A&P2 the digestive system

Types of teeth

Incisors cut, canines tear/shred, premolars/bicuspids tear/shred/crush/grind, molars crush and grind (all mechanical digestion)

Tongue functions

Manoeuvres food for chewing, shapes it into a bolus, forces the bolus to the back of the mouth for swallowing

Lingual lipase

Secreted by lingual glands; breaks down about 30% of dietary triglycerides into simpler fatty acids and diglycerides

Major salivary glands

Parotid (anterior to the ear), sublingual (under the tongue), submandibular (under the mandible/jaw)

Salivary gland secretions

Parotid = watery, high in amylase; sublingual = thick, mostly mucous, little amylase; submandibular = amylase mixed with mucous

Regions of the pharynx

Nasopharynx (respiration only), oropharynx (respiration + digestion), laryngopharynx (respiration + digestion)

Nasopharynx epithelium

Pseudostratified columnar ciliated epithelium with goblet cells; traps and transports inhaled particles

Stages of deglutition

Voluntary (tongue pushes bolus into oropharynx), pharyngeal-involuntary (soft palate/epiglottis close off airway, bolus moves through pharynx), oesophageal-involuntary (peristalsis pushes bolus to stomach)

Peristalsis

Coordinated contraction and relaxation of the circular and longitudinal muscularis layers that propels a bolus or chyme forward

Oesophageal sphincters

Upper oesophageal sphincter is skeletal muscle; lower oesophageal sphincter is smooth muscle

Stomach regions

Cardia, fundus, body, pyloric part

Stomach muscle layers

Oblique, circular, and longitudinal (oblique is unique to the stomach)

Gastric secretory cells

Surface/mucous neck cells secrete mucus; parietal cells secrete HCl and intrinsic factor; chief cells secrete pepsinogen and gastric lipase; G cells secrete gastrin into the bloodstream

HCl secretion

Carbonic anhydrase forms carbonic acid from H2O + CO2, which dissociates into H+ (pumped into the lumen by H+/K+ ATPase) and HCO3- (exchanged for Cl-, which is secreted with H+ as HCl)

Chyme and gastric emptying

Chyme is the soupy liquid formed by mixing waves; gastric emptying is the periodic release of small amounts of chyme into the duodenum through the pyloric sphincter

Vomiting complications

Prolonged vomiting can cause alkalosis (high blood pH), dehydration, and damage to the oesophagus and teeth

Small intestine regions

Duodenum (~25 cm, C-shaped), jejunum (~1 m), ileum (~2 m, longest, joins large intestine at the ileocaecal sphincter)

Absorptive surface features

Circular folds (plicae circulares), villi, and microvilli increase surface area for absorption

Villus structure

Contains an arteriole, venule, blood capillary network, and a lacteal; nutrients pass through capillary or lacteal walls into blood or lymph

Segmentation vs peristalsis

Segmentation mixes chyme with digestive juices via alternating circular contractions without propelling it; peristalsis (via migrating motility complexes) propels chyme forward after a meal is absorbed

Brush border enzymes

Digestive enzymes on microvilli membranes, e.g. dextrinase, maltase, sucrase, lactase, some peptidases, nucleotide-digesting enzymes, phosphatases

Digestion end products

Carbohydrates to monosaccharides; proteins to amino acids/dipeptides/tripeptides; triglycerides to fatty acids/glycerol/monoglycerides

Large intestine regions

Caecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, anal canal, anus

Teniae coli and haustra

Teniae coli are three thickened longitudinal muscle bands; their tonic contractions gather the colon into pouches called haustra

Large intestine digestive functions

Recovery of water and electrolytes, formation and storage of faeces, fermentation of indigestible matter by bacteria

Chemical digestion in the large intestine

None occurs; it's completed in the small intestine and the large intestine produces no digestive enzymes

Bacterial fermentation products

Short-chain fatty acids, gases like methane, and vitamins B and K

Constipation causes

Irregular eating habits, spasms or obstructions, poor abdominal/pelvic floor musculature, or Hirschsprung disease (absence of ganglion cells so peristalsis can't occur)

Diarrhoea types

Fatty (fat not digested/absorbed), inflammatory (mucosa inflamed), watery (too much water secreted — osmotic, secretory, or functional)

Crohn's disease vs ulcerative colitis

Crohn's can affect any part of the GI tract through all wall layers; ulcerative colitis is limited to the mucosa of the colon and rectum, usually with rectal bleeding