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Mastering these skills helps you understand how breathing works and why keeping the respiratory system healthy is so important in everyday life and your future career.
Compiled by: Mary Jemutai
The respiratory system plays a crucial role in human health and nutrition, serving as the primary means of gas exchange that supports cellular metabolism. For Nutrition and Dietetics professionals in Kenya, understanding this system is essential because oxygen intake and carbon dioxide removal directly influence energy production, nutrient utilization, and overall metabolic function. Knowledge of respiratory anatomy and physiology allows dietitians to appreciate how respiratory health impacts nutritional status, particularly in patients with respiratory diseases or those undergoing clinical nutrition management. This chapter explores the structure, functions, and adaptations of the respiratory system in a way that aligns with nutrition and dietetics practice.
The respiratory system brings oxygen into the body and removes carbon dioxide. It is made up of the airways that carry air, the lungs where gases cross into and out of the blood, and the muscles and bony cage that move air in and out. Every cell uses oxygen to release energy from carbohydrate, fat and protein, so a dietitian who plans feeds for a client cannot separate energy needs from how well that client breathes. A client with a chest infection, a long-standing lung disease or weak breathing muscles may eat less, may tire while eating and may burn more energy simply to breathe. How the breathing process works and which diseases affect the lungs are taught in the topics Respiration process and Disorders of the respiratory system. This topic covers the parts of the system, what they do and how they are built for the job.
The respiratory system is a continuous pathway. Air enters through the nose, passes down the throat and trachea, divides into the bronchi, and finally reaches millions of tiny air sacs in the lungs. The diaphragm and the muscles between the ribs drive the movement of air. The structures are described below in the order that air meets them.
The human respiratory system (Source: e342927be8e0e562)
The respiratory tract is divided at the voice box (larynx). The upper tract is made up of the nose, nasal cavity, paranasal sinuses and pharynx (throat). Its main task is to filter, warm and moisten the air. The lower tract is made up of the larynx, trachea, bronchi, bronchioles and the alveoli of the lungs. Its main task is to conduct air to the alveoli and to exchange gases there. The pharynx is shared with the digestive system: food and air both pass through it, and a flap of cartilage called the epiglottis folds over the entrance of the larynx during swallowing so that food goes into the oesophagus and not into the airway.
The nose is the usual entrance of the airway. The two nostrils (nares) open into the nasal cavity, which is divided into a right and a left side by the nasal septum. The front part of the cavity (the vestibule) has coarse hairs that trap large particles. Behind it, the side walls carry three shelf-like bony folds called the nasal conchae (turbinates), which make the air swirl and bring it into contact with a large moist surface.
The lining of the cavity is a mucous membrane with a rich blood supply. Goblet cells produce sticky mucus that traps dust and microorganisms, and cilia (tiny hair-like projections) sweep the mucus backwards towards the throat, where it is swallowed. The blood flowing beneath the lining warms cold air, and the moist surface adds water vapour to dry air. The roof of the cavity holds the olfactory (smell) receptors. Air spaces called paranasal sinuses open into the nasal cavity and lighten the skull.
For a dietitian, the nose matters in three ways. Smell contributes strongly to flavour, so a blocked nose or loss of smell reduces appetite. Clients who cannot swallow safely may be fed through a thin tube passed through a nostril into the stomach (a nasogastric tube), which is placed by a clinician and follows the path from the nostril through the throat and oesophagus. A client who breathes through the mouth because the nose is blocked may have a dry mouth, which makes dry foods hard to chew and swallow.
The trachea (windpipe) is a flexible tube about 10 to 12 cm long and about 2 cm wide. It starts at the lower end of the larynx in the neck and runs down into the chest in front of the oesophagus. Its wall contains 16 to 20 C-shaped rings of hyaline cartilage. The rings keep the tube open so that it cannot collapse when air pressure falls during breathing in. The open part of each C faces backwards, where a band of smooth muscle (the trachealis) completes the wall. This lets the oesophagus, which lies directly behind, bulge forwards a little when a swallowed lump of food passes.
