Gas Exchange

1. Gas Exchange vs Respiration

	Gas exchange (this chapter)	Respiration (A2, topic 1)
Nature	PHYSICAL — breathing in & out air, diffusion of O₂ / CO₂	CHEMICAL — metabolic process inside cells
Where	air tract → alveoli ↔ blood	cellular level (cells use up O₂, make CO₂)

O₂ for cellular respiration comes from the atmosphere → so gas exchange must work continuously. Alveoli = the main site of gas exchange (surrounded by a network of blood capillaries).

2. The Air Tract — Pathway of Air

Air tract (physical) — follow the air from outside to the alveoli:

• Nostrils — only part visible outside. Nasal cavity = hollow internal lining of the nose, first internal structure exposed to air.
• Pharynx = common pathway of the digestive + respiratory systems. Two exits: → trachea (air) and → oesophagus → GIT/gut (food). Guarded by the epiglottis (thin flap).
• Larynx = voice box. The lung structure (trachea onwards) starts from here.
• Trachea = single tube → splits into 2 bronchi (one per lung). s: bronchus · p: bronchi.
• Bronchi branch into bronchioles (large → terminal → respiratory), which end in alveolar duct → alveolar sac → alveoli.

The epiglottis — why we don't choke

Diameters keep getting smaller

Structure	Diameter (approx. average)
Trachea	1.8 cm
Bronchus / bronchi	1.2 cm
(Terminal) bronchioles	1 mm
Respiratory bronchioles	0.5 mm
Alveolar duct	400 µm
Alveolus	250 µm

diameter ↓ ⇒ diffusion distance ↓ ⇒ diffusion faster. (1 cm = 10 mm; 1 mm = 1000 µm — a µm is tiny!)

Alveolar sac vs alveoli (don't mix them up!)

• Alveolar sac = a "bag/packet" holding lots of alveoli. Alveolar duct = duct opening into a sac. Each bronchiole end carries bunches of sacs — not one alveolus.
• Why bunches? Alveoli = the only gas-exchange surface → many alveoli in little space ⇒ large surface area ⇒ faster diffusion.

3. Role of the Gas Exchange System

1. To clean, warm and moisten the air (mainly nasal cavity)
2. To provide a surface through which gas exchange takes place → alveolar surface
3. To minimise diffusion distance → alveoli one cell thick, capillaries one cell thick
4. Continuous blood supply + alveolar ventilation maintain an adequate diffusion gradient for gas exchange

① Clean · warm · moisten (nasal cavity)

• Clean: nasal hairs trap dust → sneeze/cough it out. Later, cilia + mucus in the trachea trap & sweep away dust and bacteria.
• Warm: nasal cavity has a blood supply at body temp 37 °C; outside air is cooler (10–30 °C). Air touching the blood vessels is heated by convection → air reaches body temperature (like a hot geyser pipe warming your hand).
• Moisten: the lining is moist → air humidified before reaching the alveoli.

4. Air Composition & the Diffusion Gradient

	Inhaled (atmospheric) air	Exhaled (expired) air
O₂	20 – 21 %	16 %
CO₂	0.04 %	4 %

O₂ falls 21 → 16 % because O₂ is used up in respiration (oxidation); CO₂ rises 0.04 → 4 % because it is the waste by-product.

Why a constant gradient? The alveolus is wrapped in a network of capillaries bringing fresh DOB (low O₂, high CO₂) from the heart; breathing keeps bringing fresh air (high O₂, low CO₂) into the alveolus. So the two sides never equalise.

5. Trachea — Transverse Section

Lumen = the hollow open space of a tube through which something passes (air in the air tract; blood in vessels; food in the gut). Think of a pipe / bottle with the cap off. A solid, packed tube couldn't carry air!

Moving away from the lumen there are 4 layers:

#	Layer	What it is / has
1	Mucosa	= the epithelium; first layer, exposed to the lumen
2	Submucosa	contains the mucous glands (+ blood vessels)
3	Cartilage	C-shaped hyaline cartilage (soft bone)
4	Adventitia	outermost covering layer

6. Layer 1 — Mucosa (the Epithelium)

Decoding the big name

Word	Meaning
Pseudo	fake / false impression (like "pseudo-science")
Stratified	layered
Ciliated	has cilia (hair-like projections made of microtubules)
Columnar	shape of the cells — tall, column/cylinder-like
Epithelium	the lining/covering layer

• It is a single layer of cells (not double, not triple) that appears like more than one layer ⇒ pseudostratified.
• Found in: trachea, bronchi, bronchioles. (Alveoli have squamous epithelium — see later.)
• Sir: listen to the recording for this explanation — very important & often asked.

