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168 Cards in this Set
- Front
- Back
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Secretory glands serve two primary functions
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1. Digestive enzymes are secreted (mouth to distal ileum) 2. Mucous glands provide lubrication and protection of all parts of AT (mouth to anus)
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What stimulus causes digestive enzymes to be secreted?
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Digestive secretions are formed in response to food in the AT, Quantity secreted is in the appropriate amount needed for proper digestion; In some parts of the AT, the types of enzymes, etc. in secretions vary according to type of food present
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Anatomical Types of Glands
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Single cell mucous glands (goblet cells), Pits (crypts of Lieberkuhn in SI), Deep tubular glands, Complex glands
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Single cell mucous glands (goblet cells)
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Function in response to local irritation of the epithelium, Extrude mucus directly onto the epithelial surface to lubricate and protect surfaces from excoriation and digestion; Extrude mucus directly onto the epithelial surface to lubricate and protect surfaces from excoriation and digestion
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Pits (crypts of Lieberkuhn in SI)
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Invaginations of epithelia into submucosa, Crypts of L. contain specialized secretory cells
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Deep tubular glands
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Stomach (oxyntic glands secrete pepsinogen) and upper duodenum
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Complex glands
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Salivary glands, Pancreas, liver – secretions for emulsification of foods, Liver is highly specialized
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Salivary and Pancreatic Glands
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Salivary glands and pancreas are compound acinous glands (lie outside the walls of AT) – contain millions of acini lined with secretory cells; acini empty into ducts that empty into AT
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Basic mechanisms of stimulation of the alimentary tract glands
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Effect of contact of food with the epithelium-function of enteric nervous stimuli; Autonomic stimulation of secretion
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Effect of contact of food with the epithelium-function of enteric nervous stimuli
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Mechanical presence of food in an area causes glands of that region (and adjacent regions) to secrete juices, Results from direct contact stimulation of the surface glandular cells by the food, Local epithelial stimulation activates the enteric nervous system via stimuli: 1. Tactile, 2. Chemical Irritation, 3. Distension of Gut wall; Nervous reflexes stimulate mucous cells and deep glands to increase their secretions
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Parasympathetic stimulation
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Mainly applies to upper GI tract (salivary glands, esophageal glands, pancreas, and brunner’s glands) innervated by glossopharyngeal and vagus nerves, Distal LI glands also stimulated by pelvic splanchnics, SI, and first two thirds of LI respond to local neural and hormonal stimuli in each segment of the gut
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Sympathetic stimulation
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Stimulation of Sym nerves causes slight increase in secretion locally, Also results in vasoconstriction, Dual effect: sym stimulation alone will increase secretion, if PS secretion is already going, sym stimulation will decrease the secretion due to vasoconstriction
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Basic Mechanism of Section by Glandular cells
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(1) Nutrients needed for formation of secretion diffuse (or active transport from blood capillaries) into base of gland cell (2) Mitochondria in gland cell base form ATP (3) ATP + substrates synthesize organic substances (occurs in ER and Golgi; ribosomes on ER form secreted proteins) (4) Secretory materials are transported through ER tubules to Golgi complex (takes about 20 minutes) (5) In Golgi complex, materials are modified, added to, concentrated, and discharged in secretory vesicles – stored at apical end of cells (6) Vesicles remain stored until nervous or hormonal control signals cause cells to extrude contents through cell surface; control signals increase cell membrane permeability to Ca2+ ions, Ca exocytosis of vesicles
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In addition to hormones/stimulation, what else is necessary for glandular secretion?
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Water and electrolytes
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Mucous
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a thick secretion composed mainly of water, electrolytes, and a mixture of several glycoproteins, which are themselves composed of large polysaccharides bound with smaller quantities of proteins
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Universal characteristics of mucous
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(1) adherence tightly to food or other particles and spreads a thin film over the surfaces (2) body that coats the wall of the gut and prevents actual contact of most food with the mucosa (3) low resistance for slippage (4) mucous causes fecal particles to adhere to one another to form the feces that are expelled during a bowel movement (5) stongly resistant to digestion by GI enzymes (6) contains glycoprotiens that are amphoteric
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Principal glands in Saliva Production
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parotid, submandibular, sublingual, small buccal
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Amount of Saliva Produced:
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Daily secretion: 800-1500 mL (avg. 1000mL)
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Two types of Salivary Secretion:
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1. Serous – contains ptyalin (enzyme for starch digestion) 2. Mucus – contains mucin for lubrication and protection
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Which gland(s) only secrete Serous?
