When our body exercises, our muscle cells are more active; they need more energy, so more aerobic respiration occurs, and more oxygen must be provided.
Muscles in our circulatory system and our respiratory system respond to the body's new requirements.
Our heart rate increases to speed up the supply of oxygenated blood to the muscles, and deoxygenated blood to the lungs.
It also increases the supply of glucose to the muscles.
Our breathing rate increases to increase the supply of air to the lungs, so that more oxygen can be absorbed.
Inside the lungs, air is replaced more quickly so that oxygen content is kept high, and carbon dioxide does not build up. This makes sure that diffusion occurs at its maximum rate because it keeps the concentration gradients high for both these gases in the alveoli .
What is/are the main muscle/muscles powering our circulatory system?
> ventricles - lower chambers of the heart
What is/are the main muscle/muscles powering our respiratory system?
> intercostal muscles of the chest & the diaphragm
How could you measure an increase in heart rate?
> pulse rate (beats per minute) would go up
What else could the heart do to increase the supply of blood?
> pump more blood at each beat (increase stroke volume)
How could you measure an increase in breathing rate?
> chest movement (breaths per minute) would go up
What else could the chest do to increase the supply of air?
> pump more air at each breath (increase depth of breathing)
If the rate of exercise is increased due to greater muscular activity, a point is reached at which not enough oxygen is supplied to meet the requirements of the muscle cells for aerobic respiration, as described earlier.
It is not just that insufficient oxygen can be absorbed from the extra amount of air breathed in, or that the heart can only increase blood flow by a certain amount, but also that haemoglobin in red blood cells can only absorb a certain amount of this oxygen, and this limits the amount it can unload into plasma in capillaries near the muscle cells. Then there is the slowness of diffusion of oxygen from the plasma into the tissue fluid, and into the mitochondria inside the muscle cells.
The actual rate limiting factor is probably the concentration of red blood cells in the blood, which explains why blood doping and the use of EPO are common in some sports events.
In vigorous exercise, it is often said that glucose which is already present in the muscle cells is converted into lactic acid, the substance produced when milk goes sour, but it is probably more accurate to say that lactate is produced, so the effect seen is not simply a direct consequence of the buildup of acidity. This conversion is less of a change than breaking down glucose all the way to carbon dioxide and water, and so less energy is released than in aerobic respiration, but the process does not require any oxygen or produce carbon dioxide. This anaerobic respiration is not actually a completely different pathway than aerobic respiration, but more of a short-term switch.
Anaerobic respiration cannot go on for more than a few seconds as the lactic acid/lactate builds up in the muscles and is said to cause cramp, or at least fatigue and soreness. In fact it is better to say that there is an equilibrium between the rate of production and the rate of removal of lactic acid/lactate.
World Records (Men)
This is typically the situation for the 100m or 200m sprint, which can take (just under) about 10 seconds per 100m, whereas moving up from 400m to 10000m takes progressively more time - over 15 second per 100m.
These longer distances rely on sustained aerobic respiration, but may still use anaerobic respiration for a sprint finish. Obviously athletes train their bodies to maximise the performance of their respiratory and circulatory system, but also to increase their tolerance to lactic acid/lactate.
After strenuous exercise has ended, the body responds by continuing to breathe deeply, even though the muscles no longer need the extra energy they did a few seconds before.
This is because the body has built up an oxygen debt. This must be repaid by the intake of oxygen to re-saturate the blood's haemoglobin, and restore a chain of oxidation reactions which replace the ATP used by the muscle for contraction.
Oxygen is also used to convert lactic acid/lactate back to glucose using oxygen. This glucose may be converted into the insoluble polysaccharide glycogen which is stored in the liver. Most of this conversion occurs in the liver, but some may occur in the muscles themselves.
Some more questions
What is a more legal method of increasing the number of red blood cells in the body?
> altitude training - more red blood cells are produced at high altitudes
It is quite difficult to reconcile the explanations given in sports and health sources with purely "scientific" biological information, and some of the information used by exam boards. This is even more so with web-based sources, which often have their own agendas.