Summary
Highlights
The respiratory system acquires oxygen and removes carbon dioxide, which is a byproduct of ATP production and can be harmful if it accumulates. Key structural components include the external nose, nasal cavity, pharynx, larynx, trachea, bronchi, and alveoli.
The upper respiratory tract extends from the nose to the larynx, while the lower respiratory tract includes the trachea, bronchi, and alveoli. The conducting zone is for air passage only, with no gas exchange, whereas the respiratory zone, primarily the alveoli, is where gas exchange occurs.
Beyond oxygen and carbon dioxide exchange, the respiratory system maintains blood pH by regulating CO2, produces chemical mediators like ACE (regulating blood pressure), facilitates voice production via the larynx, enables olfaction (smell), and offers protection against microorganisms through nose hairs and mucous membranes.
The external nose is mainly hyaline cartilage. The nasal cavity extends from the nostrils to the choana, features paranasal sinuses (lined with mucus for defense), and has conchae to increase surface area for warming and humidifying air. The pharynx (throat) is a common pathway for air and food, divided into nasopharynx, oropharynx, and laryngopharynx.
The larynx, or voice box, is located in the anterior throat. It consists of nine cartilages, including the prominent thyroid cartilage (Adam's Apple) and the epiglottis, which prevents food from entering the trachea. Vocal folds within the larynx vibrate as air passes through, producing sound, with force determining loudness and tension determining pitch.
The trachea (windpipe) is made of 16-20 C-shaped cartilage rings and lined with pseudostratified columnar epithelium. Smoking can damage its cilia. The bronchi branch from the trachea, with decreasing diameter but increasing number as they extend into the lungs. The alveoli are air sacs surrounded by capillaries, serving as the primary site for gas exchange.
Alveoli are a complex network of air-filled sacs. The respiratory membrane, where gas exchange occurs, is formed by the thin walls of the alveoli and capillaries. It consists of alveolar epithelium (simple squamous), a thin interstitial space, and capillary endothelium (also simple squamous), optimizing gas diffusion. Surfactants prevent the collapse of alveoli.
The thoracic wall, comprising bones and muscles, encloses the thoracic cavity containing the lungs. The lungs are the primary organs of respiration, cone-shaped, with the right lung having three lobes and the left lung two (accommodating the heart's apex). Deoxygenated blood is carried to the lungs via pulmonary arteries, oxygenated in pulmonary capillaries, and returns to the heart via pulmonary veins.
The lungs are encased by a double-layered pleura: the parietal pleura lines the thoracic cavity, and the visceral pleura covers the lung surface, creating a pleural cavity. Ventilation, or breathing, involves moving air in and out through muscle action (diaphragm, intercostals) and air pressure gradients. Inspiration involves increasing thoracic volume, while expiration reduces it.
Ventilation is affected by gender, age, body size, and physical fitness. Lung recoil, enabled by elastic fibers and fluid, helps exhalation. Surfactants prevent lung collapse. Gas exchange in lungs and tissues is driven by partial pressure differences of oxygen and carbon dioxide. Respiratory membrane thickness affects diffusion rate; pulmonary edema reduces it.
Hemoglobin, a protein in red blood cells, binds to oxygen. The medulla oblongata in the brain regulates the rhythm of ventilation through the medullary respiratory center, which includes dorsal (inspiration) and ventral (both inspiration and expiration) respiratory groups. The pre-Botzinger complex establishes the basic rhythm.
Respiratory rate increases with decreased oxygen (hypoxia) or increased carbon dioxide (hypercapnia), and decreases with decreased carbon dioxide (hypocapnia). Chemoreceptors detect changes in CO2 and blood pH. The Hering-Breuer reflex limits inhalation depth to prevent overinflation of the lungs.
Microscopic views show the nasal cavity with its open spaces and septum. The larynx reveals false and true vocal cords, lined with pseudostratified ciliated columnar epithelium and goblet cells. The trachea displays C-shaped hyaline cartilage rings and similar ciliated epithelium. Bronchioles have cuboidal epithelium and smooth muscles, while alveoli are composed of simple squamous epithelium for efficient gas exchange, along with alveolar macrophages for protection.