The Respiratory System

Overview of the Respiratory System

• The respiratory system is a complex network of organs and tissues designed to facilitate breathing.
• Its primary components include the airways, lungs, and blood vessels.
• The muscles that power the lungs are also considered integral parts of this system.
• The system operates by moving oxygen ($O_2$) throughout the body while removing waste gases, specifically carbon dioxide ($CO_2$).
• The four main functions of the respiratory system are:
  • Gas exchange: Facilitating the intake of $O_2$ and removal of $CO_2$.
  • Acid-base balance: Maintaining physiological pH levels.
  • Phonation: The production of sound and speech.
  • Pulmonary defense and metabolism: Protecting the body against inhaled pathogens and participating in metabolic processes.

Major Organs and Structures

• The respiratory system is composed of several key structures:
  • Nose
  • Mouth
  • Pharynx (throat)
  • Larynx (voice box)
  • Trachea (windpipe)
  • Bronchi (large airways)
  • Bronchioles (small airways)
  • Lungs
• The system is divided into two distinct tracts:
  • Upper Respiratory Tract: Consists of organs located in the head and neck, ranging from the nose through the larynx.
  • Lower Respiratory Tract: Consists of organs located within the thorax, ranging from the trachea through the lungs.

Detailed Anatomy of the Nose and Nasal Cavity

• Functions of the Nose:
  • Warming, cleansing, and humidifying inhaled air.
  • Detecting odors via sensory cells.
  • Acting as a resonating chamber to amplify the voice.
• Nasal Cavity Structure:
  • The nasal cavity is an internal chamber that lies above the cribriform plate.
  • It is divided into right and left halves known as nasal fossae by the nasal septum.
  • The roof is composed of the ethmoid and sphenoid bones.
  • The floor is formed by the palate, which separates the nasal cavity from the oral cavity, allowing for simultaneous breathing and eating.
  • The nasal septum is composed of four structures, including the perpendicular plate of the ethmoid bone and the vomer (inferior part).
• Anatomical Components:
  • Vestibule: The chamber just inside the nostril, lined with stratified squamous epithelium and vibrissae (guard hairs) that block insects and debris.
  • External Nares: The nostrils which allow air to enter from the outside.
  • Cranial Bones: A total of $12$ cranial bones contribute to the nasal cavity structure, including the paired nasal bones, maxilla, palatine, lacrimal bones, ethmoid, sphenoid, frontal, and vomer.
  • Olfaction: Odors are detected in the olfactory mucosa, a small patch of epithelium covering the roof of the nasal fossa.
• Nasal Conchae (Turbinates):
  • Three folds of tissue located posterior to the vestibule: Superior, Middle, and Inferior.
  • These scroll-shaped bony elements increase surface area and create air turbulence in the meatus.
  • Turbulence ensures air contacts the mucous membranes for rapid warming, humidification, and dust filtration.

The Pharynx and Related Structures

• The pharynx is a hollow muscular tube that extending from the base of the skull to the level of the sixth cervical vertebra ($C6$). It is divided into three sections:
  • Nasopharynx: Located dorsal to the soft palate. It receives the Eustachian tubes from the middle ear and houses the pharyngeal tonsils. Air takes a $90$-degree turn here; large particles cannot make this turn due to inertia and collide with the mucosa near the tonsils.
  • Oropharynx: The space between the soft palate and the root of the tongue, behind the oral cavity. It contains the palatine and lingual tonsils.
  • Laryngopharynx: The union of the nasopharynx and oropharynx at the level of the hyoid bone, ending at the level of the cricoid cartilage and the beginning of the esophagus.
• Secondary Structures:
  • Uvula: A fleshy hanging ball at the back of the throat that prevents food and liquid from entering the nose during swallowing.
  • Tonsils: Lymph nodes that include the sublingual, pharyngeal, and two palatine tonsils.
  • Epiglottis: A flap of tissue beneath the tongue that closes over the trachea during eating to prevent food from entering the airway.