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Parotid glands primarily serous secretion
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Which gland(s) secrete both Mucous and Serous?
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Submandibular and sublingual glands secrete both
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Which gland(s) only secrete Mucous?
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Buccal glands secrete mucous only
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pH of Saliva?
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pH of saliva between 6-7, good range for the action of ptyalin
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Secretion of ions in saliva
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Saliva contains large K+ and HCO3- concentration; lower Na+ and Cl- conc. than plasma
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Stages of Saliva Secretion
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Stage 1: acini secrete a primary secretion with ptyalin and/or mucin in a solution of ions similar in concentration to ECF; Stage 2: primary secretion flows through the ducts and two transport processes occur
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Mechanism for secretion of saliva
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(1) Na+ ions reabsorbed from all salivary ducts and K+ ions are actively excreted; thus excess Na+ reabsorption over K+ secretion which creates electrical negativity in ducts which causes chloride ions to be passively reabsorbed thus Cl- concentration falls very low in the saliva and matches duct decrease in Na+ concentration (2) HCO3- are secreted by the ductal epithelium into the lumen of the duct (in exchange for chloride and also by active transport)
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Net Result of Saliva Production
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Net result – at resting conditions, Na+ and Cl- conc. 15mEq/L which is 1/7 to 1/10 of normal plasma concentration; K+ concentration is 30mEq/L which is 7 times the normal plasma concentration; HCO3- is 50-70mEq/L which is 2-3 times normal plasma concentration
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Maximal Salivation
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At Maximal salivation – ionic concentrations change due to rate of formation of primary secretion by acini (inc. up to 20 times normal); secretion flows so rapidly through the ducts that reconditioning of secretion is reduced; Na ,Cl conc. only increase to about ½ to 2/3 of normal plasma, and K conc. is only 4 times greater
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Basal awake condition, ____ mL saliva (mucous type) secreted each minute_
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0.5
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Secretion of Saliva during sleep
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Sleep – secretion is very little
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Oral Bacteria
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Mouth contains many pathogenic bacteria that easily destroy tissues and cause dental caries
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Saliva Effect on Oral Bacteria
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Saliva prevents deteriorative processes by the following ways: 1. Flow of saliva helps wash away pathogenic bacteria and food particles 2. Saliva contains many factors that destroy bacteria (thiocyanate ions and proteolytic enzymes like lysozyme) 3. Saliva contains protein antibodies that can destroy oral bacteria, including those that cause dental caries
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Enzymes in saliva that destroy bacteria
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thiocyanate ions and proteolytic enzymes (lysozyme)
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What happens to Oral tissue without Saliva?
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Without saliva, oral tissues can become ulcerated and otherwise infected and caries of teeth become rampant
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Parasympathetic Salivary
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PS nerve pathways control salivary glands from the superior and inferior salivatory nuclei in the brain stem, Nuclei located at the junction of medulla and pons
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What stimulation causes activation of Parasympathetic Salivary response?
