Anatomy and physiology
Physical Education · GCSE · The School
During exercise
Heart rate and stroke volume rise, so CARDIAC OUTPUT increases and more oxygenated blood reaches the muscles. Breathing rate and depth rise. Body temperature rises and you sweat. Blood is redirected away from the digestive system towards working muscles, which is why exercising straight after a large meal feels unpleasant.
Over months of training
The body ADAPTS. The heart wall thickens and the chamber enlarges, so it pumps more per beat and resting heart rate falls — trained endurance athletes may sit near 40 beats per minute. Muscles gain more capillaries and mitochondria. Bones become denser and tendons stronger. Adaptations are specific to the training done.
Aerobic and anaerobic
AEROBIC respiration uses oxygen, releases much more energy per unit of glucose, and powers longer, moderate activity. ANAEROBIC respiration works without oxygen for short intense efforts, produces far less energy and generates lactic acid, which is why maximum effort cannot be sustained. Most real activity uses both, shifting between them.
A footballer jogs into position: aerobic, using oxygen, producing carbon dioxide and water, sustainable for the whole match. She sprints 30 metres for the ball: demand outruns oxygen supply, so anaerobic respiration takes over, producing lactic acid and lasting seconds. She jogs back: aerobic again, and the oxygen debt is repaid. Both systems ran the whole time; what changed was the proportion, set by INTENSITY.