Digestive System Functions and Organs

Ingestion

Taking food in, occurs in the mouth.

Mechanical digestion

Physical process that breaks food down into smaller pieces without changing the chemical nature of the food.

Chemical digestion

The process of breaking covalent bonds of molecules in food to produce smaller molecules; involves the secretion of enzymes by digestive organs.

Propulsion

Movement of food along the digestive tract, including deglutition (swallowing) and peristalsis (contractions of muscularis externa).

Absorption

Transport of nutrients from the lumen of the GI tract into the blood.

Defecation

Elimination of indigestible substances in the form of feces.

Oral cavity (Mouth)

Location for ingestion, mastication, and early stages of chemical digestion for carbohydrates and fats; voluntary phase of deglutition.

Pharynx

Muscular passageway for the second phase of deglutition.

Esophagus

A muscular tube that carries a bolus from the pharynx to the stomach during the final phase of deglutition.

Stomach

Location for churning, secretion of acidic gastric juice, and formation of chyme from a bolus.

Small intestine

Primary location of chemical digestion and nutrient absorption.

Large intestine

Absorbs water and some nutrients, forms feces, and eliminates feces in the process of defecation.

Salivary glands

Produce saliva that lubricates food, contains enzymes for chemical digestion, and aids in barrier defenses.

Pancreas

Produces digestive enzymes and bicarbonate for neutralizing chyme.

Liver

Produces bile for emulsifying lipids to aid in their digestion and absorption.

Gall bladder

Stores, concentrates, and releases bile into the small intestine.

Deciduous teeth (primary dentition)

First set of teeth, 20 in total, emerging from ages 6 to 24 months.

Permanent teeth (secondary dentition)

Second set of teeth, 32 in total, most emerging from ages 6-12 years, with the third molar emerging at 17-25 years.

Incisors

Flat, sharp front teeth for cutting when biting into food.

Cuspids (canines)

Fang-like teeth with a pointed edge to pierce and tear up tough or fleshy foods.

Bicuspids (premolars)

Broad teeth with two rounded cusps useful for mashing foods.

Molars

Posterior teeth with several pointed cusps used to crush food; the most posterior are commonly referred to as wisdom teeth.

Crown

Exposed portion of the tooth covered with enamel, the hardest substance in the body.

Neck

Contacts the gums and connects the crown to the root of the tooth.

Root

Connected to the dental alveolus by the periodontal ligament; contains the pulp cavity, dentin, and cementum.

Mucosa

Deepest layer of the alimentary canal, containing epithelial tissue lining the lumen, lamina propria, and muscularis mucosae.

Submucosa

Layer of dense irregular connective tissue containing blood vessels, lymphatic vessels, glands, and a plexus of nervous tissue.

Muscularis

Layer of muscle tissue, typically containing smooth muscle organized into inner circular and outer longitudinal layers, with a myenteric plexus in between.

Serosa (visceral peritoneum)

Layer of serous membrane covering the superficial surface of the alimentary canal and helping to anchor it to the abdominopelvic cavity walls.

Stratified Squamous

Epithelial tissue in the esophageal mucosa, forming a protective lining.

Adventitia

Dense irregular connective tissue that anchors the esophagus to its surroundings.

Cardia

Region where the esophagus empties into the stomach.

Fundus

The dome-shaped top of the stomach.

Body

The largest portion of the stomach.

Pylorus

The terminal portion of the stomach that abuts the first portion of the small intestine.

Gastric pits

Indentations of the surface of the stomach epithelium, connecting to glands that secrete gastric juice.

Duodenum

Short proximal segment of the small intestine responsible for most chemical digestion and some nutrient absorption.

Jejunum

Long middle segment of the small intestine responsible for most nutrient absorption.

Ilium

Longest and most distal segment of the small intestine responsible for some nutrient absorption and containing Peyer's patches.

Cecum

The first part of the large intestine, receiving the contents of the ileum.

Colon

The region of the intestine distal to the ileum, including the ascending, transverse, descending, and sigmoid.

Enteric nervous system

Nervous tissue in the walls of the GI tract that functions as sensory receptors, local integration centers, and motor neurons for intrinsic control mechanisms.

Salivary amylase

Initiates the breakdown of carbohydrates in the oral cavity, produced by salivary glands.

Pancreatic amylase

Completes the conversion of polysaccharides to maltose and limit dextrins in the small intestine, produced by the pancreas.

Pepsin

Enzyme that initiates chemical digestion of proteins in the stomach, produced by chief cells as the inactive precursor pepsinogen.