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Excited by taste and tactile stimuli from the tongue, mouth and pharynx
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Sour Taste
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Sour taste elicits copious secretions – 8-20 times normal secretion
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Smooth vs Rough object on Salivation
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Smooth objects cause marked salivation while rough objects cause less salivation
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Appetite area in brain
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in close proximity to PS centers of ant. Hypothalamus and responds to signals from taste and smell areas of cerebral cortex (amygdala)
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Salivation in response to reflexes in the stomach or upper small intestine
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Swallowed saliva helps remove the irritating factor in the GI tract as well as diluting and neutralizing irritating substances (in response to irritating foods or nausea)
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Sympathetic Stimulation on Salivation
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Sympathetic stimulation increases salivation slightly from superior cervical ganglia following surface vessel walls to the salivary glands
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Parasympathetic effect on blood vessel
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PS nerve signals moderately dilate blood vessels thus increased nutrition and increased secretion, Blood supply to the glands affects secretion, Glands require adequate nutrients from the blood
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Kallikrein in Saliva
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Kallikrein – secreted in saliva acts as an enzyme to split one of the blood proteins (α-2-globulin) to form bradykinin – a strong vasodilator
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Esophageal secretion
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Secretions are entirely mucous to provide lubrication for swallowing
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Glands in the esophagus
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main body is lined by many simple mucous glands; Mouth end and Gastric end also has many compound mucous glands – protects esophagus from excioration by newly entering food (proximal) and prevents digestion of gut wall by acidic gastric juices (distal)
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Gastric Glands
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the stomach is lined with mucous secreting cells; Oxyntic glands located on inside surfaces of body and fundus of stomach (proximal 80%); pyloric glands located in antral portion of stomach (distal 20%)
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Secretions oxyntic (gastric) glands
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secrete acid (HCl, pepsinogen,intrinsic factor, mucous)
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Secretion pyloric glands
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mucous for protextion of the pyloric mucosa from the stomach acid; also secrete gastrin
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Three types of cells: Oxynic glands
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(1) Mucous neck cells – secrete mucous (2) Peptic (chief) cells – secrete pepsinogen (3) Parietal (oxyntic) cells – secrete HCl and intrinsic factor
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Basic mechanism of hydrochloric acid secretion
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Parietal cells secrete acid solution isotonic (same # solutes) with body fluids with pH 0.8 (H+ ion concentration 3 million times more than arterial blood); To concentrate H+ takes 1500 calories of energy/L gastric juice; At the same time that the hydrogen ions are secreted, HCO3- ions diffuse into the blood so that gastric venous blood has a higher pH than arterial blood when the stomach is secreting acid
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Structure of a parietal cell
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Contains large branching intracellular canaliculi, HCl formed at villus-like projections inside canaliculi and conducted through them to secretory end of cell
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Main driving force for HCl secretion by parietal cells
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hydrogen-potassium (H+-K+ ATPase)
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Chemical mechanism HCl formation
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-H2O --> H+ and OH- in cell cytoplasm
-H+ secreted into canliculus in exchange for K+ (catalyzed by H+K+ ATPase) -K+ ions transported into cell by Na+K+ ATPase on the basolateral side tend to leak into lumen but are recycled back into the cell by the H+K+ ATPase -The basolateral Na+-K+ ATPase creates low intracellular Na+, which contributes to Na+ reabsorption from the lumen into the canaliculus -the pumping of H+ out of the cell by the H+-K+ ATPase permits OH- to accumulate and form HCO3- from CO2 either formed during metabolism in the cell or entering the cell from the blood; catalyzed by carbonic anhydrase -the HCO3- is then transported across the basolateral membrane into the extracellular fluid in exchange for chloride ions into the canaliculus, giving a strong solution of HCl in the canaliculus -The HCl acid is then secreted outward through the open end of the canaliculus into the lumen of the gland -Water passes into the canaliculus by osmosis because of extra ions screted into the canaliculus |
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Effect of parasympathetics on stomach secretion
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excites secretion of pepsinogen by peptic cells, hydrochloric acid by parietal cells, and mucous by mucous cells
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Effect of gastrin and histamine on stomach
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stimulate secretion of acid by parietal cells but have little effet on other cells
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Pepsinogen
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Pepsinogen is a proenzyme (no digestive activity) until it contacts HCl – splits to form pepsin. There are Different types of pepsinogen secreted but all with same function
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Pepsin
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Pepsin is an active proteolytic enzyme at low pH (1.8-3.5); above pH 5 no activity and completely inactivated in a short period of time
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Intrinsic Factor
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Essential to the absorption of B12 in the ileum, Secreted by parietal cells
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Destruction of Parietal Cells
