Pulse Data After Three Minutes of Running
During a health and exercise unit, a student records her own measurements. Sitting at rest, her pulse is 65 beats per minute, her breathing rate is 15 breaths per minute, and her stroke volume is estimated at 70 mL per beat. She then runs for three minutes. Immediately afterwards her pulse is 140 beats per minute, her breathing rate is 30 per minute, and her stroke volume is about 110 mL. Ten minutes after stopping, her pulse is 70 and her breathing rate is 16. She notices that her breathing stays deep for a minute or two after she stops, even though her leg muscles are no longer working hard, and that her skin is flushed, damp and warm. A classmate says the deep breathing proves that the muscles 'used up all the oxygen' in her blood during the run.
1. Calculate her cardiac output at rest and immediately after exercise, and the factor by which it increased.
Read the worked answer
Definition. Cardiac output = heart rate x stroke volume.
At rest: 65 beats/min x 70 mL/beat = 4550 mL/min, about 4.6 L/min. After exercise: 140 beats/min x 110 mL/beat = 15 400 mL/min, about 15.4 L/min.
Increase. 15 400 / 4550 = 3.4.
So the heart delivers about 3.4 times as much blood per minute. Note that both factors contribute: rate roughly doubles and stroke volume rises by about 1.6 times, and the product of the two is larger than either change alone. This is why quoting only the pulse rate understates the response.
2. Explain why her breathing stays deep for a minute or two after she stops running, and why her skin is warm and damp.
Read the worked answer
Oxygen debt. When the muscles work faster than oxygen can be delivered, they supplement aerobic respiration with anaerobic respiration, which produces lactic acid. Lactic acid must be broken down or converted back to glucose once the run ends, and that repair work needs oxygen. The extra deep breathing after exercise supplies this oxygen, so it is repaying a debt built up during the run, not meeting the immediate demand of resting muscles.
Heat and flushing. Respiration releases heat, so a higher metabolic rate warms the body. Blood vessels near the skin dilate to carry more blood to the surface, which is why she looks flushed, and sweating plus evaporation cools her. Both effects persist while the body works through the extra heat and the lactic acid, which is why they outlast the exercise itself.
3. Judge the classmate's claim that the muscles 'used up all the oxygen' in the blood. What does the data actually show limits her performance?
Read the worked answer
The claim is wrong. Oxygen is consumed continuously, not stored and emptied. Blood leaving an exercising muscle carries less oxygen than at rest because more has been unloaded to the tissue, but it never reaches zero: haemoglobin still holds oxygen and the venous blood is still partly saturated. If all oxygen were removed, the muscle would be unable to respire aerobically at all, and the run could not continue for three minutes.
What the data show. Cardiac output rose 3.4-fold and breathing rate doubled, so the body responded by delivering more oxygen, not by exhausting a fixed supply. What limits hard exercise is the rate at which oxygen can be delivered and used - cardiac output, ventilation, diffusion into muscle and fuel supply - together with the accumulation of lactic acid and heat. The post-exercise deep breathing is best explained as repayment of the oxygen debt, which is a much better account of the observation than 'the oxygen ran out'.