Cells need energy to fulfil their allocated function eg movement (muscle) or thinking (brain). Cells burn fats and carbohydrates by combining them with oxygen to provide energy, carbon dioxide and water. This is a sustainable situation that depends on the cell getting enough oxygen and this is dictated by the levels of carbon dioxide, not oxygen as might be expected.
When air is breathed into the lungs, oxygen in the air attaches to the haemoglobin in the blood and is transported to the tissues. Here under the influence of carbon dioxide, oxygen is released from the haemoglobin and becomes available to the cells. A hard working muscle cell produces plenty of carbon dioxide which facilitates this release of oxygen (the Bohr effect) and enables the cell to continue working aerobically in an elegant positive cycle – the harder the cell works the more carbon dioxide is produced and so the more oxygen is made available.
The blood returns to the lungs and carbon dioxide rapidly dissipates into the air in the alveoli making the haemoglobin receptive to the oxygen in the air and oxygen is taken into the body.
If carbon dioxide levels in the tissues are low (due to over-breathing), oxygen remains fixed on the haemoglobin and is unavailable to the cell. In order to obtain energy the cell has to switch to anaerobic (without oxygen) respiration and produces lactic acid rather than carbon dioxide and water, and only 5% of the energy.
The positive cycle based on carbon dioxide is lost and the cells are compromised in their efficiency. Less energy is produced and the by-product, lactic acid, instead of being useful (as are carbon dioxide and water) is acidic and needs to be detoxified using oxygen which increases the oxygen debt. Build up of lactic acid indicates that damage has occurred due to lack of oxygen. This damage is reversible if the body regains normal carbon dioxide levels.
Equine Breathing reduces the volume of air breathed which enables carbon dioxide levels to build back up. The more carbon dioxide is available the longer the cells can keep going on aerobic (oxygen based) respiration rather than having to switch to damaging and less efficient anaerobic respiration. Horses with better breathing (and therefore higher levels of carbon dioxide) will be able to maintain aerobic respiration for longer than horses with poor breathing (lower levels of carbon dioxide) during strenuous exercise and will therefore recover more quickly.
Other factors contribute to increased fitness. Carbon dioxide is a smooth muscle relaxant so at high levels of carbon dioxide the airways and the blood vessels of the circulatory system are able to relax and dilate, allowing efficient distribution of oxygen and nutrients such as glucose. Low levels of carbon dioxide are responsible for constriction of the airways and the blood vessels and consequent starving of the cells of oxygen and nutrients.