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If parietal cells destroyed (like from chronic gastritis), person develops achlorhydria (lack of stomach acid secretion) and pernicious anemia (RBCs fail to mature in absence of B12 stimulation in bone marrow)
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achlorhydria
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lack of stomach acid secretion
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Pyloric Glands
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Structure similar to oxyntic glands with few peptic cells and almost no parietal cells; contain mostly mucous cells that are identical to mucous cells in oxyntic glands that secrete a little pepsinogen and thin mucous to lubricate food movement and protect stomach from digestive enzymes; also secrete gastrin – controls gastric secretion
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Surface mucosa cells
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Secrete large quantities of viscid mucous that coats stomach 1mm thick; protects stomach wall; lubricates food transport; alkalization
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________ cells are the only cells that secrete HCl
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Parietal cells of the Oxyntic Cells
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Enterochromaffin-like (ECL) cells
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Secrete histamine
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Location ECL cells
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Lie deep in oxyntic glands and release histamine in contact with parietal cells of the glands
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Stimulation ECL stimulation
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Gastrin – formed in response to protein digestion; Ach released from stomach vagal n. endings
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Gastrin
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Hormone secreted by G cells located in the pyloric glands
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Forms of gastrin
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Gastrin has 2 forms: G-34 large – 34 amino acids; G-17 small – 17 amino acids (most abundant)
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Location G cells
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in the pyloric glands in the distal end of the stomach
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Stimulation Gastrin
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Proteins stimulate gastrin cells to release gastrin
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Signals involved in regulation of pepsinogen secretion by the peptic cells in the oxyntic glands
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(1) Stimulation of peptic cells by Ach released from vagus nerves or gastric enteric nervous plexus (2) Stimulation of peptic cell secretion in response to acid in the stomach
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Phases of gastric secretion
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(1) Cephalic phase (2) Gastric Phase (3) Intestinal Phase
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Cephalic Phase of gastric secretion
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Occurs before food enters stomach, while it is being eaten; Results from the sight, smell, thought, or taste of the food
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Neurologic signals that cause the cephalic phase
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Neurogenic signals originate in cerebral cortex in appetite centers of amygdale and hypothalamus – transmitted through dorsal motor nuclei to the vagus n. to the stomach; 30% of gastric secretion associated with eating a meal
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Gastric Phase of Gastric Secretion
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Once food enters the stomach, it excites (1) long vagovagal reflexes from the stomach to the brain and back to the stomach (2) local enteric reflexes (3) the gastrin mechanism; accounts for 60% of gastric secretion
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Intesinal Phase of Gastric Secretion
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Presence of food in upper SI (duodenum) continues to cause stomach secretion of small amounts of gastric juice; due to small amounts of duodenal gastrin secretion; 10% gastric response
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Inhibition of Gastric Secretion
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(1) Reverse enterogastric reflex (2) Presence of acid, fat, protein breakdown products, hyperosmotic or hypo-osmotic fluids, or any irritating factor in the upper SI causes release of intestinal hormones (secretin – important in control of pancreatic secretion, and opposes stomach secretion; gastric inhibitory peptide, vasoactive intestinal polypeptide, somatostatin)
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Reverse enterogastric reflex
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initiated by presence of food in the SI; transmitted through myenteric nervous system, sym and PS pathways; reflex initiated by distended small bowel, acid presence in upper intestine, presence of protein breakdown products or mucosa irritation
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Hormones that can inhibit gastrin secretion
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secretin, and to a lesser extent, GIP, vasoactive polypeptide, and somatostatin
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Functional purpose of inhibitory gastric secretion
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slow passage of chyme from stomach if SI is already full or overactive; Hormones also inhibit stomach motility
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Gastric secretion during the interdigestive period;
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Stomach secretes a few mLs of gastric juice each hour during interdigestive period (little or no digestion occurring in the gut)
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Nonoxyntic Type Secretion
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Nonoxyntic type secretion (mainly mucous, a little pepsin, and no acid)
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Emotional Stimuli Effect on Gastric Secretion
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Emotional stimuli increase gastric secretion to 50mL/hour (like cephalic phase) – causes ulcers
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Chemical composition of gastrin and other GI hormones
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Gastrin, cholecystokinin, and secretin are all large polypeptides, Last 5 amino acids in gastrin and cholecystokinin are the same
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Functional activity of gastrin, CCK, and secretin
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Functional activity of gastrin in last 4 AA, cholecystokinin in last 8 AA, all AA in secretin are essential
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Pentagastrin
