Human Respiratory System
The human respiratory system is designed for two jobs:
- Conduct air to lungs (clean, warm, moisten it)
- Exchange gases (O₂ and CO₂) at the alveoli
Two Functional Divisions
1. Conducting Part:
External nostrils → nasal cavity → pharynx → larynx → trachea → bronchi → bronchioles → terminal bronchioles
- Functions:
- Conduction: carries air to exchange site
- Filtration: removes dust (mucus + cilia)
- Humidification: adds moisture
- Warming: brings air to body temperature
2. Exchange Part:
Respiratory bronchioles + alveolar ducts + alveoli
- Function: Diffusion of O₂ into blood and CO₂ out of blood
Path of Air
Nostrils → Nasal chamber → Pharynx → Larynx → Trachea → Bronchi → Bronchioles → Alveoli
Mnemonic: N N P L T B B A (Nose–Nasal–Pharynx–Larynx–Trachea–Bronchi–Bronchioles–Alveoli)
Parts of the Human Respiratory System
1. Nasal Cavity
- Air is filtered by hairs, mucus and cilia
- Air is warmed + moistened
Key point: Nasal passage improves air quality before it reaches lungs.
2. Pharynx
- Common passage for food and air
3. Larynx (Sound Box)
- Cartilaginous structure containing vocal cords → sound production
- Opening is called glottis
- Epiglottis (elastic cartilage flap) covers glottis during swallowing → prevents food entry into trachea
Key point: Epiglottis = safety valve for airway.
4. Trachea (Windpipe)
- Extends to mid-thoracic cavity
- Divides into right & left primary bronchi
- Supported by incomplete C-shaped cartilaginous rings
Key point: C-shaped rings prevent collapse of airway during inspiration.
Trachea divides at 5th thoracic vertebra (T5).
5. Bronchi → Bronchioles (Bronchial Tree)
- Primary bronchi branch into secondary & tertiary bronchi
- Further branch into bronchioles
- Bronchioles are thinner and have less/no cartilage as they become finer
6. Alveoli (Exchange Units)
Terminal parts end in alveolar sacs made of alveoli.
Why alveoli are perfect for gas exchange?
- Huge surface area (many alveoli)
- Very thin wall (simple squamous epithelium)
- Moist lining (gases dissolve easily)
- Rich capillary network (fast transport)
Key point: Alveoli follow the diffusion rule: Thin + Moist + Large + Blood supply.
Lungs and Pleura
- Lungs are paired, spongy organs in thoracic cavity
- Covered by double-layered pleura
- Outer pleura: near thoracic lining
- Inner pleura: on lung surface
- Pleural fluid in between reduces friction and helps smooth lung movement
Key point: Pleura + pleural fluid = friction-free breathing.
Thoracic Chamber
The lungs sit in an air-tight thoracic chamber:
- Dorsal: vertebral column
- Ventral: sternum
- Lateral: ribs
- Lower: dome-shaped diaphragm
Why is it important?
- We cannot directly expand lungs like a balloon by hand.
- Instead, we change thoracic cavity volume, and lungs follow because they are tightly associated.
Key point: Change in thoracic volume → change in lung volume → air movement.
Important Notes
- Right lung: 3 lobes
- Left lung: 2 lobes (space for heart)
- Respiratory membrane is extremely thin → diffusion is easy
Human Respiratory System
Air Path: Nostrils → Nasal cavity → Pharynx → Larynx → Trachea → Bronchi → Bronchioles → Alveoli
Two Parts:
- Conducting zone: carries + filters + warms + moistens
- Exchange zone: alveoli = gas exchange
Trachea support: C-shaped cartilage rings = prevents collapse
Safety during swallowing: Epiglottis covers glottis
Alveoli features: Thin + Moist + Large surface + Rich capillaries
Pleura: Pleural fluid reduces friction
Lobes: Right 3, Left 2 (heart space)
Breathing logic: Thoracic volume change → Lung volume change → Air moves
💡 Questions and Answers
Q1. At which vertebral level does the trachea divide?
A1: The trachea divides into right and left primary bronchi at the level of the 5th thoracic vertebra (T5).
Key points:
- Trachea bifurcation = T5
- Forms two primary bronchi
Q2. Differentiate between conducting part and exchange part of respiratory system.
A2: The conducting part is like the airway pipe that brings air inside and also cleans it. The exchange part is where the real gas exchange happens.
Key points:
- Conducting: nostrils → terminal bronchioles
- carries, filters, warms, moistens air
- Exchange: alveoli (and ducts)
- O₂ enters blood, CO₂ leaves blood
Q3. What prevents the trachea from collapsing?
A3: The trachea has incomplete C-shaped cartilaginous rings, which keep it open even when we inhale strongly.
Key points:
- C-shaped cartilage rings
- Prevents airway collapse
Q4. What is the function of pleural fluid?
A4: Pleural fluid works like a lubricant. It reduces friction so lungs can expand and relax smoothly during breathing.
Key points:
- Between two pleura layers
- Reduces friction during breathing
Q5. Why is the larynx called the sound box?
A5: Because the larynx contains vocal cords which vibrate and produce sound when air passes through.
Key points:
- Larynx = voice box
- Vocal cords → sound production
Q6: Why are alveoli suitable for gas exchange?
A6: Alveoli are ideal because they have a very large surface area, thin and moist walls, and a rich capillary network, so diffusion happens fast.
Key points:
- Large surface area
- Thin + moist walls
- Rich blood supply
Q7: Why does left lung have fewer lobes than right lung?
A7: The left lung has two lobes because it has to make space for the heart on the left side.
Key points:
- Left lung 2 lobes
- Space for heart