echoesof.science · Ages 15–16
Circulation, gas exchange and cardiac output
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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.
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.
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.
Experiment: Exercise and recovery
- 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.
| Time | Heart rate (beats/min) | Cardiac output (L/min, SV = 70 mL) |
|---|---|---|
| Rest (mean) | Predict how many minutes your heart rate will take to return to resting. | |
| 0 min after | Predict how many minutes your heart rate will take to return to resting. | |
| 1 min after | Predict how many minutes your heart rate will take to return to resting. | |
| 2 min after | Predict how many minutes your heart rate will take to return to resting. | |
| 3 min after | Predict how many minutes your heart rate will take to return to resting. | |
| 4 min after | Predict how many minutes your heart rate will take to return to resting. |
Questions
Describe the route of a red blood cell from the right atrium to the aorta.
Explain why gas exchange in the alveoli is fast. Use the words concentration gradient and capillaries.
Calculate your resting and post-exercise cardiac output (SV = 70 mL). By what factor did it change?
Give two limitations of your method and one improvement.
Sources for every fact on this sheet: echoesof.science/school/heart-and-lungs/sources/