaries, which deliver oxygen and nutrients and remove waste.[4]
Cardiac output is the volume of blood each ventricle pumps per minute: CO = HR × SV. With a typical resting stroke volume of 70 mL and heart rate of 75 beats/min, CO ≈ 5.25 L/min. In hard exercise it can reach about 19.5 L/min.[5]
Most adults have a resting heart rate of about 60–100 beats/min, and fitter people often lower. A common rough estimate of maximum heart rate is 220 minus age.[6]
Try it: Exercise and recovery
You need
- Stopwatch
- A step or low bench (about 20–30 cm high), or space for marching on the spot
- Results table
- Optional: a heart-rate monitor or fitness watch
Safety
Moderate exercise only. Pupils with a heart or breathing condition, an injury or who feel unwell do not exercise; they can time and record. Stop at once if dizzy, in pain or very short of breath. Do not press on the neck to find a pulse. Results are for learning, not health checks: no comparing or ranking pupils.
What to do
- Sit quietly for 3 minutes. Find the pulse on the inside of your wrist with two fingers (not your thumb). Count beats for 15 s and multiply by 4. Repeat and take the mean as your resting heart rate.
- Step up and down (or march briskly) at a steady pace for 2 minutes.
- Immediately sit and measure your heart rate for 15 s (× 4).
- Measure again every minute until it is back within 5 beats/min of resting. Record the recovery time.
- Calculate your resting and post-exercise cardiac output, assuming SV = 70 mL at rest. Discuss why this assumption is weak during exercise.
What happens, and why?
Working muscles need more oxygen and produce more carbon dioxide, so heart rate (and stroke volume) rise to increase cardiac output, and breathing speeds up. After exercise, both fall back towards resting values; recovery time is one simple indicator that varies between people.[5][3][6]
Play: Cardiac output calculator
Work out each answer with CO = HR × SV, then pick the right box (L/min, beats/min or mL).
Your best score is kept only on this device. Nothing is sent anywhere.
Quiz
Other ages: Ages 6–7 · Ages 10–11