Transport in Humans
Structures and Functions in Living Organisms · 6 question types
Exam Frequency Analysis
Past paper frequency (2018 to 2024)
This topic accounts for approximately 15% of your exam marks.
The heart, blood vessels, and blood components are regularly tested, particularly structure-function links.
A typical adult's resting heart rate is around 60 to 80 beats per minute (bpm). The heart rate is set by a special patch of cells in the right atrium called the pacemaker, which sends out regular electrical signals that trigger each heartbeat.
Why heart rate changes with exercise
When you exercise, the muscles need extra energy for every contraction, so the muscle cells step up the rate of respiration. Faster respiration means:
- More oxygen and glucose needed at the muscles
- More carbon dioxide produced that needs removing
- More heat produced that needs distributing or losing
The nervous system responds by speeding up the heart rate, which:
- Pumps more oxygen-rich blood to the muscles per minute
- Removes carbon dioxide from the muscles faster
Each individual heartbeat also pumps more strongly (technically the stroke volume, meaning how much blood is ejected per beat). The combined effect is that cardiac output can rise to roughly five times its resting value during hard exercise.
Adrenaline
The hormone adrenaline, released by the adrenal glands during exercise or fear, also makes the heart beat faster and harder. This is part of the body's fight-or-flight response, preparing the muscles for sudden action.
Heart rate after exercise
The heart rate does not drop straight back to normal when you stop. It stays elevated for several minutes because the body is still paying back the oxygen debt built up during anaerobic respiration in the muscles. The faster the heart rate returns to normal, the fitter the person is, giving a simple way to estimate fitness from heart-rate recovery time.