The Digestive System

Digestive System

• The digestive system uses mechanical and chemical activities to break down food into absorbable substances.
• The digestive system can be divided into two main categories: organs that make up the alimentary canal and accessory digestive organs.

Alimentary Canal Organs

• The alimentary canal, also called the gastrointestinal (GI) tract or gut, is a one-way tube about 7.62 meters (25 feet) in length during life and closer to 10.67 meters (35 feet) in length after death.
• The main function of the organs of the alimentary canal is to nourish the body.
• The alimentary canal consists of the mouth, pharynx, esophagus, stomach, small and large intestines, and anus.

The Mouth

• The mouth is part of the digestive system and is also called the oral cavity.
• The inner lining of the cheek is known as the buccal mucosa.

The Pharynx

• The pharynx, commonly called the throat, is a muscular, funnel-shaped passageway inside the body.
• It connects the mouth and nose to the esophagus (leading to the stomach) and larynx (leading to the trachea and then lungs).

The Esophagus

• The esophagus is a hollow, muscular tube that passes food and liquid from the throat to the stomach.
• The primary function of the esophagus is to carry food and liquid from the mouth to the stomach.

The Stomach

• The stomach is a J-shaped organ that digests food.
• It produces enzymes and acids that break down food so it can pass to the small intestine.
• There are four main regions in the stomach: the cardia, fundus, body, and pylorus.

The Small Intestine

• The small intestine is the longest part of the alimentary canal, about 3.05 meters (10 feet) long.
• It is where most digestion occurs and where practically all absorption occurs.
• The small intestine is subdivided into three regions: the duodenum, jejunum, and ileum.
• Villi in the small intestine are specialized for absorption, having a thin wall, one cell thick, which enables a shorter diffusion path.

Accessory Structures

• Accessory digestive organs aid in the breakdown of food.
• The liver, pancreas, and gallbladder release secretions essential for digestion.
• The gallbladder stores, concentrates, and releases bile.
• The pancreas produces pancreatic juice, which contains digestive enzymes and bicarbonate ions.

Liver

• The liver is the largest gland in the body, weighing about three pounds in an adult.
• It plays important roles in metabolism and regulation.
• The digestive role of the liver is to produce bile and export it to the duodenum.

Pancreas

• The pancreas is an organ and a gland.
• It performs two main functions: exocrine function (produces substances that help with digestion) and endocrine function (sends out hormones that control the amount of sugar in the bloodstream).
• The pancreas produces lipase, amylase, and protease enzymes.

Gallbladder

• The gallbladder is a small, pear-shaped organ that stores and releases bile.
• Bile helps the digestive system break down fats.

Digestive Processes

• The processes of digestion include six activities: ingestion, propulsion, mechanical or physical digestion, chemical digestion, absorption, and defecation.
• Ingestion refers to the entry of food into the alimentary canal through the mouth.
• Propulsion refers to the movement of food through the digestive tract, which includes the voluntary process of swallowing and the involuntary process of peristalsis.
• Mechanical digestion is a purely physical process that does not change the chemical nature of the food.
• Chemical digestion starts in the mouth, and digestive secretions break down complex food molecules into their chemical building blocks.
• Absorption takes place primarily within the small intestine, where food that has been broken down enters the bloodstream, and its nutrients are put to use.
• Defecation is the final step in digestion, where undigested materials are removed from the body as feces.