Review of the Respiratory System

25.1 General Organization and Functions of the Respiratory System

• The respiratory system is essential for gas exchange, primarily facilitating the uptake of oxygen (O₂) from the atmosphere and expelling carbon dioxide (CO₂), a metabolic waste product.

• Composed of two main parts:

  • Upper Respiratory Tract: Includes the nose, nasal cavity, pharynx, and larynx—functions to filter, warm, and humidify incoming air before it reaches the lungs.

  • Lower Respiratory Tract: Comprises the trachea, bronchi, bronchioles, and lungs—where the actual gas exchange occurs.

• Functionally divided into:

  • Conducting Portion: Transports air from the external environment into the lungs without participating in gas exchange.

  • Respiratory Portion: Where the critical gas exchange occurs, consisting of the respiratory bronchioles and alveoli, where oxygen enters the blood, and carbon dioxide is removed.

25.1a Respiratory System Functions

1. Breathing (Pulmonary Ventilation):

   • Inhalation (inspiration): The diaphragm and intercostal muscles contract, expanding the thoracic cavity and allowing air to flow into the lungs through negative pressure.

   • Exhalation (expiration): Relaxation of these muscles reduces the thoracic volume, pushing air out of the lungs.

2. Gas Exchange:

   • External Respiration: The exchange of O₂ and CO₂ occurs in the alveoli—O₂ diffuses from alveoli into the blood, while CO₂ diffuses from blood into the alveoli to be exhaled.

   • Internal Respiration: Gas exchange occurs between blood and body cells—oxygenated blood delivers O₂ to tissues, and in exchange, CO₂ produced by cellular respiration enters the bloodstream.

3. Additional Functions:

   • Gas Conditioning: Air inhaled through the nose is warmed, humidified, and cleansed as it passes through the nasal cavity, which is lined with mucous membranes and rich blood supply.

     • Turbulent airflow created by nasal conchae maximizes contact with the mucosal surface for efficient conditioning.

   • Sound Production: The larynx, also known as the voice box, is instrumental in producing sound through the vibration of vocal cords as air passes through.

   • Olfaction: The sense of smell occurs via the olfactory epithelium, located in the nasal cavity, which detects airborne chemicals and sends signals to the brain.

   • Defense Mechanisms: The respiratory system traps inhaled particles and pathogens using mucus, while cilia on respiratory epithelium move mucus upwards toward the throat to be swallowed or expelled.

25.2 Upper Respiratory Tract

25.2a Nose and Nasal Cavity

• Nose: Serves as the main conducting airway, supported by bony structures (nasal bones) and flexible cartilage, ensuring the passage remains open.

• Nasal Cavity:

  • Divided by a nasal septum into left and right segments.

  • Contains nasal conchae that enhance airflow turbulence, promoting better air conditioning and filtration.

25.2b Paranasal Sinuses

• Located within the frontal, ethmoidal, sphenoidal, and maxillary bones, these air-filled spaces decrease the overall weight of the skull.

• Functions include:

  • Decreasing Skull Weight: Makes the head lighter without compromising strength.

  • Warming and Humidifying Air: Prepares air before it enters the lungs.

  • Contributing to Voice Resonance: Enhances the quality of sound produced during speech.

25.2c Pharynx

• The pharynx shares its space with the digestive system, acting as a passageway for both air and food.

• Regions:

  • Nasopharynx: Serves primarily as an air conduit and contains openings for auditory tubes that connect to the middle ear, helping to equalize pressure.

  • Oropharynx: Functions as a pathway for both food and air, located behind the oral cavity.

  • Laryngopharynx: The terminal portion, connecting to both the larynx for air and the esophagus for food intake.

25.2d Larynx

• Known as the voice box, the larynx is crucial for conducting air and producing sound.

• Contains various cartilages and intrinsic muscles that adjust tension on the vocal cords, allowing for the modulation of pitch and volume.

25.3 Lower Respiratory Tract

25.3a Trachea

• The trachea is a flexibly rigid tube that connects the larynx to the bronchi, reinforced by C-shaped cartilage rings that prevent collapse.

• It is lined with ciliated epithelium that traps and clears inhaled particles and pathogens, moving them upwards toward the pharynx.

• The trachea bifurcates into left and right main bronchi, directing air toward each lung.

25.3b Bronchial Tree

• Composed of a system of air passageways branching from the main bronchi:

  • Lobar Bronchi: Divide into segmental bronchi that lead to specific lung segments.

  • Bronchioles: Smaller branches from segmental bronchi that lead into the alveolar structures.

25.3c Respiratory Bronchioles, Alveolar Ducts, and Alveoli

• Terminal bronchioles branch into respiratory bronchioles, leading to the alveolar ducts and surrounding alveolar sacs.

• Alveoli:

  • The primary sites for gas exchange, characterized by their thin walls for efficient oxygen and carbon dioxide diffusion.

  • Type I cells: Form the alveolar walls and facilitate gas exchange.

  • Type II cells: Secrete surfactant, a substance that reduces surface tension and prevents alveolar collapse during exhalation.

25.4 Lungs

25.4a Pleura and Pleural Cavities

• The visceral pleura directly covers the lungs, while the parietal pleura lines the thoracic cavity—a double-layered membrane that allows for smooth movement during breathing.

• The pleural cavity between the two layers contains serous fluid that provides lubrication and aids in maintaining negative pressure for lung expansion.

25.4b Gross Anatomy of the Lungs

• The right lung consists of three lobes (superior, middle, inferior) while the left lung has two lobes (superior and inferior) and features a cardiac notch to accommodate the heart.

• Each lung is divided into broncho-pulmonary segments, which allows for the isolated removal of diseased portions without affecting overall lung function.

25.4c Blood Supply to and from the Lungs

• Pulmonary Circulation: Carries deoxygenated blood from the heart to the lungs where gas exchange occurs and brings oxygenated blood back to the heart.

• Bronchial Circulation: Supplies blood to the bronchi and bronchioles, ensuring that the lung tissues receive adequate oxygen and nutrition.

25.4d Lymphatic Drainage

• Lymph nodes within the lungs filter out particles and pollutants not intercepted by mucus, playing a vital role in the immune response.

• The lymphatic system drains through a network of nodes before the lymph enters the general circulation, contributing to fluid balance and immune defense in the body.