What is the human respiratory system?

The human respiratory system facilitates the continuous exchange of oxygen and carbon dioxide between the body and the environment. This vital process involves two main mechanisms: convection and diffusion. The system consists of the upper and lower airway systems. The upper airway includes the nose, paranasal cavities, pharynx, and oral cavity. The lower airway comprises the larynx, trachea, stem bronchi, intrapulmonary bronchi, bronchioles, and alveolar ducts. The diaphragm , the main respiratory muscle, along with the intercostal muscles, facilitates the pumping action of the lungs. Blood and the circulatory system are essential for gas transport.

How does the human body take in oxygen and remove carbon dioxide?

The human respiratory system facilitates the exchange of oxygen and carbon dioxide through ventilation and diffusion. Air enters the lungs, where oxygen diffuses from the alveoli into the blood and binds to hemoglobin in red blood cells. Simultaneously, carbon dioxide moves from the blood into the alveoli to be exhaled. This diffusion is driven by differences in partial pressures: oxygen moves from the air into the blood, while carbon dioxide moves from the blood into the air. The blood transports these gases between the lungs and the body's tissues.

What are the main organs involved in breathing?

The primary organs involved in breathing work together to supply the body with oxygen and expel carbon dioxide. Lungs The lungs are the main organs for gas exchange. Within the lungs, oxygen and carbon dioxide move between the air and the blood across the respiratory exchange surfaces. Diaphragm This dome-shaped muscle located below the lungs is essential for breathing. When it contracts, the diaphragm increases the height of the thoracic cavity, which lowers the pressure inside and causes air to flow into the lungs. Trachea The trachea, or windpipe, is a tube that carries air from the larynx to the lungs. Bronchi The trachea divides into two main bronchi that enter the lungs. These bronchi further divide into smaller and narrower airways called bronchioles, which distribute air to the gas-exchanging zone of the lung.

What is the process of gas exchange in the lungs?

In the lungs, gas exchange occurs across respiratory exchange surfaces between air and blood. The structure of the human lung provides a large internal surface area with a thin membrane that enhances gas exchange between the alveoli and pulmonary capillaries. The process of diffusion drives gas exchange, moving gases from areas of higher partial pressure to lower partial pressure. Oxygen and carbon dioxide are transported between tissue cells and the lungs via blood. The efficiency of gas exchange relies on the uniform distribution of blood flow and inspired air throughout the lungs.

How do the diaphragm and intercostal muscles help with breathing?

The diaphragm and intercostal muscles facilitate breathing through mechanical actions that change the volume of the chest cavity. The diaphragm , a dome-shaped muscle separating the chest and abdominal cavities, contracts and moves downward to increase the chest cavity's height, which lowers the internal pressure and draws air into the lungs. The external intercostal muscles contract, lifting the ribs up and outward to expand the chest circumference. During expiration, these muscles relax, and the chest wall and lungs' elastic recoil force air out. The internal intercostal muscles depress the rib cage to decrease chest circumference during labored breathing or exercise.

What controls the rate and depth of breathing in the body?

Breathing rate and depth are automatically controlled by neuron networks in the hindbrain, specifically the pons and medulla . These networks manage the muscles in the thorax and abdomen, creating pressure gradients for air movement in and out of the lungs. The respiratory system adjusts breathing patterns based on internal and external changes via sensors throughout the body. Chemoreceptors identify changes in blood oxygen levels and acidity, while mechanoreceptors monitor lung expansion and muscle contraction. The rate and depth of breathing increase with the activity of chemoreceptors caused by hypoxia or increased carbon dioxide, which helps to restore usual oxygen and carbon dioxide levels.

human respiratory system

Introduction
The design of the respiratory system
- Morphology of the upper airways
  - The nose
  - The pharynx
- Morphology of the lower airways
  - The larynx
  - The trachea and the stem bronchi
- Structural design of the airway tree
- The lungs
  - Gross anatomy
  - Pulmonary segments
  - The intrapulmonary conducting airways: bronchi and bronchioles
  - The gas-exchange region
  - Blood vessels, lymphatic vessels, and nerves
  - Lung development
Control of breathing
- Central organization of respiratory neurons
- Chemoreceptors
  - Peripheral chemoreceptors
  - Central chemoreceptors
  - Muscle and lung receptors
- Variations in breathing
  - Exercise
  - Sleep
The mechanics of breathing
- The lung–chest system
- The role of muscles
- The respiratory pump and its performance
Gas exchange
- Transport of oxygen
- Transport of carbon dioxide
- Gas exchange in the lung
- Abnormal gas exchange
Interplay of respiration, circulation, and metabolism
Adaptations
- High altitudes
- Swimming and diving

References & Edit History Related Topics

Images, Videos & Interactives

Medical illustration of the human chest showing the lungs, diaphragm, ribs, trachea, bronchus, and bronchioles, with one lung cut away to reveal internal structures.

Passage of air through the respiratory tract explained

Medical illustration showing a sagittal section of the human head focused on the nasal cavity, with labeled parts including the frontal and sphenoid sinuses, ethmoid bone, conchae, nasal vestibule, oral cavity, soft palate, anterior nasal spine, and tongue.

human body systems interactive - respiratory

A swimmer wearing goggles and a swim cap turns their head to breathe while swimming in open water, emphasizing the act of breathing during exercise.

