The neural regulators and feedback mechanisms which control the rhythmic nature of breathing in mammals (and many other organisms) are currently the subject of intense research.

Mammalian central pattern generators producing rhythmic movements exhibit robust but flexible behavior. However, brainstem network architectures that enable these features are not well understood.

Source Respir Physiol Neurobiol. 2009 Aug 31; 168(1-2): 19–25.

It's known that the 'Respiratory Center' is located in the medulla oblongata and pons in the brainstem. But the exact mechanisms which generate and regulate the rhythms are unknown.

One area of interest is the pre-Bötzinger Complex (preBötC) which is a cluster of inter-neurons in the ventral respiratory group of the medulla of the brainstem. It has been suggested that it may be crucial for the generation of the respiratory rhythm in mammals, though it's currently unclear how this system is regulated.

The location of neurons generating the rhythm of breathing in mammals is unknown.
[…] the respiratory rhythm in the mammalian neonatal nervous system may result from a population of conditional bursting pacemaker neurons in the pre-Bötzinger Complex.

Source : Science. 254(5032): 726–729.

Also see : Carotid Body Mechanismplugin-autotooltip__plain plugin-autotooltip_bigCarotid Body Mechanism

The level of oxygen present in mammalian blood - critical for survival - is controlled by a complex feedback system triggering increased or decreased breathing, and alterations in blood pressure etc.. It's dependent on 'sensors', one primary sensor is called the Carotid Body. and Yawningplugin-autotooltip__plain plugin-autotooltip_bigYawning

The physical and psychological sequences which occur during the yawning process have been widely studied. Nevertheless, the reasons for yawning are disputed and unclear - as is identifying an evolutionary reason to explain the need for it.

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