Chapter 1
Anatomy and Physiology: The Respiratory System

The respiratory system is a complex and vital component of the human body responsible for the exchange of gases between the external environment and our internal environment. The respiratory system plays a crucial role in maintaining the body's homeostasis by ensuring an adequate supply of oxygen to cells and removing waste carbon dioxide.

It is essential to understand the fundamentals to assess and care for patients with respiratory conditions effectively. This chapter explores the anatomy and physiology of the respiratory system, providing readers with a foundational understanding of its structure and function. In the following chapters, respiratory assessments, common disorders and interventions to provide comprehensive, holistic, respiratory care are outlined.

Cells can only survive if they receive a continuous supply of oxygen. As cells use oxygen, a waste gas, carbon dioxide, is produced. If this gas is allowed to build up, carbon dioxide can disrupt cellular activity and interrupt homeostasis. The key function of the respiratory system, therefore, is to ensure that the body extracts sufficient oxygen from the atmosphere and disposes of the excess carbon dioxide. The collection of oxygen and removal of carbon dioxide is known as respiration. Respiration involves the following four distinct processes:

1. Pulmonary ventilation
2. External respiration
3. Transport of gases
4. Internal respiration

All four are examined in this chapter; only pulmonary ventilation and external respiration are the sole responsibility of the respiratory system. As oxygen and carbon dioxide are transported around the body in blood, effective respiration is also reliant upon a fully functioning cardiovascular system.

The Respiratory System

The respiratory system is separated into the upper and lower respiratory tract (see Figure 1.1). Those structures found below the larynx are part of the lower respiratory tract. The respiratory system can also be said to be divided into conduction and respiratory regions. The upper respiratory tract and the uppermost section of the lower respiratory tract form the conduction region, where air is conducted through a series of tubes and vessels. The respiratory region is the functional part of the lungs, where the oxygen diffuses into blood. The structures within the respiratory region are microscopic, they are very fragile and easily damaged by infection. For this reason, the upper and lower respiratory tracts are fashioned in such a way as to fight off any invading airborne bacterial or viral pathogens.

Figure 1.1 The upper and lower respiratory system

The Upper Respiratory Tract

Air enters the body via the nasal and oral cavities. The nasal cavity is divided into two equal sections, separated by the nasal septum, which is a structure formed out of the ethmoid bones and the vomer of the skull. The space where air enters the nasal cavity just inside the nostrils is called the vestibule. Beyond each vestibule, the nasal cavities are subdivided into three air passageways, the meatuses, these are three shelf-like projections called the superior, middle and inferior nasal conchae (see Figure 1.2). The region around the superior conchae and upper septum contains olfactory receptors, which are responsible for the sense of smell. The pharynx connects the nasal and oral cavity with the larynx. The pharynx is divided into three regions called the nasopharynx, the oropharynx and the laryngopharynx. The nasopharynx is located behind the nasal cavity and contains two openings that lead to the auditory (eustachian) tubes. The oropharynx and laryngopharynx are found underneath the nasopharynx and behind the oral cavity. The oropharynx and oral cavity are divided by the fauces. Both the oropharynx and the laryngopharynx are passageways for food and drink and also air. To protect these structures from abrasion by food particles and to prevent moisture loss, they are lined with non-keratinised stratified squamous epithelium.

Figure 1.2 Structures of the upper respiratory system

As well as providing the sense of smell, another function of the upper respiratory tract is to ensure that air entering the lower respiratory tract is warm, damp and clean. The vestibule is lined with coarse hairs (cilia) that filter incoming air, ensuring large dust particles do not enter the airways. The conchae are lined with a mucous membrane made of pseudostratified ciliated columnar epithelium, which contains a network of capillaries and a plentiful supply of mucus-secreting goblet cells. The blood flowing through the capillaries warms the passing air, while the mucus moistens it and traps any passing dust particles. The mucus-covered dust particles are propelled by the cilia towards the pharynx, where they are swallowed or expectorated.

