Digestion, Absorption, and Transport: The Basics of Human Nutrition

The Digestive System

Overview

The digestive system is responsible for breaking down food, absorbing nutrients, and eliminating waste. It consists of the gastrointestinal (GI) tract and accessory organs that aid in digestion.

• Digestion: The process of breaking down food into molecules small enough to be absorbed through the intestinal wall.

• Absorption: The movement of nutrients from the GI tract into the circulatory system.

• Transport: The distribution of absorbed nutrients throughout the body via the circulatory and lymphatic systems.

• Elimination: The excretion of undigested and unabsorbed food as feces.

Organs of the GI Tract

• Mouth: Begins mechanical and chemical digestion.

• Esophagus: Transports food to the stomach.

• Stomach: Mixes food with gastric juices, forming chyme.

• Small Intestine: Major site of digestion and absorption; consists of duodenum, jejunum, and ileum.

• Large Intestine: Absorbs water and ions; forms feces.

• Sphincters: Muscular valves that regulate passage of food between organs.

• Accessory Organs: Liver, pancreas, and gallbladder, which secrete digestive juices.

Processes of Digestion

Mechanical and Chemical Digestion

• Mechanical Digestion: Physical breakdown of food by chewing, grinding, and muscular movements (peristalsis and segmentation).

• Chemical Digestion: Enzymatic breakdown of food molecules.

• Saliva: Contains amylase, which begins carbohydrate digestion.

• Bolus: The ball of food formed for swallowing.

The Role of the Epiglottis

• Epiglottis: Directs food to the esophagus and prevents entry into the trachea.

• Esophagus: Transports food from the pharynx to the stomach.

Sphincters at Work

• Lower Esophageal Sphincter (LES): Relaxes to allow bolus into the stomach; contracts to prevent reflux.

Stomach Function and Anatomy

Stomach's Role

• Stomach: Churns and mixes food with gastric juices, forming chyme.

• Chyme: Semiliquid, partially digested food that enters the small intestine.

• Pyloric Sphincter: Controls passage of chyme into the small intestine.

Anatomy of the Stomach

• Includes gastric pits and glands, chief cells, parietal cells, and mucous cells.

Small Intestine: Digestion and Absorption

Structure and Function

• Segments: Duodenum, jejunum, ileum.

• Villi and Microvilli: Increase surface area for absorption.

• Contact Time: 3 to 10 hours; fats take the longest to digest.

Structures of the Small Intestinal Wall

• Circular Folds: Slow movement and increase absorption.

• Villi: Finger-like projections containing blood and lymph vessels.

• Microvilli: Microscopic extensions that further increase surface area.

Propulsion of Food Through the GI Tract

Muscular Contractions

• Peristalsis: Wave-like contractions that move food forward.

• Segmentation: Back-and-forth contractions that mix and increase contact with absorptive surfaces.

Large Intestine: Water and Ion Absorption

Function and Anatomy

• Segments: Cecum, colon, rectum.

• Absorption: Water and ions (Na+, K+, Cl-).

• Bacteria: Break down remaining material, producing methane.

• Feces Formation: Occurs over 12-70 hours.

Accessory Organs and Their Secretions

Salivary Glands

• Dissolve food particles; produce 1 quart of saliva per day.

• Saliva contains water, mucus, electrolytes, and enzymes.

Liver

• Largest internal organ; essential for metabolism and detoxification.

• Produces bile salts for fat digestion.

• Site of alcohol metabolism.

Alcohol and the Liver

• Alcohol impairs liver function and can lead to fatty liver, hepatitis, and cirrhosis.

• 2 or more drinks per day can cause permanent damage.

Gallbladder

• Receives, concentrates, and releases bile into the small intestine.

Pancreas

• Endocrine: Releases hormones for blood glucose regulation.

• Exocrine: Secretes digestive enzymes into the small intestine.

