The primary stimulus for breathing is the bronchioles in the lungs reacting to a build-up of carbon dioxide. When carbon dioxide levels are too high, the body reacts by taking in oxygen and expelling carbon dioxide.
Breathing can be manually controlled by the body to a point. When carbon dioxide levels rise too high and reach the medulla oblongata, the body involuntarily takes a breath. Cellular respiration produces carbon dioxide at a faster rate than oxygen production. This causes the lungs to respond through increased ventilation, providing an additional oxygen supply.
Carotid bodies in the carotid artery respond to changes in environmental oxygen loss when the carbon dioxide levels remain unchanged. They help regulate breathing to provide the necessary amount of oxygen to survive in oxygen-sparse environments.