The inside of the trachea is lined with ciliated cells and mucus-producing goblet cells. Mucus traps particles that escaped the nose, and the cilia beat upwards to carry the mucus to the throat, where it is swallowed or coughed out. This cleaning action is called the mucociliary escalator. At its lower end, the trachea divides at a ridge called the carina into the right and left main bronchi.
Because the airway and the food pipe lie side by side, a client with a weak or poorly coordinated swallow (for example an elderly client or one with a nerve disease) may let food or fluid slip into the trachea. This is called aspiration and it can cause coughing, choking and chest infection. Dietitians therefore work with the speech therapist and doctor to choose food and fluid textures that the client can swallow safely.
The right and left main bronchi carry air from the trachea into the two lungs. Their walls contain plates of cartilage, smooth muscle and the same ciliated, mucus-producing lining as the trachea. The right main bronchus is wider, shorter and more vertical than the left, so a swallowed or inhaled object is more likely to enter the right lung. The left main bronchus is narrower, longer and slopes more sideways because the heart lies to the left of the middle line.
Inside each lung the bronchi divide again and again, like the branches of a tree:
As the tubes become narrower, the cartilage disappears and the lining becomes thinner. The smooth muscle of the bronchioles is the part that tightens during an asthma attack, narrowing the airway and making breathing out difficult.
The lungs are two cone-shaped, spongy, elastic organs that fill most of the chest (thoracic) cavity, one on each side of the heart. Each lung has a narrow top (apex) that reaches just above the collar bone, a broad base that rests on the diaphragm, a rib-facing (costal) surface and an inner (mediastinal) surface where the hilum lies. The right lung is slightly larger and has three lobes, the upper, middle and lower, separated by two fissures. The left lung has two lobes, the upper and lower, and a notch (the cardiac notch) that makes room for the heart.
Each lung is covered by a double-layered membrane called the pleura. The inner layer (visceral pleura) is attached to the lung surface and the outer layer (parietal pleura) lines the chest wall and the diaphragm. A thin film of pleural fluid between the two layers lets them slide over each other without friction and keeps them together, so that the lung follows the movement of the chest wall.
The working tissue of the lung is made up of about 300 million alveoli, tiny thin-walled air sacs clustered like bunches of grapes at the ends of the smallest airways. Each alveolus is wrapped in a net of capillaries. Two main cell types form the alveolar wall: type I cells, which are very flat and form the thin barrier across which gases move, and type II cells, which make surfactant. Macrophages (cleaning cells) move around inside the alveoli and engulf dust and microorganisms that reach them.
Repair of lung tissue and the work of the breathing muscles both need enough energy and protein. A person who is severely underweight loses muscle, including the muscles of breathing, and has a weaker cough, so lung infection is more likely and harder to clear.
The diaphragm is a large dome-shaped sheet of skeletal muscle with a central tendon. It forms the floor of the chest cavity and the roof of the abdominal cavity, so it separates the lungs and heart from the stomach, liver and intestines. It is the main muscle of breathing. It is supplied by the phrenic nerve, which arises from the neck part of the spinal cord. When the diaphragm contracts, it flattens and moves downwards, the chest cavity becomes larger and air is drawn in. When it relaxes, it returns to its dome shape and air leaves the lungs.
Three openings pass through the diaphragm: one for the oesophagus, one for the aorta (the main artery) and one for the inferior vena cava (the large vein returning blood from the lower body). The oesophageal opening is called the hiatus. If a part of the stomach pushes upwards through it (a hiatus hernia), the client may have heartburn and discomfort after large meals, and a dietitian advises smaller, more frequent meals. A full stomach or a swollen abdomen presses on the diaphragm and makes breathing shallow, which is one reason some clients cannot finish a large meal.
