Digestive System Structure and Function

Digestive System Structure and Function

• The digestive system is a series of organs that process food, extract nutrients, and eliminate waste.
• The GI tract is responsible for ingestion, propulsion, secretions, mechanical digestion, chemical digestion, absorption, and elimination.

Functions of the GI Tract

• Ingestion: taking food into the mouth
• Propulsion: moving food through the GI tract
• Secretion: release of fluids, mucus, and enzymes
• Mechanical digestion: breaking food into smaller pieces
• Chemical digestion: breaking down food molecules with enzymes
• Absorption: moving nutrients into the bloodstream
• Elimination: removing waste products from the body

Organs of the GI Tract

• Oral cavity: chewing, mixing with saliva
• Teeth: mechanical digestion
• Tongue: taste buds, moving food
• Salivary glands: saliva production
• Pharynx: swallowing
• Esophagus: transportation to the stomach
• Stomach: storage, mixing, breakdown of food
• Liver: bile production, nutrient storage, detoxification
• Gallbladder: bile storage and concentration
• Pancreas: digestive enzymes, bicarbonate production
• Small intestine: enzymatic digestion and absorption
• Large intestine: water absorption, waste compaction and elimination
• Rectum: storage of waste
• Anus: elimination of waste

The Gastrointestinal Tract

• Mouth: reservoir for chewing and mixing food with saliva; contains water, mucus, sodium, bicarbonate, chloride, potassium and salivary amylase.
• Tongue: covered in taste buds; involved in food manipulation and swallowing
• Esophagus: peristalsis transports food bolus to stomach
• Stomach: involved in mechanical and chemical breakdown of food; produces chyme;
• Small intestine: absorption of digested food through villi and microvilli
• Large intestine: absorption of water and compaction of waste; segments include cecum, ascending, transverse, descending and sigmoid colon
• Peritoneum: membrane surrounding organs of abdomen and pelvic cavity; visceral peritoneum covers organs and parietal peritoneum lines body walls.

Intestinal Microbiome

• Bacteria increase along the length of the GI tract, highest in the colon
• Multiple factors affect bacterial flora composition including - genetics, diet, environment and medications.
• The immune system is alerted to infection in the gut
• Intestines are initially sterile but are colonized after birth.
• Bacteria number and diversity decreases with age.
• Stomach acid inhibits bacterial growth.

Intestinal Digestion & Absorption

• Digestion begins in the stomach with hydrochloric acid and pepsin.
• In the small intestine, pancreatic enzymes, intestinal enzymes and bile salts further break down food.
• Nutrients are absorbed via active transport, diffusion, or facilitated diffusion.
• Carbohydrates, proteins, and fats are important nutrient groups.

Accessory Organs of Digestion

• Liver (hepatocytes): produces bile; involved in nutrient processing.
• Gallbladder: stores bile
• Pancreas: exocrine and endocrine - producing enzymes and hormones.

Bile

• Bile is an alkaline, bitter-tasting liquid, made in the liver to assist in fat digestion and absorption and is stored in the gallbladder.
• Bile contains bile salts, cholesterol, and bilirubin.

Liver Functions

• Stores blood, and aids in clotting factor synthesis (vitamin K)
• Processes nutrients (carbohydrates, fats, and proteins).
• Detoxifies metabolic wastes
• Stores minerals and vitamins.

Aging and the GI System

• Oral Cavity & Esophagus: Enamel deterioration, reduced taste buds, and salivary secretion decrease which can affect chewing and swallowing in older age.
• Stomach: Changes in gastric motility, hydrochloric acid production and protective mucosal barrier, which increase the risk of ulcers.
• Liver: Reduced ability to detoxify medications, and decreased blood flow affecting medication metabolism.
• Intestines: Change in microbiota, weakened mucosal immunity, shorter and less effective villi, reduced motility and weakened anal sphincter can cause constipation, fecal impaction and incontinence
• Pancreas & Gallbladder: Fibrosis, fatty deposits and pancreatic atrophy can reduce enzyme production - causing decreased digestive enzyme production and increased gallstones.