Respiratory System

Original Editor - Lucinda hampton Top Contributors -

Introduction[edit | edit source]

Respiratory System.png

The Lungs and Respiratory System allow us to breathe.

They bring oxygen into our bodies (called inspiration, or inhalation) and send carbon dioxide out (called expiration, or exhalation)[1].

  • The respiratory system of animals is crucial for the life as it allows the exchange of gases between an organism and the environment.[2]
  • This exchange of oxygen and carbon dioxide is called respiration.
  • The average human exhales 0.35 L of water each day. The amount varies with air temperature, relative humidity, and level of activity, so the range is probably 0.3 L to 0.45 L per day[3].
  • Dysfunction of the respiratory system ultimately leads to hypoxia. There are four classifications of hypoxia etiology: hypoventilation, right-to-left-shunt, V/Q mismatch, and diffusion limitations[4].

Parts of the Respiratory System[edit | edit source]

The human respiratory system comprises of

  • Nose and Mouth: Air enters the respiratory system through the nose or the mouth. If it goes in the nostrils, the air is warmed and humidified. Cilia (tiny hairs) protect the nasal passageways and other parts of the respiratory tract, filtering out dust and other particles that enter the nose through the breathed air.
  • Pharynx (throat): The nasal cavity and the mouth openings meet at the pharynx at the back of the nose and mouth. The pharynx is part of the digestive system as well as the respiratory system because it carries both food and air. At the bottom of the pharynx, this pathway divides in two, the esophagus, which leads to the stomach and the other for air. The epiglottis, a small flap of tissue, covers the air-only passage when we swallow, keeping food and liquid from going into the lungs.
  • The Larynx (voice box): Top part of Trachea. This short tube contains a pair of vocal cords, which vibrate to make sounds.

Tracheobronchial tree: Network of alveoli, bronchioles, bronchi and trachea.[1][5].

  • Trachea:  The walls of the trachea are strengthened by stiff rings of cartilage to keep it open. The trachea is also lined with cilia, which sweep fluids and foreign particles out of the airway so that they stay out of the lungs. These 2 procedure occur at the trachea 1. Endotracheal tube (intubation) 2.Tracheostomy.[5]
  • Carina: The angle made between the two primary bronchi when they diverge at the tracheal bifurcation; it is richly innervated with sensory nerve endings to respond to the arrival of any aspirated material by initiating a cough reflex; it may be visualised as a ridge within the bronchial tree when using a bronchoscope[5].
  • Bronchi: At its bottom end, the trachea divides into left and right air tubes called bronchi, which connect to the lungs. Within the lungs, the bronchi branch into smaller bronchi and even smaller tubes called bronchioles.
  • Lungs: The functional units of respiration and are key to survival (each lung weighing approximately 1.1 kg). The structure of the lung is well suited for efficient exchange of respiratory gases.
    • Through the airway and vascular trees, fresh gases and venous blood are delivered to and removed from a large alveolar capillary surface area.
    • In an adult, inhaled air enters the trachea and is delivered to the alveoli with a surface area of ∼140 m2, roughly the size of a tennis court.
    • Similarly, the pulmonary vascular tree begins as the main pulmonary artery and repeatedly bifurcates into arterioles and capillaries that cover 85–95% of the alveolar surface.
    • An exceptionally thin membrane of only 1 μm separates the alveolar gas and blood compartments, allowing gases to diffuse rapidly between them.
    • Due to the relatively large blood volume within the alveolar capillaries, blood flow slows and the transit time for blood increases, normally to 0.25–0.75 s, allowing more time for gas exchange.
    • The fantastic design that allows this gas exchange within the thoracic cavity has been highlighted by comparing this engineering feat to that of folding a letter so that it fits into a thimble[6].
    • Lungs are affected by a wide range of pathology that results in a diverse range of illnesses.[7]
  • Alveoli: Bronchioles end in tiny air sacs called alveoli, where the exchange of oxygen and carbon dioxide actually takes place. Each person has hundreds of millions of alveoli in their lungs.

The Thorax houses the bronchial tree, lungs, heart, and other structures.