The diaphragm works together with the external intercostal muscles, which lie between the ribs and lift the ribcage up and outwards. Together they enlarge the chest cavity in all directions.
The thoracic cage is the bony frame that protects the lungs and heart. It is formed by the twelve pairs of ribs, the sternum (breastbone) in front and the thoracic vertebrae behind. It is flexible because the ribs join the sternum through cartilage. Besides the diaphragm and external intercostal muscles, the internal intercostals and the abdominal muscles help during forced breathing out, and the neck muscles help during hard breathing in. Weak breathing muscles in a malnourished client reduce the ability to breathe deeply and to cough effectively, which is why nutritional rehabilitation forms part of the care of such clients.
The lungs have two blood supplies. The pulmonary arteries carry blood that is low in oxygen from the right side of the heart to the alveoli, and the pulmonary veins return oxygen-rich blood to the left side of the heart. The bronchial arteries branch from the aorta and bring oxygen-rich blood to feed the lung tissue itself. Breathing is controlled by nerves: the phrenic nerve supplies the diaphragm, the intercostal nerves supply the muscles between the ribs, and the autonomic nerves, including the vagus, adjust the width of the airways. Damage to these nerves, for example after a spinal injury, weakens or stops breathing movements and changes the way the client can be fed.
The first task of the system is gas exchange, but it also helps to regulate blood acidity, defend the body, produce sound and detect smells. These tasks link directly to nutrition: the oxygen taken in is used to release energy from food, and the carbon dioxide released is a waste product of that process.
Oxygen from the air diffuses across the alveolar wall into the blood, where red blood cells carry it to the tissues. Carbon dioxide made by the cells travels back in the blood and diffuses into the alveoli to be breathed out. Inside the cells, oxygen is used to release energy (as ATP) from glucose and other nutrients. When a client has poor gas exchange, cells receive less oxygen and the client tires quickly, which lowers the amount of food the client can prepare, eat and digest. Dietitians therefore pay attention to the client's energy intake and set meal sizes the client can manage.
Carbon dioxide dissolves in blood and forms carbonic acid, so the level of carbon dioxide affects the acidity of the blood. When breathing is faster or deeper, more carbon dioxide leaves and the blood becomes less acidic. When breathing slows, carbon dioxide builds up and the blood becomes more acidic. The lungs work together with the kidneys and the blood buffers to keep blood pH within narrow limits. For example, a client with kidney disease may develop acid in the blood and breathe deeply and rapidly to blow off carbon dioxide and compensate.
The airways form a defence barrier. Hairs, mucus and cilia in the nose and trachea trap particles and microorganisms, and the cilia carry them out of the airway. Sneezing and coughing are protective reflexes that expel irritants forcefully. In the alveoli, macrophages engulf particles that reach the deepest part of the lung, and the lining secretes substances that help to fight infection. A client with a poor nutritional state has weaker defences, so respiratory infections are more frequent and more severe, and each infection increases energy needs and may reduce appetite. Good hygiene and adequate feeding therefore protect the lungs.
Phonation is the production of sound. Air breathed out passes through the larynx, where it makes the vocal cords vibrate. The tongue, lips, teeth and palate shape the sound into speech. Clear speech helps the dietitian to counsel, and a client with a weak voice or breathlessness may find a long counselling session tiring, so short sessions and written guidance are useful.
The sense of smell depends on receptors in the roof of the nasal cavity. Odours from food reach these receptors both through the nostrils and from the back of the throat while chewing. Smell therefore gives food much of its flavour and starts the flow of saliva and digestive juices. A client who has lost the sense of smell after an upper respiratory infection may lose interest in food. The dietitian can then advise on foods with distinct textures, temperatures, colour and taste to keep meals attractive.
Air breathed out is warm and saturated with water vapour. The lungs therefore lose a small but steady amount of water and heat with every breath. The loss rises when breathing is fast, as during fever or exercise, and it adds to the fluid that a client needs to take in, especially a feverish or breathless client who is also eating poorly.
