Respiratory System – Introduction

Introduction

The respiratory system comprises organs and structures that facilitate breathing, including the lungs, airways, and diaphragm—working together to oxygenate blood, eliminate carbon dioxide, and regulate pH balance essential for cellular function and homeostasis.

Overview of the Respiratory System

The respiratory system comprises a series of organs and structures that work collectively to facilitate the exchange of oxygen and carbon dioxide between the body and the environment. Its primary physiological role is to supply oxygen to the bloodstream and remove carbon dioxide, a waste product of metabolism. The respiratory system also plays roles in speech, olfaction (sense of smell), and maintaining acid-base balance in the body.

The major components of the respiratory system include the upper and lower respiratory tracts, the lungs, the airways, the alveoli, and accessory structures such as the diaphragm and intercostal muscles. Each part has a specific anatomical structure and function, contributing to the overall efficiency of respiration.

General Structure and Major Components

• Upper respiratory tract: Nose, nasal cavity, pharynx, and larynx
• Lower respiratory tract: Trachea, bronchi, bronchioles, and lungs
• Accessory structures: Diaphragm, intercostal muscles, thoracic cage

Upper Respiratory Tract

Nose and Nasal Cavity

The nose serves as the main external opening for the respiratory system and consists of the external nose and the nasal cavity. The primary functions of the nose include filtering, warming, and humidifying inspired air, as well as housing the olfactory receptors responsible for smell.

• External Nose: Composed of bone and cartilage, the external nose shapes the nostrils (nares) through which air enters.
• Nasal Cavity: A large air-filled space behind the nose, divided by the nasal septum. The cavity is lined by a mucous membrane containing cilia and goblet cells, which trap dust and pathogens.
• Turbinates (Conchae): Three bony projections on the lateral walls of the nasal cavity (superior, middle, and inferior conchae) increase the surface area for air filtration and humidification.
• Sinuses: Air-filled cavities in the skull bones surrounding the nasal cavity; they lighten the skull and contribute to voice resonance.

Pharynx: Structure and Function

The pharynx is a muscular tube approximately 12–14 cm long, extending from the base of the skull to the oesophagus. It is divided into three regions:

• Nasopharynx: Located behind the nasal cavity, it serves as an air passage and contains the pharyngeal tonsil (adenoid).
• Oropharynx: Located behind the oral cavity, it serves as a passage for both air and food and contains the palatine tonsils.
• Laryngopharynx (Hypopharynx): The lowest part, located behind the larynx, it communicates with the oesophagus and larynx.

The pharynx plays a key role in ensuring that air passes into the respiratory tract while food and liquids are directed into the digestive tract.

Larynx: The Voice Box

The larynx, or voice box, is a cartilaginous structure situated between the pharynx and the trachea. It is responsible for producing sound and protecting the lower airways during swallowing.

• Cartilages: The larynx is made up of several cartilages, the largest of which are the thyroid cartilage (Adam’s apple), cricoid cartilage, and epiglottis. The epiglottis acts as a lid, preventing food from entering the trachea during swallowing.
• Vocal Cords (Vocal Folds): Bands of muscle and connective tissue that vibrate to produce sound when air passes through.
• Glottis: The opening between the vocal cords, which regulates airflow into the trachea.

The larynx also serves as a critical point for airway protection, as the epiglottis covers the glottis during swallowing to prevent aspiration.

Lower Respiratory Tract

Trachea: The Windpipe

The trachea is a flexible, cylindrical tube about 10–12 cm long and 2 cm in diameter, extending from the larynx to the level of the fifth thoracic vertebra, where it divides into the right and left main bronchi. The trachea is supported by C-shaped rings of hyaline cartilage, which keep the airway open while allowing flexibility during breathing and swallowing.

The inner lining of the trachea contains ciliated epithelial cells and goblet cells that trap and move dust and pathogens upward towards the pharynx, a mechanism known as the mucociliary escalator.

Bronchi and Bronchial Tree

The trachea divides into the right and left main (primary) bronchi at the carina. Each main bronchus enters the respective lung and further divides into secondary (lobar) bronchi, tertiary (segmental) bronchi, and progressively smaller bronchioles, forming the bronchial tree.

• Main Bronchi: The right main bronchus is wider, shorter, and more vertical, making it more likely for aspirated objects to enter the right lung.
• Lobar Bronchi: Three on the right (for each lobe), two on the left.
• Segmental Bronchi: Supply the bronchopulmonary segments of each lobe.
• Bronchioles: Small airways less than 1 mm in diameter, lacking cartilage but surrounded by smooth muscle; further divide into terminal and respiratory bronchioles.

