The respiratory system is a remarkable network that enables us to breathe and sustain life. Whilst we often think of respiration as simply breathing, it encompasses a complex process of gas exchange that occurs both externally at the lungs and internally at the cellular level throughout our body.
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External respiration occurs between the outside environment and the capillary bloodstream of the lungs. Oxygen is inhaled (air contains about 21% oxygen) into the air sacs of the lungs and immediately passes into tiny capillary blood vessels. Simultaneously, carbon dioxide passes from the blood into the air spaces to be exhaled (exhaled air contains about 16% oxygen).
Internal (cellular) respiration is the exchange of gases at the cells within all organs of the body. Oxygen passes from the bloodstream into tissue cells, whilst carbon dioxide passes from tissue cells into the bloodstream to be carried back to the lungs for exhalation.
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Air enters through the nose and passes through the nasal cavity, lined with mucous membrane and fine hairs (cilia) that filter foreign bodies whilst warming and moistening the air.
Hollow, air-containing spaces within the skull communicate with the nasal cavity, providing lubricating mucus, lightening skull bones, and helping produce sound.
After the nasal cavity, air reaches the pharynx (throat), which has three divisions: nasopharynx, oropharynx, and hypopharynx (laryngopharynx).
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The larynx contains the vocal cords and is surrounded by pieces of cartilage for support. The thyroid cartilage is the largest and is commonly referred to as the Adam's apple. Sounds are produced as air is expelled past the vocal cords, causing them to vibrate.
A flap of cartilage called the epiglottis covers the opening to the larynx (glottis). During swallowing, when food and liquid move through the throat, the epiglottis closes off the larynx to prevent these substances from entering the airway.

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A vertical tube about 4.5 inches long and 1 inch in diameter, kept open by 16-20 C-shaped rings of cartilage separated by fibrous connective tissue.
In the mediastinum region, the trachea divides into two branches called bronchi. Each bronchus leads to a separate lung and subdivides into smaller tubes.
The smallest bronchial branches are called bronchioles, resembling the finest branches of a tree structure.
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At the end of the bronchioles are clusters of air sacs called alveoli (singular: alveolus). Each alveolus is made of a one-cell layer of epithelium, creating a very thin wall that allows efficient gas exchange.
Capillaries surround and come in close contact with each alveolus. Blood flowing through these capillaries accepts oxygen from the alveolus and deposits carbon dioxide into it for exhalation. Oxygen binds to haemoglobin protein in erythrocytes and is carried throughout the body.
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Each lung is enveloped in a double-folded membrane called the pleura, moistened with serous (thin, watery) secretion.
The outer layer of pleura, nearest the ribs and wall of the thoracic cavity.
The inner layer, closest to the lung tissue, facilitating smooth lung movements within the chest.
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The apex is the uppermost part, whilst the base is the lower area.
The remaining lobes continue to function normally after removal.
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The diaphragm contracts and descends, enlarging the thoracic cavity area and reducing internal air pressure. Air flows into the lungs to equalise pressure.
When the lungs are filled with air, the diaphragm prepares to relax and return to its resting position.
The diaphragm relaxes and elevates, making the thoracic cavity smaller and increasing air pressure. Air is expelled from the lungs to equalise pressure.
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Air enters and is filtered, warmed, and moistened
Air passes through the throat and voice box
Air travels down the windpipe and into bronchial branches
Air reaches the smallest airways and air sacs
Gas exchange occurs with the bloodstream
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Collections of lymph tissue in the nasopharynx, also called pharyngeal tonsils
Air sac in the lung where gas exchange occurs
Branch of the trachea acting as a passageway into the lung's air spaces
Thin hairs attached to mucous membrane epithelium lining the respiratory tract
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Listening to sounds within the body using a stethoscope to detect abnormalities in breathing patterns and lung sounds.
Tapping on a surface to determine the difference in density of the underlying structure, helping identify fluid or air accumulation.
Identifying abnormal sounds such as pleural rub, rales (crackles), rhonchi, and stridor to diagnose respiratory conditions.
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Grating sound produced by the motion of pleural surfaces rubbing against each other, indicating inflammation.
Abnormal crackling sounds heard during inspiration when there is fluid in the alveoli.
Abnormal, rumbling sounds heard during expiration, often indicating mucus in the airways.
A sustained, high-pitched, noisy breathing associated with obstruction of the larynx or bronchus.
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Acute respiratory syndrome in children and infants characterised by laryngeal obstruction, barking cough, and stridor.
Acute infection of the throat and upper respiratory tract caused by Corynebacterium diphtheria bacteria.
Nosebleed, which can result from various causes including trauma, dry air, or underlying conditions.
Bacterial infection caused by Bordetella pertussis affecting the pharynx, larynx, and trachea. Also known as whooping cough.
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Spasm and narrowing of bronchi leading to bronchial airway obstruction, causing difficulty breathing and wheezing.
Cancerous tumours arising from a bronchus; the most common form of lung cancer.
Inflammation of the bronchi persisting for a long time, often associated with smoking or environmental irritants.
Inherited disease of exocrine glands affecting the pancreas, sweat glands, and respiratory tract mucous membranes, leading to airway obstruction.
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Incomplete expansion of alveoli; collapsed, functionless, airless lung or portion of a lung.
Hyperinflation of air sacs with destruction of alveolar walls, reducing gas exchange efficiency.
Abnormal condition caused by dust in the lungs, with chronic inflammation, infection, and bronchitis.
Acute inflammation and infection of alveoli, which fill with pus or inflammatory products.
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A large collection of pus resulting from bacterial infection in the lungs, requiring aggressive treatment.
Swelling and fluid accumulation in the air sacs and bronchioles, often related to heart failure or other conditions.
A clot or other material lodges in vessels of the lung, blocking blood flow and potentially causing serious complications.

