Anaerobic respiration

Cells can release energy from nutrient molecules without using oxygen. They do this through a series of chemical reactions called anaerobic respiration. The released energy can be used by the cell.

Anaerobic respiration releases much less energy per glucose molecule than aerobic respiration, which uses oxygen. The products of anaerobic respiration depend on the cells involved.

Anaerobic respiration in yeast

In yeast cells, glucose is broken down to produce alcohol and carbon dioxide:

Anaerobic respiration in muscles

During vigorous exercise, muscle cells can release energy from glucose by anaerobic respiration. In these cells, glucose is broken down to produce lactic acid:

Cardiorespiratory responses to exercise

During exercise, muscles contract more and need more energy. They therefore need more oxygen for aerobic respiration, the process that releases energy from glucose using oxygen.

The heart and breathing respond together to increase the uptake of oxygen and its delivery to the active muscles:

• Heart rate increases: the heart beats more times each minute, helping deliver more oxygenated blood to the muscles.
• Breathing rate increases: more breaths are taken each minute.
• Breath volume increases: more air is moved in and out with each breath.

The changes in breathing move more air through the lungs each minute, increasing oxygen uptake into the blood. The increased heart rate helps deliver this oxygen to the muscles for aerobic respiration.

Oxygen debt

During exercise, muscles may need energy faster than the oxygen supply can support. When this happens, some glucose is broken down by anaerobic respiration: it is incompletely oxidised, and lactic acid accumulates in the muscles.

The body needs extra oxygen after exercise to help remove and process the accumulated lactic acid and return towards resting conditions. This additional oxygen requirement is called the oxygen debt.

During recovery, breathing and heart rates can remain elevated. Faster breathing brings more oxygen into the body, while a faster heart rate helps deliver it in the blood. As recovery continues and the need for extra oxygen decreases, breathing and heart rates return towards their resting levels.

Muscle fatigue

During a long period of vigorous activity, muscles may become less able to sustain effective contractions. This temporary decline in muscle performance is called muscle fatigue.

As fatigue develops, repeated contractions may produce less force or become less effective. The activity therefore becomes harder to maintain at the same level.

Muscle fatigue does not necessarily mean that the muscle has been permanently damaged. With rest, its ability to contract can recover.