Digestive System: Functions and Organs

• The digestive system breaks down food into smaller molecules.
• The smaller molecules are absorbed and used for energy, growth, and repair.
• The digestive system consists of a series of organs working together.

Main Functions

• Ingestion occurs when food is taken into the body through the mouth.
• Digestion breaks down food into smaller molecules using mechanical and chemical processes.
• Absorption transports digested molecules from the digestive tract into the bloodstream.
• Elimination removes undigested and unabsorbed material from the body as feces.

Organs of the Digestive System

• The digestive system has two main components.
• The alimentary canal (or digestive tract) is one component.
• Accessory organs are the second component.
• The alimentary canal is a continuous tube from the mouth to the anus.
• Accessory organs aid digestion but are not part of the alimentary canal.

Mouth

• The mouth is the entry point for food.
• Mechanical digestion starts in the mouth with chewing (mastication).
• Chewing increases the surface area of food.
• Saliva, secreted by salivary glands, begins chemical digestion.
• Saliva contains amylase, which breaks down starch into sugars.
• The tongue manipulates food, forming a bolus.
• A bolus is a soft, rounded mass that is easy to swallow.

Esophagus

• The esophagus connects the pharynx (throat) to the stomach.
• The esophagus transports the bolus to the stomach via peristalsis.
• Peristalsis involves wave-like muscle contractions that propel food along the digestive tract.
• The lower esophageal sphincter (LES) prevents stomach contents from refluxing back into the esophagus.

Stomach

• The stomach is a J-shaped organ that mixes and stores food.
• Gastric juice, containing hydrochloric acid (HCl) and pepsin, is secreted by the stomach.
• HCl lowers the pH of the stomach, creating an acidic environment.
• The acidic environment kills bacteria and denatures proteins.
• Pepsin is a protease that breaks down proteins into smaller peptides.
• The stomach’s muscular walls churn and mix food with gastric juice, forming chyme.
• The pyloric sphincter controls the release of chyme into the small intestine.

Small Intestine

• The small intestine is the primary site of nutrient absorption.
• The small intestine consists of the duodenum, jejunum, and ileum.
• The duodenum receives chyme from the stomach.
• The duodenum receives digestive enzymes from the pancreas.
• The duodenum receives bile from the liver and gallbladder.
• The pancreas secretes pancreatic juice containing enzymes that digest carbohydrates, proteins, and fats.
• Bile, produced by the liver and stored in the gallbladder, emulsifies fats, which aids digestion.
• The jejunum and ileum absorb nutrients into the bloodstream.
• The inner lining of the small intestine is highly folded and covered with villi and microvilli.
• Villi and microvilli increase the surface area for absorption.

Large Intestine

• The large intestine absorbs water and electrolytes from undigested material.
• The large intestine consists of the cecum, colon, rectum, and anus.
• The cecum is a pouch that receives waste material from the small intestine.
• The colon compacts and stores undigested material, creating feces.
• The rectum stores feces until they are eliminated through the anus.
• The large intestine contains a diverse community of gut bacteria that aid digestion and immunity.

Accessory Organs

• The liver produces bile, aiding fat digestion and absorption, and plays a role in detoxification and metabolism.
• The gallbladder stores and concentrates bile produced by the liver, releasing it into the small intestine.
• The pancreas secretes pancreatic juice containing digestive enzymes and bicarbonate ions to neutralize stomach acid.

Digestive Processes

• Mechanical digestion involves the physical breakdown of food into smaller pieces, including chewing and churning.
• Chemical digestion involves the enzymatic breakdown of food into smaller molecules.
• Enzymes like amylase, pepsin, and lipase break down carbohydrates, proteins, and fats, respectively.

Absorption Mechanisms

• Nutrients are absorbed into the bloodstream through active transport, passive transport, facilitated diffusion, and endocytosis.
• Active transport requires energy to move nutrients against their concentration gradient.
• Passive transport allows nutrients to move down their concentration gradient without energy.
• Facilitated diffusion involves nutrients moving down their concentration gradient with the help of membrane proteins.
• Endocytosis involves the cell membrane engulfing larger molecules.

Regulation of Digestion

• Hormonal and nervous mechanisms regulate the digestive system.
• Hormones like gastrin, secretin, and cholecystokinin (CCK) regulate gastric secretion, pancreatic enzyme release, and gallbladder contraction.
• The enteric nervous system controls motility and secretion within the digestive tract.
• The autonomic nervous system (parasympathetic and sympathetic branches) influences digestive function.

Common Digestive Disorders

• Gastroesophageal Reflux Disease (GERD) occurs when stomach acid refluxes into the esophagus, causing heartburn and inflammation.
• Peptic ulcers are sores that develop in the lining of the stomach or small intestine.
• Peptic ulcers are often due to bacterial infection (Helicobacter pylori) or prolonged use of NSAIDs.
• Inflammatory Bowel Disease (IBD) involves chronic inflammation of the digestive tract, including Crohn's disease and ulcerative colitis.
• Irritable Bowel Syndrome (IBS) affects the large intestine, causing abdominal pain, bloating, and changes in bowel habits.
• Celiac Disease is an autoimmune disorder triggered by gluten, leading to damage of the small intestine and malabsorption of nutrients.
• Gallstones are hard deposits that form in the gallbladder, causing pain and inflammation.