The process of breathing in is called inhalation. The process of breathing out is called exhalation. Breathing is a vital function of life. The lungs add oxygen to the blood and remove carbon dioxide in a process called gas exchange.

In addition to the lungs, your respiratory system includes airways, muscles, blood vessels, and tissues that help make breathing possible. Your brain controls your breathing based on your body’s need for oxygen.

Your lungs lie on each side of your breastbone and fill the inside of your chest cavity. The right lung is divided into three main sections called lobes, and the left lung has two lobes to allow space for the heart. Your left lung is slightly smaller than your right lung.

The airways are pipes that carry oxygen-rich air to your lungs. They also carry carbon dioxide, a waste gas, out of your lungs. The airways include your:

• Mouth
• Nose and linked air passages called the nasal cavity and sinuses
• Larynx, or voice box
• Trachea, or windpipe
• Tubes called bronchial tubes, or bronchi, and their branches
• Small tubes called bronchioles that branch off of the bronchial tubes

Air first enters your body through your nose or mouth, which wets and warms the air. Cold, dry air can irritate your lungs. The air then travels past your voice box and down your windpipe. The windpipe splits into two bronchial tubes that enter your lungs. A tough tissue called cartilage helps the bronchial tubes stay open.

Within the lungs, your bronchial tubes branch into thousands of smaller, thinner tubes called bronchioles. The muscular walls of the bronchioles are different from the bronchial tubes. The bronchioles do not have cartilage to help them stay open, so the walls can widen or narrow to allow more or less airflow through the tubes.

The thousands of bronchioles end in clusters of tiny round air sacs called alveoli. Your lungs have about 150 million alveoli. Normally, your alveoli are elastic, meaning that their size and shape can change easily. Surfactant coats the inside of the sacs or alveoli and helps the air sacs stay open.

Each of these alveoli is covered in a mesh of tiny blood vessels called capillaries. The space where the alveoli come into contact with the capillaries is called the lung interstitium. The capillaries connect to a network of arteries and veins that move blood through your body.

The pulmonary artery and its branches deliver blood rich in carbon dioxide and lacking in oxygen to the capillaries that surround the air sacs. Carbon dioxide moves from the blood into the air inside the alveoli. At the same time, oxygen moves from the air into the blood in the capillaries.

The pleura and the muscles used for breathing

The lungs are enclosed by the pleura, a membrane that has two layers. The space between these two layers is called the pleural cavity. The membrane’s cells create pleural fluid, which acts as a lubricant to reduce friction during breathing.

The lungs are like sponges; they cannot move on their own. Muscles in your chest and abdomen contract, or tighten, to create space in your lungs for air to flow in. The muscles then relax, causing the space in the chest to get smaller and squeeze the air back out.

These muscles include the:

• Diaphragm, which is a dome-shaped muscle below your lungs. It separates the chest cavity from the abdominal cavity. The diaphragm is the main muscle used for breathing.
• Intercostal muscles, which are located between your ribs. They also play a major role in helping you breathe.
• Abdominal muscles. They help you breathe out when you are breathing fast, such as during physical activity.
• Muscles of the face, mouth, and pharynx. The pharynx is the part of the throat right behind the mouth. These muscles control the lips, tongue, soft palate, and other structures to help with breathing. Problems with these muscles can cause sleep apnea.
• Muscles in the neck and collarbone area. These muscles help you breathe in when other muscles involved in breathing are not working well or when lung disease impairs your breathing.

Damage to the nerves in the upper spinal cord can interfere with the movement of your diaphragm and other muscles in your chest, neck, and abdomen. This can happen due to a spinal cord injury, a stroke, or a degenerative disease such as muscular dystrophy. The damage can cause respiratory failure. Ventilator support or oxygen therapy may be necessary to maintain oxygen levels in the body and protect the organs from damage.

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The larynx is composed of 3 large, unpaired cartilages (cricoid, thyroid, epiglottis); 3 pairs of smaller cartilages (arytenoids, corniculate, cuneiform); and a number of intrinsic muscles (see the image and video below). The hyoid bone, while technically not part of the larynx, provides muscular attachments from above that aid in laryngeal motion

Anatomical Structure

The larynx is formed by a cartilaginous skeleton, which is held together by ligaments and membranes. The laryngeal muscles act to move the components of the larynx for phonation and breathing. More information about each of these structures can be found in their respective sections.