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Synthetic gastrin – terminal 4 of natural gastrin + alanine – has same physiologic properties as gastrin – called pentagastrin
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The pancreatic digestive enzymes are secreted by _________
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pancreatic acini
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Large volumes of sodium bicarb solution are secreted by ______ in the pancreas
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small ductules and large ducts leading from the pancreatic acini
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Flow of panceatic juices
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Pancreatic acini --> pancreatic duct --> emptiesinto duodenum through papilla of Vater, surrounded by sphincter of Oddi
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Stimuli for pancreatic secretion
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presence of chyme in the upper portions of the SI; Juice characteristics determined by types of food in the chyme
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Difference between pancreatic juice secretion and insulin secretion
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Insulin is not secreted into the SI; rather, it is secreted into the blood directly by the islets of langerhans
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Pancreatic Enzymes
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Contains multiple enzymes for digesting all three major types of food (proteins, carbohydrates, and fats), Also large quantities of bicarbonate ions – important to neutralize acidity of chyme from stomach
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Pancreatic Digestive Enzymes for Protein digestion
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Most important enzymes for protein digestion: trypsin (most abundant), chymotrypsin, carboxypolypeptidase
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Function: Trypsin and chymotrypsin
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split proteins into peptides of different lengths – does not cause release of individual amino acids
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Function: carboxypolypeptidase
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cleaves peptides into single AAs
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Pancreatic Amylase
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enzyme for carbohydrates – hydrolyzes starches, glycogen, etc. doesn’t break down cellulose
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Pancreatic Enzyme for Fat Digestion
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Enzymes for fat digestion: pancreatic lipase (hydrolyzes neutral fat into fatty acids and monoglycerides), cholesterol esterase (hydrolysis of cholesterol esters); phospholipase (splits fatty acids from phospholipids)
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Function: Pancreatic lipase
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hydrozyes neutral faty into fatty acids and monoglycerides
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Function: cholesterol esterase
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hydrolysis of cholesterol esters
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Function: Phopholipase
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splits fatty acids from phospholipids
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Proenzymes
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Proenzymes (trypsinogen, chymotrypsinogen, and procarboxypolypeptrypsinogen) are released first (inactive state) – Trypsinogen becomes active only after secretion into GI tract by enterokinase (secreted by intestinal mucosa when chyme enters intestine). Important that enzymes not become active until it is in the intestine because they would digest the pancreas itself; Chymotrypsinogen is activated by trypsin to form chymptrypsin and procarboxypolypeptrypsinogen is activated in a similar manner
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Purpose of Trypsin Inhibitor
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Secretion of trypsin inhibitor prevents digestion of the pancreas itself. Same cells that secrete enzymes secrete trypsin inhibitor – prevents activation of trypsin in secretory cells, acini, and pancreatic ducts; Trypsin inhibitor is formed in the cytoplasm of the glandular cells, and it prevents activation of trypsin both inside the secretory cells and in the acini and ducts of the pancreas
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Pancreatic Block
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If pancreas becomes severely blocked or damaged, secretions may become pooled and trypsin inhibitor is overwhelmed causing enzymes to become activated and causes digestion of the pancreas(acute pancreatitis)
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Where are bicarb ions and water secreted from the pancreas?
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epithelial cells of the ductules and ducts that lead from the acini
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Function: Bicarb secretion
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Neutralizes HCl from stomach as it enters Duodenum
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Cellular mechanism for secretion of HCO3-
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(1) CO2 diffuses from blood into cell, carbonic anhydrase converts CO2 + water to H2CO3, then dissociates into H+ and HCO3-, HCO3- transported with Na+ through cell into lumen (2) H+ ions are exchanged for Na+ from blood via active transport; Na+ secreted with HCO3- into lumen of duct (3) Movement of ions creates osmotic pressure gradient causing osmosis of water into duct – isosmotic bicarbonate solution
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The Three Basic stimuli that cause pancreatic secretion
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Acetylcholine, Cholecystokinin, and Secretin
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Acetylcholine and CCK effect on Pancreas
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Acetylcholine – stimulate acinar cells; causes production of pancreatic enzymes (small amount of water and electrolytes – without water enzymes are temporarily stored in acini and ducts until water solution is secreted in larger quantities)
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What secretes Acetylcholine at Pancreas?
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Released from vagus n. endings in enteric nervous system;
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What secretes Cholecystokinin?
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Secreted by duodenal and upper jejunal mucosa when food enters the SI
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Secretin effect on Pancreas
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Secretin – stimulates HCO3- secretion by ducts;
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What secretes Secretin?