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Tuberculosis (TB) is an infectious disease caused by Mycobacterium tuberculosis. Whilst the lungs are usually involved, any organ in the body may be affected by this bacterial infection.
TB remains a significant global health concern, particularly in developing countries. Early detection and proper treatment are essential for controlling the spread of this disease.
Airborne through respiratory droplets
Persistent cough, fever, night sweats, weight loss
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A malignant tumour arising in the pleura, composed of mesothelium (epithelium covering membrane surfaces such as pleura, peritoneum, and pericardium).
Escape of fluid into the pleural cavity, which can result from various conditions including infection, heart failure, or cancer.
Inflammation of the pleura causing sharp chest pain that worsens with breathing or coughing.
Accumulation of air or gas in the pleural cavity, causing lung collapse and requiring immediate medical attention.
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X-ray pictures of the chest can be taken in the frontal (coronal) plane (anteroposterior view) or in the sagittal plane (lateral view) to visualise lung structures.
A series of x-rays showing pictures of the chest region at various depths, able to detect small masses not visible on regular films.
A computerised reconstruction of x-ray pictures taken as slices in a transverse (axial or cross-sectional) plane, providing detailed images.
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Dye is injected into a blood vessel, and x-rays are taken of the arteries or veins in the lung to visualise blood flow and detect blockages or abnormalities.

Magnetic resonance imaging uses magnetic waves to create detailed images of the chest in the frontal, sagittal, and transverse planes without radiation exposure.

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Radioactive material is injected intravenously or inhaled, and images are recorded of its distribution in lung tissue. This test shows both air flow (ventilation) and blood supply (perfusion) to the lung.
The V/Q scan is particularly useful for detecting pulmonary embolism and assessing regional lung function.
Air flow assessment
Blood supply evaluation
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Bronchoscopy involves examination of the bronchial tubes by passing a lighted, flexible fiberoptic tube through the nose, throat, larynx, and trachea into the bronchi. This procedure allows direct visualisation and specimen collection.
Specimens can be obtained through the tube for cytological and bacterial studies by aspiration of bronchial secretions or bronchial alveolar lavage.
Biopsies can be performed on suspicious areas using forceps or brush (bronchial brushing) inserted through the bronchoscope.
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A tube is placed through the nose or mouth, through the pharynx and larynx, and into the trachea to establish an airway. This typically occurs during surgery or in the ICU.
Visual examination of the larynx using a lighted flexible tube passed through the mouth or nose into the larynx.
Creation of an opening into the trachea through the neck and insertion of a tube to create an airway. May be permanent or used as an emergency device.
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Visual examination of the mediastinum performed in the operating room with local and general anaesthesia. A suprasternal (above the breastbone) incision is made to access the mediastinal space.
This procedure is used to remove biopsy samples of lymph nodes, tumours, and to diagnose conditions such as tuberculosis. It provides valuable diagnostic information about structures in the central chest area.
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This group of tests measures ventilation (breathing) mechanics of the lungs, specifically the quantity of air moved into and out of the lungs under normal conditions. A spirometer measures the air taken in and out of the lungs.
Measured by asking the patient to inspire to the maximum and then exhale into the spirometer as forcefully and rapidly as possible.
Used to diagnose and monitor conditions such as asthma, COPD, and other respiratory disorders affecting lung capacity.
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Insertion of a needle or catheter through the skin and between the ribs into the pleural space to obtain fluid for analysis, drain pleural effusions, or reexpand a collapsed lung.
A major surgical procedure in which the chest is opened by an incision for diagnostic or therapeutic procedures. Necessary for lung biopsy and lung resections (lobectomy, pneumonectomy).
A chest tube is passed through a small incision in the skin to continuously drain pleural spaces after thoracotomy.
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Agents such as PPD (purified protein derivatives) are applied to the surface of the skin with multiple punctures (Heaf's and tine tests) or by intradermal injection (Mantoux's test).
A local cutaneous inflammatory reaction (redness, swelling) is observed in persons who are sensitive to the test substance.
A positive test indicates prior or present infection with tuberculosis. However, it does not distinguish between active disease and past exposure.
Further testing is required to confirm active TB infection and determine appropriate treatment.
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Understanding the respiratory system is fundamental to healthcare practice. From the intricate anatomy of airways to the complex physiology of gas exchange, mastering these concepts prepares you for excellence in patient care.
Covered in this comprehensive guide
Essential clinical techniques
Common respiratory disorders
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Understanding the Respiratory System