  • The top and sides of the thorax are formed by the ribs and attached muscles, and the bottom is formed by the diaphragm.
  • The chest walls form a protective cage around the lungs and other contents of the chest cavity[1].

Tracheobronchial tree[edit | edit source]

Upper respiratory system 2.jpg

The tracheobronchial tree is a branching structure of tubes of an ever-decreasing diameter that start at the larynx and end in the alveoli. It can broadly be divided into conduction and respiratory zones.

  1. Conduction zone
  • Gas is warmed and humidified as it is conducted from the oropharynx to the functional portion of the lung where gas exchange occurs.
  • The conduction zone is composed of the trachea, bronchi, bronchioles and terminal bronchioles.
  • Their function is to optimise gas delivery to the functional portion of the lung.
  • Their walls contain cilia to remove particulates from the inspired gas and cartilage to ensure that they do not collapse in expiration.

2. Respiratory zone

  • The respiratory zone is an extension of the tracheobronchial tree at the level of the terminal bronchioles.
  • It is composed of respiratory bronchioles, alveolar ducts and alveoli, and is the location of gas transfer within the lung[7].

Alveoli[edit | edit source]

An alveolus, is an anatomical structure that has the form of a hollow cavity. Mainly found in the lung, the pulmonary alveoli are spherical outcroppings of the respiratory bronchioles and are the (1).png

The alveoli (singular: alveolus) are tiny hollow air sacs that comprise the basic unit of respiration.

Alveoli are found within the lung parenchyma and are found at the terminal ends of the respiratory tree, clustered around alveolar sacs and alveolar ducts.  Each alveolus is approximately 0.2 mm in diameter.  There are around 300 million to 1 billion alveoli in the human lungs, covering an area of 70 square metres.

  • The alveolar walls are comprised of collagen and elastic fibres which facilitate expansion during inspiration and return to the original shape during expiration.  There are numerous capillaries within the alveolar walls where gas exchange occurs. Pores of Kohn are also located within the walls.
  • Alveoli contain two major types of epithelial cells. The most abundant, type 1 pneumocytes (95%) are squamous cells in which gas exchange occurs. The remaining 5%, type 2 pneumocytes, are granular cells which secrete surfactant.  Surfactant is a lipoprotein with a high phospholipid content which reduces surface tension.  This increases pulmonary compliance, prevents atelectasis and aids recruitment of collapsed airways.
  • Alveolar macrophages are also located in the alveoli. They protect the alveoli from foreign material by engulfing it, including bacteria, dust and carbon particles[8].

Gas exchange occurs in the lungs between alveolar air and blood of the pulmonary capillaries. For effective gas exchange to occur, alveoli must be ventilated and perfused. Ventilation (V) refers to the flow of air into and out of the alveoli, while perfusion (Q) refers to the flow of blood to alveolar capillaries.

  • Individual alveoli have variable degrees of ventilation and perfusion in different regions of the lung.
  • Collective changes in ventilation and perfusion in the lungs are measured clinically using the ratio of ventilation to perfusion (V/Q).
  • Changes in the V/Q ratio can affect gas exchange and can contribute to hypoxemia[9].

How the Respiratory System Works[edit | edit source]

949 937 muscles-of-respiration.jpg

The cells in our bodies need oxygen to stay alive. Carbon dioxide is a by-product of respiration. The lungs and respiratory system allow oxygen in the air to be taken into the body, while also letting the body get rid of carbon dioxide in the air breathed out.