3 types of cell in the mucosa

Ciliated columnar cells	Goblet cells	Basal cells
hair-like projections = cilia; role: waft (sweep) mucus up toward the larynx	produce mucin, a glycoprotein that makes up the mucus lining the epithelium	(named only — for syllabus you need goblet + ciliated)

Goblet cell & mucin — how it's made

Recap from cell structure: RER makes & folds the protein → packaged in vesicles → Golgi = protein modification (adds a short polysaccharide chain) → glycoprotein (mucin) → secretory vesicles → released.

Role of mucus & cilia

1. Mucus is sticky ⇒ traps bacteria & dust particles that were inhaled.
2. The thickness of the mucus lining increases the distance between the bacteria and the epithelial cells ⇒ bacteria can't reach / infiltrate the epithelium (acts as a separating barrier).

Role of cilia: responsible for wafting (sweeping) the mucus up towards the larynx; the mucus entraps dust particles + bacteria (like sweeping a carpet).

Mucus moved upward to the pharynx ⇒ either swallowed (→ oesophagus → stomach, where HCl kills the bacteria) or coughed out.

7. Layer 2 — Submucosa (Mucous Glands)

• Mucous glands are present in the submucosa. Glands = involved in secretion.
• The ducts of the mucous glands open on the surface of the respiratory epithelium (the mucosa).
• They produce a serous fluid — transparent, watery — which dissolves the glycoprotein mucin to form the mucus.

Word	What it is
Mucin	glycoprotein made by goblet cells
Mucus	the sticky lining fluid = serous fluid + mucin ✓
Mucous gland	gland in the submucosa → makes serous fluid
Mucosa	the first layer (epithelium) exposed to the lumen ✓

8. Layers 3 & 4 — Cartilage, Smooth Muscle, Adventitia

③ C-shaped cartilage

• Cartilage = soft bone (hyaline cartilage). Cells = chondrocytes (cyte = cell: erythrocyte = RBC, leucocyte = WBC, osteocyte = bone cell, chondro = cartilage).
• Role: keeps the gas-exchange tubes patent (open) — stops them collapsing under air pressure when we breathe in, so the whole system never collapses.

Why a "C" and not a full ring?

• The oesophagus lies behind the trachea; food goes through it when swallowing.
• A full cartilage ring would press on the oesophagus → swallowing difficult / painful (food would "choke"). Cartilage never contracts or relaxes — it only pushes.
• So on the oesophagus side there is smooth muscle: it contracts (lets food bulge through) then relaxes after the food has passed.

Smooth muscle

• Contraction → lumen constricts (narrows). Relaxation → airway widens.
• Controls/regulates air movement → ensures ventilation. Air entering → airway widens; after the air passes, it returns to original size.
• Contraction needs ATP — an energy-consuming process (muscle contraction — A2).

④ Adventitia

The outermost layer of the trachea wall — the covering/outer protective layer (4th layer, farthest from the lumen).

9. Spot the Trachea vs Oesophagus (micrographs)

• In the photomicrograph the highlighted band across the open side of the C is the fibroelastic membrane (with smooth muscle), separating trachea from oesophagus. (Photomicrograph = micrograph taken from an elderly person — shows trachea next to oesophagus at the base of the neck.)
• Tracheostomy diagram on board: larynx → thyroid cartilage → trachea → tracheostomy tube → thyroid gland → oesophagus behind.

10. Bronchus and Bronchioles

Bronchus (plural: bronchi)

• Almost everything seen in the trachea is also in the bronchus: mucosa (pseudostratified ciliated columnar epithelium with ciliated + goblet cells + basal cells), submucosa with mucous glands, smooth muscle, elastic fibres, blood vessels.
• Key difference: bronchus cartilage = irregular plates (chondrocytes), NOT a complete C-ring.
• Smooth muscle present — contracts/relaxes to maintain airflow; airway must widen when air enters.