The respiratory system is built in a way that suits its job. Each feature below helps either the exchange of gases or the protection and delivery of air.
The lungs contain hundreds of millions of alveoli. The total alveolar surface has been estimated at roughly 50 to 100 square metres, many times larger than the surface of the skin, and a very large area is available for oxygen and carbon dioxide to cross. This allows the body to take in enough oxygen even during hard physical work or when it is recovering from illness. When disease destroys alveoli or fills them with fluid, the surface area shrinks and the client becomes breathless on slight effort.
The wall between the air in an alveolus and the blood in a capillary is less than one micrometre thick. It is made up of the very flat alveolar cell, a thin basement membrane and the capillary wall. Gases cross a thin barrier quickly, so oxygen and carbon dioxide can move between air and blood in the short time that blood spends in the lung capillaries. Infection, fluid or scarring that thickens this membrane slows gas exchange.
A thin film of fluid covers the inner surface of each alveolus. Gases must dissolve in this fluid before they can cross the membrane, so a moist surface supports diffusion. A client who is dehydrated may have thicker mucus that is harder to clear, which is one reason fluid intake is checked in clients with chest conditions.
Surfactant is a mixture of fats and proteins made by the type II alveolar cells. It lowers the surface tension of the fluid film, which would otherwise pull the alveolar walls together and make the small air sacs collapse at the end of every breath. Surfactant therefore keeps the alveoli open and makes the lungs easy to inflate. A baby born too early may not yet have made enough surfactant because production matures late in pregnancy, and such a baby can develop breathing difficulty that needs specialised care and carefully planned feeding.
Each alveolus is surrounded by a dense network of capillaries. The large volume of blood passing close to the air sacs carries oxygen away and brings carbon dioxide for removal. Blood flow and airflow are matched, so well-ventilated alveoli receive plenty of blood.
Elastic fibres in the lungs recoil when the breathing muscles relax, which pushes air out without effort during quiet breathing. Cartilage rings and plates in the trachea and bronchi keep the larger airways open. A ciliated, mucus-secreting lining cleans and moistens the air, and the nasal conchae and blood-rich lining warm it before it reaches the delicate alveoli.
Chemoreceptors in the brainstem and in the large arteries monitor the carbon dioxide level, and to a smaller extent the oxygen level, in the blood, and nerve centres in the brainstem (the medulla oblongata and pons) change the rate and depth of breathing to match the body's demand. Breathing therefore becomes faster and deeper during exercise, fever or stress. When a client has a chronic lung condition, the energy spent on breathing is higher than normal, so the dietitian bases the client's energy requirements on this higher demand and not on the usual figures for a healthy person of the same size.
Describe the main structural components of the respiratory system and explain their roles in supporting respiratory function. (10 marks)
Explain five key functions of the respiratory system and their relevance to nutritional health. (10 marks)
Discuss five adaptations of the respiratory system that enhance its efficiency in gas exchange. (10 marks)
How does the respiratory system contribute to the regulation of blood pH, and why is this important in clinical nutrition? (5 marks)
In what ways can respiratory system impairments affect nutritional status in patients? Provide examples relevant to Kenyan healthcare settings. (10 marks)
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Create a free accountThis chapter provides a comprehensive overview of the respiratory system, detailing its structure and key components including the nose, trachea, bronchi, lungs, and diaphragm. It explains the primary functions of the respiratory system, such as facilitating gas exchange and maintaining oxygen supply to the body. The chapter also explores how the respiratory system is adapted to perform its functions efficiently, highlighting structural features that support breathing and oxygen transport. The process of respiration is described step-by-step, covering both external breathing and internal cellular respiration. Finally, common disorders affecting the respiratory system are discussed, emphasizing their impact on health and the importance of early detection and management. Overall, the chapter integrates anatomical knowledge with physiological processes to provide a clear understanding of how the respiratory system supports life.
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