Anesthesiologists require an extensive knowledge of respiratory physiology to care for patients in the operating room and the intensive care unit. The respiratory system may be regarded as a collapsible elastic sac (the lungs) surrounded by a semirigid cage (the thorax) with a piston at one end (the diaphragm) supplied through a branching set of semirigid tubes (the airway and bronchial tree). The periodic exchange of alveolar gas with the fresh gas from the upper airway reoxygenates desaturated blood and eliminates CO2. This exchange is brought about by small cyclic pressure gradients established within the airways. Control of breathing during spontaneous ventilation is the result of rhythmic neural activity in respiratory centers within the brain stem originates in the medulla. Two medullary groups of neurons are generally recognized: a dorsal respiratory group, which is primarily active during inspiration; and a ventral respiratory group, which is active during expiration. This activity regulates pulmonary muscles to maintain normal tensions of O2 and CO2 in the body. The basic neuronal activity is modified by inputs from other areas in the brain, volutional and autonomic, as well as central receptors The most important of these sensors are chemoreceptors that respond to changes in hydrogen ion concentration and peripheral receptors (sensors) Peripheral Chemoreceptors: the carotid bodies and the aortic bodies and Lung Receptors: stretch receptor are distributed in the smooth muscle of airways. There is different types of current models of anesthesia machine ventilators for example: Dräger Divan ventilator, The Fabius GS; Apolo ventilator, GE ADU ventilator; GE-Aisys, GE- Avance, GE Aespire