Human Respiratory System

The respiratory system facilitates the exchange of gases between the body and the environment. The primary function of the human respiratory system is the introduction of oxygen into the body and the removal of carbon dioxide from the body. The system also warms, humidifies, and filters the air we breathe, while also detecting and expelling foreign particles. The organs of the respiratory system include the nose, sinuses, throat, larynx, trachea, bronchi, and lungs. The larynx contains the vocal cords, which enable sound production and speech.

Respiratory System Anatomy

The respiratory system is divided into the upper and lower respiratory tracts. The upper tract extends from the nose to the larynx and functions largely as an air conduit and filter. The lower tract, extending from the trachea to the lungs both conducts air and performs pulmonary gas exchange.

Upper Respiratory Tract

• Nose and Nasal Cavities: The nose is the primary entry point of air into the body. Air passes through the nostrils into the nasal cavity where air is humidified and filtered by nose hairs and a mucosal lining. Irritation of the nasal mucosa triggers the sneeze reflex.
• Paranasal Sinuses: The sinuses warm, humidify, and filter inhaled air. They produce mucus that plays a critical role in capturing and removing harmful particles and germs. The sinuses also play an important role in sound resonance, affecting the tone and quality of voice production.
• Pharynx: The pharynx (throat) is a muscular tube divided into three sections: the nasopharynx, the oropharynx, and the laryngopharynx (hypopharynx). The pharynx serves both the respiratory and digestive systems, as the laryngopharynx enters both the esophagus and the larynx.
• Larynx: The larynx, also called the voice box, connects the pharynx and the trachea. Its functions include airway protection, sound production, and air conduction. The epiglottis, a cartilaginous flap at the top of the larynx, depresses during swallowing and covers the glottis; this helps prevent ingested food from entering the trachea. The larynx contains the vocal cords, which vibrate to produce sound (phonation). Additionally, the larynx includes nerve endings that trigger the cough reflex.

Lower Respiratory Tract

• Trachea: The trachea, also called the windpipe, is a tube that extends from the larynx to the main bronchi. It has multiple C-shaped rings of hyaline cartilage provide stability, flexibility, and maintain an open airway during pressure changes in the respiratory cycle. The mucus lining of the trachea traps foreign particles, which are moved upward by cilia toward the upper airway so they can be expelled. Alternately, foreign particles in the trachea may trigger a cough reflex for more rapid expulsion. The esophagus runs parallel to and immediately behind the trachea.
• Bronchi: The trachea ends at the carina, where it splits into the right and left mainstem bronchi. The combination of the trachea and bronchi is called the tracheobronchial tree. The bronchi continue branching into secondary lobar bronchi, tertiary segmental bronchi, and sequentially smaller bronchioles within the lungs.
• Lungs: The lungs are a pair of lobed organs located in the thoracic cavity that are the primary organs of pulmonary respiration.
  • Bronchioles: Small branches (1 mm or less in diameter) of the bronchi leading to the alveolar ducts and, ultimately, the alveoli.
  • Alveoli: Balloon-like air sacs at the distal end of the bronchial tree. Alveoli have thin walls and blood vessels that allow gas exchange via pulmonary blood flow. The lungs contain roughly 480 million alveoli.¹

The diaphragm is a dome-shaped muscle separating the thoracic and abdominal cavities. It plays a crucial role in breathing, as it contracts and relaxes to change the volume of the thoracic cavity, creating a pressure differential that causes air to flow into and out of the lungs. The process is also aided by the movement of external intercostal muscles in the ribcage that further expand the thoracic cavity.

Respiratory System Physiology

Air enters the body and is conducted via negative pressure ventilation to the alveoli of the lungs, where pulmonary gas exchange occurs. This includes ventilation and respiration.

Ventilation

Ventilation is the movement of air into and out of the lungs through the airways. The process is controlled by the nervous system. Neurons in the brain stem detect levels of carbon dioxide and oxygen in the bloodstream, as well as the blood’s pH level. Electrical signals are then sent to the diaphragm and intercostal muscles to direct the rate and depth of breathing.

• Inspiration (breathing in): Inspiration is the passage of air from the environment into the lungs. During inspiration, the diaphragm and external intercostal muscles contract, expanding the thoracic cavity space, causing a negative pressure gradient that causes air to flow into the lungs.
• Expiration (breathing out): During expiration, also called exhalation, the diaphragm and external intercostal muscles relax. The diaphragm returns upwards to its default dome shape, and external intercostal muscles also relax, releasing the ribcage inward. This reduces the volume of the thoracic cavity, which increases intrapulmonary pressure, causing air to be expelled from the lungs. Expiration is generally a passive process. However, during strenuous activity, the intercostal muscles and accessory muscles of respiration play a more substantial role in ventilation to force air out more rapidly.

At rest, the average adult takes 12-15 breaths per minute (respiratory rate), inhaling about 500 mL of air per breath. During normal breathing, the diaphragm accounts for roughly 75% of air movement.²

Respiration

Respiration includes the exchange of gases (oxygen and carbon dioxide) within the lungs and tissues throughout the body.

• External Respiration: Gas exchange between the blood and the external environment occurs in the lungs, where oxygenated air reaches the alveoli via inspiration. The thin alveolar walls are rich with blood vessels, allowing gases to diffuse across the pulmonary membrane.
• Internal Respiration: A function of the circulatory system, the exchange of gases between the blood and the body’s cells takes place at the cellular level through diffusion across the cell membrane. Oxygen is delivered to the cells for energy production and the waste product of that energy production, carbon dioxide, is removed.

Respiratory Diseases

A number of acute and chronic conditions can affect the respiratory system. These can be broadly categorized into malformations, infectious or inflammatory conditions, and malignancies. Common respiratory conditions include asthma, acute bronchitis, chronic obstructive pulmonary disease, and pneumonia. Asthma is a condition that can be present in both children and adults. It is characterized by airway inflammation and narrowing of the bronchi, which result in difficulty breathing. Acute bronchitis is inflammation of the airways that can lead to coughing and chest congestion. It is most often the result of a viral or bacterial infection. Chronic obstructive pulmonary disease (COPD) is a group of progressive lung diseases that make it difficult to breathe. The main subtypes of COPD are emphysema and chronic bronchitis. These conditions are usually caused by smoking. Pneumonia is an infection of the lungs that can be caused by viruses, bacteria, or parasites and can lead to difficulty breathing and chest pain.

Prevention and treatment of respiratory diseases may include lifestyle changes, such as smoking cessation and exercise, vaccinations, medications, and in some cases, surgical interventions or supportive therapies like oxygen therapy or mechanical ventilation. Regular monitoring of lung function can help detect and manage respiratory diseases early, reducing the risk of complications and improving overall lung health.

Contributors:

• Steve Jarzembowski, MD
• Teresa Roberts, DNP

Reviewers:

• Brian Sullivan, MD

References:

1. Ochs, Matthias, et al. The Number of Alveoli in the Human Lung. American Journal of Respiratory and Critical Care Medicine. 2003 Aug.
2. Rubenstein, D., Yin, W., & Frame, M. D. (2021). Biofluid mechanics: An introduction to fluid mechanics, macrocirculation, and microcirculation (3rd ed.). Academic Press. p401.

Published: March 30, 2023