The urge to inhale in humans result from (a) rising pCO2 (b) rising pO2 (c) falling pCO2 (d) falling pO2

p stands for partial pressures. I am guessing the answer is falling pCO2.... but I wanted what is the answer and the reason for it?

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    $\begingroup$ It sounds like a homework question, so I added the tag. Feel free to correct me if I am wrong. $\endgroup$ – skymningen Apr 7 '16 at 8:34
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    $\begingroup$ As a homework question, guessing doesn't suffice @MukilanSC $\endgroup$ – rg255 Apr 7 '16 at 8:37
  • $\begingroup$ @skymningen This was not an homework question. It was a question given by my biology faculty and he said the answer was fallin pO2.. I wanted to know the reason behind this to counter my faculty and to make the class aware of the real reason.... So you are wrong and I did not violate the community guidelines? $\endgroup$ – Mukilan S C Apr 7 '16 at 16:35
  • $\begingroup$ In the help section on how to ask, the first sentence is "Have you thoroughly searched for an answer before asking your question? Sharing your research helps everyone. Tell us what you found and why it didn’t meet your needs. This demonstrates that you’ve taken the time to try to help yourself, it saves us from reiterating obvious answers, and above all, it helps you get a more specific and relevant answer!" You simply guessed. @rg255 is correct. $\endgroup$ – anongoodnurse Apr 7 '16 at 16:46
  • $\begingroup$ @MukilanSC I don't think we have a general difference here to distinguish between (written) homework questions and "classwork" or "in class" questions/ exam questions and so on. $\endgroup$ – skymningen Apr 11 '16 at 8:23

The correct answer would be a) rising pCO2. The rising CO2 level in the blood causes an urge to inhale and take in more O2.

For how it works, this is controlled by respiratory center:

The respiratory centers (RC) are located in the medulla oblongata and pons, which are part of the brain stem. The RCs receive controlling signals of neural, chemical and hormonal nature and control the rate and depth of respiratory movements of the diaphragm and other respiratory muscles ... In healthy individuals the presence of elevated carbon dioxide levels in the blood is the stimulant that the RC responds to in order to signal the respiratory muscles to breathe. Chemoreceptors found in carotid bodies and aortic bodies are responsible for detecting decrease in blood pH by this carbon dioxide.

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