Introduction
- The respiratory system is responsible for gas exchange, enabling the body to take in oxygen for metabolic processes and expel carbon dioxide, a by-product of metabolism.
- It is crucial for maintaining homeostasis, acid-base balance, and energy production.
- The respiratory system works closely with the cardiovascular system to ensure efficient oxygen delivery and carbon dioxide removal.
Parts of the Respiratory System
The respiratory system can be divided into two main parts: structurally and functionally.
Structural Division
Upper Respiratory Tract
a. Nose and Nasal Cavity
- Structure: The external nose has a cartilaginous framework, while the nasal septum divides the nasal cavity. It contains nasal conchae (turbinates) and is lined by respiratory and olfactory epithelium.
- Functions:
- Filters air using nasal hairs and mucus.
- Warms and humidifies incoming air to prevent drying of lower airways.
- Contains olfactory receptors for the sense of smell.
- Acts as a resonating chamber for speech.
b. Pharynx
- Structure: A muscular tube that is divided into three parts:
- Nasopharynx (behind the nasal cavity)
- Oropharynx (behind the oral cavity)
- Laryngopharynx (opens into the larynx and esophagus)
- Functions:
- Serves as a common pathway for air and food.
- The nasopharynx contains the adenoids (pharyngeal tonsils) for immune defense.
- Assists in equalizing pressure between the middle ear and the atmosphere via the Eustachian tube.
c. Larynx (Voice Box)
- Structure: A cartilaginous structure containing the vocal cords and between the pharynx and trachea.
- Major cartilages: Thyroid (Adam’s apple), cricoid, and epiglottis.
- Functions:
- Protects the airway during swallowing (via the epiglottis, which prevents food from entering the trachea).
- Produces sound by vibrating the vocal cords during exhalation.
- Allows the passage of air between the pharynx and trachea.
Lower Respiratory Tract
a. Trachea (Windpipe)
- Structure: A cylindrical tube supported by C-shaped cartilage rings lined with ciliated pseudostratified columnar epithelium.
- Functions:
- Conducts air to the bronchi.
- Cilia and mucus trap dust, pathogens, and particles, preventing them from reaching the lungs (mucociliary escalator mechanism).
b. Bronchi
- Structure: The trachea divides into the right and left primary bronchi, which enter each lung. They further branch into secondary (lobar) and tertiary (segmental) bronchi.
- Functions:
- Serve as airways that direct air into the lungs.
- The branching structure ensures the distribution of air to all lung segments.
c. Bronchioles
- Structure: Smaller airways lacking cartilage but lined with smooth muscle and epithelium. Terminal bronchioles lead to respiratory bronchioles.
- Functions:
- Control airflow and resistance within the lungs via smooth muscle contraction and relaxation.
- Serve as transitional passages for air moving toward the alveoli.
d. Alveoli
- Structure: Tiny, sac-like structures surrounded by a dense capillary network. Two types of cells line the alveoli:
- Type I alveolar cells: Thin, flat cells that allow gas exchange.
- Type II alveolar cells: Produce surfactant, which reduces surface tension and prevents alveolar collapse.
- Functions:
- The primary site for gas exchange (oxygen diffuses into the blood, and carbon dioxide diffuses out).
- Surfactant maintains alveolar stability, especially during exhalation.
e. Lungs
- Structure: A pair of spongy organs divided into lobes (3 on the right, 2 on the left). Encased by a pleural membrane, the lungs have a large surface area to maximize gas exchange.
- Functions:
- Facilitate oxygen absorption and carbon dioxide removal.
- Regulate blood pH by altering CO₂ levels through ventilation.
Other Supporting Structures
Diaphragm
- Structure: A dome-shaped skeletal muscle that separates the thoracic and abdominal cavities.
- Functions:
- The primary muscle of respiration. Contracts during inhalation create negative pressure that draws air into the lungs.
Intercostal Muscles
- Structure: Located between the ribs, divided into external and internal intercostal muscles.
- Functions:
- Assist in breathing by elevating (external) or depressing (internal) the ribcage during inhalation and exhalation.
Pleura
-
- Structure: A double-layered membrane surrounding the lungs. The parietal pleura lines the chest wall, while the visceral pleura covers the lungs. The pleural cavity contains a thin layer of fluid.
- Functions:
- Reduces friction during lung expansion.
- Maintains lung expansion by creating negative pressure.
Functional Division
- Conducting Zone: Includes the nose, pharynx, larynx, trachea, bronchi, and bronchioles. Its function is to filter, warm, and transport air.
- Respiratory Zone: Includes respiratory bronchioles, alveolar ducts, and alveoli. This is where gas exchange occurs.
Functions of the Respiratory System
- Gas Exchange
- Oxygen is transported into the blood while carbon dioxide is removed. This exchange occurs in the alveoli.
- Regulation of Blood pH
- Maintains acid-base balance by regulating carbon dioxide levels (via bicarbonate buffer system).
- Vocalization
- The larynx produces sound for speech.
- Protection
- Mucus, cilia, and immune cells protect against pathogens and particulate matter.
- Olfaction
- The nasal cavity houses receptors for the sense of smell.
- Heat and Water Exchange
- Air is warmed and humidified during inhalation, which helps regulate body temperature.
Clinical Aspects
- Common Disorders
- Asthma: Chronic inflammation and airways narrowing, causing breathing difficulty.
- Chronic Obstructive Pulmonary Disease (COPD): Includes emphysema and chronic bronchitis, reducing airflow and gas exchange.
- Pneumonia: Infection causing inflammation of the alveoli.
- Tuberculosis (TB): Bacterial infection affecting the lungs caused by Mycobacterium tuberculosis.
- Lung Cancer: Malignant growths in lung tissues.
- Pulmonary Embolism: A blockage in the pulmonary artery, often caused by a blood clot.
- Diagnostic Tests
- Spirometry: Measures lung capacity and airflow.
- Chest X-ray: Detects abnormalities like infections or tumors.
- Arterial Blood Gas (ABG): Assesses oxygen and carbon dioxide levels in the blood.
- Surgical and Medical Interventions
- Intubation and Mechanical Ventilation: Support for patients with respiratory failure.
- Bronchoscopy: Visual examination of the airways.
- Lung Transplant: For end-stage lung disease.
- Impact of Lifestyle
- Smoking, pollution, and occupational hazards significantly increase the risk of respiratory diseases.
- Emerging Conditions
- Post-COVID-19 syndrome has highlighted long-term respiratory issues, including fibrosis and reduced lung capacity.