🫁 Physiology of the Lungs
1. Introduction – Why do we even need lungs?
Every cell in our body needs oxygen to produce energy and must get rid of carbon dioxide.
The lungs handle this exchange quietly and continuously—about 20,000 breaths a day, without
us thinking about it.
In simple terms, lungs keep us alive by managing breathing and gas exchange.
2. Basic anatomy (just what we need to know)
The lungs are two spongy organs present in the chest cavity.
Right lung → 3 lobes
Left lung → 2 lobes (space for the heart)
Air travels in this order:
Nose → Trachea → Bronchi → Bronchioles → Alveoli
Alveoli are the real heroes—this is where gas exchange happens.
3. Alveoli – The functional units of lungs
Tiny, thin-walled air sacs
Surrounded by capillaries
Very large surface area
Walls are just one cell thick
This structure makes it easy for:
Oxygen to enter blood
Carbon dioxide to leave blood
Think of alveoli like tiny balloons wrapped in blood vessels.
4. Breathing (Pulmonary ventilation)
Breathing has two phases:
a) Inspiration (Breathing in)
Diaphragm contracts and moves downward
Ribs move upward and outward
, Chest cavity volume increases
Pressure inside lungs decreases
Air moves into the lungs
Air always moves from high pressure to low pressure.
b) Expiration (Breathing out)
Diaphragm relaxes and moves upward
Ribs move downward and inward
Chest cavity volume decreases
Pressure inside lungs increases
Air moves out of the lungs
Normal expiration is a passive process.
5. Respiratory muscles
Main muscles involved in breathing:
Primary muscles:
Diaphragm – most important
External intercostal muscles
Accessory muscles (used in deep or forced breathing):
Sternocleidomastoid
Scalene muscles
Abdominal muscles (during forced expiration)
6. Exchange of gases (Respiration)
Gas exchange occurs in two places:
a) External respiration (in lungs)
Oxygen moves from alveoli → blood
Carbon dioxide moves from blood → alveoli
Happens by diffusion
Driven by difference in partial pressure of gases.
b) Internal respiration (in tissues)
Oxygen moves from blood → tissues
Carbon dioxide moves from tissues → blood
1. Introduction – Why do we even need lungs?
Every cell in our body needs oxygen to produce energy and must get rid of carbon dioxide.
The lungs handle this exchange quietly and continuously—about 20,000 breaths a day, without
us thinking about it.
In simple terms, lungs keep us alive by managing breathing and gas exchange.
2. Basic anatomy (just what we need to know)
The lungs are two spongy organs present in the chest cavity.
Right lung → 3 lobes
Left lung → 2 lobes (space for the heart)
Air travels in this order:
Nose → Trachea → Bronchi → Bronchioles → Alveoli
Alveoli are the real heroes—this is where gas exchange happens.
3. Alveoli – The functional units of lungs
Tiny, thin-walled air sacs
Surrounded by capillaries
Very large surface area
Walls are just one cell thick
This structure makes it easy for:
Oxygen to enter blood
Carbon dioxide to leave blood
Think of alveoli like tiny balloons wrapped in blood vessels.
4. Breathing (Pulmonary ventilation)
Breathing has two phases:
a) Inspiration (Breathing in)
Diaphragm contracts and moves downward
Ribs move upward and outward
, Chest cavity volume increases
Pressure inside lungs decreases
Air moves into the lungs
Air always moves from high pressure to low pressure.
b) Expiration (Breathing out)
Diaphragm relaxes and moves upward
Ribs move downward and inward
Chest cavity volume decreases
Pressure inside lungs increases
Air moves out of the lungs
Normal expiration is a passive process.
5. Respiratory muscles
Main muscles involved in breathing:
Primary muscles:
Diaphragm – most important
External intercostal muscles
Accessory muscles (used in deep or forced breathing):
Sternocleidomastoid
Scalene muscles
Abdominal muscles (during forced expiration)
6. Exchange of gases (Respiration)
Gas exchange occurs in two places:
a) External respiration (in lungs)
Oxygen moves from alveoli → blood
Carbon dioxide moves from blood → alveoli
Happens by diffusion
Driven by difference in partial pressure of gases.
b) Internal respiration (in tissues)
Oxygen moves from blood → tissues
Carbon dioxide moves from tissues → blood
