AQA GCSE Biology: Response to Exercise

During exercise the muscles need much more energy, so the body works to deliver more oxygen and glucose to them. This guide explains how the heart and lungs respond, what happens when oxygen runs short, and the Higher-tier ideas of oxygen debt and the liver's role in recovery. The theme is meeting a rising demand for energy.

Responding to increased demand

When you exercise, your muscles respire faster and need more oxygen and glucose. The body responds in three ways, and AQA expects all three.

• Heart rate increases, pumping blood faster, so more blood reaches the muscles and lungs.
• Breathing rate increases, so air is exchanged more often.
• Breath volume increases, so each breath brings in more air.

Together these deliver more blood, carrying more oxygen and glucose, to the working muscles, allowing a higher rate of aerobic respiration to release more energy for contraction.

It is worth seeing why both the lungs and the heart have to change, because questions often ask for the reason rather than the response. Breathing harder loads more oxygen into the blood. A faster heart rate delivers that blood to the muscles more quickly. One without the other achieves little: well-oxygenated blood sitting in the lungs is no use to a leg muscle, and blood arriving quickly but carrying no extra oxygen is no use either.

Muscles also hold a store of glucose in the form of glycogen, which is converted back into glucose and respired during exercise. Glycogen is covered properly in the metabolism guide.

When oxygen runs short

During very intense or prolonged exercise, the muscles may not get enough oxygen to meet their energy demand. Anaerobic respiration then happens alongside aerobic respiration. Because glucose is only partly broken down in anaerobic respiration, lactic acid is produced. A build-up of lactic acid causes an oxygen debt and makes the muscles fatigued, so they stop contracting efficiently.

Two details matter here. Anaerobic respiration adds to aerobic respiration rather than replacing it, so a sprinting athlete is doing both at once. And muscle fatigue has a specific meaning in the mark scheme: the muscles stop contracting efficiently. Saying the muscles "get tired" or "cramp up" is not the same thing and usually scores nothing.

The phrase AQA uses for why lactic acid appears is the incomplete oxidation of glucose. Learn that wording. The glucose is not fully broken down, so less energy is released from each molecule and a product is left over.

Oxygen debt

(Higher tier only.) The oxygen debt is the extra oxygen the body needs after exercise to react with the built-up lactic acid and remove it from the cells. This is why heart rate and breathing rate stay high for a while after you stop exercising, supplying the extra oxygen to repay the debt.

The name is a good description of what happens, and the analogy is worth holding onto. Your muscles needed energy faster than oxygen could be delivered, so they carried on working on credit, and the debt falls due once you stop. Repaying it takes real oxygen, which is why you stand there breathing hard for several minutes after a sprint even though you are no longer moving.

The usual mistake is defining oxygen debt as the oxygen the body was short of during exercise. It is the extra oxygen needed afterwards, to deal with the lactic acid that shortage produced.

The role of the liver

(Higher tier only.) Blood flowing through the muscles carries the lactic acid away to the liver, where it is converted back into glucose.

Note that the muscle does not deal with its own lactic acid. The blood transports it to the liver, and the liver does the conversion. Questions that ask you to describe the fate of lactic acid are looking for that journey as well as the destination.

How this fits together

Respiration and its response to exercise are part of the wider set of reactions that keep an organism alive. The whole set of these reactions is called metabolism, the subject of the metabolism guide.