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Respiratory system

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Respiratory system consists of the organs that carry out the process of respiration. The body needs oxygen for its metabolism without which life is incompatible.

Respiratory system
Respiratory system


The body cells need a continuous supply of oxygen for the metabolic processes that are necessary to maintain life through breathing of air.

Cardiopulmonary system

The respiratory system works with the circulatory system to provide this oxygen and to remove the waste products of metabolism. It also helps to regulate pH of the blood.

What is respiration?

Respiration is the sequence of events that results in the exchange of oxygen and carbon dioxide between the atmosphere and the body cells. Every 3 to 5 seconds, nerve impulses stimulate the breathing process, or ventilation, which moves air through a series of passages into and out of the lungs. After this, there is an exchange of gases between the lungs and the blood. This is called external respiration.

Transport of oxygen

The blood transports the gases to and from the tissue cells. The exchange of gases between the blood and tissue cells is internal respiration.

Respiratory system
Respiratory system

Cellular respiration

Finally, the cells utilize the oxygen for their specific activities: this is called cellular metabolism, or cellular respiration. Together, these activities constitute respiration.

Process of respiration

The process of respiration includes ventilation, external respiration, transport of gases such as oxygen and carbon dioxide, internal respiration, and cellular respiration.

Physics of breathing

The three pressures responsible for pulmonary ventilation are atmospheric pressure, intraalveolar pressure, and intrapleural pressure. Under normal conditions, the average adult takes 12 to 15 breaths a minute. A breath is one complete respiratory cycle that consists of one inspiration and one expiration. Respiratory (pulmonary) volumes are an important aspect of pulmonary function testing because they can provide information about the physical condition of the lungs.


A spirometer is used to measure respiratory volumes and capacities. These measurements provide useful information about the condition of the lungs.

Pulmonary alveolus
Pulmonary alveolus

Respiratory anatomy

The respiratory conducting passages are divided into the upper respiratory tract and the lower respiratory tract. The upper respiratory tract includes the nose, pharynx, and larynx. The lower respiratory tract consists of the trachea, bronchial tree, and lungs.


The two lungs, which contain all the components of the bronchial tree beyond the primary bronchi, occupy most of the space in the thoracic cavity. The lungs are soft and spongy because they are mostly air spaces surrounded by the alveolar cells and elastic connective tissue.

They are separated from each other by the mediastinum, which contains the heart. The only point of attachment for each lung is at the hilum, or root, on the medial side. This is where the bronchi, blood vessels, lymphatics, and nerves enter the lungs.

Anatomy of the respiratory system

  • The frontal, maxillary, ethmoidal, and sphenoidal sinuses are air-filled cavities that open into the nasal cavity.
  • The pharynx, commonly called the throat, is a passageway that extends from the base of the skull to the level of the sixth cervical vertebra.
  • The larynx, commonly called the voice box, is the passageway for air between the pharynx above and the trachea below.
  • The trachea, commonly called the windpipe, is the main airway to the lungs.
  • The trachea divides into the right and left primary bronchi, which branch into smaller and smaller passageways until they terminate in tiny air sacs called alveoli.
  • The two lungs contain all the components of the bronchial tree beyond the primary bronchi.
  • The right lung is shorter, broader, and is divided into three lobes.
  • The left lung is longer, narrower, and is divided into two lobes.

Glossary of respiratory system

partial pressure of oxygen in the alveoli (usually around 100 mmHg)

duct that extends from the terminal bronchiole to the alveolar sac

structure consisting of two or more alveoli that share a common opening

how much air is in the alveoli

(plural: alveoli) (also, air sac) terminal region of the lung where gas exchange occurs

