Summary
Highlights
The main role of the lungs is gas exchange—bringing oxygen into the body and expelling carbon dioxide. During inhalation, the diaphragm and chest muscles contract, creating a vacuum effect. Exhalation involves muscle relaxation, allowing the lungs to return to their normal size, pushing air out.
Air enters through the nostrils into the nasal cavity, which is lined with mucus-secreting cells. This mucus, along with nose hairs, traps particles, dust, pollen, and bacteria. The nasal cavity connects to four paranasal sinuses (frontal, ethmoid, sphenoid, and maxillary) that warm and moisten inspired air and amplify voice resonance.
Clean, warm, and moist air moves from the nasal cavity into the pharynx (throat), which includes the nasopharynx, oropharynx, and laryngopharynx. The soft palate and uvula prevent food from entering the nasopharynx during eating. At the top of the larynx, the epiglottis acts as a lid, sealing off the airway when swallowing, directing food towards the esophagus. A cough reflex protects the larynx from anything other than air.
Air continues from the larynx down the trachea (windpipe), which splits into two mainstem bronchi at the carina. The right lung has three lobes, while the left has two. The right mainstem bronchus is wider and more vertical, making it more susceptible to foreign objects. These mainstem bronchi further divide into smaller bronchi, supported by cartilage rings.
The bronchi walls contain smooth muscle with autonomic nervous system nerves. Sympathetic nerves (fight or flight) stimulate beta 2 adrenergic receptors, increasing airway diameter. Parasympathetic nerves (rest and digest) stimulate muscarinic receptors, decreasing airway diameter. The large airways are lined with ciliated columnar cells and goblet cells that secrete mucus. This mucus traps particles, which are then moved towards the pharynx by the cilia, a process known as the mucociliary escalator.
Beyond the third generation of bronchi, airways become narrower, forming bronchioles, which don't require cartilage. Conducting bronchioles, forming about 15-20 generations, are lined by ciliated columnar cells, goblet cells, and club cells. Club cells secrete glycosaminoglycans, protecting the bronchiolar epithelium, and can transform into ciliated columnar cells to aid in regeneration. Conducting bronchioles receive oxygenated blood from the bronchial arteries.
The last part of the conducting bronchioles are the terminal bronchioles, followed by respiratory bronchioles, which feature tiny outpouchings called alveoli. There are approximately 500 million alveoli in the lungs. When an airway consists solely of alveoli, it becomes an alveolar duct, the final destination for inhaled air.
The alveolar wall lacks cilia and smooth muscle, instead featuring thin pneumocytes. Most are type I pneumocytes, but type II pneumocytes secrete surfactant, which reduces surface tension and keeps alveoli open. Like club cells, type II pneumocytes can regenerate into type I pneumocytes. Alveolar macrophages engulf particles that reach deep into the lungs, traveling up the mucociliary escalator to be expelled. Air in the alveoli is separated from the blood in capillaries by the blood-gas barrier, composed of the alveolar wall (pneumocytes), basement membrane, and capillary wall (endothelial cells).
Deoxygenated blood from the pulmonary arteries flows through the capillaries surrounding the alveoli. Carbon dioxide diffuses from the blood into the alveoli to be exhaled, while oxygen from the inhaled air diffuses into the blood. This newly oxygenated blood travels to the pulmonary veins, then to the heart, and finally to the body's tissues.
In summary, the respiratory system facilitates gas exchange. Oxygen enters through the pharynx, larynx, trachea, large airways, conducting bronchioles, respiratory bronchioles, and alveoli, eventually reaching the capillaries to be distributed to the body. Carbon dioxide follows the reverse path to be exhaled.