Lesson 2 - Upper Digestive System

Question 1

What Happens in the Mouth

Answer 1

Digestion begins in the mouth

When we chew, our food is physically broken down by our teeth. This is a type of mechanical digestion

The presence of food in the mouth (and often
just the sight or smell of food!) triggers the
salivary glands to secrete a watery fluid called
saliva. Saliva contains water, mucus and
enzymes.

Enzymes are proteins which increase the rate
of chemical reactions

One of these enzymes is amylase which breaks down starch into smaller disaccharides. This is a form of chemical digestion (converts food into its molecular components)

Once food has been chewed and mixed with saliva in the mouth, it forms a bolus (ball). The tongue pushes it to the back of the mouth where it is swallowed

Question 2

What Happens in the Pharynx and Larynx?

Answer 2

As the food is pushed into the
pharynx, and the soft palate is raised
to prevent food from entering the nasal passages

At the same time, the larynx is raised
against a flap of soft tissue called the
epiglottis. This covers the entrance to the trachea to prevent food from entering the lungs

Question 3

What Happens in the Stomach?

Answer 3

The movement of food into and out of the stomach is controlled by a circular muscular tube that passes food from the muscles called sphincters

When relaxed, the gastroesophageal sphincter
allows food to enter the stomach. When contracted, it prevents food from moving back into the esophagus

Fun Fact: When this process fails, acid from the stomach can enter the esophagus. This causes a burning sensation in the lower throat that we know as heartburn, or acid reflux!

Layers of the Stomach:

Mucosa: innermost layer
- Folded
- Secretes gastric juices (digestive enzymes,
acid, mucus)
- Epithelial cells divide rapidly (lining replaced every 3 days!

Submucosa: connective tissue (contains
nerves and blood vessels)

Muscularis: muscle layer
- Frequent contractions mix the food and
gastric juices to produce a semi-liquid called
chyme

Serosa: outermost layer
- Smooth
- Holds the stomach in place
- Secretes a lubricating fluid that eliminates
friction between organs

The nerves in the submucosa detect when food is present and initiate the release of a hormone called Gastrin into the bloodstream.

Gastrin travels to the gastric cells in the stomach where it stimulates the release of gastric juices which is largely mucus (to protect the lining of the stomach) but also contains hydrochloric acid and pepsinogen.

Stomach acid has a pH of 2-3 (very acidic) which:
- Kills many harmful micro-organisms that are
ingested with our food
- Stops the action of amylase
- Provides the necessary pH to activate other
digestive enzymes

pepsinogen + HCl -> pepsin (active form which begins the breakdown of proteins into separate amino acids)

Fun Fact: Secreting the inactive enzyme pepsinogen is a safety mechanism that prevents damage to the stomach tissue. If gastric glands were to make pepsin instead of pepsinogen, the stomach would digest itself!