7.1 Structure and function of the respiratory system

The respiratory system is a complex network of organs and structures that work together to facilitate breathing and gas exchange. From the nose to the alveoli, each part plays a crucial role in ensuring oxygen reaches our cells and carbon dioxide is expelled.

Understanding the anatomy and function of the respiratory system is key to grasping how we breathe. This knowledge forms the foundation for exploring respiratory physiology, diseases, and treatments in the broader context of human health and medicine.

Respiratory System Anatomy

Nasal Cavity and Pharynx

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• The nose and nasal cavity warm, humidify, and filter inhaled air, and contain olfactory receptors for the sense of smell
• The pharynx is a muscular tube that connects the nasal and oral cavities to the larynx and esophagus, serving as a passageway for both air and food
  • Nasopharynx: upper portion connected to the nasal cavity
  • Oropharynx: middle portion connected to the oral cavity
  • Laryngopharynx: lower portion connected to the larynx and esophagus

Larynx, Trachea, and Bronchi

• The larynx, or voice box, contains the vocal cords and connects the pharynx to the trachea, preventing food and liquid from entering the lower respiratory tract
  • Epiglottis: a flap of cartilage that covers the opening of the larynx during swallowing
  • Vocal cords: two folds of mucous membrane that vibrate to produce sound
• The trachea, or windpipe, is a flexible tube reinforced with C-shaped cartilage rings that connects the larynx to the bronchi and allows for the passage of air
  • Approximately 12 cm long and 2.5 cm in diameter
  • Lined with pseudostratified ciliated columnar epithelium and goblet cells
• The bronchi are two main branches of the trachea that lead into the lungs, where they divide into smaller bronchioles and terminal bronchioles
  • Right main bronchus is wider, shorter, and more vertical than the left main bronchus

Lungs and Pleura

• The lungs are paired, spongy organs divided into lobes (three in the right lung, two in the left) that house the bronchi, bronchioles, and alveoli for gas exchange
  • Right lung: upper, middle, and lower lobes
  • Left lung: upper and lower lobes, with a cardiac notch to accommodate the heart
• The pleura is a double-layered serous membrane that surrounds each lung, with the parietal pleura lining the thoracic cavity and the visceral pleura covering the lungs
  • Pleural fluid: a thin layer of fluid between the parietal and visceral pleura that allows the lungs to move smoothly during breathing

Pulmonary Ventilation Mechanics

Inspiration and Expiration

• Pulmonary ventilation is the process of moving air into and out of the lungs, consisting of inspiration (inhalation) and expiration (exhalation)
• During inspiration, the diaphragm and external intercostal muscles contract, increasing the volume of the thoracic cavity and decreasing the pressure, causing air to flow into the lungs
  • Boyle's Law: as volume increases, pressure decreases ($P_1V_1 = P_2V_2$)
• During expiration, the diaphragm and external intercostal muscles relax, decreasing the volume of the thoracic cavity and increasing the pressure, causing air to flow out of the lungs
  • Passive process during quiet breathing, as elastic recoil of the lungs and chest wall forces air out

Respiratory Muscles

• The diaphragm, a dome-shaped muscle located beneath the lungs, is the primary muscle of respiration, responsible for about 75% of the change in thoracic volume during normal breathing
  • Innervated by the phrenic nerve (C3-C5)
  • Contracts and flattens during inspiration, increasing vertical dimension of the thoracic cavity
• The external intercostal muscles, located between the ribs, assist in inspiration by elevating the ribs and sternum, further increasing thoracic volume
  • Innervated by intercostal nerves
  • Contraction causes the ribs to move upward and outward, increasing the anterior-posterior and transverse dimensions of the thoracic cavity
• During forced expiration, the internal intercostal muscles and abdominal muscles contract, pushing the diaphragm upward and forcing air out of the lungs more rapidly
  • Internal intercostal muscles: pull the ribs downward and inward
  • Abdominal muscles (rectus abdominis, external and internal obliques, transversus abdominis): compress the abdominal contents, pushing the diaphragm upward