Bronchioles

1. No goblet cells ⇒ no mucin production ⇒ no mucus lining. (Bronchioles are very narrow; a sticky gel lining would choke them and block continuous airflow.)
2. No cartilage.

• ★ Smooth muscle contraction + relaxation controls the amount of airflow towards the alveoli (regulation of airflow into alveoli). Very important here because bronchioles lead to alveoli.
• Very few layers; ciliated epithelium with very few ciliated cells; elastic fibres help stretch & recoil (air volume suddenly expands, then pipe springs back).
• No gas exchange with blood in trachea, bronchi or (most) bronchioles — alveoli are the only surface where gas exchange happens between blood and incoming air.

Comparison table (board): ✓ = present, ✗ = absent

Features	Trachea	Bronchi	Bronchioles
(1) Pseudostratified ciliated columnar epithelium	✓	✓	✓
(2) Goblet cells	✓	✓	✗
(3) Smooth muscle	✓	✓	✓
(4) Cartilage	✓ (C-shaped ring)	✓ (irregular plates)	✗
(5) Elastin	✓	✓	✓

Identifying structures in micrographs

• Bronchiole: folded (scalloped) epithelium around a lumen, thin smooth-muscle layer (sm), no cartilage; surrounded by alveoli.
• Respiratory bronchiole / alveolar duct / alveoli: large open spaces with thin walls; blood vessels nearby; alveolar walls = very thin (small arrows on board show thin wall = short diffusion distance).
• Constricted bronchiole (labelled micrograph): lumen narrowed by contracted smooth muscle, epithelium folded; the image also labels alveoli, simple columnar epithelium, hyaline cartilage and a seromucous gland. Scale bar = 100 µm.

11. Alveoli — the Site of Gas Exchange

• Elastin (a fibrous protein) covers/wraps the surface of the alveoli (a network of elastin fibres) → allows alveoli to stretch and recoil during each breath → air is refreshed (fresh O₂ in, CO₂ out).
• Alveolus = one cell thick: single layer of cells, squamous (flattened) epithelium. Capillaries around it = endothelium, also one cell thick.

"One cell thick" — what it really means

• NOT "made of one cell" — the thickness (width) equals the size of one cell; one single layer of cells.
• Fewer layers ⇒ shorter diffusion distance ⇒ faster, more efficient diffusion. (Hurdle race: more hurdles → more time; one hurdle → quick.)

Cell types of the alveolar epithelium (pneumocytes)

Cell	Share of alveolar epithelium	Role
Type I pneumocyte	≈ 95 %	thin flat (squamous) cells — gas exchange surface
Type II pneumocyte	≈ 5 %	secretory cells → secrete pulmonary surfactant
Alveolar macrophage	—	(on board, shown within alveolus) — engulfs dust/microbes that reach the alveoli

Pneumocyte = name of an alveolar cell (like RBC = erythrocyte). Sir: type I/II isn't heavily tested but know it in case a case-study / unseen question comes.

Pulmonary surfactant & surface tension

1. Alveoli are lined with a thin film of moisture (water + air interface).
2. Water molecules have cohesive forces (attract each other) → surface molecules pull together, shrink to occupy the least surface = surface tension.
3. High surface tension would pull the alveolus shut ⇒ collapse ⇒ gas exchange system fails.
4. Surfactant gets between the water molecules, weakens the pull ⇒ surface tension ↓ ⇒ alveoli stay open.

Epithelium types — where is each?

Epithelium	Where (our system)
Simple squamous (flattened cells)	alveoli
Pseudostratified columnar (ciliated)	trachea, bronchi, bronchioles

Other epithelia on the board chart (not in this system): simple cuboidal, simple columnar, stratified squamous / cuboidal / columnar, transitional.

12. Adaptations of Alveoli for Gaseous Exchange

1. Numerous small alveoli → large surface area for gaseous exchange (↑ SA:V)
2. Thin diffusion distance — alveoli are one cell thick ⇒ greater rate of diffusion
3. Thin film of moisture lining the alveolus dissolves the gases before they diffuse
4. Surfactant production by type II pneumocytes decreases the surface tension of the fluid lining the alveolus, preventing alveolar collapse
5. Maintenance of a persistent diffusion gradient via alveolar ventilation & constant blood flow from the network of capillaries surrounding the alveoli