Inhalation (or inspiration) is the process of drawing air or other gases into the respiratory tract, primarily for the purpose of breathing and oxygen exchange within the body. It is a fundamental physiological function in humans and many other organisms, essential for sustaining life. Inhalation is the first phase of respiration, allowing the exchange of oxygen and carbon dioxide between the body and the environment, vital for the body's metabolic processes. This article delves into the mechanics of inhalation, its significance in various contexts, and its potential impact on health.

Physiology

The process of inhalation involves a series of coordinated movements and physiological mechanisms. The primary anatomical structures involved in inhalation are the respiratory system, which includes the nose, mouth, pharynx, larynx, trachea, bronchi, and lungs. Here is a brief overview of the inhalation process:

1. Inspiration: Inhalation begins with the contraction of the thoracic diaphragm, a dome-shaped muscle that separates the chest cavity from the abdominal cavity. The diaphragm contracts and moves downward, increasing the volume of the thoracic cavity.
2. Air entry: When a person or animal inhales, the diaphragm, located below the lungs, contracts, and the intercostal muscles between the ribs expand the chest cavity. This expansion creates a lower pressure inside the chest compared to the atmosphere, causing air to flow into the lungs.
3. Air filtration: The nasal passages and the mouth act as entry points for air. These passages are lined with tiny hair-like structures called cilia and mucus-producing cells that help filter and humidify the incoming air, removing particles and debris before it reaches the lungs.
4. Gas exchange: Once the air enters the lungs, it travels through a branching network of tubes known as the bronchial tree, ultimately reaching tiny air sacs called alveoli. In the alveoli, oxygen from the inhaled air diffuses into the bloodstream, while carbon dioxide, a waste product of metabolism, is released from the blood into the alveoli for exhalation.
5. Expiration: Exhalation is a passive process, primarily driven by the relaxation of the diaphragm and the elastic recoil of the lungs. This expels carbon dioxide from the body.^[1][2]

Other substances – accidental

Examples of accidental inhalation includes inhalation of water (e.g. in drowning), smoke, food, vomitus and less common foreign substances^[3] (e.g. tooth fragments, coins, batteries, small toy parts, needles).

Other substances – deliberate

Recreational use

Legal^{[citation needed]} – helium, nitrous oxide ("laughing gas")

Illegal^{[citation needed]} – various gaseous, vaporised or aerosolized recreational drugs, called inhalants

Medical use

Diagnostic

Various specialized investigations use the inhalation of known substances for diagnostic purposes. Examples include pulmonary function testing (e.g. nitrogen washout test, diffusion capacity testing (carbon monoxide, helium, methane)) and diagnostic radiology (e.g. radioactive xenon isotopes).

Therapeutic

Gases and other drugs used in anaesthesia include oxygen, nitrous oxide, helium, xenon, volatile anaesthetic agents. Medication for asthma, croup, cystic fibrosis and some other conditions.

Mechanism

Inhalation begins with the contraction of the muscles attached to the rib cage; this causes an expansion in the chest cavity. Then takes place the onset of contraction of the thoracic diaphragm, which results in expansion of the intrapleural space and an increase in negative pressure according to Boyle's law. This negative pressure generates airflow because of the pressure difference between the atmosphere and alveolus.

The inflow of air into the lungs occurs via the respiratory airways. In health, these airways begin with the nose.^[4][5] It is possible to begin with the mouth, which is the backup breathing system. However, chronic mouth breathing leads to, or is a sign of, illness.^[6][7][8] They end in the microscopic dead-end sacs(alveoli) always opened, though the diameters of the various sections can be changed by the sympathetic and parasympathetic nervous systems. The alveolar air pressure is therefore always close to atmospheric air pressure (about 100 kPa at sea level) at rest, with the pressure gradients that cause air to move in and out of the lungs during breathing rarely exceeding 2–3 kPa.^[9][10]

Other muscles that can be involved in inhalation include:^[11]

• External intercostal muscles
• Scalene muscles
• Sternocleidomastoid muscle
• Trapezius muscle

Hyperinflation

Hyperinflation or hyperaeration is where the lung volume is abnormally increased, with increased filling of the alveoli. This results in an increased radiolucency on X-ray, a reduction in lung markings and depression of the diaphragm. It may occur in partial obstruction of a large airway, as in e.g. congenital lobar emphysema, bronchial atresia and mucus plugs in asthma.^[12]

Yoga

Yogis such as B. K. S. Iyengar advocate both inhaling and exhaling through the nose in the practice of yoga, rather than inhaling through the nose and exhaling through the mouth.^[13][14][15] They tell their students that the "nose is for breathing, the mouth is for eating."^{[14][16][17][13]}