Respiratory System

Original Editor - Lucinda hampton Top Contributors - Lucinda hampton, Uchechukwu Chukwuemeka, Stacy Schiurring, Jess Bell, Ammar Suhail, Tarina van der Stockt, Kim Jackson, Khloud Shreif, Ewa Jaraczewska and Rishika Babburu

Introduction

The primary role of the respiratory system is gas exchange—the intake of oxygen and expulsion of carbon dioxide— and maintaining bodily homeostasis by regulating blood pH through breathing. In addition, it is involved in vocalisation, olfaction, immunity, and temperature regulation. The respiratory system also plays a role in regulating heart rate, cardiac output, and blood pressure due to its neurological and physiological connection with the cardiovascular system.

This article explores the respiratory system, covering major anatomical structures and components, respiratory physiology and function. It also includes resources for commonly encountered respiratory diagnoses.

Respiratory System Components and Function

Upper Respiratory Tract

Nasal cavity: breathing begins when air is drawn into the nasal cavity via the nostrils (anterior nares) which function to filter inspired dust and particulates using tiny hairs called cilia. The nasal cavity is formed by craniofacial bones. It is lined with a mucous membrane, or mucosa, which warms and moistens air that passes over it, and further filters airborne particles.

Nose breathing versus mouth breathing: nose breathing is the natural and preferred method of breathing as it better prepares inspired air for respiration and results in improved oxygenation. Prolonged mouth breathing may result in headache, forward head posture, fatigue and drowsiness, decreased lung function and activation of the muscles of respiration, and increased risk of low oxygen saturation in the brain.

Pharynx: the pharynx is a muscular tube lined with mucosa. It is the pathway to both the respiratory system and gastrointestinal tract. It consists of three sections: (1) nasopharynx, (2) oropharynx, and (3) laryngopharynx (also known as the hypopharynx). The nasopharynx receives air from the nasal cavity. It also has a connection to the middle ears via the auditory tubes (Eustachian or pharyngotympanic tubes). This connection acts to equalise pressure and facilitate secretion drainage from the middle ears. The oral cavity joins at the oropharynx, which moves food and liquids down toward the oesophagus. The laryngopharynx is clinically significant as it is where the pharynx divides anteriorly into the larynx of the lower airways and posteriorly into the oesophagus. It, therefore, plays an important role in both swallowing and speech.

Lower Respiratory Tract

Larynx: the larynx is a cartilaginous segment that marks the beginning of the lower respiratory tract. Its primary function is to protect the lower respiratory tract from aspirating food and liquids into the trachea and lungs below. The larynx contains six different cartilages: (1) thyroid cartilage, (2) cricoid cartilage, (3) epiglottis, (4) paired arytenoid cartilages, (5) paired corniculate cartilages, and (6) paired cuneiform cartilages. The thyroid cartilage is the largest. It provides protection to the anterior larynx and forms the laryngeal prominence, also known as the Adam’s apple. The epiglottis is a leaf-shaped cartilaginous flap that covers the opening into the larynx and trachea. During swallowing, the epiglottis and larynx are drawn upward. This closes the trachea and allows food/liquid to enter the oesophagus.

The larynx also functions as the "voice box". The laryngeal cartilages involved in vocalisation are the thyroid, cricoid, and arytenoid cartilages. The thyroid cartilage provides the anterior framework and attachment points for muscles controlling vocal cord tension. The cricoid cartilage forms the base of the larynx and supports the arytenoid cartilages, which are crucial for vocal cord movement because of their ability to rotate and slide. The vocal cords themselves consist of two distinct structures: (1) the false vocal cords (vestibular folds) located superiorly, are primarily protective and do not vibrate during normal speech; (2) the true vocal cords (vocal folds) located inferiorly, are the actual sound-producing structures. The true vocal cords are muscular folds that stretch between the thyroid cartilage anteriorly and the vocal processes of the arytenoid cartilages posteriorly. Coordinated movement of these cartilages through intrinsic laryngeal muscle action allows precise control of vocal cord length, tension, and position. This enables the vibration of the true vocal cords, which is necessary for phonation and speech production.

Tracheobronchial tree: 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. The conduction zone is composed of the trachea, bronchi, and bronchioles. Their function is to optimise gas delivery to the functional portion of the lung. Their walls contain a mucosa layer and cilia, hair-like projections that beat upwards to move inhaled particles out of the lungs, and cartilage for support against expiration. The respiratory zone is composed of respiratory bronchioles, alveolar ducts, and alveoli, and is the location of gas transfer within the lung.

