A&P 2 Exam 1 - Digestive System

General Functions

Take in food, break it down into nutrient molecules, absorb molecules into the bloodstream, and rid body of any indigestible remains.

Divisions

Alimentary Canal (GI Tract or Gut), Accessory Digestive Organs.

Alimentary Canal

Digests food (breaks down into smaller fragments); continuous muscular tube that run from mouth to anus; absorbs fragments through lining into blood; organs include mouth, pharynx, esophagus, stomach, small intestine, large intestine, and anus.

Accessory Digestive Organs

Teeth (oral cavity), tongue (oral cavity), gallbladder, digestive glands: salivary glands, liver, pancreas.

Digestive Processes

Ingestion, propulsion (swallowing and peristalsis), mechanical breakdown (movement such as crushing, segmentation), digestion (chemical), absorption, and defecation.

Ingestion

Eating.

Propulsion

Movement of food through the alimentary canal, which includes swallowing and peristalsis (major means of propulsion of food that involves alternating waves of contraction and relaxation)(muscular contractions).

Mechanical Breakdown

Includes chewing, mixing food with saliva, churning food in stomach, and segmentation; segmentation is local constriction of intestine that mixes food with digestive juices.

Digestion (chemical)

Series of catabolic steps that involve 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.

Peritoneum

Serous (serous means fluid or lubricated) membranes of abdominal cavity that consists of visceral (organ) peritoneum and parietal peritoneum.

Visceral Peritoneum

Membrane on external surface of most digestive organs.

Parietal Peritoneum

Membrane that lines body wall.

Peritoneal Cavity

Fluid-filled space between two peritoneums; fluid lubricates mobile organs.

Mesentery

Double layer of peritoneum; layers are fused back to back; holds organs in place and also stores fat; are double layers of peritoneum in abdominal cavity; continuations of visceral and parietal peritoneum.

Retroperitoneal Organs

Located outside, or posterior to, the peritoneum.

Layers of Alimentary Canal

Mucosa, submucosa, muscularis externa, serosa.

Mucosa

Tunic (wall) layer that lines lumen (cavity inside organ); functions include secreting mucus, digestive enzymes, and hormones, absorbing end products of digestion, and protesting against infectious disease; made up of three sublayers: epithelium, lamina propria, muscularis mucosae.

Submucosa

Consists of areolar connective tissue (loose); contains blood and lymphatic vessels, lymphoid follicles, and submucosal nerve plexus that supply surrounding GI tract tissues; has abundant amount of elastic tissue that help organs to regain shape after storing large meals.

Muscularis Externa

Muscle layer responsible for segmentation and peristalsis; contains inner circular muscle layer and outer longitudinal layers; circular layer thickens in some areas to form sphincters (valves).

Serosa

Outermost layer, which is made up of the visceral peritoneum; formed from areolar connective tissue covered with mesothelium (single layer of squamous epithelium) in most organs.

Mouth

Where food is chewed and mixed with enzyme-containing saliva, beginning the process of digestion,, and swallowing process is initiated; Also called oral (buccal) cavity; Oral orifice is the anterior opening; mouth walls lined with stratified squamous epithelium; cells of gums, hard palate, and part of tongue are keratinized for extra protection.

Palate

Forms roof of mouth and has two distinct parts.

Hard Palate

Formed by palatine bones and palatine processes of maxillae with midline ridge called raphe; mucosa is slightly corrugated to help create friction against tongue.

Soft Palate

Fold formed mostly of skeletal muscle; closes off nasopharynx during swallowing; the uvula is a fingerlike projective that faces downward from free edge of soft palate.

Tongue

Occupies floor of mouth; composed of interlacing bundles of skeletal muscle; functions include: gripping, repositioning, and mixing, formation of bolus (mix of food and saliva), and initiating swallowing, speech, and taste.

Salivary Glands

Functions of saliva; cleanses mouth, dissolves food chemicals for taste, moistens food, etc.; breaks down starch with enzyme amylase; glands include parotid, submandibular, sublingual; saliva made mostly of water (97-99.5%) and electrolytes (ions); salivary amylase and lingual lipase; chemically break down carbs and lipids; salivary glands activated by parasympathetic nervous system.

