Mechanisms of Digestion

The Digestive System The Digestive System • Acts as “disassembly line,” enabling us to acquire nutrients from what we eat • Nutrients can be energy-yielding or not, but all are necessary for body maintenance, growth, metabolism, & reproduction • Major aspects: – Feeding (food acquiition & ingestion) – Digestive processes Food can only be broken down into forms usable by the body via chemical digestion. 1. True 2. False Mechanisms of Digestion • Mechanical digestion – Breakdown of food into smaller bits via teeth/tongue (chewing, or mastication), churning action of stomach, & rhythmic contractions of the small intestine • Chemical digestion (SEE NOTES) – Breakdown of food into smaller bits via digestive enzymes which break (by hydrolysis) chemical bonds holding basic food units together, e.g. carbohydrates  monosaccharides, proteins  amino acids Basic Processes of Digestion – Motility – Secretion – Digestion – Absorption Will be discussed as we cover the digestion pathway Motility (movement) • Muscular movements w/in digestive tract – Occurs via contractions of smooth muscles around digestive tract (autorythmic – responds to stretching, etc) – Influenced by neuronal & hormonal controls • Basic types: – Peristalsis – Segmentation • Peristalsis – Propels contents forward via waves of contractions in adjacent segments (waves of peristaltic contractions) • Segmentation – Mixes contents via simultaneous contractions of multiple non-adjacent segments mechanical digestion • Fig. 25.26: Types of digestive tract motility (segmentation vs. peristalsis) Basic Anatomy (Fig. 25.1) • Digestive system includes the digestive tract plus all accessory organs Accessory Organs/Structures • • • • • • Teeth Tongue Salivary glands Liver Gall bladder Pancreas Digestive Tract • Mouth • Esophagus • Gastrointestinal (GI) tract – Stomach – Small intestine – Large intestine (colon) • Rectum • Anus Journey through the Digestive System Overview of Pathway Mouth Esophagus Stomach Liver Pancreas Small intestine Large intestine out Mouth • Presence of food stimulates salivation • Saliva - secreted by salivary glands Fig. 25.9: the extrinsic salivary glands – located outside mouth, secretions enter mouth thru ducts Also have intrinsic salivary glands in tongue, inside lips, & inside cheeks What is Saliva? pH 6.8 – 7 Mostly water Electrolytes Mucus – lubricates food for easy passage down esophagus • Antibodies & antibacterial agents • Lingual lipase – inactive until encounters hi acidity in stomach • Salivary amylase – initiates carbohydrate digestion • • • • Functions of Saliva • Assists in swallowing by moistening food • Begins first stage of carbohydrate digestion • Kills some bacteria • Allows taste sensation (taste must happen in soltn for taste buds) • Neutralizes food acids (via HCO3-) Esophagus • In mouth, food formed into a bolus, then swallowed • Esophagus = muscular tube w/ primary function of getting bolus to stomach – NO digestion occurs here! • Swallowing is complex process coordinated by swallowing center in medulla oblongata – Bolus enters esophagus  stimulates stretch receptors  (afferent signals) medulla  (efferent impuses) initiate peristalsis  propels bolus toward stomach Swallowing (Fig. 25.11) • Misdirection of bolus prevented by tongue (blocks oral cavity), soft palate (blocks nasal cavity), & epiglottis (blocks trachea) Don’t Try to Swallow & Talk at the Same Time • Inhaling, talking, or laughing while swallowing prevents epiglottis from blocking entry into larynx  triggers cough reflex people.eku.edu/ritchisong/epiglottis • Bolus from esophagus enters stomach when gastroesophageal sphincter opens • Stomach functions: – Food storage (capable of lots of stretch) – Continues mechanical & chemical digestion (initiated in mouth) Stomach (Fig. 25.12) Stomach Capacity • Stomach is capable of extensive stretching (up to 80x its empty volume!) – Empty: 50 ml volume – Average meal: 1–1.5L – Max. capacity: 4L • Why many people who’ve had stomach bypass surgery end up gaining back much of the weight they initially lose • Outer layers (serosa) – smooth muscle • Inner layer (mucosa) – lots of invaginations which increase SA for lots of gastric glands Stomach Wall (Fig. 25.13) Gastric Glands of Stomach • Contain specialized cells w/ specific functions • Mucous