A labeled medical illustration shows a sagittal section of the pharynx, highlighting anatomical features such as the tonsils, palate, tongue, epiglottis, and related muscles. An inset diagram identifies the nasal, oral, and laryngeal regions of the pharynx in color.

Educational diagram showing the human voice-producing apparatus, including a side view of the head and neck with labeled tongue, larynx, vocal cords, and trachea, plus a top-down view of the vocal cords during inhalation and sound production. Labels explain the roles of resonators, vocal cord positioning, and cartilage structures in speech.

Medical image showing a detailed view of healthy human lungs, highlighting the trachea, bronchi, and branching bronchioles within the chest.

At a Glance

respiratory system summary

human respiratory system

physiology

Written by

Neil S. CherniackAll

Fact-checked by

Britannica Editors

Last updated

Sep. 26, 2026 •History

The design of the respiratory system

The human gas-exchanging organ, the lung, is located in the thorax, where its delicate tissues are protected by the bony and muscular thoracic cage. The lung provides the tissues of the human body with a continuous flow of oxygen and clears the blood of the gaseous waste product, carbon dioxide. Atmospheric air is pumped in and out regularly through a system of pipes, called conducting airways, which join the gas-exchange region with the outside of the body. The airways can be divided into upper and lower airway systems. The transition between the two systems is located where the pathways of the respiratory and digestive systems cross, just at the top of the larynx.

The upper airway system comprises the nose and the paranasal cavities (or sinuses), the pharynx (or throat), and partly also the oral cavity, since it may be used for breathing. The lower airway system consists of the larynx, the trachea, the stem bronchi, and all the airways ramifying intensively within the lungs, such as the intrapulmonary bronchi, the bronchioles, and the alveolar ducts. For respiration, the collaboration of other organ systems is clearly essential. The diaphragm, as the main respiratory muscle, and the intercostal muscles of the chest wall play an essential role by generating, under the control of the central nervous system, the pumping action on the lung. The muscles expand and contract the internal space of the thorax, the bony framework of which is formed by the ribs and the thoracic vertebrae. The contribution of the lung and chest wall (ribs and muscles) to respiration is described below in The mechanics of breathing. The blood, as a carrier for the gases, and the circulatory system (i.e., the heart and the blood vessels) are mandatory elements of a working respiratory system (see blood; cardiovascular system).

Morphology of the upper airways

The nose

The nose is the external protuberance of an internal space, the nasal cavity. It is subdivided into a left and right canal by a thin medial cartilaginous and bony wall, the nasal septum. Each canal opens to the face by a nostril and into the pharynx by the choana. The floor of the nasal cavity is formed by the palate, which also forms the roof of the oral cavity. The complex shape of the nasal cavity is due to projections of bony ridges, the superior, middle, and inferior turbinate bones (or conchae), from the lateral wall. The passageways thus formed below each ridge are called the superior, middle, and inferior nasal meatuses.

Male muscle, man flexing arm, bicep curl.

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On each side, the intranasal space communicates with a series of neighbouring air-filled cavities within the skull (the paranasal sinuses) and also, via the nasolacrimal duct, with the lacrimal apparatus in the corner of the eye. The duct drains the lacrimal fluid into the nasal cavity. This fact explains why nasal respiration can be rapidly impaired or even impeded during weeping: the lacrimal fluid is not only overflowing into tears, it is also flooding the nasal cavity.

The paranasal sinuses are sets of paired single or multiple cavities of variable size. Most of their development takes place after birth, and they reach their final size toward age 20. The sinuses are located in four different skull bones—the maxilla, the frontal, the ethmoid, and the sphenoid bones. Correspondingly, they are called the maxillary sinus, which is the largest cavity; the frontal sinus; the ethmoid sinuses; and the sphenoid sinus, which is located in the upper posterior wall of the nasal cavity. The sinuses have two principal functions: because they are filled with air, they help keep the weight of the skull within reasonable limits, and they serve as resonance chambers for the human voice.

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The nasal cavity with its adjacent spaces is lined by a respiratory mucosa. Typically, the mucosa of the nose contains mucus-secreting glands and venous plexuses; its top cell layer, the epithelium, consists principally of two cell types, ciliated and secreting cells. This structural design reflects the particular ancillary functions of the nose and of the upper airways in general with respect to respiration. They clean, moisten, and warm the inspired air, preparing it for intimate contact with the delicate tissues of the gas-exchange area. During expiration through the nose, the air is dried and cooled, a process that saves water and energy.

Two regions of the nasal cavity have a different lining. The vestibule, at the entrance of the nose, is lined by skin that bears short thick hairs called vibrissae. In the roof of the nose, the olfactory bulb with its sensory epithelium checks the quality of the inspired air. About two dozen olfactory nerves convey the sensation of smell from the olfactory cells through the bony roof of the nasal cavity to the central nervous system.

human respiratory system

The design of the respiratory system

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Morphology of the upper airways

The nose