Anatomically, the internal cavity of the larynx can be divided into three sections:

• Supraglottis – From the inferior surface of the epiglottis to the vestibular folds (false vocal cords).
• Glottis – Contains vocal cords and 1cm below them. The opening between the vocal cords is known as rima glottidis, the size of which is altered by the muscles of phonation.
• Subglottis – From inferior border of the glottis to the inferior border of the cricoid cartilage.

The interior surface of the larynx is lined by pseudostratified ciliated columnar epithelium. An important exception to this is the true vocal cords, which are lined by a stratified squamous epithelium.

Vasculature

The arterial supply to the larynx is via the superior and inferior laryngeal arteries:

• Superior laryngeal artery – a branch of the superior thyroid artery (derived from the external carotid). It follows the internal branch of the superior laryngeal nerve into the larynx.
• Inferior laryngeal artery –  a branch of the inferior thyroid artery (derived from the thyrocervical trunk). It follows the recurrent laryngeal nerve into the larynx.

Venous drainage is by the superior and inferior laryngeal veins. The superior laryngeal vein drains to the internal jugular vein via the superior thyroid, whereas the inferior laryngeal vein drains to the left brachiocephalic vein via the inferior thyroid vein.

Innervation

The larynx receives both motor and sensory innervation via branches of the vagus nerve:

• Recurrent laryngeal nerve – provides sensory innervation to the infraglottis, and motor innervation to all the internal muscles of larynx (except the cricothyroid).
• Superior laryngeal nerve – the internal branch provides sensory innervation to the supraglottis, and the external branch provides motor innervation to the cricothyroid muscle.

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The trachea begins at the inferior end of the larynx in the base of the neck. It is located along the body’s mid line, anterior to the esophagus and just deep to the skin, so that it is possible to feel the larynx through the skin of the neck. From its origin at the larynx, the trachea extends inferiorly into the thorax posterior to the sternum.

In the thorax, the trachea ends where it splits into the left and right bronchi, which continue onward toward the lungs.

Viewed in cross section, the trachea is about one inch (2.6 cm) in diameter. It has a thin, membranous wall with C-shaped rings of cartilage embedded into it. Between sixteen and twenty cartilage rings are stacked along the length of the trachea, with narrow membranous regions spaced between the cartilage rings. The open ends of the cartilage rings face the posterior of the trachea near the esophagus.

Four layers of tissues make up the walls of the trachea:

• The mucosa is the innermost layer and consists of ciliated pseudostratified columnar epithelium with many goblet cells. Goblet cells produce sticky mucus to coat the inner lining of the trachea and catch any debris present in inhaled air before it reaches the lungs. On the surface of the columnar cells, long, hair-like cilia beat together to push mucous away from the lungs like a microscopic conveyor belt. Mucus from the trachea, along with any trapped contaminants, makes its way to the larynx, where it is either expelled during coughing or swallowed and digested in the stomach.
• Deep to the mucosa is the submucosa layer, which is made of areolar connective tissue containing blood vessels and nervous tissue. Many collagen, elastin and reticular protein fibers give soft support and elasticity to the wall of the trachea, while blood vessels and nerves support the other layers of the tracheal wall. Longitudinal smooth muscle fibers are present in the posterior trachea between the ends of the cartilage rings. This smooth muscle tissue allows the trachea to adjust its diameter as needed.
• Surrounding the submucosa is a layer of hyaline cartilage that forms the supportive rings of the trachea. Hyaline provides a strong, yet flexible structure that maintains an open airway and is resistant to external stresses.
• The outermost layer of the trachea is the adventitia, a layer of areolar connective tissue that loosely anchors the trachea to the surrounding soft tissues.

While the trachea plays a vital role as a passive air passageway, it also performs several other important functions as well. The trachealis muscle in the posterior wall allows the trachea to contract and reduce its diameter, which makes coughs more forceful and productive. During the process of swallowing food, the esophagus expands into the space normally occupied by the trachea. The incomplete cartilage rings of the trachea allow it to narrow and permit the esophagus to expand into its space. Finally, the loose connection of the adventitia allows the trachea to move within the neck and thorax, aiding the lungs in their expansion and contraction during breathing.

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