cells: secrete mucus that protects mucosa (inner lining) • Chief cells (most abundant): secrete pepsinogen (inactive form of pepsin, a protease) – Zymogen =any inactive form of an enzyme (see notes) Fig. 25.13 • Enteroendocrine cells (e.g. G cells): secrete hormones regulating digestion (don’t need to know specific enteroendocrine cells- but know group) • Parietal cells: secrete hydrochloric acid (HCl) & Intrinsic factor Gastric Glands of Stomach (cont’d.) – Intrinsic factor: needed for vitamin B12 absorption (thus, Hb synthesis) in small intestine Fig. 25.13 Functions of HCl in Stomach Secretions • Activates pepsinogen  pepsin • Activates lingual lipase to begin some lipid digestion (about 10%) • Destroys ingested bacteria and other pathogens • Helps liquefy food by breaking up plant cell walls & animal connective tissue (along w/ mixing actions)  chyme • Converts ingested Fe+3  absorbable Fe+2 needed for Hb synthesis Gastric (Stomach) Motility • Stomach movement • Under hormonal & neural control • As you begin to swallow, swallowing center in medulla oblongata signals stomach to relax as it prepares to receive food  arriving bolus stretches stomach  brief contraction followed by relaxation of smooth muscles to accommodate more food …. Gastric Motility (cont’d.) • Stomach undergoes peristaltic contractions & churning actions which enhance mechanical & chemical digestion of contents, turning contents into chyme – This “gastric mixing” primarily occurs in the antrum of stomach Gastric Motility (cont’d.) • Typical meal emptied from stomach (through opened pyloric sphincter) in 4 h (less if more liquids, more if hi fat) • Animation… • Occurs when gastroesophageal sphincter relaxes while stomach & upper small intestine (duodenum) contract spasmodically w/ help from strong abdominal & diaphragm contractions • Regulated by brain (vomiting center of medulla) • Causes: overstretching of stomach, bacterial toxins, intense pain, psychological stimuli, etc. Vomiting Gastric (Peptic=general name) Ulcers • Gastritis (inflammation of stomach mucosa) can cause HCl & pepsin to “breach” gel-like, protective mucous barrier & erode stomach wall • Left untreated, can lead to organ perforation & fatal bleeding • Causes: bacterium that destroys mucous & epithelial lining, smoking, aspirin, NSAIDS (min. mucous coat) At this stage in the digestive journey… • Chyme leaves stomach w/ – proteins broken down into smaller peptides via action of pepsin – small proportion of fats (10%) broken down into monoglycerides & fatty acids via salivary lipase – carbohydrates (starch f/ plant & glycogen f/ animal) broken down into disaccharide (maltose) via continued action of salivary amylase • Divided into 3 sections (Fig. 25.24) Small Intestine (SI) – Duodenum: (small ~ 10in) begins at pyloric sphincter (ends at duodenojejunal flexure); receives stomach contents, pancreatic juices, & bile from liver – Jejunum – Ileum SI Histology • Form follows function: SI microanatomy important to understanding its function – SI completes digestion of food, & most of all nutrient absorption occurs here SI Histology (cont’d.) • Outer serosa contains smooth muscle layers • Middle submucosa contains Brunner’s glands – secrete HCO3 rich mucus to help neutralize stomach acids • Inner mucosa (= intestinal mucosa) - arranged in circular folds faces lumen =intestinal folds SI Histology (cont’d.) – Fig. 25.25 • Intestinal mucosa arranged in simple columnar epithlm • Each villus contains different cells & structures… SI Histology (cont’d.) – Fig. 25.25 • Each villus made up of – Simple columnar epithelium made up of absorptive cells (enterocytes) & govlet cells (secrete mucus) – Central lacteal (lymph vessel) – Capillaries (blood supply) Recall This? • Fig. 5.6 • Mucosa of small intestine • Note columnar cells (enterocytes), goblet cells, basement membrane, & “brush border” The Enterocyte • Intestinal mucosa also called “brush border” due to microvilli of ea. enterocyte • Microvilli An Enterocyte – Gives “brushy” appearance – Invaginations of enterocyte plasma membrane – Lots of SA for enzymes (= brush border enzymes) • Structure of intestinal mucosa allows for a 600x greater surface area than if a flat surface – Intestinal folds  3x increase in SA – Villi  10X increase in SA – Microvilli  20x increase Total 600x