(also, anatomical shunt) region of the lung that lacks proper ventilation/perfusion due to an anatomical block

ion created when carbonic acid dissociates into H+ and (HCO−3)(HCO3−)

system in the blood that absorbs carbon dioxide and regulates pH levels

airway that extends from the main tertiary bronchi to the alveolar sac

(plural: bronchi) smaller branch of cartilaginous tissue that stems off of the trachea; air is funneled through the bronchi to the region where gas exchange occurs in alveoli

molecule that forms when carbon dioxide binds to hemoglobin

enzyme that catalyzes carbon dioxide and water into carbonic acid

exchange of chloride for bicarbonate into or out of the red blood cell

measurement of the elasticity of the lung

area in the lung that lacks proper ventilation or perfusion

domed-shaped skeletal muscle located under lungs that separates the thoracic cavity from the abdominal cavity

property of the lung that drives the lung tissue inward

work conducted by the intercostal muscles, chest wall, and diaphragm

amount of additional air that can be exhaled after a normal exhalation

ratio of how much air can be forced out of the lung in one second to the total amount that is forced out of the lung; a measurement of lung function that can be used to detect disease states

work of breathing performed by the alveoli and tissues in the lung

(also, forced vital capacity) measure of how much air can be forced out of the lung from maximal inspiration over a specific amount of time

expiratory reserve volume plus residual volume

amount of air that can be forcibly exhaled after taking the deepest breath possible

centralized iron-containing group that is surrounded by the alpha and beta subunits of hemoglobin

molecule in red blood cells that can bind oxygen, carbon dioxide, and carbon monoxide

tidal volume plus inspiratory reserve volume

amount of additional air that can be inspired after a normal inhalation

muscle connected to the rib cage that contracts upon inspiration

space between the layers of pleura

voice box, a short passageway connecting the pharynx and the trachea

measurement of two or more lung volumes (how much air can be inhaled from the end of an expiration to maximal capacity)

measurement of air for one lung function (normal inhalation or exhalation)

complex glycoprotein found in mucus

sticky protein-containing fluid secretion in the lung that traps particulate matter to be expelled from the body

opening of the respiratory system to the outside environment

disease (such as emphysema and asthma) that arises from obstruction of the airways; compliance increases in these diseases

curve depicting the affinity of oxygen for hemoglobin

amount of oxygen that can be transported in the blood

amount of pressure exerted by one gas within a mixture of gases

small particle such as dust, dirt, viral particles, and bacteria that are in the air

throat; a tube that starts in the internal nares and runs partway down the neck, where it opens into the esophagus and the larynx

(also, physiological shunt) region of the lung that lacks proper ventilation/perfusion due to a physiological change in the lung (like inflammation or edema)

tissue layer that surrounds the lungs and lines the interior of the thoracic cavity

painful inflammation of the pleural tissue layers

(also, main bronchus) region of the airway within the lung that attaches to the trachea and bifurcates to each lung where it branches into secondary bronchi

process of opening airways that normally remain closed when the cardiac output increases

amount of air remaining in the lung after a maximal expiration

measurement of lung obstruction

terminal portion of the bronchiole tree that is attached to the terminal bronchioles and alveoli ducts, alveolar sacs, and alveoli

disease that arises from a deficient amount of surfactant

ratio of carbon dioxide production to each oxygen molecule consumed

number of breaths per minute

disease that results from a restriction and decreased compliance of the alveoli; respiratory distress syndrome and pulmonary fibrosis are examples

genetic disorder that affects the shape of red blood cells, and their ability to transport oxygen and move through capillaries

method to measure lung volumes and to diagnose lung diseases

detergent-like liquid in the airways that lowers the surface tension of the alveoli to allow for expansion

region of bronchiole that attaches to the respiratory bronchioles

rare genetic disorder that results in mutation of the alpha or beta subunits of hemoglobin, creating smaller red blood cells with less hemoglobin

amount of air that is inspired and expired during normal breathing

sum of the residual volume, expiratory reserve volume, tidal volume, and inspiratory reserve volume

cartilaginous tube that transports air from the larynx to the primary bronchi

partial pressure of carbon dioxide in the veins (40 mm Hg in the pulmonary veins)

partial pressure of oxygen in the veins (100 mm Hg in the pulmonary veins)

region of the lung that lacks proper alveolar ventilation (V) and/or arterial perfusion (Q)

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