Lipase

Enzyme that catalyzes the hydrolysis of triglycerides to produce fatty acids and monoglycerides.

Bile

Emulsifies fats in the duodenum, produced by the liver and stored in the gall bladder.

Autonomous single-unit smooth muscle

Interstitial cells of cajal display rythmic, spontaneous variations in membrane potential

Chemo/Mechanoreceptors

Located in the alimentary canal. Detect presence of food to stimulate intrinsic and extrinsic control mechanisms to regulate the organs of the digestive system.

Enteric motor neurons

coordinate activity of smooth muscle and exocrine glands

excitatory neurons

Release ACTH

Inhibitory neurons

Release variety of neurotransmitters (ex. adenosine)

Extrinsic nerves

Reflexes responding to stimuli within or outside the GI tract

Parasympathetic efferent fibers

Release ACTH to stimulate secretions and contractions

Sympathetic division

Inhibits secretions and contractions

G cells

Produce gastrin to stimulate churning and secretion of gastric acid in the stomach. (HCl)

S cells

Produce secretin to inhibit churning and secretion of gastric acid. (-) feedback in gastric secretion. (+) feedback in pancreatic secretion of HCO-3

I cells

Produce CCK to inhibit churning and secretion. (-) feedback in gastric secretion. (+) feedback in pancreatic secretion of enzymes and bile secretion.

Intrinsic control mechanism

Enteric neurons release ACh to stimulate churning and secretion

Cephalic phase of gastric secretion

Extrinsic mechanisms stimulated by brain when thinking about food. Produced by the vagus (CNX) nerve

Gastric phase of gastric secretions

Intrinsic mechanisms stimulated when a bolus enters the stomach

Intestinal phase of gastric secretions

Extrinsic mechanism stimulated when chyme enters duodenum

Positive feedback mechanism of intestinal phase

G cells in duodenum release intestinal gastrin to stimulate secretion

Enterogastric reflex

Decreases release of Ach by parasymp. fibers to decrease secretion.

Parotid glands

The largest salivary glands located lateral to the masseter. Produces thin watery saliva containing enzymes.

Sublingual glands

Produces thick, slipper mucous saliva. Most anterior salivary glands located in the floor of the mouth inferior to the tongue.

Submandibular salivary gland

Located posterior to sublingual glands in the floor of the mouth. Produces thick, slippery mucous containing some enzymes.

Salivatory nuclei

In the brain stem regulate autonomic efferent fibers traveling to the salivary glands to regulate the volume of saliva production

Parasympathetic division regulating saliva production

ACH stimulates larger volume of saliva

Sympathetic division regulating saliva

Postganglionic fibers release NOE stimulating smaller volume of saliva to lubricate mouth.

Simple reflex

ingested food—> gustatory receptors—> salivary center—>parasympathetic efferent fibers release acetylcholine in salivary glands

Conditioned reflex

thought of food—> salivary center—> parasympathetic efferent fibers release ACh

Bicarbonate

(HCO3-) reacts with H+ to neutralize acidity of chyme

Brush border enzymes

Catalyze the hydrolysis of limit dextrins & disaccharides to monosaccharides

Sodium glucose linked transporters

A family of symporter proteins; cotransport sodium and glucose into cells. Transports glucose and galactose into enterocytes across apical surface

Glut

Enables facilitated diffusion of glucose and other water-soluble nutrients.

Hepatocytes

Epithelial cells of the liver constantly produce bile acids and secret them into bile canaliculi to empty ducts.

hepatopancreatic sphincter

At the base of common bile duct; prevents bile from entering duodenum between meals.

Regulation of bile release

Fatty chyme enters the small intestine to stim release of CCK for contraction of gallbladder and relaxation of sphincter to release bile into duodenum

Bile salts

Formed when bile acids react with a base. 5% lost in feces

Enterohepatic circulation

Bile salts reabsorbed back into blood in ileum and returned to liver by hepatic portal system.

Absorption of fat

Lipid soluble nutrients are absorbed by simple diffusion. Fatty acids and monoglycerides—> triglycerides.

Chylomicrons

Lipoprotein particles released into lamina propria of the mucosa—> lympathic lacteals through exocytosis

Parietal cells

Located primarily in the middle region of gastric pits. Release HCl

HCl

responsible for high acidity of gastric juice and activates pepsin. Kills bacteria from food and denatures proteins

Intrinsic factor

Glycoprotein necessary for absorption of Vitamin B12 in small intestine

Chief cells

Basal region of gastric pits. Secretes pepsinogen.