1.4.2 Digestion in Stomach

Introduction

After food is swallowed, it travels down the oesophagus and enters the stomach. The stomach is a J-shaped, muscular organ that acts as a mixing tank and storage bag. Here, food undergoes both mechanical and chemical digestion. This topic explains what happens in the stomach during digestion.

Definition

Digestion in the stomach refers to the breakdown of food that takes place inside the stomach. It includes:

• Mechanical digestion — churning and mixing of food by the muscular wall of the stomach.
• Chemical digestion — breakdown of food molecules by enzymes in gastric juice.

Main Idea

In the stomach:

• The cardiac sphincter relaxes to allow food in.
• Food stimulates gastric glands to secrete gastric juice.
• Gastric juice contains mucous, HCl, pepsin, and rennin.
• The stomach wall churns and mixes food into a soupy mixture called chyme.
• When chyme reaches a certain degree of acidity, the pyloric sphincter relaxes.
• A little food passes into the duodenum.

Core Concept

Gastric Juice and Its Components

Detailed Explanation

1. Entry of Food into the Stomach

On the peristaltic reflex, the cardiac sphincter relaxes to allow food into the stomach.

What is the cardiac sphincter?

• A ring of muscle between the oesophagus and the stomach.
• It opens to let food in and closes to prevent food from going back up.

Simple explanation: The cardiac sphincter is like a gate that opens to let food into the stomach and then closes behind it.

2. Gastric Juice Secretion

The food in the stomach stimulates the gastric glands to secrete gastric juice.

What is gastric juice?

• A digestive fluid produced by gastric glands in the stomach wall.
• It consists of mucous, HCl, pepsin, and rennin.

Simple explanation: When food enters the stomach, the stomach wall releases gastric juice to help digest it.

3. Components of Gastric Juice

(a) Mucous

What it does:

• Mucous forms a coating over the stomach.
• It prevents the digestion of the stomach wall by the enzyme pepsin.

Simple explanation: Mucous is like a protective layer that stops the stomach from digesting itself.

Why this is important:

• Pepsin is a protein-digesting enzyme.
• The stomach wall is made of protein.
• Without mucous, pepsin would digest the stomach wall, causing ulcers.

(b) Hydrochloric Acid (HCl)

What it does:

HCl performs the following functions:

• Stops the action of salivary amylase — salivary amylase works at neutral pH; HCl makes the stomach acidic, so salivary amylase stops working.
• Converts inactive pepsinogen to active pepsin — HCl activates the inactive form of the enzyme.
• Provides acidic medium — suitable for the action of enzymes (pepsin works best in acidic conditions).
• Kills many microorganisms — the acidic environment destroys bacteria and other microbes that enter with food.

Simple explanation: HCl makes the stomach acidic. This acidity:

• Stops starch digestion (which started in the mouth).
• Activates pepsin.
• Kills germs.

Why this is important:

• Without HCl, pepsin would not become active.
• Without HCl, harmful bacteria could survive and cause infection.

(c) Pepsin

What it is:

• Pepsin is secreted as inactive pepsinogen.
• HCl or already activated pepsin converts pepsinogen to pepsin.

What it does:

- Pepsin digests proteins to polypeptides (chain of many amino acids) and peptides (chain of few amino acids).

Simple explanation: Pepsin is a protein-digesting enzyme. It cuts long protein chains into shorter chains.

Why it is secreted in inactive form:

• If pepsin were secreted in active form, it would digest the stomach wall.
• By secreting it as pepsinogen (inactive), the stomach protects itself.
• HCl activates pepsinogen only when it is in the stomach cavity.

(d) Rennin

What it is:

- Rennin is secreted only in infants.

What it does:

- It helps in the digestion of milk protein.

Simple explanation: Rennin is a special enzyme for babies. It helps them digest the protein in milk.

Why it is only in infants:

• Infants drink milk as their main food.
• Rennin curdles milk protein (casein) so it can be digested slowly.
• Adults do not need rennin because they eat a variety of foods.

4. Churning Action of the Stomach

By the churning action of the stomach wall, the food gets mixed into a soupy mixture called chyme.

