The Respiratory System

The major respiratory structures span the nasal cavity to the diaphragm.
Unit 9 figure 1 Respiratory System Overview

The respiratory system is a system that plays several roles in the physiology of our bodies. It helps us to move air in and out of our lungs (a process called ventilation) and enables our bodies to exchange gases between our blood and the air that we breathe (a process called respiration). Oxygen (O2) is exchanged for carbon dioxide (CO2) in the millions of tiny air sacs called alveoli that fill our lungs. Our respiratory system enables us to produce sound through our vocal chords and to smell along with receptors in our nasal passages. The respiratory system also heats and humidifies the air that we breathe. Cilia, which are part of the mucociliary escalator (a lining of cilia along the bronchi and trachea), help to trap tiny particles that we may inhale and move them upwards towards our mouth so that the air that reaches the alveoli is free of dust or dirt.

The pharynx is divided into three regions: the nasopharynx, the oropharynx, and the laryngopharynx.
Unit 9 figure 2 Divisions of the Pharynx
The structure of the respiratory system can be divided into the upper and lower respiratory tracts. The nasal cavity, oral cavity, and pharynx make up the upper respiratory system. The larynx, or voice box, separates the upper from lower respiratory system. The trachea, bronchi, and lungs make up the lower respiratory system. We often designate respiratory infections based on this classification. For example, when we talk about upper airway infection (URI), we refer to infection in the parts of the respiratory system from the mouth or nose to the larynx.
Photo of the keel of the ship "Mariah" next to a photo of the primary bronchi cross-sectional posterior view.
Unit 9 figure 3 Coat Keel and Carina in Trachea

The trachea divides into two primary bronchi (one for each lung, right and left) at an anatomical feature called the carina, after the Latin word for boat keel.

Bronchioles lead to alveolar sacs in the respiratory zone, where gas exchange occurs.
Unit 9 figure 4 Respiratory Zone

Oxygen and carbon dioxide are exchanged in the lower respiratory system. The main bronchi branch into smaller air pathways called bronchioles and then terminal bronchioles and respiratory bronchioles which eventually terminate at the small air sacs called alveoli. We divide respiration (the exchange of gases such as O2 and CO2) into two categories: external respiration and internal respiration. External respiration is the exchange of gases between the pulmonary capillaries and the alveoli which contain atmospheric air. CO2 diffuses from the pulmonary capillaries into the alveoli; O2 diffuses from the alveoli into the pulmonary capillaries. Internal respiration is the exchange of gases between the tissues and the systemic capillaries. Oxygen diffuses from the systemic capillaries into the tissues; carbon dioxide diffuses from the tissues into the bloodstream. Carbon dioxide is carried back to the lungs, first by venules, then veins, then one of the two venae cavae, to be exchanged with oxygen. As we exhale, we release that carbon dioxide back into the air.

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