The Digestive System and its Processes

The Digestive System

Liver

The liver, the largest internal organ, contains ducts called hepatocytes. These hepatocytes collect bile secreted by liver cells. The liver plays a crucial role in the metabolism of carbohydrates, lipids, and proteins.

Gallbladder

The gallbladder is a pear-shaped structure located under the liver. Its primary function is to store bile.

Pancreas

The pancreas is a long, narrow gland with two ducts, located on the left side of the abdomen.

Small Intestine

The small intestine starts at the pylorus and ends at the ileocaecal valve, where it joins the large intestine. It has two main parts:

• Duodenum: Secretes bile and pancreatic juice into this section.
• Jejunum and Ileum: Form the rest of the intestine. The small intestine is covered in projections called intestinal villi. There are also intestinal glands that secrete intestinal juice.

Large Intestine

The large intestine is connected to the small intestine and does not have villi. It secretes lubricating mucus to help substances pass through.

How Does Our Digestive System Work?

Food goes through a process of digestion, which can be categorized into two types:

• Mechanical Digestion: Food is ground, cut up, and mixed with digestive juices.
• Chemical Digestion: Food is broken down by hydrolytic enzymes.

1. Preparation

The teeth and lips are used to move food, initiating the process of mastication:

• Mastication: A mechanical process carried out by the teeth. The tongue moves food into position.
• Insalivation: Food is mixed with saliva, aided by the tongue. Pieces of food are moistened and joined together to form a bolus. Saliva contains three main components: salivary amylase, mucin, and lysozyme.

2. Deglutition

Deglutition is the movement of food. The tongue pushes the bolus back toward the pharynx, forcing the epiglottis to close. The food bolus then passes into the esophagus and is moved downwards as the two muscle layers contract and relax.

3. Gastric Digestion

Food traveling down the esophagus enters the stomach through the cardiac sphincter. The entering bolus stimulates the production of gastric acid, which has two main components:

• Hydrochloric Acid: Breaks down food and eliminates most microorganisms.
• Pepsin: An enzyme that breaks the bonds between amino acids.

4. Intestinal Digestion

Intestinal digestion involves three main components:

• Pancreatic Juice: A watery solution containing sodium and digestive enzymes such as pancreatic amylase and lipase.
• Bile: Helps digestion by breaking down fats.
• Intestinal Juice: Secreted by glands on the intestinal wall.

5. Intestinal Absorption

Nutrients are absorbed by the intestinal villi. The villi are covered in an epithelium made up of a single layer of cells. There is a large network of capillaries surrounding a blind-ended lymph vessel.

6. Egestion

Substances fermented by intestinal flora are compacted to form feces and are temporarily stored in the rectum before being eliminated from the body.