Organ level

At an organ level, the respiratory system consists of two zones: the conducting zone and the respiratory zone. The conducting zone consists of the nasal cavity, pharynx, and larynx, as well as the trachea, bronchi. The trachea is a tube-like structure that goes from the bottom of the larynx to where it splits into two bronchi which enter the lungs and branch further into smaller bronchioles. The bronchioles are the point where the conducting zone leads into the respiratory zone of the respiratory system. The bronchioles branch off and become smaller and smaller until they terminate at small spherical structures called alveoli. Alveoli look like bunches of grapes at the end of their respective bronchioles, and they are covered in capillaries (tiny blood vessels). The walls of the alveoli are only one cell thick which allows gases (like the oxygen and carbon dioxide that we breath in and out) to easily diffuse into the adjacent blood vessels. Functionally, the conducting zone is tasked with conducting (transporting) oxygen from the environment to respiratory zone where it can diffuse into the blood, oxygenating the red blood cells.

Image sourced from: https://serc.carleton.edu/details/images/37789.html

Pharynx: (aka throat) air from the nasal cavity passes through

Larynx: contains vocal cords, which vibrate when air passes through the larynx, allowing us to make sounds.

Nasal cavity: contains projections that increase the surface area and filters, warms, and moistens air before it enters lungs. It also contains smell receptors and is a resonating chamber for speech sounds. The nasal cavity also contains hairs and mucus that trap incoming dust.

Epiglottis: closes the trachea during swallowing so that food and liquid don’t enter the lungs

Trachea: (aka windpipe) carries air to and from the lungs, lined with mucous membrane and cells with cilia that beat to move mucous and trapped particles in an upwards direction away from the respiratory zone of the lungs.

Bronchi: The two primary bronchi branch from the trachea and then divide into secondary bronchi and then further divide into tertiary bronchi

Bronchioles: very fine tubes with walls of smooth muscle. The finest of them end in groups of alveoli.

Alveoli: tiny sacs that make up most of the lungs and are the site of gas exchange.

Diaphragm: the muscle separating the chest cavity from the abdomen. It contracts and flattens downwards to increase the volume of the chest when inhaling.

Ribs: curved bones that form the framework of the chest.

Intercostal Muscles: muscles between ribs that move the ribcage upwards and outwards to increase volume of the chest cavity and lungs when breathing in.

The lungs: The lungs occupy all the space in the chest cavity (except for the space taken up by the heart and blood vessels).

Pleural Membrane: a membrane that covers the surface of the lungs and lines the inside of the chest cavity. Contains a thin layer of pleural fluid between the membrane layers which holds the lungs inside the chest wall and allows them to move, expanding when they take in air.

Tissue level

Different areas of the respiratory system have different tissue and cell types to perform their unique functions. The tissue in the nasal cavity is columnar epithelium. The cells lining the trachea have cilia (have small finger-like projections, as pictured) and produce mucus. The mucus functions to trap incoming particles that haven’t been caught by the hairs in the nasal cavity and the cilia beat in an upwards direction, pushing the mucus up and out of the lungs.

The Mechanics of Breathing

Air is moved into and out of the lungs in a process called ventilation (or breathing). Air flows from places of higher pressure to places of lower pressure. This is utilised by the lungs to pull air in and push it out.

Inspiration is process by which air enters the lungs. The pressure in the lungs must be less than atmospheric pressure, so the volume of the lungs is increased by the contraction of the diaphragm. The diaphragm becomes flatter, and the rib cage moves upwards and outwards. Lungs follow this expansion because they are adhered to the pleura and air flows into the nose and trachea until the pressure is equalised. When breathing heavily, the external intercostal muscles also contract to further expand the lungs and draw more air in.

Expiration is the exact opposite of inspiration. The diaphragm relaxes and bulges into the chest cavity reducing the volume. This causes the pressure in the lungs to be greater than the atmospheric pressure and air will flow out of the lungs until the pressure equalises. Expiration is usually a passive process when breathing quietly but the intercostal muscles get involved when breathing heavier.