The Larynx and Trachea

• Larynx (Voice Box):
  • A hollow muscular organ that serves as an air passage and holds the vocal cords.
  • Primary function: Keeping food and drink out of the airway.
  • Secondary function: Sound production.
• Trachea (Windpipe):
  • A tube approximately $4$ inches long and less than $1$ inch in diameter.
  • Positioned anterior to the esophagus and runs behind the breastbone (sternum).
  • Supported by $16$ to $20$ C-shaped rings of hyaline cartilage to prevent collapse. The open part of the "C" faces posteriorly to allow esophageal expansion during swallowing.
  • Lined with pseudostratified ciliated columnar epithelium and mucus-producing cells.
  • Carina: The point at which the trachea divides into the right and left primary bronchi.

Anatomy of the Lungs and Bronchial Tree

• Gross Anatomy:
  • Lungs are spongy, air-filled organs located in the thoracic cavity on either side of the heart.
  • Apex: The superior portion located slightly above the clavicle.
  • Base: The inferior portion resting on the diaphragm.
  • Hilum: A concave region on the mediastinal surface where primary bronchi, blood vessels, lymphatic vessels, and nerves enter the lung.
• Lobe Structure:
  • Right Lung: Shorter than the left because the liver sits higher on the right. It has three lobes: Superior, Middle, and Inferior, separated by two fissures.
  • Left Lung: Includes a cardiac impression/notch where the heart sits. It has two lobes: Superior and Inferior.
• Pleurae: The lung surface is covered by the visceral pleura, while the chest wall is lined by the parietal pleura. The space between them is the pleural cavity, containing pleural fluid.
• Bronchial Tree:
  • A highly branched system of air tubes extending from the primary bronchi.
  • Branching levels: Primary bronchi → Secondary (lobar) bronchi → Tertiary (segmental) bronchi → Bronchioles.
  • There are approximately $65,000$ terminal bronchioles. Each terminal bronchiole divides into two or more respiratory bronchioles where alveoli begin to bud.

Alveoli and Gas Exchange

• Alveoli: Tiny air sacs at the end of the bronchial tree (approximately $600$ million in total). Collectively, their surface area is as large as a tennis court.
• Type I Alveolar Cells (Squamous Alveolar Cells): Thin cells that form the structure of the alveolar walls and enable rapid gas diffusion between air and blood.
• Gas Diffusion: Occurs between the alveoli and the network of capillaries surrounding them.
  • Blood enters the capillaries via the pulmonary artery and leaves via the pulmonary vein.
  • $O_2$ moves from the lungs into the bloodstream; $CO_2$ moves from the blood into the lungs.

Mechanics of Respiration

• Four Types of Respiration:
  • Pulmonary Ventilation: Movement of air into and out of the lungs (breathing).
  • External Respiration: Gas exchange between lungs and blood.
  • Transport: Movement of $O_2$ and $CO_2$ between lungs and tissues.
  • Internal Respiration: Diffusion of gases between systemic capillaries and tissue cells.
• Pulmonary Ventilation Phases:
  • Inspiration (Inhalation): Muscles contract; the diaphragm flattens and external intercostal muscles elevate the ribs, expanding the thoracic cavity to pull air in.
  • Expiration (Exhalation): Muscles relax, and air is pushed out of the lungs.
  • Rest: The normal adult respiration rate at rest is $12$ to $20$ breaths per minute.
• Lung Volumes:
  • Tidal Volume: The amount of air inhaled or exhaled during a normal respiratory cycle.
  • Vital Capacity ($VC$): The maximum amount of air a person can expel from the lungs after maximum inspiration. It is the sum of tidal volume, inspiratory reserve volume, and expiratory reserve volume, typically ranging from $3$ to $5$ liters.

Clinical Definitions and Conditions

• Epistaxis: A nosebleed, typically caused by trauma to the lower nasal septum or inferior nasal conchae.
• Deviated Septum: A condition where the nasal septum is off-center or leans to one side.
• Tonsillitis: Infection or inflammation of the tonsils caused by viruses or bacteria.
• Tonsil Stones (Tonsilloliths): Hard lumps of bacteria and debris in the tonsil nooks; often causes bad breath.
• Epiglottitis: A life-threatening condition where the epiglottis swells, potentially blocking the airway. It can be spread by saliva or mucus droplets.
• Tracheostomy: A procedure creating a temporary opening in the trachea inferior to the larynx to insert a tube for airway obstruction or long-term ventilation.
• Pneumonectomy: Surgical removal of an entire lung due to cancer or trauma.
• Diaphragm: The dome-shaped muscular partition separating the thoracic cavity from the abdominal cavity; it is the primary muscle of respiration.