To provide further protection, the upper respiratory tract is lined with irritant receptors. When they are stimulated by invading particles (for example, dust or pollen), they force a sneeze, ensuring the offending material is ejected through the nose or mouth. The pharynx contains five tonsils. The two tonsils visible when the mouth is open are the palatine tonsils; located behind the tongue are the lingual tonsils and the pharyngeal tonsil or adenoid sits on the upper back wall of the pharynx. Tonsils are lymph nodules and part of the body's defence system. The epithelial lining of their surface has deep folds, known as crypts. Inhaled bacteria or particles become entangled within the crypts, and they are then engulfed and destroyed.

The Lower Respiratory Tract

This aspect of the respiratory tract includes the larynx, the trachea, the right and left primary bronchi and all the constituents of both lungs (see Figure 1.3). The lungs are two cone-shaped organs almost filling the thorax. They are protected by a framework of bones, the thoracic cage, which consists of the ribs, sternum (breastbone) and vertebrae (spine). The tip of each lung, the apex, extends just above the clavicle (collarbone) and their wider bases sit just above a concave muscle known as the diaphragm. The larynx (voice box) connects the trachea and the laryngopharynx. The remainder of the lower respiratory tract divides into branches of airways. For this reason, the structure of the lower respiratory tract is often referred to as the bronchial tree.

Figure 1.3 The lower respiratory tract

Larynx

This is made up of nine pieces of cartilage tissue: three single pieces and three pairs (see Figure 1.4). The single pieces of cartilage are the thyroid cartilage, the epiglottis and the cricoid cartilage. The thyroid cartilage is more commonly known as the Adam's apple and, together with the cricoid cartilage, protects the vocal cords. The cricothyroid ligament, connecting the thyroid and cricoid cartilage, is the landmark of an emergency airway or tracheostomy (Wheeldon 2020). The epiglottis is a leaf-shaped piece of elastic cartilage attached to the top of the larynx. It protects the airway from food and liquids entering. On swallowing, the epiglottis blocks entry to the larynx and food and liquids are diverted towards the nearby oesophagus. If foreign substances are inhaled, they can block the lower respiratory tract and cut off the body's supply of oxygen. This is a medical emergency known as aspiration and requires the swift removal of the offending substance.

Figure 1.4 The larynx

The three pairs of cartilage are the arytenoid, cuneiform and corniculate cartilages (see Figure 1.4). The arytenoid cartilages are the most significant as they influence the movement of the mucous membranes (true vocal folds) that generate the voice. Speaking is, therefore, reliant upon a fully functioning respiratory system. Many obstructive lung disorders, for example, asthma, reduce a person's ability to utter a full sentence without having to draw a new breath (Wheatley 2018).

The Trachea

The trachea (or windpipe) is a tubular vessel. It carries air from the larynx down towards the lungs. The trachea is lined with pseudostratified ciliated columnar epithelium, ensuring that any inhaled debris is trapped and propelled upwards towards the oesophagus and pharynx where it is swallowed or expectorated. The trachea and the bronchi also contain irritant receptors, which stimulate a cough, forcing larger invading particles upwards. The outermost layer of the trachea is made up of connective tissue, reinforced by a series of 16-20 C-shaped cartilage rings. These rings stop the trachea from collapsing during an active breathing cycle.

The Lungs

The lungs are divided into distinct regions called lobes. There are three lobes in the right lung and two in the left. The heart, along with its major blood vessels, occupies a space between the two lungs called the mediastinum. The lungs are surrounded by two thin protective membranes called the parietal and visceral pleura (see Figure 1.3). The parietal pleura lines the wall of the thorax and the visceral pleura lines the lungs themselves. The space between the two pleurae is the pleural space or cavity, which is tiny and contains a thin film of lubricating fluid. This reduces friction between the two pleurae, allowing the two layers to slide over one another during breathing. The fluid also helps the visceral and parietal pleura to adhere to each other, in the same way, two pieces of glass stick together when they are wet....