Biology of Disease

released by its action are perceived by specific taste receptors allowing fatty
foods, and therefore energy rich nutrients, to be detected.

In swallowing, the soft palate is elevated, which seals off the nasal cavity and
pushes against the back wall of the pharynx. This, in turn, triggers an auto-
matic reflex action in muscles that raise the larynx, pushing its opening, the
glottis, against a flap of tissue called the epiglottis. These actions prevent food
entering the trachea of the respiratory system and ensure food is expelled
from the mouth into the esophagus. Rhythmic waves of contraction of smooth
muscle tissue in the walls of the GIT, called peristalsis, forces food along the
GIT. Peristalsis in the esophagus ensures that food is propelled to the stomach
even if the person is upside down.

DIGESTION

CZhhVg6]bZY!BVjgZZc9Vlhdc!8]g^hHb^i]:YLddY '-(

Parotid gland

Epiglottis

Trachea

Submandibular

gland

Sublingual

gland

Tongue

Esophagus

Figure 11.6The submandibular, parotid and sublingual salivary glands.
Figure 11.7Molecular model of a salivary
amylase molecule. PDB file 1SMD.

O O O

O O O O

O OH O

O O O O

H 2 O

HO

Figure 11.8Schematic outlining the digestion (hydrolysis) of carbohydrates (polysaccharide).

Stomach

The stomach is a sac-like region of the GIT (Figure 11.1(A) and (B)). Its inner
surface is highly folded (Figure 11.9) allowing it to expand up to eightfold from
its empty volume of about 50 cm^3 following a meal. Muscular activities of the
stomach wall mix the food with gastric juice secreted by gastric glands located
in the mucosal lining of the stomach. Gastric glands produce about 1 to 2 dm^3