RESPIRATORY SYSTEM: A TEACHER’S COMPREHENSIVE GUIDE
Welcome, future healthcare professionals!
The respiratory system forms the pathway through which air travels from the nose to the lungs. Its major function is the exchange of gases — taking in oxygen (O₂) and removing carbon dioxide (CO₂). Apart from this, it also filters, warms, and humidifies the inhaled air. The respiratory system also includes the vocal cords (for sound production), the lungs (which help maintain body pH), and the olfactory bulbs (for the sense of smell).
Dpharmguru’s exam insights:
In my years of teaching respiratory physiology, I have observed that students often confuse the conducting zone with the respiratory zone. Remember: The conducting zone (nose to bronchioles) moves air but does not exchange gases. The respiratory zone (alveolar ducts to alveoli) is where gas exchange occurs. This is a very common exam question!
EVENTS DURING RESPIRATION
- Ventilation of lungs – movement of air in and out.
- Exchange of gases – between blood and alveolar air.
- Excretion of water vapour.
- Supplying air to the larynx – for voice production.
PARTS OF THE RESPIRATORY SYSTEM
The human respiratory system includes the following parts:
- Nose
- Pharynx (throat)
- Larynx (voice box)
- Trachea (windpipe)
- Bronchi and bronchioles
- Lungs
- Alveoli
Based on Function
- Conducting Zone (Nose to Bronchioles): Provides a pathway for air to reach the alveoli. It also filters, warms, and moistens air while removing foreign particles.
- Respiratory Zone (Alveolar Duct to Alveoli): The site where actual gas exchange (O₂ and CO₂) occurs between alveoli and blood capillaries.
Based on Anatomy
- Upper Respiratory Tract: Includes organs outside the chest cavity – nose, pharynx, and larynx.
- Lower Respiratory Tract: Includes organs inside the chest cavity – trachea, bronchi, bronchioles, alveolar ducts, and alveoli.
NOSE
The nose is located between the forehead and upper lip. It is the first organ of the respiratory tract that receives inhaled air and passes it to the nasal cavity.
Anatomy of the Nose
The nose has two parts – external and internal (nasal cavities). The nasal cavity is an irregular space divided into two halves by a septum. The posterior part of the septum is bony (formed by the perpendicular plate of the ethmoid and vomer bones). The anterior part is cartilaginous (formed by hyaline cartilage). The bony framework is made up of the frontal, nasal, and maxilla bones. The external part is supported by septal cartilage, lateral cartilages, and alar cartilages (forming the nostrils).
Functions of the Nose
- Respiration: The main pathway for inhaled air.
- Air Conditioning: Incoming air is warmed to 31–37°C and fully humidified before reaching the lungs.
- Defence: The mucociliary system traps 95% of dust and microbes. IgA antibodies and enzymes in mucus destroy bacteria. Sneezing expels irritants.
- Voice Resonance: The nose adds tone to the voice. Paranasal sinuses act as sound resonators.
Dpharmguru’s exam insights:
Remember the three functions of the nose: Air conditioning (warming and humidifying), Defence (filtering and trapping particles), and Voice resonance. This is a common exam question!
PHARYNX
The pharynx (throat) is a funnel-shaped muscular tube that extends from the internal nares to the oesophagus and larynx. It has both respiratory and digestive functions.
Anatomy of Pharynx
- Nasopharynx: Lies behind the nasal cavity. Bounded above by the base of skull and below by the soft palate.
- Oropharynx: Lies behind the oral cavity. Contains tonsils and tonsillar pillars.
- Laryngopharynx (Hypopharynx): Lies below the oropharynx and above the oesophagus. Found at the level of 3rd–6th cervical vertebrae.
Functions of Pharynx
- Passage for both air and food.
- Assists in taste sensation.
- Warms and moistens inhaled air.
- Equalizes air pressure through the auditory tube for proper hearing.
- Protects against infection (tonsils act as guards).
- Helps in speech by resonating sound from the larynx.
LARYNX
The larynx, commonly known as the voice box, is a triangular chamber located in the upper front part of the neck. A prominent elevation called the Adam’s apple can be seen on its front surface. The wall of the larynx is made up of nine cartilages.
Cartilages of the Larynx
Single Cartilages:
- Thyroid Cartilage: Largest of all cartilages (larger in males). Forms the front and sides of the larynx. The Adam’s apple is part of this cartilage.
- Cricoid Cartilage: Ring-shaped cartilage situated below the thyroid cartilage. Narrow in the front and broader at the back.
- Epiglottis: A leaf-shaped cartilage located above the laryngeal inlet. Acts as a protective flap, closing the trachea during swallowing.
Paired Cartilages:
- Arytenoid Cartilages: Pyramid-shaped structures located on top of the cricoid cartilage. Help in movement of the vocal cords.
- Corniculate Cartilages: Small, conical nodules sitting on top of arytenoid cartilages.
- Cuneiform Cartilages: Small, rod-shaped cartilages providing support to the laryngeal wall.
Functions of Larynx
- Voice Production (Phonation): The vocal cords vibrate to produce sound.
- Protection of Airway: The epiglottis closes over the trachea during swallowing.
- Control of Airflow: The glottis widens during inhalation and narrows during exhalation.
- Cough and Sneezing Reflexes: The larynx helps in these reflexes to clear foreign particles.
Dpharmguru’s exam insights:
The epiglottis is a very important structure — it prevents food from entering the trachea during swallowing. A common exam question is: “Which cartilage acts as a flap to cover the trachea during swallowing?” The answer is the epiglottis.
