Topic 6 Gas exchange

Physiological respiration

exchange of O₂ and CO₂ between body and environment. 

Cellular respiration

mitochondria use O₂ to make ATP and release CO₂. 

Gas exchange depends on

respiratory medium (air or water) and respiratory surface (skin, gills, lungs). 

Respiratory surface

thin epithelial layer; gases cross by diffusion. 

Diffusion rate ↑ with

• Larger surface area 

• Shorter distance 

• Greater concentration gradient 

• Higher temperature 

Small animals (sponges, worms) use

body surface

Large animals have

specialized structures: gills, lungs, or tracheae

Gases must

dissolve in water to cross cells

Gills

external extensions (in aquatic animals)

Lungs

internal sacs (in terrestrial animals) to prevent drying

Ventilation

flow of air/water over respiratory surface

Perfusion

 flow of blood to that surface. 

Gills

branched filaments that ↑ surface area. 

Lungs/tracheae

folded or branched internal surfaces

Water 

less O₂, requires more energy to move

Air

more O₂, easier to ventilate, but causes water loss.

(Insects) Network of air tubes (tracheae)

reinforced by chitin. 

Air enters via (insects)

spiracles (openings)

Amphibians positive pressure

gulp air

Reptiles & mammals negative pressure

pull air in by expanding chest

Air path

Nose → Pharynx → Larynx → Trachea → Bronchi → Bronchioles → Alveoli 

Alveoli (thin-walled sacs surrounded by capillaries)

main site of gas exchange

Cilia & mucus in bronchi

trap dust and bacteria

Diaphragm 

main breathing muscle separating thoracic and abdominal cavities

Inhalation

diaphragm contracts (moves down), intercostal muscles lift ribs → chest volume ↑, pressure ↓, air flows in

Exhalation

diaphragm relaxes → elastic recoil expels air

Forceful exhalation 

abdominal & internal intercostal muscles contract

O₂ and CO₂ diffuse from

high → low partial pressure

O₂ enters plasma →

diffuses into erythrocytes → binds to hemoglobin (Hb).

Bohr effect

Low pH (acidic, from CO₂) → Hemoglobin releases more O₂. 

Active tissues produce CO₂ → form carbonic acid → lower pH → ↓ Hb affinity → more O₂ released.