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secreted by duodenal and jejunal mucosa when high acid food enters SI; stimulates water solution production
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Multiplicative effects of different stimuli on Pancreas
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When all stimuli occur at once, secretion is greater than sum of stimuli; stimuli multiply one another; Normal pancreatic secretion is from the combination of multiple stimuli
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Phases of pancreatic secretion
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Cephalic Phase, Gastric Phase, and Intestinal Phase
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Cephalic Phase of Pancreatic Secretion
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same stimuli that cause stomach secretions cause acetylcholine release by vagus n. endings in pancreas; causes enzymes to be secreted into pancreatic acini – 20% of total pancreatic secretion
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Gastric Phase
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nervous stimulated secretion continues for another 5-10% of secretions – only small amounts reach duodenum because of small fluid amounts
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Intestinal Phase
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once chyme enters duodenum, hormone secretin stimulates copious pancreatic secretion
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Secretin
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Secretin – polypeptide with 27 amino acids secreted as prosecretin in S cells of duodenum and jejunum; Acid chyme below pH 4.5-5 enters duodenum – duodenal mucosa releases and activates secretin – then absorbed into blood; net reaction HCl + NaHCO3 → NaCl + H2CO3
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Why is neutralizing the juices in the duodenum so important?
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Prevents duodenal ulcers
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Stimuli for CCK secretion
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Food in upper SI causes I cells (in mucosa of duodenum and upper jejunum) to release cholecystokinin – 33 amino acid peptide; Especially from presence of proteoses, peptones, and long-chain fatty acids in chyme from stomach
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Function of CCK
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Enters pancreas through blood – causes secretion of more pancreatic digestive enzymes by acinar cells (similar to vagal stimulation); 70-80% of total secretion of pancreatic enzymes
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Total pancreatic secretion
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1L / Day
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Amount of Bile Secreted / Day
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Liver secretes bile – 600-1000mL/day
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Functions of Bile Acids:
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(1) Aids Fat digestion and absorption by emulsifying large fat particles into smaller particles that can then be attacked by lipase enzymes (2) Aid in absorption of digested fat through intestinal wall (3) Excretion of waste products like bilirubin and excess cholesterol
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2 Stages of Bile Secretion
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(1) Hepatocyte secretion – initial secretion contains bile acids, cholesterol, other organic constituents, secreted into bile canaliculi (2) Bile flows from canaliculi to interlobular septa – canaliculi empty into terminal bile ducts into larger ducts until they empty into the hepatic duct then common bile duct; from here bile either goes into duodenum or into cystic duct to gallbladder
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Where is Bile stored and produced?
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Bile secreted continually by liver and Stored in gall bladder until needed
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Maximum Galbladder Volume?
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Max GB volume is 30-60mL; 450mL secreted bile is concentrated (5x-20x) by GB mucosal removal of water and electrolytes, leaving @30-60 mL of bile salts, cholesterol, lecithin, and bilirubin
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Gallbladder absorption
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Most absorption by active transport of Na through GB epithelium followed by absorption of chloride, water, and other constituents
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Most abundant substance secreted in the bile
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bile salts, bilirubin, cholesterol, lecithin, and the usual electrolytes of plasma
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Which substances are reabsorbed in the gallbadder during the concentrating process
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water and large portions of electrolytes (except calcium ions)
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Mechanism of gallbladder emptying
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contractions of the wall of the gallbadder ~ 30 minutes after a meal + relaxation of the sphincter of Oddi
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Most potent stimulus for gallbladder contractions
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CCK - secreted in response to the presence of fatty foods in the duodenum
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Less potent stimuli for gallbladder
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Stimulated less strongly by acetylcholine
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Gallbladder stimulated most by proteins, fats, or carbs?
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Fats!
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The liver synthesize about ____ grams of bile salts daily
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6
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Precursor to bile salts is ______
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cholesterol
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Mechanism: Cholesterol --> Bile Salts
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(1) Cholesterol first converted to cholic acid or chenodeoxycholic acid (2) Acids combine with glycine or taurine to form conjugated bile acids (3) Salts of these acids are secreted in bile
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Function of Bile Salts?
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(1) Detergent action on fat particles in food, Decreases surface tension of particles, Allows agitation in GI tract to break down fats. (2) Bile salts help in absorption of fatty acids, monoglycerides, cholesterol, and other lipids done by forming micelles (small lipid complexes)
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What are Micelles?
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Micelles are semisoluble in chyme because bile salts have e- charge
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Fate of Bile salts after secretion
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94% of bile salts are reabsorbed into blood from SI (1/2 by diffusion in early SI and 1/2 by action transport through the mucosa of the distal ileum) --> Enter portal blood, pass back to liver, absorbed into hepatic cells and resecreted
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Salts make the circuit __ times before joining feces
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17
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Quantity of salts secreted by liver every day dependent on....
