P1 Biology p6 (the lungs+digestive system)

Alveoli – 4 adaptations for gas exchange

1) Very large combined surface area

2) Walls only one cell thick → short diffusion distance

3) Surrounded by many capillaries → good blood supply

4) Moist lining → gases dissolve easily

Cilia + mucus

Mucus traps dirt + bacteria; cilia (tiny hair‑like structures) sweep mucus up and out of the lungs.

Gas that moves in to blood / out of blood

Oxygen diffuses into blood; 

carbon dioxide diffuses out of blood into alveoli

Trachea and bronchi: rings of cartilage

Stop the air‑ways collapsing when breathing in.

Path air takes

Nose → trachea → bronchi → bronchioles → alveoli

Mouth

Teeth physically break food down; saliva has amylase to start digesting starch.

Oesophagus

Tube that pushes food down to the stomach by peristalsis (muscle contractions)

Stomach

Churns food mechanically; makes protease and hydrochloric acid – acid kills bacteria and gives right pH for stomach protease

Small intestine

Where digestion finishes; nutrients absorbed into blood – has villi for large surface area.

Villi adaptations

Large surface area, thin one‑cell wall, good blood supply → fast absorption.

Gall‑bladder

Stores bile before releasing it into the small intestine