Chapter3

Digestion vs. absorption

Digestion breaks down foods into nutrients. Absorption moves nutrients through the cells of the small intestine so they can be transported to other body cells.

Path of food through the digestive tract

Mouth → esophagus → upper and lower esophageal sphincters → stomach → pyloric sphincter → small intestine → ileocecal sphincter → large intestine → rectum → anus.

Peristalsis vs. segmentation

Peristalsis uses wavelike contractions to move food along. Segmentation uses periodic squeezing to mix digestive contents and help keep them moving at a reasonable pace.

Digestive secretions and enzymes

Fluids and digestive enzymes from the salivary glands, stomach, pancreas, liver (via the gallbladder), and small intestine help digest food.

Salivary glands: secretion and function

Secrete saliva, which eases swallowing. Salivary enzymes begin breaking down some carbohydrates.

Gastric glands: secretion and function

Secrete gastric juice into the stomach. Hydrochloric acid uncoils proteins, enzymes digest proteins, fluid mixes with the food, and mucus protects stomach cells.

Pancreas: secretion and function

Secretes pancreatic juice into the small intestine. Bicarbonate neutralizes acidic stomach contents, while pancreatic enzymes digest carbohydrates, fats, and proteins.

Liver and gallbladder: bile

The liver produces bile, which is stored in the gallbladder until needed.

Gallbladder: function

Releases bile into the small intestine. Bile emulsifies fat, allowing digestive enzymes to access and digest it.

Intestinal glands: secretion and function

Secrete intestinal juice containing enzymes that digest carbohydrate, fat, and protein fragments. Mucus protects the intestinal wall.

Fat digestion and lipases

Saliva and gastric juice also contain lipases, but most fat breakdown occurs in the small intestine.

Chyme

A semiliquid mixture of partially digested food that travels through the digestive tract.

Sphincters

Muscular rings that regulate the passage of digestive contents between sections of the digestive tract.

Bile’s main function

Emulsifies fats into smaller droplets so enzymes can digest them more effectively.

Bicarbonate’s function

Neutralizes acidic gastric juices when they enter the small intestine.

Fibre and digestion

Fibre passes through the gastrointestinal tract mostly undigested and unabsorbed.

Small intestine: absorption

Most nutrient absorption occurs in the small intestine. Its many folds and villi increase surface area, making absorption more efficient.

Villi

Projections lining the small intestine that increase surface area for nutrient absorption.

How nutrients enter circulation

Nutrients pass through intestinal villi and enter either the blood or lymph, depending on their properties.

Nutrients absorbed into the blood

Water-soluble nutrients and small fat fragments enter the blood.

Nutrients absorbed into the lymph

Fats and fat-soluble vitamins enter the lymphatic system first.

Blood route after nutrient absorption

Blood leaving the digestive system travels to the liver before nutrients are transported to the body's cells.

Lymph route after nutrient absorption

Nutrients absorbed into the lymph bypass the liver initially. They eventually enter the vascular system through the thoracic duct, which opens into the subclavian vein.

Lymphatic system and nutrient transport

Transports absorbed fats and fat-soluble vitamins before they enter the bloodstream.

Gut bacteria and GI health

A diverse and abundant population of bacteria supports gastrointestinal health.

Regulation of digestion and absorption

The hormonal system and nervous system coordinate and regulate gastrointestinal processes.

Gastrin, secretin, and cholecystokinin

These are hormones involved in regulating digestive processes.

Four main sections of digestion and absorption

3.1 Digestion; 3.2 Absorption; 3.3 Transportation through the circulatory systems; 3.4 Health and regulation of the GI tract.