Digestive System: Overview and GI Tract Layers Quiz

Overview of the Digestive System

• The alimentary canal, also known as the gastrointestinal (GI) tract, is a 25 ft long tube that extends from the mouth to the anus.
• The GI tract is composed of four layers: mucosa, submucosa, muscularis, and serosa.
• The digestive system breaks down food to release and absorb nutrients.

GI Tract Layers

• Mucosa: the innermost layer, composed of a mucous membrane epithelium that comes into contact with food, and a lamina propria with blood and lymphatic vessels.
• The mucosa contains epithelial cells, goblet cells, and enteroendocrine cells.
• Muscularis mucosa: a layer of smooth muscle that is always in a state of tension.

Layers

• Submucosa: a layer of dense connective tissue between the mucosa and muscularis, containing blood and lymphatic vessels, and a collection of nerves called the submucosal plexus.
• Muscularis: a layer of smooth muscle that is responsible for mechanical digestion and peristalsis.

Digestive Processes

• Mechanical digestion: the physical breakdown of food into smaller particles.
• Chemical digestion: the breakdown of complex food molecules into their building blocks using enzymes, acid, and salts.
• Absorption: the process of taking nutrients into the bloodstream or lipids into the lymphatic system.
• Defecation: the removal of undigested materials from the body.

Regulatory Mechanisms

• Neural control: the regulation of digestive processes through sensors and reflexes.
• Hormonal control: the regulation of digestive processes through hormones such as gastrin, secretin, and CCK.

Mouth, Pharynx, and Esophagus

• Mouth: the oral cavity, responsible for mechanical and chemical digestion, and respiration.
• Tongue: involved in ingestion, mechanical and chemical digestion, swallowing, and vocalization.
• Pharynx: a tube that connects the mouth to the esophagus.
• Esophagus: a muscular tube that propels food into the stomach.

The Small and Large Intestine

• Small intestine: the primary site of digestion and absorption, with a huge surface area.
• The small intestine is composed of three regions: duodenum, jejunum, and ileum.
• Large intestine: responsible for the final absorption of water and electrolytes, and the formation and elimination of feces.

Mechanical Digestion

• Peristalsis: the movement of chyme through the intestine, stimulated by the hormone motilin.
• Segmentation: the back and forth contractions of the intestine to mix chyme with digestive juices.

Chemical Digestion

• Finishes the digestion of carbohydrates and proteins.
• Most lipids are undigested and require bile and pancreatic lipase for digestion.
• Most water is absorbed through osmosis.

Large Intestine

• Composed of four regions: cecum, colon, rectum, and anal canal.
• The large intestine is responsible for the final absorption of water and electrolytes, and the formation and elimination of feces.
• Bacterial flora: trillions of nonpathogenic bacteria live in the large intestine, facilitating digestion, absorption, and synthesis of certain vitamins.

Bile and Pancreatic Juice

• Bile: stored in the gallbladder, and released into the duodenum to aid in fat digestion.
• Pancreatic juice: released into the duodenum to aid in carbohydrate, protein, and fat digestion.

Digestion and Absorption

• Digestion is accomplished through enzymes and hydrolysis, requiring ATP.
• The body absorbs monosaccharides, breaks down disaccharides and polysaccharides, but cannot break down fiber.
• Carbohydrate digestion begins in the mouth and continues in the small intestine.
• Protein digestion begins in the stomach and continues in the small intestine.
• Lipid digestion occurs mainly in the small intestine.
• Minerals are absorbed and balanced by the sodium-potassium pump, requiring ATP.