Chemical control of breathing, via variations in blood chemistry, affect activity of respiratory center and adjust ventilation proportionate to actual metabolic demands. Two control mechanisms ensure chemical control of breathing. Peripheral chemoreceptors are located in the carotid and aortic bodies. They are complex sensors are respond to both: hydrogen ion concentration and to the partial pressure of oxygen in arterial blood. Central chemoreceptors are located near ventrolateral surface of medulla oblongata and sense H+ ion concentration in their local environment. Measuring the chemoreflexes can provide very important information describing the behavior of the regulatory system not only in the normal but also in the pathological conditions due to respiratory instability.