134 Respiratory Zone

Bronchi and Subdivisions

A bronchus is a passage of airway in the respiratory tract that conducts air into the lungs and divides into terminal bronchioles.

A bronchus (plural bronchi, adjective bronchial) is a passage of airway in the respiratory tract that conducts air into the lungs. The bronchus branches into smaller tubes called bronchioles.

The bronchi and bronchioles are considered anatomical dead space, like the trachea and upper respiratory tract, because no gas exchange takes place within this zone.

Anatomy of the Bronchi

The human trachea divides into two main bronchi (also called mainstem bronchi), that extend laterally (but not symmetrically) into the left and right lung respectively, at the level of the sternum. The point where the trachea divides into the bronchi is called the carina.

The right main bronchus is wider, shorter than the left main bronchus, which is thinner and longer. The right main bronchus subdivides into three lobar bronchi, while the left main bronchus divides into two. The lobar bronchi (also called secondary bronchi) divide into tertiary bronchi, each of which supplies air to a different bronchopulmonary segment.

A bronchopulmonary segment is a distinct region of the lung separated from the rest of the lung by connective tissue. Each bronchopulmonary segment forms a discrete functional unit in the lung that is independent of the other segments. This property allows a bronchopulmonary segment to be surgically removed without affecting other segments.

There are 10 segments in the right lung and 8 to 9 segments in the left lung due to anatomical differences. The segmental bronchi divide into many primary bronchioles that divide into terminal bronchioles. Each terminal bronchiole then gives rise to several respiratory bronchioles, which go on to divide into two to 11 alveolar ducts.

There are five or six alveolar sacs associated with each alveolar duct. The alveolus is the smallest anatomical unit of the lung, and the site of gas exchange between the lung and the bloodstream.

Histology

The histology of the bronchi are largely similar to that of the trachea. There is hyaline (transparent and consisting of collagen) cartilage present in the bronchi, in rings that are more irregular than those in the trachea.

There are also small plates and islands of hyaline cartilage in the primary and terminal bronchioles. Smooth muscle is present continuously around the bronchi (similar to the trachealis muscle of the trachea) and is innervated with the parasympathetic nervous system.

The amount of bronchial smooth muscle increases as the amount of hyaline cartilage decreases as the bronchi become smaller further into the lungs. The mucous membrane lining the bronchi also undergoes a transition—from ciliated pseudostratified columnar epithelium to simple cuboidal epithelium to simple squamous epithelium further into the lungs.

Physiology of the Bronchi

Like the trachea, the bronchi and bronchioles are part of the conducting zone, so they moisten and warm air and contribute to the volume of anatomical dead space. The bronchi and bronchioles are also part of the mucociliary escalator that removes mucus and pathogens from the lungs.

A unique characteristic of the bronchi and bronchioles is bronchoconstriction, in which the smooth muscle of the bronchi or bronchioles tightens. This leads to coughing, wheezing, and dyspnea (shortness of breath).

It is caused by activation of the parasympathetic nervous system and release of acetylcholine in the bronchi, as well as by overproduction of mucus or allergic reactions and inflammation. It is a symptom of diseases such as bronchitis (chronic inflammation and mucus production in the bronchi) and asthma (an acute attack of bronchoconstriction, often allergic). Both cause obstruction of the airways and make it more difficult to breathe.

Bronchoconstriction is treated with anti-inflammatory drugs, such as corticosteroids, and prevented by maintaining lung health, such as through avoiding smoking, air pollution, and airborne allergens.

The complete respiratory system: This figure details the respiratory system including the bronchi and its many subdivisions.

Alveoli

Alveoli are hollow cavities in the lung that perform gas exchange with the blood.

An alveolus is an anatomical structure that has the form of a hollow cavity. Its plural is alveoli, from the Latin alveolus, meaning little cavity.

Found in the lung parenchyma, the pulmonary alveoli are the terminal ends of the respiratory tree that outcrop from either alveolar sacs or alveolar ducts; both are sites of gas exchange with blood.

The alveolar membrane is the gas-exchange surface. Carbon-dioxide-rich blood is pumped from the rest of the body into the alveolar blood vessels where, through passive diffusion, it releases its carbon dioxide and absorbs oxygen into the blood vessels.

Anatomy of the Alveoli

Pulmonary alveolus: A diagram of the pulmonary alveolus.

The alveoli are located in the respiratory zone of the lungs, at the distal termination of the alveolar ducts. These air sacs are at the end points of the respiratory tract.

There are approximately 700 million alveoli in the lungs, covering a total surface area of about 70 m², which is a considerably larger surface area relative to volume. The large surface area makes gas exchange with the bloodstream more efficient.

The alveoli are highly elastic, so the alveoli can stretch as they are filled with air during inhalation. They then spring back during exhalation in order to expel the carbon-dioxide-rich air.

Histology

The alveoli consist of an extremely thin epithelial layer and an extracellular matrix (a fluid space made of collagen and elastin that contains no cells); it is surrounded by many capillaries, the tiniest type of blood vessel.

The fluid extracellular matrix supports the structure of the alveoli and allows gases to dissolve in water, making passive diffusion of those gases with the capillaries possible. In some alveolar walls there are pores between alveoli called the pores of Kohn, that connect alveoli in order to equalize air pressure between the different sacs of an alveolus.

There are two major types of epithelial cells found in alveoli (pneumocytes):

• Type I (Squamous Alveolar) cells: These form the structure of an alveolar wall. They are extremely thin, and permeable, which facilitates gas exhange with the capillaries. They can’t undergo mitosis, making them vulnerable to injury.
• Type II (Great Alveolar) cells: These are the site of surfactant production in the lungs, making them critical for maintaining the elastic recoil of the lung. They are more common than type I cells and can undergo mitosis, and may even proliferate into new type I cells when necessary.

Besides these epithelium cells, there are many macrophages found in the alveoli that provide immune system defense of the alveoli from pathogens and foreign material.

Physiology of the Alveoli

The surfactant produced by type II epithelial cells is very important for maintaining the elastic recoil of the lungs. It is a lipoprotein with hydrophilic and hydrophobic ends that reduce the amount of surface tension from water in the lungs. Without surfactant, the surface tension would cause the lungs to collapse during exhalation, making normal breathing impossible.

Surfactant is first produced by human lungs between 24 and 28 weeks in the womb, and many infants born prematurely do not have enough surfactant to breathe on their own after birth. Surfactant replacement therapy is necessary to save the lives of these premature births.

The alveoli are the site of alveolar ventilation, and are not normally considered dead space. However, alveoli that are injured and can no longer contribute to gas exchange become alveolar dead space.

This is a common occurrence in people with lung diseases like COPD (chronic pulmonary obstructive disorder, i.e., emphysema and bronchitis) or restrictive lung diseases like pulmonary fibrosis, in which scarring of the lung tissue hinders gas exchange in the alveoli, or lung infections like pnuemonia.

Physiological dead space is the sum of normal anatomical dead space and alveolar dead space, and can be used to determine the rate of ventilation (gas exchange) in the lungs. When any type of dead space increases, the rate of ventilation in the lungs will decrease.