The respiratory system is an outgrowth of the foregut ventral wall.

The right lung has three lobes and the left lung has two lobes.

The lingula lobe on the left is the equivalent of the right middle lobe.

The left lung is smaller than the right lung.

The average person takes 16 breaths in a minute, between 17,280 and 23,040 breaths a day.

Unconscious breathing is triggered by the brainstem.

The cerebral cortex controls conscious breathing.

Adult life-course of the respiratory system is characterized by maximal lung function in young adulthood and then decline with advancing age.
The magnitude of the peak function and rate of subsequent decline are indicated for overall health.

A lower peak lung function and accelerated loss of lung function or associated with future expiratory airflow obstruction as well as non-respiratory comorbid conditions such as diabetes and cardiovascular disease.

The lungs take in as much as 5000 gallons of air every 24 hours and with that air comes bacteria, viruses, mold, fungus and pollutants.

In a healthy young adult tidal volume is approximately 500 mL per inspiration, or  7 mL per kilogram, and at 12 to 14 breaths per minute they process 10,000 L of air each day, or nearly 4,000,000 L each year.

Throughout the respiratory pathway, pseudostratified epithelium cilia line trachea, bronchi and bronchioles.

The large in small airways remove tremendous amounts of inhaled toxicants by mucocilliary mechanisms and molecular detoxication processes carried out by the pulmonary epithelium and the immune system.

Depending upon the patient,, the toxicant and patients genetic architecture, their defense mechanisms of airway reactivity and parenchymal changes can cause fibrosis and result in maladaptive processes that lead to disease.

Goblet cells produce mucus, which traps harmful particles and cilia move the mucus away from the lungs to be coughed up.

Alveoli have alveolar macrophages, which are immune system cells that ingest foreign particles and digest bacteria and organisms.

The right mainstem bronchus is more vertical and more in the direct line with the trachea than is the left main stem bronchus, resulting in higher likelihood of developing aspiration of blood, vomit and foreign bodies on the right side.

Bronchi branch into smaller airways and are accompanied double arterial supply to the lungs with pulmonary and bronchial arteries.

The human respiratory tree may consist of, on average, 23 such branchings into progressively smaller airways.

Branching of bronchi into bronchioles which are distinguished from bronchi with a lack of cartilage and submucosal glands within their walls.

Bronchopulmonary Segment:

These are independent units of the lung supplied by a (tertiary) segmental bronchus, a tertiary branch of the pulmonary artery, and a branch of the bronchial artery.

There are ten bronchopulmonary segments in the right and 8 in the left lung, respectively.

A bronchopulmonary accessory segment may be present in a certain percentage of populations.

The right lung has oblique and horizontal fissures dividing it into three lobes with ten segments.

Right Upper Lobe

Apical segment 

Posterior segment 

Anterior segment 

Right Middle Lobe 

Lateral segment 

Medial segment

Right Lower Lobe

There are around 30,000 bronchioles in each lung.

The acinus is the part of the lung distal to the terminal bronchiole, it is a spherical area about 7 mm in diameter.

The acinus is composed of respiratory bronchioles which give off alveoli from their sides.

Respiratory bronchioles proceed into alveolar ducts, which brand into alveolar sacs, the blind end of respiratory passages which are the site of gas exchange.

All alveoli are open and have incomplete walls.

Pulmonary lobule refers to a cluster of three to five terminal bronchioles, each with its appended acinus.

The respiratory tree is lined by pseudostratified, columnar, ciliated epithelial cells with a mixture of cartilaginous airways and mucus secreting goblet cells.

Submucosal mucus secreting cells line the walls of the trachea and bronchi, but not the bronchioles.

The bronchial walls also contain neuroendocrine cells with neurosecretory granules with serotonin, calcitonin, and gastrin releasing peptide.

The alveolar septa have a capillary endothelium lining, a basement membrane with interstitial tissue separating the endothelium from the alveolar lining epithelial cells, an alveolar epithelial lining and alveolar macrophages.

Tiny capillaries, the diameter of one red blood cell, form a web like structure over the alveoli.

The walls of the alveoli and of the capillaries are each one cell thick, so the bloodstream is in close contact with the air, and that allows for the exchange of oxygen and carbon dioxide, the primary function of the lungs.

Ventilation is driven by complex neural circuits located in the brainstem, including a central pattern generator and chemosensor along with feedback from peripheral sensory nerves located in the carotid bodies, aortic arch, muscles, and lungs.


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