  • When you breathe in, the diaphragm moves downward toward the abdomen, and the rib muscles pull the ribs upward and outward (making the chest cavity bigger and pulling air through the nose or mouth into the lungs). See muscles of Respiration.
  • In exhalation, the diaphragm moves upward and the chest wall muscles relax, which causes the chest cavity to get smaller and push air out of the respiratory system through the nose or mouth.
  • With each inhalation, air fills a large portion of the millions of alveoli. Oxygen diffuses from the alveoli to the blood through the capillaries lining the alveolar walls. Once in the bloodstream, oxygen gets picked up by the hemoglobin in red blood cells. This oxygen-rich blood then flows back to the heart, which pumps it through the arteries to oxygen needy tissues throughout the body.
  • In the capillaries of the body tissues, oxygen is freed from the hemoglobin and moves into the cells.
  • Carbon dioxide produced moves out of the cells into the capillaries, where most of it dissolves in the plasma of the blood.
  • Blood rich in carbon dioxide then returns to the heart via the veins.
  • From the heart, this blood is pumped to the lungs, where carbon dioxide passes into the alveoli to be exhaled[1].

Control of Respiratory Rate[edit | edit source]

Respiratory system control.png

Breathing is an automatic and rhythmic act produced by networks of neurons in the hindbrain (the pons and medulla). Complex procedure see image R.

  • The neural networks direct muscles that form the walls of the thorax and abdomen and produce pressure gradients that move air into and out of the lungs.
  • The respiratory rhythm and the length of each phase of respiration are set by reciprocal stimulatory and inhibitory interconnection of these brain-stem neurons.

An important characteristic of the human respiratory system is its ability to adjust breathing patterns to changes in both the internal and the external environment.[10]

  • Ventilatory rate (minute volume) is tightly controlled and determined primarily by blood levels of carbon dioxide as determined by metabolic rate.
  • Chemoreceptors can detect changes in blood pH that require changes in involuntary respiration to correct.The apneustic (stimulating) and pnuemotaxic (limiting) centers of the pons work together to control rate of breathing.
  • The medulla sends signals to the muscles that initiate inspiration and expiration and controls nonrespiratory air movement reflexes, like coughing and sneezing.

The motor cortex within the cerebral cortex of the brain controls voluntary respiration (the ascending respiratory pathway).

  • Voluntary respiration may be overridden by aspects of involuntary respiration, such as chemoreceptor stimulus, and hypothalamus stress response.
  • The phrenic nerves, vagus nerves, and posterior thoracic nerves are the major nerves involved in respiration.
  • Voluntary respiration is needed to perform higher functions, such as voice control.[11]

Evolution - Final Note[edit | edit source]


Respiration in animals is a necessity as it allows the exchange of respiratory gases that are required for survival.

  • There are huge variations in the designs of respiratory systems and each has evolved due to selective pressures in environments.
  • The evolution from aquatic environments to terrestrial environments created a cause for the fast development of newly designed systems in order to support air breathing as oppose to gaining oxygen from water using gills.
  • Lungfish (image at R) first developed lungs, and the ability to breathe air instead of water, whilst living in aquatic environments. The appearance of air-breathing in fish is the major foundation for terrestrialization.
  • Which ultimately led to we humans living on Earth and having a wonderful Respiratory System[2].

References[edit | edit source]

  1. 1.0 1.1 1.2 1.3 Teenhealth Lungs and Respiratory System Available from: (last accessed 1.10.2020)
  2. 2.0 2.1 AU essays Evolution of Respiratory Systems in Animals Available from: (last accessed 1.10.2020)
  3. Socrates Water loss in RS Available from: (last accessed 2.10.2020)
  4. Haddad M, Sharma S. Physiology, Lung. StatPearls [Internet]. 2020 Jul 22.Available from: (last accessed 2.10.2020)
  5. 5.0 5.1 5.2 Respiratory system (last accessed 1.10.2020)
  6. Petersson J, Glenny RW. Gas exchange and ventilation–perfusion relationships in the lung.Available from: (last accessed 2.10.2020)
  7. 7.0 7.1 Radiopedia The Lungs Available from: (last accessed 1.10.2020)
  8. Radiopedia Alveoli Available from: (last accessed 1.10.2020)
  9. Powers KA, Dhamoon AS. Physiology, Pulmonary, Ventilation and Perfusion.2019 Available from: (last accessed 2.10.2020)
  10. Britannica Control of Breathing Available from: (last accessed 2.10.2020)
  11. Boundless Anatomy and Physiology Neural Mechanisms (Respiratory Center) Available from: (last accessed 2.10.2020)