ABSTRACT
The respiratory system conducts inspired air from the environment to the lung alveoli, where gas exchange takes place and conducts the expired air containing carbon dioxide out of the body. The upper respiratory tract starts at the nostrils and the nasal cavity, preparing the air (humidity, temperature, filtration) to go through the pharynx, larynx, trachea, and lungs. Within the lungs, the air passes from large bronchi, supported by rings of cartilage, smaller bronchi, and bronchioles, with smooth muscle in their walls. Finally, the air reaches the alveoli, where the thin wall makes gas exchange occurs. Birds, reptiles, and amphibians show specialised respiratory systems, and fish show distinct structures for respiration such as gills.
