Respiratory Anatomy

The respiratory system exists to do one essential thing: get oxygen into the blood and carbon dioxide out. Every cell in your body needs a constant supply of oxygen to generate energy — even a few minutes without it causes irreversible damage. The system is divided into two parts: Upper respiratory tract — the nose, nasal cavity, pharynx (throat), and larynx (voice box). These structures warm, humidify, and filter incoming air before it reaches the lungs. Lower respiratory tract — the trachea (windpipe), bronchi, bronchioles, and alveoli. These structures conduct air to the lungs and perform gas exchange. The fundamental process — moving air in and out — is called ventilation. The actual exchange of gases between air and blood is called respiration (or more specifically, gas exchange). The two terms are often used interchangeably in everyday speech but mean different things to clinicians.

The nose and nasal cavity Air normally enters through the nose, which is far better than the mouth for conditioning air. The nasal cavity has three functions: 1. Filtering — coarse nasal hairs (vibrissae) trap large particles. The mucous membrane lining traps finer particles. 2. Warming — the richly vascular nasal mucosa warms air to body temperature before it reaches the lungs. 3. Humidifying — the mucous membrane adds moisture to dry air. The nasal cavity drains into the paranasal sinuses — air-filled spaces in the skull bones around the nose. These lighten the skull, add resonance to the voice, and produce mucus. They drain into the nasal cavity, which is why sinus infections cause a runny nose. The pharynx (throat) A muscular tube behind the nasal and oral cavities. It is a shared passageway for both air and food — which creates a choking hazard. The pharynx has three parts: - Nasopharynx — behind the nose, conducts air only. Contains the adenoids and the openings to the Eustachian tubes (connecting to the middle ear — why throat infections can cause ear pain). - Oropharynx — behind the mouth, conducts both air and food. Contains the tonsils. - Laryngopharynx — the lowest part, where the airways and food passage finally separate. The larynx (voice box) A cartilaginous structure that sits on top of the trachea. It has three key roles: 1. Conducting air into the trachea 2. Voice production — the vocal cords (vocal folds) vibrate as air passes over them 3. Protecting the airway — the epiglottis is a flap of cartilage that folds down over the larynx during swallowing, preventing food from entering the trachea. If it fails — you choke. The glottis is the opening between the vocal cords. During normal breathing it is open. During swallowing it closes, and the epiglottis folds down for double protection.

The trachea (windpipe) A 10–12 cm tube made of 15–20 C-shaped rings of cartilage. The cartilage keeps the airway permanently open so it never collapses when you breathe in (unlike flexible tubes, which would be sucked shut by negative pressure). The open back of each C-ring faces the oesophagus — allowing the oesophagus to bulge slightly when swallowing. The inside of the trachea is lined with pseudostratified ciliated columnar epithelium with goblet cells. Goblet cells secrete mucus that traps particles and pathogens. Cilia beat constantly in a coordinated upward wave — the mucociliary escalator — moving the mucus up towards the throat where it is swallowed. Cigarette smoke paralyses the cilia, destroying this defence mechanism. The bronchial tree At the level of the 4th–5th thoracic vertebra, the trachea divides into the right and left main bronchi (singular: bronchus) at a ridge called the carina. The carina is highly sensitive to touch — stimulating it triggers a powerful cough reflex. The right main bronchus is wider, shorter, and more vertical than the left. This matters clinically: inhaled foreign objects (coins, peanuts, teeth) tend to fall into the right bronchus. The bronchi enter the lungs and continue to divide — like an upside-down tree: Main bronchi → Lobar bronchi (one per lobe) → Segmental bronchi → Bronchioles → Terminal bronchioles → Respiratory bronchioles → Alveolar ducts → Alveoli As the airways get smaller, cartilage disappears (bronchioles have no cartilage) and smooth muscle increases. This is why bronchospasm — contraction of smooth muscle in asthma — narrows the airways and causes breathing difficulty.

Structure of the lungs The lungs are the main organs of the respiratory system — two large, spongy organs in the thoracic (chest) cavity. They are separated by the mediastinum (the central compartment containing the heart, great vessels, trachea, and oesophagus). The right lung is larger, with three lobes (upper, middle, lower). The left lung has only two lobes (upper, lower) — the space normally occupied by a middle lobe is taken up by the cardiac notch, an indentation that accommodates the heart. Each lung is surrounded by a double-layered membrane called the pleura: - Visceral pleura — tightly covers the lung surface - Parietal pleura — lines the inner chest wall Between the two layers is the pleural space (pleural cavity) — a potential space containing a thin film of fluid that lubricates the lungs as they slide against the chest wall during breathing. Normally this space contains no air. Air in the pleural space (pneumothorax) collapses the lung. Fluid in the pleural space (pleural effusion) compresses the lung. Alveoli: The gas exchange units Alveoli (singular: alveolus) are tiny, thin-walled, grape-like sacs at the ends of the respiratory bronchioles. There are approximately 300–500 million alveoli in the two lungs, giving a total surface area of roughly 70 square metres — about the size of a tennis court. This enormous surface area is essential for efficient gas exchange. The alveolar wall is just one cell thick — made of two cell types: - Type I pneumocytes — flat cells forming the vast majority of the wall surface. So thin that gases diffuse across them in milliseconds. - Type II pneumocytes — cube-shaped cells that produce surfactant — a mixture of lipids and proteins that reduces surface tension in alveoli, preventing them from collapsing when you breathe out. Premature babies lack surfactant — causing respiratory distress syndrome (RDS), a life-threatening condition. Gas exchange: oxygen diffuses from the alveolar air (high concentration) across the alveolar wall and into blood in the surrounding capillaries. Carbon dioxide diffuses in the opposite direction — from the blood (high concentration) into the alveoli to be exhaled.

Understanding respiratory anatomy makes clinical medicine make sense. Pneumonia — infection in the alveoli. Bacteria or viruses invade the alveoli, causing inflammation and filling them with fluid and pus. Gas exchange fails in the affected area. On a chest X-ray, infected areas appear as white patches (consolidation) because fluid replaces air. Asthma — inflammation and bronchospasm in the small airways (bronchioles). The smooth muscle in their walls contracts, narrowing the lumen and making it hard to breathe. The classic expiratory wheeze occurs because air can enter but struggles to escape through the narrowed tubes. COPD (Chronic Obstructive Pulmonary Disease) — destruction of alveoli (emphysema) and narrowing of small airways (chronic bronchitis), almost always caused by smoking. Loss of alveoli = loss of surface area = reduced gas exchange. The chest becomes barrel-shaped as patients use accessory muscles and trap air to maintain airway patency. Pulmonary embolism (PE) — a blood clot (usually from a deep vein thrombosis in the leg) travels to the pulmonary arteries and blocks blood flow to part of the lung. The alveoli in that area ventilate normally but receive no blood — a ventilation-perfusion (V/Q) mismatch. Presents with sudden breathlessness, chest pain, and haemoptysis (coughing blood). Intubation — in emergencies, a tube is inserted through the mouth into the trachea (endotracheal intubation) to maintain the airway. Knowledge of the anatomy — the position of the cords, the angle of the trachea, the carina — is essential.