Phys M13

⭐ Digestion

Digestion is the process of converting food into absorbable nutrients through both mechanical and chemical digestion.

⭐ Mechanical vs. Chemical Digestion

Mechanical digestion physically breaks food into smaller pieces (chewing, churning), while chemical digestion uses enzymes and acids to break food into absorbable molecules.

⭐ Alimentary Canal vs. Accessory Organs

The alimentary canal directly handles food, while accessory organs (salivary glands, liver, gallbladder, pancreas) aid digestion by producing secretions.

⭐ Peristalsis

Peristalsis consists of wave-like muscular contractions that move food forward through the digestive tract from the esophagus to the rectum.

⭐ Segmentation

Segmentation is back-and-forth mixing in the small intestine that increases contact between food, digestive enzymes, and the intestinal wall for absorption.

⭐ End Products of Digestion

Carbohydrates become monosaccharides (mainly glucose), proteins become amino acids, fats become monoglycerides and free fatty acids, and nucleic acids become nitrogenous bases, sugars, and phosphates.

⭐ Site of Nutrient Absorption

Most nutrient absorption occurs in the small intestine.

⭐ Tongue Functions

The tongue positions food for chewing, helps form a bolus, contains taste buds that stimulate digestion, secretes lingual lipase, and is essential for swallowing (deglutition).

⭐ Mastication

Mastication is the first step of mechanical digestion. Chewing increases the surface area of food so digestive enzymes can work more efficiently.

Teeth Functions

Incisors cut food, canines tear food, and premolars and molars grind and crush food.

⭐ Salivary Glands

The parotid, submandibular, and sublingual glands produce saliva to lubricate food and aid swallowing.

⭐ Saliva Components

Salivary amylase begins starch digestion, mucins thicken saliva to form a bolus, bicarbonate buffers pH, and lysozyme with IgA provides immune protection.

⭐ Digestion in the Mouth

Mechanical digestion occurs through chewing, chemical digestion begins with salivary amylase and lingual lipase, and very little absorption occurs except for some drugs absorbed under the tongue.⭐ Pharynx

⭐ Three Phases of Deglutition

Swallowing occurs in three phases: voluntary (tongue pushes the bolus into the pharynx), pharyngeal (reflexes close the nasopharynx and the epiglottis blocks the airway), and esophageal (the bolus enters the esophagus).

⭐ Esophagus

The esophagus transports food from the pharynx to the stomach through peristalsis and does not perform chemical digestion.

⭐ Esophageal Sphincters

The upper esophageal sphincter opens to allow food into the esophagus, while the lower esophageal sphincter (LES) opens to let food enter the stomach and closes to prevent acid reflux.

⭐ Stomach Mechanical Digestion

The stomach mechanically digests food through strong muscular contractions that churn food into chyme while mixing it with gastric secretions.

⭐ Chyme

Chyme is the semi-liquid mixture of partially digested food and gastric secretions that leaves the stomach through the pyloric sphincter into the duodenum.

⭐ Gastrin

Gastrin is a hormone released by the stomach when food enters. It stimulates HCl and pepsinogen secretion, increases gastric motility, and increases gastric juice production. Gastrin secretion decreases when the stomach is empty or the pH becomes too low.

⭐ Hydrochloric Acid (HCl)

Hydrochloric acid denatures proteins, activates pepsinogen into pepsin, and creates the acidic environment needed for protein digestion.

⭐ Pepsin & Gastric Lipase

Pepsin begins protein digestion, while gastric lipase begins lipid digestion within the stomach.

⭐ Mucus in the Stomach

Mucus protects the stomach lining from damage by hydrochloric acid and digestive enzymes.

⭐ Intrinsic Factor

Intrinsic factor is produced by parietal cells, binds vitamin B₁₂, protects it from digestion, and allows its absorption in the ileum.

⭐ Parietal vs. Chief Cells

Parietal cells produce hydrochloric acid and intrinsic factor, while chief cells produce pepsinogen, which is activated into pepsin by HCl.⭐ Duodenum

⭐ Digestion in the Duodenum

Mechanical digestion occurs through segmentation, while chemical digestion continues with pancreatic enzymes and bile.

⭐ Pancreatic Enzymes

Pancreatic enzymes digest carbohydrates, proteins, and fats after chyme enters the duodenum.

⭐ Bile

Bile emulsifies fats, breaking large fat globules into smaller droplets so lipase can digest them more efficiently.

⭐ Early Nutrient Absorption

The duodenum begins absorption of nutrients, including iron, calcium, magnesium, and fatty acids.

⭐ Secretin

Secretin is released when acidic chyme enters the duodenum. It stimulates the pancreas to release bicarbonate, which neutralizes stomach acid.

⭐ Cholecystokinin (CCK)

CCK stimulates the pancreas to release digestive enzymes, causes the gallbladder to release bile, and slows gastric emptying.

⭐ Gastric Inhibitory Peptide (GIP)

GIP slows stomach activity and stimulates insulin release to prepare the body for nutrient absorption.

⭐ Hormones of Digestion Together

Secretin neutralizes acid, CCK promotes enzyme and bile release, and GIP slows stomach emptying while promoting insulin secretion.

⭐ Pancreatic Juice

Pancreatic juice contains bicarbonate to neutralize acidic chyme and digestive enzymes that break down carbohydrates, proteins, and fats.

⭐ Segmentation vs. Peristalsis Review

Segmentation mixes chyme to maximize digestion and absorption, while peristalsis propels contents forward through the digestive tract.

⭐ Digestive System Summary

Food undergoes mechanical and chemical digestion, nutrients are primarily absorbed in the small intestine, and digestive hormones coordinate enzyme secretion, bile release, and gastric activity.