The respiratory system's structured pathways enable breathing by carrying oxygen to the lungs and filtering out harmful substances, explains WebMD. Respiratory organs also expel carbon dioxide from the body and transport oxygen to the bloodstream so it can be delivered to the cardiac system and cell tissue.
Air inhaled through the nose and mouth passes through the larynx, or voice box, and down a tubular windpipe in the neck and chest, known as the trachea, states the National Heart, Lung, and Blood Institute. The trachea branches off to the left and right bronchi, both of which branch off to several smaller airways, or bronchioles. The lungs expand as they fill up with air, allowing an individual to breathe in and out. The bronchioles inside each lung feed into tiny air sacs, or alveoli, which are lined with blood vessels designed to absorb oxygen into the bloodstream. The pulmonary artery carries blood back and forth between the heart and the vessels surrounding the alveoli. When the body is ready to expel carbon dioxide, the pulmonary artery transfers carbon-rich blood back into the respiratory system, where the carbon is extracted and exhaled.
During inhalation, the respiratory tract keeps its airways unobstructed and protected from infection and inflammation by filtering out foreign matter, notes WebMD. The nose and airways are lined with hair-like protrusions, or cilia, that "comb" out harmful particles. The trachea and bronchi are also lubricated with mucus to trap particles and maintain a healthy level of moisture.