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availability of salts – higher amt. in circulation, greater the rate of secretion
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Bile fistula
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If bile fistula empties salts to exterior (cannot be reabsorbed) liver increases it production of bile salts 6-10x
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Role of secretin in helping to control bile secretion
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Can more than double bile secretion; Mostly increases secretion of watery bicarbonate solution by epithelial cells of ducts (assists in neutralizing HCl from stomach)
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Causes of gallstones
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(1) too much absorption of water from bile (2) too much absorption of bile acids from bile (3) too much cholesterol in bile (4) Inflammation of epithelium
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Solubility of cholesterol
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Cholesterol insoluble in water but bile salts and lecithin form micelles that are soluble and thus keep cholesterol in solution
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Inflammation of GB epithelium
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(from infection, etc) may change absorptive characteristics of GB mucosa – may cause excessive absorption of water and bile salts leaving cholesterol in greater concentrations; Cholesterol precipitates forming crystals then larger stones
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What and where are the Brunner's glands?
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Brunner’s glands – complex mucous glands in wall of duodenum between pylorus and ampulla of Vater
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What do Brunner's glands secrete?
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Secrete large amounts of alkaline mucous in response to irritating stimuli, vagal stimuli – increase secretion, and GI hormones (secretin)
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What is the function of the Brunner's gland secretion?
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Mucous protects duodenal wall from high acid content of chyme from stomach (also contains bicarbonate to neutralize acid)
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What inhibits Brunner's glands?
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sympathetic stimulation
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Sympathetic effect on Brunner's Glands?
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Inhibited by sympathetic stimulation (excitable people may leave duodenal bulb unprotected thus causing peptic ulcers – 50% of ulcers)
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Crypt's of Lieberkuhn
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Small pits found over entire surface of SI between intestinal villi; secretion of digestive juices
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Two cells of Small Intestine lumen
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Goblet cells & Enterocytes
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What are Goblet Cells?
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secrete mucous to lubricate and protect GI wall
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What are Enterocytes?
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Enterocytes – secrete large amounts of water and electrolytes, also reabsorb water and electrolytes as well as digestion products
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Primary Function of Small Intestines
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Primary function of SI is to absorb nutrients and digestive products into the blood
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Two active secretory processes of Small Intestines:
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1. Active secretion of Cl- into crypts 2. Active secretion of HCO3- ; Secretions cause Electro Negative voltage potential and Na follows fluid and causes the osmotic movement of water
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Digestive enzymes in the SI secretion
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(1) Peptidases – split small peptides to amino acids (2) Sucrase, maltase, isomaltase, and lactase – to split disaccharides into monosacharrides (3) Intestinal lipase – splits neutral fats into glycerol and fatty acids
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Replacement of cells in crypts
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Cells in crypts of L continually undergo mitosis – new cells migrate upward out of the crypts to tips of villi; replaces villus epithelium and forming new enzymes; Life cycle of intestinal cell is 5 days – allows for rapid repair of excoriations
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Most important means for regulating small intestine secretion
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local enteric nervous reflexes
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Large Intestine Histology
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Many crypts of Lieberkuhn; no villi; no enzymes
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Mucus Secretion of Large Intestine
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Mainly mucus cells that secrete only mucus – contains some bicarbonate
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Nervous stimulation of Mucus secretion in Large Intestine by:
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Stimulation of pelvic nerves – parasympathetics to distal ½ of LI – causes increase in mucus secretion with increased peristalsis
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Rate of secretion in large intestine regulated by
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direct, tacile, stimulations of the epithelial cells lining the large intestine and by local nervous reflexes of the muous cells in the crypts
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Emotional Disturbance Effect on Mucus Secretion
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Emotional disturbances can cause extreme PS stimulation – so much mucus secreted that bowel movement is ropy mucus as often as every 30 minutes with little or no fecal material
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Mucus Effects in Large Intestines
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Mucus protects LI wall and provides medium to hold fecal matter together, also protects against bacterial activity in feces. Mucus provides a barrier to keep acids in feces from harming intestinal wall
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Diarrhea of Large Intestines
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Diarrhea caused by excess secretion of water and electrolytes in response to irritation. When segment of colon is irritated (like when bacterial infection becomes rampant like in enteritis) – mucosa secretes large quantities of water and electrolytes. Dilutes irritating factors to cause rapid movement of feces – diarrhea. Causes loss of large quantities of water and electrolytes while washing away irritants – thus earlier recovery from the disease
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