In humans and most mammals, there are three parts in the respiratory system.
The first is the series of conductive tubes that carry air from the atmosphere to the lungs. The second part consists of the muscles of respiration, the diaphragm and the intercostal muscles in the ribs. The lungs make up the third part.
The diaphragm is a dome-shaped muscle that curves upward toward the lungs. When it contracts, it flattens and, therefore, increases the volume of the thoracic cavity. Similarly, the contraction of the external intercostal muscles moves the ribs up and out.
This increase in volume leads to a drop in pressure within the lungs, allowing air to passively flow into the airways. Gas exchange occurs in the alveoli until these muscles relax, reversing the process.
The airways can be divided into the conduction and breathing zones. The conduction zone begins in the nose and finishes in smaller bronchioles, and these conduits take air towards the internal hollows of the lungs.
The respiratory zone contains the terminal bronchioles and the alveoli, the sites where gas exchange occurs.
The nose and mouth form the main external openings and mark the beginning of the airway conduction zone or the respiratory tract. The nasal cavity behind the nose contains hair and filters and humidifies the air.
Most of the large environmental pollutants are trapped in the mucus secreted by the cells of the nose and nasal cavity.
The mouth can not reproduce all the functions of the nasal cavity and acts as a secondary opening when the nose is blocked or there is an immediate need for large amounts of air.
That is why we take large amounts of air while we gasp or sleep with our mouths open when our nose is congested.
The air then passes to the pharynx, which is also involved in swallowing. The epiglottis prevents the movement of food into the respiratory tract and the incorrect direction of air into the esophagus. When the epiglottis does not work properly, small particles can enter the trachea. These are eliminated through coughing.
If food is lodged or trapped in the airway, it may be necessary to remove it quickly by abdominal compressions, also known as the Heimlich maneuver.
The larynx follows the pharynx and its main function is the production of sound.
The air flow through this region can influence tone and volume. Then, the air enters the trachea, a long tube that is covered by a series of cartilaginous rings, which help this tubular structure retain its shape during inhalation and exhalation.
The trachea is covered by pseudostratified columnar epithelia with goblet cells that secrete mucins and help to form mucus.
The trachea divides to form two primary bronchi, called the left and right bronchi . Each of these leads to a lung and then branches repeatedly to produce secondary and tertiary bronchi and bronchioles, with successively smaller diameters.
When the bronchioles are less than a millimeter in diameter, they are called terminal bronchioles, whose purpose is to end up in vascularized alveoli. As the bronchi begin to branch out, their internal structure changes.
Cartilage is more common in the larger airways, and a single epithelial layer is common in the smaller parts of the conduction zone and respiratory zone. The bronchi and bronchioles contain smooth muscles that can contract during rest or dilate during exercise.
The lungs are made of spongy tissue that contains many vascular tissues and much of the airway that appears after the trachea. A pleural membrane allows these paired organs to expand and contract with minimal friction.
The left lung is smaller than the right because of the location of the heart on the left side of the thoracic cavity.