Chapter 40 Flashcards
Ingestion
The process of taking food into the mouth, starting on its journey through the digestive tract
Digestion
A group of processes that break complex nutrients into simpler ones, thus facilitating their absoption
Mechanical
Physically breaks large chunks of food into smaller bits and pieces
Chemical
Breaks molecules apart; chemically changes food
Mechanical digestion caused by
Caused by ‘movements’ of the digestive tract.
Changes ingested food from large particles into tiny particles, facilitating chemical digestion
Churns contents of the GI lumen to mix with digestive juices and ensure contact with the surface of the intestinal mucosa, facilitating absorption
Propels food along the alimentary tract, eliminating digestive waste from the body
Mechanical digestion involves
Mastication (chewing) Deglutition (swallowing) Peristalsis and Segmentation Regulation of motility Intestinal Motility
Chewing and swallowing
Mastication (chewing): reduces size of food particles and mixes food with saliva to prepare it for deglutition
Deglutition (swallowing): a complex process involving several stages (See Figure in Textbook):
Oral stage
Pharyngeal stage
Esophageal stage
Oral stage
Mouth-> oropharynx
under voluntary control
food bolus is formed, pushed against palate by tongue, then moved back into the oropharynx
Pharyngeal stage
Oropharynx -> esophagus
Involuntary
Requires blockage of mouth, nasopharynx and larynx
Accomplished by contractions & gravity
Esophageal stage
Esophagus -> stomach
Involuntary
Accomplished by contractions and gravity
Peristalsis and segmentation are
two main types of motility produced by smooth muscle of the GI tract.
Peristalsis
wave-like ripple of the muscle layer of a hollow organ that moves matter forward in the GI tract (See Figure in Textbook). Triggered by the presence of food.
Segmentation
: a forward-and-backward mixing movement within a single segment of the GI tract. Helps breakdown food, mixes food with digestive juices and brings it in contact with the intestinal mucosa where absorption takes place (See Figure in Textbook)
Gastric (STOMACH) mobility
Food in stomach is churned and mixed (via peristaltic contractions called propulsion and retropropulsion) with gastric juices to form chyme , which exits the stomach about every 20 seconds. Emptying of stomach after a meal takes about 2 to 6 hours and is controlled by:
Hormonal mechanisms – the presence of fat in the duodenum stimulates a hormone that slows down the passage of chyme into the duodenum
Nervous mechanisms – presence of acid and distention picked up by receptors in duodenal mucosa impulses travel through vagus nerve inhibits gastric peristalsis
Intestinal motility
Includes peristalsis and segmentation
Segmentation
O ccurs in duodenum and 1st part of jejunum; mixes chyme with digestive juices from pancreas, liver, & intestinal mucosa
Peristalsis
moves chyme through rest of small intestine large intestine.
Takes 5 hours for chyme to travel through length of small intestine
Regulated by stretch reflexes and the hormone CCK (cholecystokinin)
Chemical changes in food result from
Hydrolysis
Digestive enzymes
Function in lumen of digestive tract (extracellular enzymes)
All digestive enzymes = hydrolases breakdown food molecules using water
Specific in action (lock-and-key)
Require specific pH and temperature for optimal function
Digestive enzyme reactions do not go in reverse!