The bronchi and bronchioles ensure that air is distributed evenly throughout the lungs and that foreign particles are removed before reaching the delicate alveoli.

Lungs: Gross Anatomy and Microscopic Structure

Gross Anatomy of the Lungs

The lungs are paired, spongy organs located within the thoracic cavity, separated by the mediastinum. Each lung is conical in shape, with an apex (top), base (bottom), costal surface (against the ribs), and medial surface (facing the mediastinum). The right lung is slightly larger and consists of three lobes (superior, middle, inferior), while the left lung has two lobes (superior and inferior) and a cardiac notch to accommodate the heart.

Lobes and Segments

• Right Lung: Three lobes (superior, middle, inferior) and ten bronchopulmonary segments
• Left Lung: Two lobes (superior, inferior) and eight to ten bronchopulmonary segments

Each lobe is further divided into bronchopulmonary segments, which are functionally and anatomically discrete units supplied by their own segmental bronchus and artery. This segmentation is clinically significant, as diseases may affect individual segments, and surgical removal of a segment is possible without compromising other regions.

Pleura: The Protective Membrane

The lungs are enveloped by a double-layered serous membrane called the pleura:

• Visceral Pleura: Covers the surface of the lungs.
• Parietal Pleura: Lines the thoracic cavity, diaphragm, and mediastinum.
• Pleural Cavity: The potential space between the visceral and parietal pleura, containing a small amount of lubricating fluid that reduces friction during breathing movements.

Microscopic Structure of the Lungs

Within the lungs, the smallest bronchioles terminate in clusters of alveoli. The lung tissue is composed of millions of alveoli, elastic fibres, blood vessels, nerves, and lymphatics. The alveoli are the sites of gas exchange, while the interstitium provides structural support and contains immune cells.

Alveoli and Gas Exchange

Structure of Alveoli

Alveoli are tiny, balloon-like sacs with thin walls, surrounded by capillaries. Each lung contains approximately 300–500 million alveoli, providing a large surface area (about 70 square metres) for efficient gas exchange.

• Alveolar Wall: Composed of a single layer of squamous epithelial cells (type I pneumocytes), type II pneumocytes (which secrete surfactant to reduce surface tension), and alveolar macrophages (which remove debris).
• Respiratory Membrane: The thin barrier (about 0.5 microns) formed by the alveolar wall, the capillary endothelium, and their fused basement membranes. This facilitates rapid diffusion of gases.

Mechanism of Gas Exchange

Gas exchange occurs across the respiratory membrane by simple diffusion:

• Oxygen from inhaled air diffuses from the alveoli into the capillary blood, where it binds to haemoglobin in red blood cells.
• Carbon dioxide, produced by cellular metabolism, diffuses from the blood into the alveoli to be exhaled.

The efficiency of gas exchange depends on the integrity of the alveolar walls, the surface area available, and the matching of ventilation (air flow) and perfusion (blood flow).

Blood Supply and Innervation

Pulmonary and Bronchial Circulation

• Pulmonary Circulation: The right ventricle pumps deoxygenated blood into the pulmonary arteries, which branch into arterioles and capillaries surrounding the alveoli. Oxygenated blood then returns to the left atrium via the pulmonary veins.
• Bronchial Circulation: The bronchial arteries, branches of the aorta, supply oxygenated blood to the lung tissue itself (except the alveoli). Bronchial veins drain some blood back to the systemic circulation.

The dual blood supply ensures that the lung tissue receives adequate oxygen and nutrients, and enables efficient gas exchange.

Nerve Supply (Innervation) of the Lungs

The lungs and airways are innervated by both the autonomic and sensory nervous systems:

• Parasympathetic Nerves (Vagus Nerve): Cause bronchoconstriction and increased mucus secretion.
• Sympathetic Nerves: Cause bronchodilation and decreased mucus secretion.
• Sensory Nerves: Detect irritants and initiate reflexes such as coughing.

Understanding the nerve supply is crucial for nurses, especially when managing patients with respiratory diseases or those receiving medications that affect airway tone.

Lymphatic Drainage

Lymphatic Pathways of the Lungs

The lymphatic system helps maintain fluid balance and protects against infection by draining excess fluid, proteins, and foreign particles from the lungs.

• Superficial (Subpleural) Lymphatics: Drain the lung surface and pleura.
• Deep Lymphatics: Drain the bronchial tree, blood vessels, and connective tissue.
• Lymph ultimately drains into the hilar (bronchopulmonary) and mediastinal lymph nodes, and then into the thoracic duct or right lymphatic duct.

