Overview of the Human Digestive System

Ingestion

the act of eating

Propulsion

movement of food through the alimentary canal

Mechanical breakdown

includes chewing, mixing food with saliva, churning food in the stomach, and segmentation

Segmentation

local constriction of intestine that mixes food with digestive juices

Digestion

series of catabolic steps that involves enzymes that break down complex food molecules into chemical building blocks

Absorption

passage of digested fragments from lumen of GI tract into blood or lymph

Defecation

elimination of indigestible substances via anus in form of feces

Hydrolytic enzymes

catalyze the digestion of each of the classes of macromolecules found in food.

What do hydrolytic enzymes break nucleotides down into?

Nucleosides, nitrogenous bases, sugars, and phosphates.

Extracellular digestion

the breakdown of food outside cells within compartments that are continuous with the outside of the body

Alimentary canal

the digestive tube that food ingested through the mouth and pharynx passes through

Esophagus

the tube that leads food from the mouth to the stomach

Small intestine

the site where final digestion and nutrient absorption occur over a period of 5 to 6 hours

Large intestine

the part of the digestive system where undigested material passes through in 12 to 24 hours

Caecum

a pouch in the lower abdominal cavity that receives undigested food material from the small intestine

Appendix

a small structure attached to the caecum

Sphincters

- Muscular ringlike valves that regulate the passage of material between specialized chambers of the canal

- Two sphincters, one involuntary and one voluntary, are located between the rectum and the anus.

Digestive juices

secretions from accessory glands that aid in the digestion process

ATP production

the process by which the animal uses digested molecules as fuel

Polysaccharides

complex carbohydrates that are split into simple sugars during digestion

Fats

broken down into glycerol and fatty acids during digestion

Proteins

broken down into amino acids during digestion

Nucleic acids

cleaved into nucleotides during digestion

Time for food passage

it takes 5 to 10 seconds for food to pass down the esophagus to the stomach

Time in stomach

food spends 2 to 6 hours being partially digested in the stomach

Time in large intestine

any undigested material passes through the large intestine in 12 to 24 hours

Accessory glands

Include the salivary glands, the pancreas, the liver, and the gallbladder.

Oral cavity

Initiates food processing.

Salivation

May occur in anticipation due to learned associations between eating and stimuli.

Saliva

Contains mucin, buffers, antibacterial agents, and salivary amylase.

Mucin

A slippery glycoprotein that protects the mouth lining and lubricates food.

Salivary amylase

An enzyme that begins the digestion of starch in the oral cavity by hydrolyzing starch and glycogen into smaller polysaccharides and maltose.

Tongue

Tastes food, manipulates it during chewing, and helps shape it into a bolus.

Bolus

A ball of food formed by the tongue for swallowing.

Pharynx

Also called the throat; a junction that opens to both the esophagus and the trachea.

Epiglottis

A cartilaginous flap that blocks the trachea during swallowing.

Uvula

Secretes saliva, may play a role in speech, and may prevent food from entering the nasopharynx.

Esophageal sphincter

Muscle that is contracted when not swallowing, allowing airflow through the trachea.

Peristalsis

Waves of contraction that move the bolus to the stomach.

Stomach

Stores food and performs preliminary digestion with a capacity of about 2 L.

Gastric juice

Digestive fluid secreted by the stomach epithelium with a pH of about 2.

Includes enzymes, hydrochloric acid, and mucus.

Pepsin

An enzyme in gastric juice that begins the hydrolysis of proteins.

Rugae

Accordion-like folds in the stomach that allow it to stretch.

Chief cells

Cells that secrete pepsinogen, the inactive precursor of pepsin.

Parietal cells

Cells that secrete H+ and Cl- to form hydrochloric acid in gastric juice.

Hydrochloric acid (HCl)

Acid in gastric juice that activates pepsin and digests food.

pH of gastric juice

About 2, which is acidic enough to digest iron nails.

Extracellular matrix

Structure that is disrupted by acid in gastric juice, aiding digestion.

Pepsinogen

Inactive form of pepsin secreted by specialized chief cells in gastric pits.

HCl

Secreted by parietal cells, converts pepsinogen to active pepsin in the stomach lumen.

Positive feedback

Activated pepsin can activate more pepsinogen molecules.

Enteroendocrine cells

Cells in the stomach that secrete chemical messengers.

Serotonin

Chemical messenger that causes contraction of stomach muscle.

Histamine

Chemical messenger that activates parietal cells to release HCl.