Teeth

Uses mastication (process of chewing that tears and grinds food into smaller fragments; primary detention consists of 20 deciduous teeth or milk or baby teeth; permanent teeth include: incisors, canines, premolars, molars.

Pharynx and Esophagus

One allows passage of food, fluids, and air while the other is a flat muscular tube that runs from laryngopharynx to the stomach; gastroesophageal sphincter keeps orifice closed when food is not being swallowed.

Deglutition (swallowing)

3 phases: buccal (oral cavity), pharyngeal, esophageal; Buccal is voluntary, tongue pushes food into oropharynx; Pharyngeal is involuntary, all routes blocked except esophagus; Esophageal is involuntary.

Peristalsis

Wavelike contractions that alternate between the longitudinal and circular muscle layers move the food into the stomach.

Stomach

Temporary storage (compartments) tank that starts chemical breakdown of protein digestion; converts bolus into paste-like chyme (soup); when empty, stomach mucosa forms many folds called rugae; muscularis externa has circular and longitudinal smooth muscle layers, also has a third layer called inner oblique layer which allows stomach not only to churn, mix, and move chyme but also to pummel (beat) it; mucosa layer also modified: Consists of simple columnar epithelium but also dotted with gastric pits, leading into gastric glands; gastric glands produce gastric juices.

Cells within Gastric Glands

Mucous neck cells (secrete thin, acidic mucus), parietal cells (secretes hydrochloric acid (HCI) which activates pepsin that breaks down protein and intrinsic factor which glycoprotein is required for absorption of vitamin B12), chief cells (secretes pepsinogen which is an inactive enzyme that’s activated to pepsin by HCI and lipases which digests 15% of lipids), and enteroendocrine cells (secrete chemical messengers into lamina propria in which some act as paracrines (gastrin)).

Liver

Filters blood, manufactures plasma proteins (lipid hormones), stores and recycles vitamins, minerals and nutrients and activates vitamin D, and processes most of the nutrients absorbed in the GI tract; produces bile.

Bile

Hepatocytes are liver cells that produce this; transported down the hepatic duct to the gallbladder, which only stores it; contains bilirubin which is a waste product from breaking down the RBC’s and bile salts which function in fat emulsification (breaks down into droplets) and absorption.

Pancreas

Exocrine function is producing pancreatic juice; cells involved are acini cells (clusters of secretory cells that produce enzymes), ducts (secrete to duodenum via main pancreatic duct); pancreatic juice is a watery, alkaline solution (pH 8) to neutralize acidic chyme coming from stomach, also includes electrolytes and digestive enzymes; proteases (prevents self-digestion, includes trypsin and chymotrypsin), amylase (for carbs), lipases (for lipids), and nucleases (for nucleic acids, DNA RNA)

Small Intestine

Major organ for digestion and absorption (90% of absorption happens here); duodenum, jejunum, and ileum; structural modifications provide huge surface area for nutrient absorption; modifications include circular folds, villi, and microvilli.

Circular Folds

Permanent folds that force chyme to slowly spiral through lumen, allowing more time for nutrient absorption.

Villi

Fingerlike projections of mucosa with a core that contains dense capillary bed and lymphatic capillary for absorption.

Microvilli

Cytoplasmic extensions of mucosal cell that give fuzzy appearance called the brush border.

Main Types of Cells Found in Villi and Crypts

Enterocytes (make up bulk of epithelium, simple columnar absorptive cells bound by tight junctions and contain microvilli, produce intestinal juice), goblet cells (secrete mucus), enteroendocrine cells (source of enterogastrones), paneth cells (secrete antimicrobial agents that destroy bacteria), and stem cells (continuously divide to produce other cell types).

Large Intestine

Subdivisions include cecum, appendix (bacterial storehouse capable of recolonizing gut when necessary), colon, rectum, and anal canal; digestive processes include: residue remains in large intestine for 12-24 hours, no food breakdown occurs except what enteric bacteria digests, vitamins (made by bacterial microbiota), water, and electrolytes are reclaimed, and major functions is propulsion of feces to anus and defecation.