Clinical Significance

Lymphatic drainage is important in the spread of lung infections, cancer metastasis, and immune responses. Enlarged lymph nodes may be detected in cases of infection, malignancy, or inflammatory diseases, and can be identified through imaging or clinical examination.

Accessory Structures: Diaphragm, Intercostal Muscles, and Thoracic Cage

Diaphragm

The diaphragm is a dome-shaped skeletal muscle separating the thoracic and abdominal cavities. It is the primary muscle of respiration. During inspiration, the diaphragm contracts and flattens, increasing thoracic cavity volume and drawing air into the lungs. During expiration, the diaphragm relaxes and resumes its dome shape, reducing thoracic volume and expelling air.

Intercostal Muscles

Located between the ribs, the intercostal muscles assist with breathing by elevating (external intercostals) or depressing (internal intercostals) the rib cage. Their coordinated action, along with the diaphragm, is essential for effective ventilation.

Thoracic Cage

The thoracic cage, formed by the ribs, sternum, and thoracic vertebrae, protects the lungs and provides attachment points for respiratory muscles. Its flexibility allows for expansion and contraction during breathing.

Clinical Relevance for Nurses

Common Respiratory Conditions

A sound knowledge of respiratory anatomy enables nurses to understand, recognise, and manage common respiratory conditions such as:

• Asthma: Characterised by bronchoconstriction, airway inflammation, and mucus hypersecretion. Recognising signs of respiratory distress and administering bronchodilators is crucial.
• Chronic Obstructive Pulmonary Disease (COPD): Involves chronic bronchitis and emphysema, leading to airflow limitation. Nurses play a key role in monitoring oxygen therapy and teaching breathing exercises.
• Pneumonia: Infection of the lung parenchyma, resulting in alveolar inflammation and consolidation. Nurses monitor vital signs, administer antibiotics, and support airway clearance.
• Pulmonary Embolism: Blockage of a pulmonary artery by a blood clot, causing sudden respiratory distress. Nurses must act quickly to support oxygenation and prepare for emergency interventions.
• Pneumothorax: Air in the pleural cavity causing lung collapse. Nurses are involved in monitoring chest drains and recognising complications.

Implications for Nursing Care

• Assessment of respiratory function using inspection, palpation, percussion, and auscultation.
• Monitoring and interpreting arterial blood gases (ABGs) and pulse oximetry.
• Assisting with airway management, including suctioning, oxygen therapy, and mechanical ventilation.
• Educating patients and families about respiratory health, breathing exercises, and infection prevention.
• Recognising early signs of respiratory compromise and implementing timely interventions.

Key Points

• The respiratory system comprises the upper and lower respiratory tracts, lungs, airways, alveoli, and accessory structures.
• The upper respiratory tract filters, warms, and humidifies air, while the lower tract conducts air to the lungs and facilitates gas exchange.
• The lungs are divided into lobes and segments, each supplied by its own bronchus and blood vessels.
• Alveoli are the primary sites of gas exchange, relying on a thin respiratory membrane for efficient diffusion of gases.
• Blood supply, nerve supply, and lymphatic drainage are essential for lung function and defence against infection.
• Accessory structures like the diaphragm and intercostal muscles are vital for effective ventilation.
• Knowledge of respiratory anatomy is fundamental for nurses, forming the basis for assessment, intervention, and patient education in respiratory care.

REFERENCES

1. Ross and Wilson, Anatomy and Physiology in Health and Illness, Fourteenth Edition, 1 July 2022, ISBN-13: 978-0323834612.
2. Roger Watson, Anatomy and Physiology for Nurses, 14th Edition, 12-06-2018, ISBN: 9780702077418
3. P.R Asha Latha, Text Book of Applied Anatomy & Physiology for Nurses, 7th Edition,3 January 2024, ISBN-13: 978-9356968622.
4. Bryan H. Derikson, Tortora’s Principles of Anatomy and Physiology, 16th Edition, August 2023, ISBN: 978- 1119400066.
5. Standring S, ed. Gray’s Anatomy: The Anatomical Basis of Clinical Practice. 42nd ed. London: Elsevier; 2020. pp. 75–102. ISBN 978-0702077050.
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7. Dr. Peter Parkinson, Simplemed, Respiratory System, https://simplemed.co.uk/subjects/respiratory/anatomy-of-the-respiratory-system
8. Anatomy.co.uk, Respiratory System, Last updated on April 24, 2025, https://anatomy.co.uk/Respiratory-system

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