Somatostatin

Chemical messenger with various inhibitory functions.

Gastrin

Chemical messenger with stimulating functions, including HCl secretion.

Mucus

Coating secreted by epithelial cells that protects the stomach lining.

Stomach epithelium

Continually eroded and completely replaced by mitosis every three days.

Gastric ulcers

Lesions in the stomach lining caused by the acid-tolerant bacterium Heliobacter pylori.

Acid chyme

Nutrient-rich broth formed from recently swallowed meals mixed in the stomach.

Cardiac orifice

Opening from the esophagus to the stomach that dilates when a bolus arrives.

Pyloric sphincter

Regulates the passage of chyme into the small intestine.

Mesentery

Fold of tissue that attaches organs to the body wall, supplying them with blood vessels, nerves, and lymphatics.

Pancreas

Produces several hydrolytic enzymes and an alkaline solution rich in bicarbonate.

Bile

Produced by the liver, stored in the gallbladder, aids in the digestion and absorption of fats.

Emulsification

Process by which bile acids break fats into small drops to increase surface area for lipase action.

Bile pigments

By-products of red blood cell destruction in the liver, eliminated from the body with feces.

Pancreatic amylases

Hydrolyze starch, glycogen, and smaller polysaccharides into disaccharides in the small intestine.

Disaccharidases

Enzymes that hydrolyze disaccharides into monomers, built into the membranes of the intestinal epithelium.

Maltase

Enzyme that splits maltose into glucose monomers.

Sucrase

Enzyme that splits sucrose into glucose and fructose monomers.

Digestion of proteins

Completed in the small intestine, continuing the process begun by pepsin.

Enzymes in the duodenum

Dismantle polypeptides into their amino acids or into small peptides.

Trypsin and chymotrypsin

Attack peptide bonds adjacent to specific amino acids, breaking larger polypeptides into shorter chains.

Dipeptidase

Attached to the intestinal lining, splits smaller chains.

Carboxypeptidases and aminopeptidase

Split off one amino acid from the carboxyl or amino end of a peptide, respectively.

Inactive form enzymes

Trypsin, Chymotrypsin, and Carboxypeptidase are secreted in inactive form by the pancreas.

Enteropeptidase

An intestinal enzyme that converts inactive trypsinogen into active trypsin.

Active trypsin

Activates chymotrypsin and carboxypeptidase and further converts trypsinogen to trypsin.

Aminopeptidase

Secreted by the intestinal epithelium.

Digestion of nucleic acids

A team of enzymes called nucleases hydrolyzes DNA and RNA into their component nucleotides.

Fat digestion

Nearly all the fat in a meal reaches the small intestine undigested.

Bile salts

Secreted by the gallbladder into the duodenum, coat tiny fat droplets and keep them from coalescing.

Lipase

An enzyme that hydrolyzes fat molecules into glycerol, fatty acids, and monoglycerides.

Chylomicrons

Water-soluble globules formed from triglycerides coated with phospholipids, cholesterol, and proteins.

Lacteal

A lymphatic vessel at the core of each villus that transports chylomicrons.

Nutrient absorption

Occurs mainly in the jejunum and ileum, where nutrients pass the lining of the digestive tract.

Surface area of small intestine

300 m2, roughly the size of a tennis court, greatly increases the rate of nutrient absorption.

Villi

Fingerlike projections in the small intestine that increase surface area for absorption.

Microvilli

Many microscopic projections on each epithelial cell of a villus that are exposed to the intestinal lumen.

Capillaries

Microscopic blood vessels that penetrate the core of each villus.

Transport of nutrients

Nutrients are absorbed across the intestinal epithelium and then across the unicellular epithelium of capillaries or lacteals.

Passive transport

Nutrients move down their concentration gradients from the lumen of the intestine into the epithelial cells.

Active transport

Nutrients are pumped against concentration gradients by epithelial membranes, allowing higher absorption rates.

Hepatic portal vessel

The vessel that drains nutrients away from the villi and leads directly to the liver.

Hepatic portal vein

Carries nutrient-rich blood from the capillaries of the villi to the liver, then to the heart.

Liver

Regulates nutrient distribution, interconverts many organic molecules, and detoxifies many organic molecules.

Glucose concentration in blood

Blood exiting the liver usually has a glucose concentration very close to 0.1%, regardless of carbohydrate content of the meal.

Insulin and glucagon

Hormones that regulate glucose levels in the blood.

Absorption efficiency in developed countries

Typical diets lead to 80 to 90 percent absorption of the organic material in their food.