What is chyme?

• A semi-liquid, soupy mixture of food and gastric juice.
• It is acidic due to HCl.

Simple explanation: The stomach squeezes and mixes food like a washing machine. The result is a thick, soupy liquid called chyme.

5. Passage of Chyme to the Duodenum

When the chyme reaches a certain degree of acidity, the pyloric sphincter relaxes and a little food passes into the duodenum.

What is the pyloric sphincter?

• A ring of muscle between the stomach and the duodenum.
• It controls the exit of chyme from the stomach.

Simple explanation: The pyloric sphincter is like a gatekeeper. It opens a little at a time to let chyme into the small intestine.

Why this is important:

• If too much chyme enters the duodenum at once, it cannot be digested properly.
• The pyloric sphincter ensures that chyme enters gradually.

Intermediate Level

Digestion in the stomach involves:

Mechanical digestion:

• Churning and mixing by the muscular wall of the stomach.
• Converts food into chyme.

Chemical digestion:

• HCl stops salivary amylase.
• HCl activates pepsinogen to pepsin.
• Pepsin digests proteins → polypeptides and peptides.
• Rennin digests milk protein (only in infants).

Protection of stomach wall:

• Mucous forms a protective coating.
• Prevents pepsin from digesting the stomach wall.

Regulation of exit:

- Pyloric sphincter controls the release of chyme into the duodenum.

Important Points

• - The cardiac sphincter relaxes to allow food into the stomach.
• - Gastric glands secrete gastric juice.
• - Gastric juice contains mucous, HCl, pepsin, and rennin.
• - Mucous protects the stomach wall from pepsin.
• - HCl stops salivary amylase.
• - HCl converts pepsinogen to pepsin.
• - HCl provides acidic medium for enzyme action.
• - HCl kills microorganisms.
• - Pepsin digests proteins into polypeptides and peptides.
• - Rennin digests milk protein (only in infants).
• - Churning action of the stomach wall mixes food into chyme.
• - Pyloric sphincter relaxes to allow chyme into the duodenum.

Remember

• - Stomach = J-shaped, muscular organ
• - Cardiac sphincter = entrance
• - Pyloric sphincter = exit
• - Gastric juice = mucous + HCl + pepsin + rennin
• - Mucous = protects stomach wall
• - HCl = stops salivary amylase, activates pepsin, kills germs
• - Pepsin = digests proteins
• - Rennin = digests milk protein (infants only)
• - Chyme = soupy mixture
• - Pyloric sphincter = controls chyme release

Common Mistakes

• 1. Thinking protein digestion starts in the mouth.
• - Protein digestion starts in the stomach with pepsin.
• 2. Believing HCl digests food.
• - HCl does not digest food. It activates pepsin, kills germs, and provides acidic medium.
• 3. Confusing pepsin and pepsinogen.
• - Pepsinogen = inactive form.
• - Pepsin = active form.
• - HCl converts pepsinogen to pepsin.
• 4. Thinking mucous is unnecessary.
• - Mucous protects the stomach wall from being digested by pepsin.
• 5. Forgetting that rennin is only in infants.
• - Rennin is secreted only in infants to digest milk protein.
• 6. Mixing up bolus and chyme.
• - Bolus = chewed food ball in the mouth.
• - Chyme = soupy mixture in the stomach.
• 7. Thinking the pyloric sphincter is always open.
• - The pyloric sphincter opens periodically to release small amounts of chyme.

Quick Revision

60-Second Revision

• Stomach = J-shaped, muscular organ
• Cardiac sphincter = entrance (oesophagus → stomach)
• Pyloric sphincter = exit (stomach → duodenum)
• Gastric juice = mucous + HCl + pepsin + rennin
• Mucous = protects stomach wall
• HCl = stops salivary amylase, activates pepsinogen → pepsin, kills germs
• Pepsin = digests proteins → polypeptides and peptides
• Rennin = digests milk protein (infants only)
• Chyme = soupy mixture
• Warning: Do not confuse bolus (mouth) with chyme (stomach).