TRACHEA
The trachea, or windpipe, is a 10–11 cm long tube extending downward from the larynx to the 5th thoracic vertebra, where it divides into the right and left bronchi at a ridge called the carina. It lies in front of the oesophagus and serves as the main airway to the lungs.
Functions of Trachea
- Prevents collapse of airway during movement of head and neck.
- Acts as a mucociliary escalator, where cilia move trapped dust and microbes upward.
- Warms, humidifies, and filters the inhaled air.
- Generates cough reflex when irritated.
- Main passage for air to and from the lungs.
BRONCHI AND BRONCHIOLES
The bronchi are the airways that branch from the trachea and carry air into the lungs, while the bronchioles are the smaller branches that lead to the alveoli. They do not participate in gas exchange but serve as conducting passages.
Functions of Bronchi and Bronchioles
- Provide a passage for air to reach the alveoli.
- Regulate air volume entering the lungs by changing diameter of airways.
- Warm and humidify inspired air.
- Filter out foreign particles.
- Trigger cough reflex when irritated.
LUNGS
The lungs are two soft, spongy, cone-shaped organs located in the thoracic cavity on either side of the heart. They are separated by the mediastinum and covered by a pleural membrane. The right lung is larger and shorter, while the left lung is smaller because of the heart’s position.
Functions of Lungs
- Exchange of gases (O₂ intake, CO₂ removal).
- Regulation of blood pH by controlling CO₂ levels.
- Filtering of small blood clots and air bubbles.
- Activation of hormones – conversion of angiotensin I to angiotensin II.
- Provide immunological defence via IgA secretion.
- Act as a protective cushion for the heart.
ALVEOLI
The alveoli are tiny, balloon-like air sacs in the lungs where gas exchange takes place. Each lung contains about 300 million alveoli, giving a large surface area for oxygen and carbon dioxide diffusion.
Cells of Alveoli
- Type I (Squamous) cells: Thin, flat cells for rapid gas exchange.
- Type II (Great) cells: Secrete surfactant that reduces surface tension and prevents alveolar collapse.
- Alveolar macrophages (Dust cells): Engulf dust, debris, and microbes by phagocytosis.
Dpharmguru’s exam insights:
Type II alveolar cells secrete surfactant, which reduces surface tension and prevents the alveoli from collapsing. A common exam question is: “What is the function of surfactant?” The answer is to reduce surface tension and prevent alveolar collapse. This is especially important in premature babies!
MECHANISM OF RESPIRATION
Respiration is the process by which air moves in and out of the lungs, allowing oxygen to enter the body and carbon dioxide to be removed. The entire breathing process has two main stages: Inhalation (Inspiration) and Exhalation (Expiration).
Inhalation (Inspiration)
- The diaphragm contracts and flattens downward.
- The external intercostal muscles contract, pulling the rib cage upward and outward.
- The lungs expand, and intrapulmonary pressure drops below atmospheric pressure.
- Air flows from outside into the lungs.
Exhalation (Expiration)
- The diaphragm and external intercostal muscles relax.
- The thoracic and lung volumes decrease.
- Intrapulmonary pressure rises above atmospheric pressure.
- Air is pushed out of the lungs.
Regulation of Respiration
The act of breathing is controlled by the respiratory centre located in the medulla oblongata and pons of the brainstem. These centres control the rate and depth of breathing and respond to changes in carbon dioxide concentration, oxygen levels, and blood pH.
RESPIRATORY VOLUMES AND CAPACITIES
| Parameter | Definition | Formula | Average (mL) |
|---|---|---|---|
| Tidal Volume (TV) | Normal breath | – | 500 |
| IRV | Extra inspired air | – | 2500–3000 |
| ERV | Extra expired air | – | 1000–1100 |
| RV | Air left after forceful expiration | – | 1100–1200 |
| IC | Max air inspired after normal expiration | TV + IRV | 3500 |
| EC | Max air expired after normal inspiration | TV + ERV | 1600 |
| FRC | Air left after normal expiration | ERV + RV | 2300 |
| VC | Max air exhaled after deep inhalation | IRV + TV + ERV | 4600 |
| TLC | Total lung capacity | VC + RV | 5800 |
Dpharmguru’s exam insights:
The respiratory system is essential for gas exchange and maintaining acid-base balance. In exams, focus on the anatomy of the respiratory tract, the mechanism of breathing, and the lung volumes and capacities. Remember: Tidal Volume is the normal breath, Vital Capacity is the maximum air you can exhale after a deep breath, and Residual Volume is the air that always remains in the lungs. These concepts are almost guaranteed to appear in exams!
REFERENCES AND FURTHER READING
- Tortora, G. J., & Derrickson, B. H. (2017). Principles of Anatomy and Physiology (15th ed.). John Wiley & Sons.
- Marieb, E. N., & Hoehn, K. (2019). Human Anatomy & Physiology (11th ed.). Pearson Education.
- Standring, S. (2020). Gray’s Anatomy: The Anatomical Basis of Clinical Practice (42nd ed.). Elsevier.
- Hall, J. E., & Guyton, A. C. (2020). Guyton and Hall Textbook of Medical Physiology (14th ed.). Elsevier.
- National Institutes of Health (NIH). (2022). Respiratory System Resources. Retrieved from https://www.nih.gov.
Disclaimer: This article is for educational purposes only and does not constitute medical advice. Always consult qualified healthcare professionals for medical concerns.
written by:
Dr. N. Sujith Kumar
