Question by YenLum 2nd Feb 2010:

can you explain to me what is the graph trying to say?

This is a graph showing an experiment where the subject is given a gas mixture with increasing carbon dioxide content. (pCO₂ measures the partial pressure of CO₂) At the same time, his or her breathing rate is recorded. The breathing rate is given as ventilation arbitrary units. The arbitrary unit of 1 being normal, means 1 here stands for about 12-18 breaths per minute. (Ventilation is the amount of gas flowing through the lungs... so it can be physically represented by the breathing rate.) As the carbon dioxide content is increased, the ventilation rate increased, to almost 10 times it's normal value. (Imagine breathing at 120 - 180 breaths per minute!) This shows that CO₂ has an effect of increasing breathing rate so as to supply more oxygen to the subject to counteract the increase in carbon dioxide in his air supply. (Increasing the partial pressure of O₂ was found not to have an opposite effect on breathing. Probably because the body's chemoreceptors are tuned to concentration of carbon dioxide and pH more than oxygen.) We should also note that pH increases. This is an abnormal response due to the high rate of ventilation (referred to as hyperventilation) Blood responds to the Haldane effect ( which states that as the amount of oxygen increases (due to hyperventilation), the affinity of haemoglobin to carbon dioxide decreases.) The Haldane effect is opposite to the Bohr effect (which states that as amount of carbon dioxide increases, the affinity of haemoglobin to oxygen decreases.) In the tissues, Bohr effect is dominant, whereas in the lungs, Haldane effect is dominant. Increase in oxygen affinity to haemoglobin tends to increase pH (that is why the pH is rising.) The blood is becoming more alkaline (alkadosis). An increase in pH makes the body think that it is receiving more oxygen - so the body constricts its blood vessels. Unfortunately, constricting blood vessels (especially the carotid artery to the brain) supplies less blood to the brain. After some time, the subject feels dizzy, and experiences headaches. If this continues, the brain does not have enough oxygen (hypoxia) and the subject might black-out or faint. This probably is what happens if we continue this increase - the pH suddenly "drops" - subject could be unconscious by then. (Note that in this case, even if the affinity for oxygen is increased in haemoglobin, the low partial pressure of oxygen cannot increase pH any higher than 4- there is just not enough oxygen around- so actually the subject is not receiving enough oxygen.)

Hope this helps in the understanding of  what this graph is trying to say.