Physical digestion:
- It is the breakdown of food into smaller pieces without a chemical change to the food molecules.
- It increases the surface area of food for the action of enzymes in chemical digestion.
- This process occurs mainly in the mouth and the stomach of the alimentary canal.
- Physical digestion in the mouth is carried out by the teeth.
- This process of physical digestion in the mouth is also known as mastication.

Structure of human teeth:
- The part of a tooth that is visible above the gum line is called the crown.
- The gum is the tissue that overlays the jaws.
- The rest of the tooth that cannot be seen is called the root.
- The surface of the crown is covered by a very hard layer called enamel.
  - Enamel is non-living, and its function is to protect the teeth and maintain their hardness.
- Enamel is replaced by cement in the root.
  - Cement enables the teeth to grip to their bony socket in the jaw.
- Below the enamel, there is a layer of dentine.
  - Dentine is softer than enamel. It is a living part, similar to the bone.
- And finally, inside the dentine is the pulp cavity.
  - It contains the nerves and the blood vessels.

Types of human teeth:
- There are four types of teeth in humans.
- They are incisors, canines, premolars, and molars.
- Incisors have spade-like structures, similar to a chisel.
  - They are single-rooted teeth.
  - They are used to cut and bite pieces off the food.
- Canines are more pointed than the incisors.
  - They are slightly larger but have similar functions as the incisors. They are also single-rooted teeth.
- Premolars and molars are similar in shape and function.
  - Their function is to crush the food into smaller pieces.
  - They are double-rooted teeth.
  - The surface of the premolar has two projections or cusps while the surface of the molar has four projections or cusps.

Physical digestion in the stomach:
- Stomach is a bag-like organ with a thick muscular wall.
- Physical digestion occurs in the stomach by means of muscular contractions.
- After food enters the stomach, waves of muscular contraction (peristalsis) start to occur.
- Peristalsis mixes and softens the food with gastric juice to create a semi-fluid mass called chyme.
- The initial peristaltic movements are relatively gentle, but they are followed by more intense contractions that start at the body of the stomach and increase in force as they near the duodenum.
- Peristalsis forces a small volume of chyme at a time into the duodenum. This process is called gastric emptying.
- Gastric emptying is regulated by both the stomach and the duodenum.

7.03 - Physical Digestion

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