2.7 Absorption of Materials

2.7 Absorption of Materials — SQA National 5 Biology notes: Explain how respiratory alveoli in lungs and intestinal villi in the digestive tract are adapted for rapid and efficient absorption of nutrients and gases.

• All specialised exchange and absorption surfaces share three fundamental adaptations to maximise the rate of diffusion:
  1. 1. Large surface area — Folded or branching architecture (e.g. millions of alveoli or villi) provides a vast area across which molecules can diffuse simultaneously.
  2. 2. Thin walls (short diffusion distance) — Epithelial barriers are only one cell thick, minimising the distance that gases and dissolved nutrients must travel.
  3. 3. Extensive capillary blood network — Dense blood capillary supply continuously transports absorbed materials away, maintaining a steep concentration gradient.

Absorption Organ	Specialised Structure	Absorbed Substances	Specific Structural Adaptations
Lungs	Alveoli (air sacs)	Oxygen diffuses into blood; Carbon dioxide diffuses into lungs	Moist inner lining allows gases to dissolve; thin squamous epithelium (one cell thick); surrounded by dense pulmonary capillary network
Small Intestine	Villi (finger-like projections)	Glucose, amino acids, fatty acids, glycerol	Each villus has a central lacteal and a surrounding blood capillary network; lined with single-cell-thick epithelium with microvilli

• Fate of digested products in a villus:
  • Blood capillaries: Absorb water-soluble breakdown products — glucose and amino acids — which are transported directly to the liver via the hepatic portal vein.
  • Central lacteal: Part of the lymphatic system; absorbs water-insoluble fat digestion products — fatty acids and glycerol.