Alveoli Structure and Function

Alveolar Anatomy

• Alveoli are tiny, thin-walled, sac-like structures clustered at the ends of terminal bronchioles, providing a large surface area for gas exchange between the lungs and the bloodstream
  • Approximately 300 million alveoli in the adult lungs
  • Total surface area of 70-80 square meters, about the size of a tennis court
• Each alveolus is surrounded by a network of pulmonary capillaries, allowing for the diffusion of gases between the alveolar air and the blood
  • Close proximity of alveolar air and blood (0.2-0.5 μm) facilitates rapid gas exchange
• The alveolar wall consists of a thin layer of epithelial cells (type I and type II pneumocytes) and a basement membrane, minimizing the distance for gas diffusion
  • Type I pneumocytes: squamous epithelial cells that form the main structure of the alveolar wall, providing a thin, permeable barrier for gas exchange
  • Type II pneumocytes: cuboidal epithelial cells that secrete pulmonary surfactant and serve as progenitor cells for type I pneumocytes

Gas Exchange and Surfactant

• Oxygen diffuses from the alveolar air into the blood, while carbon dioxide diffuses from the blood into the alveolar air, driven by concentration gradients across the alveolar-capillary membrane
  • Fick's Law of Diffusion: rate of diffusion is proportional to the surface area and concentration gradient, and inversely proportional to the thickness of the membrane
• Type II pneumocytes secrete pulmonary surfactant, a mixture of phospholipids and proteins that reduces surface tension in the alveoli, preventing collapse during expiration
  • Dipalmitoylphosphatidylcholine (DPPC): the main component of surfactant, reduces surface tension by forming a monolayer at the air-liquid interface
  • Surfactant proteins (SP-A, SP-B, SP-C, SP-D): facilitate the spreading and stability of the surfactant layer, and contribute to innate immunity in the lungs

Upper vs Lower Respiratory Tracts

Upper Respiratory Tract

• The upper respiratory tract includes the nose, nasal cavity, pharynx, and larynx, and is responsible for filtering, humidifying, and warming inhaled air before it reaches the lower respiratory tract
• The nose and nasal cavity contain hair and mucus-producing goblet cells that trap inhaled particles and pathogens
  • Nasal conchae: scroll-like bony structures that increase the surface area for warming and humidifying air
  • Olfactory epithelium: specialized sensory epithelium in the upper nasal cavity responsible for the sense of smell
• The pharynx and larynx act as a passageway for both air and food, with the epiglottis preventing aspiration during swallowing
• The upper respiratory tract serves as a first line of defense against inhaled particles and pathogens, trapping them in the mucus produced by goblet cells and removing them via the mucociliary escalator
  • Mucociliary escalator: the coordinated beating of cilia on the surface of respiratory epithelial cells that propels mucus and trapped particles toward the pharynx to be swallowed or expectorated

Lower Respiratory Tract

• The lower respiratory tract consists of the trachea, bronchi, bronchioles, and alveoli, and is primarily responsible for the conduction of air and gas exchange
• The trachea and bronchi act as conducting airways to transport air to and from the alveoli
  • Pseudostratified ciliated columnar epithelium lines the trachea and bronchi, with goblet cells secreting mucus to trap particles
  • C-shaped cartilage rings in the trachea and plates in the bronchi provide structural support and prevent collapse during breathing
• The bronchioles are smaller, more numerous airways that lead to the alveoli
  • Smooth muscle in the walls of bronchioles allows for changes in airway diameter, regulating airflow and distribution
  • Respiratory bronchioles contain a small number of alveoli, marking the beginning of the respiratory zone
• The alveoli are the primary site of gas exchange in the lower respiratory tract
• The lower respiratory tract is sterile under normal conditions, with the trachea and bronchi acting as conducting airways to transport air to and from the alveoli, where gas exchange occurs
• The transition from the upper to the lower respiratory tract occurs at the larynx, which acts as a valve to prevent food and liquid from entering the trachea and lungs during swallowing