All mammals use their mouths and noses to breathe air into their lungs, according to Boundless. When a monkey inhales air, its lungs expand. Oxygen spreads across the inner surface of each lung, where it enters the bloodstream.
All mammalian lungs are located near the heart, explains Wikipedia. Skeletal muscles and the muscular diaphragm at the bottom of the monkey's thorax enable the lungs to constantly expand and contract. Like all other mammals, a monkey's lungs filter oxygen into the bloodstream while filtering carbon dioxide out. Air enters the nasal and oral cavities, and then flows through the pharynx, larynx and trachea. The air then enters the lungs and is filtered through the alveoli.
Wikipedia states that alveoli are millions of tiny, hollow air sacs found only in the lungs of mammals such as monkeys. Vertebrates other than mammals have different structures inside their lungs. This is one of the most prominent differences between the lungs of mammals and other animals such as birds and fish. Without such an efficient filtering system, monkeys and all other mammals would enter respiratory failure due to a lack of oxygen and an overabundance of carbon dioxide in their system, according to the National Heart, Lung and Blood Institute.