The Digestive System Notes

Overview of the Digestive System

The digestive system functions to break down foods, release nutrients, and absorb nutrients.
Alimentary Canal Organs: Mouth, pharynx, esophagus, stomach, small and large intestines.
Accessory Digestive Organs: Teeth, tongue, salivary glands, gallbladder, liver, and pancreas which aid in breakdown of food.
The alimentary canal has four tissue layers: mucosa, submucosa, muscularis, and serosa.
The enteric nervous system provides intrinsic innervation.
The autonomic nervous system provides extrinsic innervation.

Six Activities: Ingestion, propulsion, mechanical digestion, chemical digestion, absorption, and defecation.
Ingestion: Entry of food into the alimentary canal through the mouth.
Propulsion: Movement of food through the digestive tract; includes swallowing (voluntary) and peristalsis (involuntary).
Mechanical Digestion: Physical breakdown of food, increasing surface area (e.g., mastication, churning).
Chemical Digestion: Enzymes break down food molecules into building blocks (e.g., proteins into amino acids).
Absorption: Nutrients enter the bloodstream, primarily in the small intestine. Lipids are absorbed into lacteals.
Defecation: Undigested materials removed from the body as feces.
Neural and hormonal mechanisms regulate these processes.

Mouth: Mechanical digestion (teeth, tongue) and chemical digestion (saliva).
Pharynx: Involved in both digestion and respiration. During swallowing, the soft palate rises to close off the nasopharynx, and the epiglottis folds over the glottis.
Esophagus: Muscular tube connecting the pharynx to the stomach, which uses peristalsis to move food and secretes mucus to lubricate the passage.
The upper esophageal sphincter regulates food movement from the pharynx.
The lower esophageal sphincter controls passage into the stomach.

Four Regions: Cardia, fundus, body, and pylorus.
The stomach churns food, secretes gastric juices, and absorbs certain substances.
Begins protein digestion and continues carbohydrates and fats digestion.
Food is stored as chyme and released into the small intestine through the pyloric sphincter.
Cells and Secretions:
- Parietal cells secrete HCl and intrinsic factor.
- Chief cells secrete pepsinogen.
- Mucous neck cells secrete acidic mucus.
- Enteroendocrine cells secrete hormones (e.g., gastrin).
The stomach is protected from self-digestion by a mucosal barrier.

Small Intestine:
- Three regions: duodenum, jejunum, and ileum.
- Most digestion and absorption occur here.
- Structural adaptations increase surface area (circular folds, villi, microvilli).
- Segmentation: mixes chyme.
- Migrating Motility Complexes: propel chyme.
Large Intestine:
- Regions: cecum, colon, and rectum.
- Absorbs water, forms feces, and eliminates feces.
- Bacterial flora break down carbohydrates and synthesize vitamins.
- The mucosa is rich with goblet cells that secrete mucus.
- The defecation reflex is activated by feces in the rectum.

Liver: Produces bile, which emulsifies fats through bile salts.
Pancreas: Secretes pancreatic juice containing digestive enzymes and bicarbonate ions , thus buffering of the chyme.
Gallbladder: Stores and concentrates bile and releases it into the small intestine.

The small intestine is the primary site.
Intestinal brush border enzymes and pancreatic enzymes facilitate chemical digestion.
Absorption Mechanisms: Active transport, passive diffusion, facilitated diffusion, co-transport, and endocytosis.
Lipids are emulsified by bile, forming micelles, which facilitate absorption.
Water-soluble nutrients enter blood capillaries and are transported to the liver.
Lipid-soluble nutrients enter lacteals and are transported via lymphatic vessels.
Electrolytes are absorbed via active transport. Absorption of iron and calcium depends on different requirements of the body.
Fat-soluble vitamins are absorbed with dietary lipids; most water-soluble vitamins by diffusion.
Most water is absorbed in the small intestine, some in the colon.