Continually synthesized since they are continually destroyed or eliminated from body – even though they are not used up in the reaction they catalyze/assist
Most are sythesized and secreted as inactive proenzymes
Carbohydrates
Saccharide compounds
Begins in the mouth (salivary amylase)
None occurs in stomach
Most occurs in small intestine
Polysaccharides are hydrolzed by
Amylase to form disaccharides
Carbohydration digestion happens in the
Small intestine
Maltose
Glucose + glucose
Sucrose
Glucose + fructose
Lactose is
Glucose + galactose
Proteins
Amino acids
Begins in stomach; completed in small intestine
Main proteases are:
Pepsin: Gastric juice, partially digests proteins
Trypsin and chymotrypsin: Pancreatic enzymes, continue digestion of proteins
Peptidases: Intestinal enzymes, complete digestion of protein amino acids
Fat
Glycerol + fatty acids
Fats must be emulsified by bile before being digested
Bile - contains no enzymes but emulsifies fats
Pancreatic lipase - changes emulsified fats to fatty acids and glycerol in small intestine
Digestive secretions
T he release of various substances from the exocrine glands that serve the digestive system, e.g.: Saliva Gastric juice Pancreatic juice Bile Intestinal juice
Saliva
secreted by salivary glands mucus + enzymes (mostly amylase) mucus lubricates and mixes with food main enzyme in saliva is amylase amylase functions best in an alkaline pH
Gastric juice
Secreted by exocrine glands in stomach
Pepsin – protease that begins digestion of proteins
HCl – decreases pH of chyme to a level that activates and optimizes pepsin activity (See Figure in Textbook)
Intrinsic factor – required for vitamin B12 absorption (See Figure in Textbook)
Mucus & water – lubricate, protect, and help with mixing of chyme
Pancreatic juice
Secreted by exocrine part of pancreas
Secretes numerous enzymes: proteases, lipases, nucleases, amylase
Secretes sodium bicarbonate
Buffers stomach contents that enter duodenum
Increase pH for optimum enzyme function
Plays a role in maintaining blood pH
Bile
Secreted by liver; stored by gallbladder
Contains lecithin and bile salts which emulsify fat in preparation for digestion by lipases
Unique property: hydrophobic and hydrophillic micelles
Release of bile
Initiated by the presence of fat in chyme
Detection of fat in chyme stimulates release of the hormone cholecystokinin (CCK) from duodenum’s intestinal mucosa
CCK contraction of GB bile into the duodenum
Intestinal juice
Secreted by cells of intestinal exocrine glands
Mucus and water - lubricate and aid with mixing of chyme
Sodium bicorbonate increase pH to optimize enzyme function
Salivary secretion
Controlled by reflex mechanism
Chemical and mechanical stimuli from presence of food in the mouth
Olfactory and visual stimuli also involved
Three phases of gastric secretion
Cephalic
Gastric
Intestinal
Cephalic phase
Sensations of thoughts of food are sent to the brainstem
Parasympathetic signals are sent to the gastric mucosa via vagus nerve
Gastric juice secretion is stimulated
Gastrin secretion is stimulated
Gastric phase
Presence of food in stomach distention
Triggers reflexes that increase secretion of gastric juices and gastrin
Intestinal phase
Occurs as food moves into/through duodenum
Presence of fats, carbs, proteins & acid stimulates hormonal and nervous reflexes that inhibit stomach activity
Pancreatic secretion
Stimulated by several hormones released by intestinal mucosa
Secretin stimulates production of pancreatic fluid that is low in enzymes and high in bicarbonate
CCK
stimulates contraction of gallbladder
Bile secretion
Continually secreted by liver; stored in GB (CCK stimulated GB to squeeze out bile)
Intestinal secretion
Mechanism of regulation is uncertain
Absorption
Movement of digested food molecules (amino acids, glucose, fatty acids, glycerol) from the mucous membrane of the intestine into blood or lymph. Occurs in:
Small intestine: foods and most water
Plicae – villi - microvilli – bumps! (See Figure in Textbook)
Various nutrients are absorbed at different locations along digestive tract (See Figure in Textbook)
Large intestine: Some water and vitamins synthesized in the large intestine
Elimination/defecation
“Expulsion of feces from the digestive tract”
Distention of rectum receptors in rectal mucosa are stimulated reflex
It takes 3 to 5 days for fecal matter to pass through L.I.
constipation – prolonged time in L.I. leads to more water being absorbed feces becomes more solid, harder
diarrhea – fecal matter rushes through the L.I. before the water is absorbed
