Human Digestive System: Organs, Functions, and Processes

Human Nutrition and Body Systems

The digestive, respiratory, excretory, and circulatory systems all play a role in nutrition.

• Digestive system: Introduces foods into the body, and prepares and transforms it to enable its use by cells.
• Respiratory system: Obtains the oxygen cells need and eliminates carbon dioxide produced during cell metabolism.
• Circulatory system: Transports nutrients and oxygen to cells and releases waste substances.
• Excretory system: Releases waste substances outside the organism.

The Digestive System: Function and Structure

The digestive system breaks nutrients into parts small enough for your body to absorb and use for energy, growth, and cell repair. It is made up of the gastrointestinal (GI) tract and the accessory glands: the liver, pancreas, and salivary glands.

Digestive Functions

• Ingestion: Entry of food into the digestive tract.
• Digestion: Transformation of food into simple nutrients.

Types of Digestion

• Mechanical Digestion: Physical transformations such as chewing, squeezing, and mixing to reduce the size of the food and make it easier for chemical digestion.
• Chemical Digestion: Transformation of complex nutrients from food into simple nutrients through chemical reactions. This is carried out by enzymes in digestive juices such as stomach acid and bile.
• Absorption: The passing of nutrients from the digestive tract to the blood or lymph, done by the small intestine.
• Defecation: The expulsion of non-digested or non-usable parts of food.

Parts of the Gastrointestinal Tract

The GI/Digestive tract is a series of hollow organs joined in a long, twisting tube from the mouth to the anus where food moves through. The hollow organs that make up the GI tract are:

1. Mouth
2. Pharynx
3. Esophagus
4. Stomach
5. Small intestine
6. Large intestine
7. Anus

GI Tract Tissue Layers

The GI Tract is made up of three layers of tissue:

• Mucosa: Internal layer, lined with glands.
• Muscularis: Middle layer, two layers of smooth muscle.
• Serosa: Connective tissue layer, joins the digestive tract to other organs.

Digestive Glands and Chemical Digestion

Digestive glands help carry out chemical digestion by secreting digestive juices into the digestive tract. These include salivary glands, gastric glands, intestinal glands, the liver, and the pancreas.

Organs involved in chemical digestion and their secretions:

• Mouth (saliva)
• Stomach (stomach acid & digestive enzymes)
• Pancreas (pancreatic juice)
• Liver (bile)
• Gallbladder (stores bile)
• Small intestine (digestive juice)

Food Movement: Peristalsis

Food moves through the digestive tract through a process called peristalsis. The hollow organs of the GI tract have a layer of muscle which allows them to move, pushing and mixing the food forward.

Path of food: Mouth → Esophagus → Stomach → Small Intestine → Large Intestine → Rectum

Functions of Teeth

• Incisors: Biting and cutting food.
• Canines: Ripping and tearing food.
• Premolars: Crushing food.
• Molars: Chewing, crushing, and grinding food.

The Digestive Process: Key Organs

The Stomach

The stomach is an organ connected to the small intestine via the pylorus, a valve that is usually closed. The cardia is the valve at the entrance of the stomach which is always open.

Food stays in the stomach for 3–4 hours, mixing with gastric juice, creating chyme.

Gastric Juice Contents

• Pepsin: An enzyme that begins the digestion of proteins.
• Hydrochloric Acid (HCl): Activates pepsin, facilitates its action, and destroys bacteria.

The inside of the stomach contains mucus which protects the stomach wall and prevents it from being attacked by hydrochloric acid and enzymes. When chyme becomes very acidic, the pylorus opens, and it moves into the small intestine. Gastric acid is produced through involuntary processes.

The Small Intestine

A tube around 6 or 7 meters long. It is located between the pylorus and the ileocecal valve, which separates it from the large intestine. Here, chyme comes into contact with digestive juices secreted by the liver and pancreas.

The small intestine is divided into 3 parts:

1. Duodenum: 1st section, U-shaped, around 25 cm long.
2. Jejunum: 2nd and longest section.
3. Ileum: Final section, leads to the large intestine.

The Liver and Bile

The liver is a large organ in the top-right of the abdomen. It has a reddish-brown color because it contains blood. It produces bile, which enters the duodenum. The liver also carries out non-digestive functions, such as destroying toxins.

Bile Characteristics

• Stored in the gallbladder and only released when food enters the intestine.
• Does not contain digestive enzymes.
• Contains bile salts, which help to digest fats by emulsifying them.