Trachea: the larynx becomes the trachea at the level of the C6 vertebra and bifurcates at the level of T4 into left and right primary bronchi. The angle of the right and left primary bronchi is different due to the position of the heart—the primary bronchus on the left is more horizontal. Aspirated substances that make it past the larynx are more likely to end up in the right lung because of this positioning. Within the lungs, the primary bronchi branch into secondary bronchi, which divide into tertiary segmental bronchi, after which the airways continue to branch out into bronchioles. Bronchioles end in tiny air sacs called alveoli, where gas exchange occurs.

Lungs: the lungs are located within the thoracic cavity. They occupy most of the space in the chest except for the mediastinum, which contains the heart, great vessels, the primary bronchi, the oesophagus, and the thymus gland. Each lung is housed in its own pleural cavity, which is lined by a double-layered serous membrane called the pleura. The visceral pleura directly covers the lung surface, while the parietal pleura lines the inner wall of the thoracic cavity, diaphragm, and mediastinum. Between these two pleural layers is the pleural space, which contains a small amount of pleural fluid. This allows the lungs to glide smoothly during breathing while maintaining the surface tension that keeps the lungs expanded against the chest wall. The lungs extend from just above the clavicles at their apex down to the diaphragm at their base. The diaphragm serves as the primary muscle of respiration and it separates the thoracic cavity from the abdominal cavity below.

The right lung is slightly larger than the left and is divided into three lobes, the superior, middle, and inferior lobes. The left lung only has two lobes (superior and inferior) to accommodate the heart's position. Each lobe is supplied by a secondary bronchus. Each lobe is then further divided into bronchopulmonary segments, which are supplied by a tertiary bronchus. The right lung has ten segments: three in the superior lobe, two in the middle lobe, and five in the inferior lobe. The left lung has between eight and ten segments: four to five in the upper lobe and four to five in the lower lobe.

Alveoli: the alveoli are specialised air sacs that make up the basic unit of respiration. They are located in the lung parenchyma at the tips of the bronchial tree and are grouped around alveolar sacs and ducts. The human lungs contain around 500 million alveoli, covering an area of 70 square metres. The alveolar walls are made up of collagen and elastic fibres, which expand during inspiration and return to their former shape during expiration. There are many capillaries within the alveolar walls that facilitate gas exchange.

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If you would like to learn about respiratory-related diagnostics, procedures, and medical diagnoses, please see the following optional reading pages:

• Arterial Blood Gases (ABG)
• Aspiration Pneumonia
• Asthma
• Atelectasis
• Chronic Obstructed Pulmonary Disease (COPD)
• Dyspnoea
• Hypoaemia
• Incentive Spirometry
• Mechanical Ventilation
• Pneumonia
• Pneumothorax
• Pulmonary Embolism
• Pulmonary Fibrosis
• Pulmonary Oedema
• Respiratory Failure
• Sleep Apnea
• Tracheostomy

Mechanics of Breathing

Pulmonary ventilation (breathing) is the movement of air in and out of our lungs. This occurs due to differences in air pressure between the atmosphere and lungs, which are created by the muscles involved in breathing.

"Breathing in" is referred to as inspiration (or inhalation). During this phase, the diaphragm contracts and flattens to pull the lungs down, and the external intercostal muscles elevate the ribs; the combination of these actions increases the thoracic cavity volume. This reduces the air pressure within the lungs, making the atmospheric air pressure higher. Since air wants to flow from a region of higher pressure to a region with lower pressure, air flows into the lungs.  

"Breathing out" is referred to as expiration (or exhalation). During quiet breathing (breathing at rest), expiration is a passive process. The muscles of inspiration relax and the lungs go back to their original size due to their elastic recoil property. This causes the pressure inside the lungs to become greater than the atmospheric pressure, so air flows out. Expiration can be an active process as well, where the abdominal and internal intercostal muscles contract to actively decrease the thoracic cavity volume. This increases pressure within the lungs and moves air out.

Each inhalation delivers air to the 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 haemoglobin in red blood cells. This oxygen-rich blood then flows back to the heart, which pumps it through the arteries to tissues throughout the body.

In the capillaries of the body tissues, oxygen is freed from the haemoglobin and moves into the cells. Carbon dioxide 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. This blood circulates from the heart to the lungs, where carbon dioxide moves into the alveoli and is eliminated.

Additional Resources

• Muscles of Respiration

• The following video overviews the process of respiration

• The following extended video features an interview with Mary Massery, PT, DPT, DSc on the role of the diaphragm in breathing, posture, and balance.