Chemical Digestion: Enzymes and Secretions

Digestive Enzymes

Enzymes catalyze the breakdown of macronutrients. Their actions are summarized below:

Organ or Gland	Enzyme	Action	Nutrient
Salivary glands	Salivary amylase	Begins digestion of starch	Carbohydrates
Stomach	Pepsinogen → Pepsin	Hydrolyzes polypeptides	Protein
Stomach	Gastric lipase	Begins digestion of lipids	Lipids
Pancreas	Pancreatic amylase	Digests starch	Carbohydrates
Pancreas	Trypsinogen → Trypsin	Hydrolyzes proteins	Protein
Pancreas	Chymotrypsinogen → Chymotrypsin	Hydrolyzes proteins	Protein
Pancreas	Carboxypeptidase	Hydrolyzes peptide chains	Protein
Small intestine	Sucrase, Maltase, Lactase	Digest sugars	Carbohydrates
Small intestine	Peptidase, Tripeptidase	Digest peptides	Protein
Small intestine	Lipase	Digests monoglycerides	Lipids

GI Tract Secretions

Secretion	Pathway	Action
Saliva	Salivary glands	Moistens food, contains amylase
Hydrochloric acid (HCl)	Parietal cells in stomach	Denatures protein, activates pepsinogen
Intrinsic factor	Parietal cells in stomach	Needed for vitamin B12 absorption
Mucus	GI tract lining	Lubricates and protects mucosa
Intestinal juice	Crypts in small intestine	Contains digestive enzymes
Bile	Liver/gallbladder	Emulsifies fats
Bicarbonate ions	Pancreas	Raises pH, neutralizes stomach acid

Absorption of Nutrients

Mechanisms of Absorption

• Passive Diffusion: Movement from high to low concentration; no energy required.

• Facilitated Diffusion: Movement from high to low concentration via carrier protein; no energy required.

• Active Transport: Movement from low to high concentration via carrier protein; energy required.

• Endocytosis: Cell engulfs nutrient in a vesicle.

Fluid Absorption in the Large Intestine

• Water absorbed by osmosis.

• Sodium absorbed by active transport.

Regulation of Digestion: Nervous and Hormonal Control

Nervous System

• Signals when to eat, drink, and stop.

• Enteric Nervous System: Controls digestive organ function and secretion.

Hormonal Regulation

• Ghrelin: Stimulates hunger.

• Gastrin: Stimulates HCl and gastric enzyme release.

• Secretin: Stimulates pancreatic bicarbonate release.

• Cholecystokinin (CCK): Stimulates pancreatic enzyme and bile release; slows gastric motility.

• Gastric Inhibitory Peptide (GIP): Inhibits gastric motility and secretion.

• Peptide YY: Slows appetite.

• Leptin: Suppresses appetite.

Transport of Nutrients Throughout the Body

Cardiovascular and Lymphatic Systems

• Water-soluble nutrients: Absorbed into the bloodstream via the hepatic portal vein to the liver (includes carbohydrates, amino acids, water-soluble vitamins).

• Fat-soluble nutrients: Absorbed into the lymphatic system (includes fat-soluble vitamins, long-chain fatty acids).

Common Digestive Disorders

Overview

• Gastroesophageal Reflux Disease (GERD): Stomach acid enters esophagus.

• Esophageal Cancer: Often linked to smoking and alcohol.

• Belching: Caused by rapid eating, soda, anxiety, smoking.

• Foodborne Illness: Result of contaminated food.

• Ulcer: Sore in stomach/duodenum lining, often due to H. pylori.

• Gallstones: Cholesterol stones in bile duct/gallbladder.

• Celiac Disease: Autoimmune gluten intolerance.

• Flatulence: Intestinal gas from carbohydrate fermentation.

• Diarrhea: Watery stool, often from pathogens.

• Constipation: Dry, hard stool; requires more fiber, water, exercise.

• Hemorrhoids: Swollen veins in rectum/anus.

• Irritable Bowel Syndrome (IBS): Overresponsive colon.

• Ulcerative Colitis: Chronic colon inflammation, can be genetic.

• Crohn's Disease: Inflammation throughout GI tract.

• Colon Cancer: Third leading cause of cancer death; starts with polyps.

Progression of Alcohol Liver Disease

• Normal Liver → Fatty Liver (from overconsumption) → Cirrhosis (permanent damage and scarring)

Additional info: These notes expand on the provided slides with definitions, examples, and academic context for each process and disorder.