Presentation on theme: "Digestion. Types of digestion systems in unicellular and primitive multicellular organisms intracellular digestion in more developed multicellular organisms."— Presentation transcript:
Types of digestion systems in unicellular and primitive multicellular organisms intracellular digestion in more developed multicellular organisms – extracellular digestion diverse extracellular digestion systems exist in animals – there are three basic types based on the functioning of the „reactor” –intermittent, stirred – saclike; one portion in, digestion, undigested remnants out (e.g. hydra) –continuous flow, stirred – continuous intake, content mixed, continuous output (e.g. ruminant forestomach) –plug-flow, unstirred – continuous input, continuous output, tubelike reactor, composition depends on place, but not on time (e.g. small intestine in vertebrates) 2/23 Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 15-13.
Alimentary canal in vertebrates topologically external to the body entrance and exit are protected by sphincters and other devices ingested material is subjected to various mechanical, chemical and bacterial effects digestive juices break down the ingested material chemically, nutrients are absorbed, undigested, unabsorbed material is expelled with the feces tubular organization allows for functional specialization (i.e. acidic and alkaline environment) parts of the alimentary canal: headgut, foregut, midgut, hindgut 3/23 Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 15-15.
Headgut food enters here – structures related to feeding and swallowing: mouth-parts, buccal (oral) cavity, pharynx, bills, teeth, tongue, salivary glands, additional structures to direct the flow of ingested materials and inspired water or air most multicellular organisms have salivary glands to help swallowing (mucin - mucopolysaccharide) saliva may contain: enzymes, toxins, anticoagulants (vampires, leeches, etc.) tongue from chordates on – mechanical digestion, swallowing, grasping food (chameleon, anther), chemoreception (taste buds) snakes take olfactory samples from the air and wipe the samples in the Jacobson’s organ (vomeronasal organ) 4/23
Foregut in most species it consists of the esophagus and the stomach esophagus carries food from headgut to stomach in infrequently feeding animals it can contain a saclike expanded section – crop (leeches) to store food, birds might use it to feed nestlings digestion starts in the stomach in most vertebrates: pepsinogen and HCl monogastric stomach in omnivorous and carnivorous vertebrates invaginations with gastric pits with gland cells digastric stomach in ruminants: fermentative (rumen + reticulum - cellulose) and digestive (omasum + abomasum [enzymes only here]) parts camel, llama, alpaca, vicuñas: similar stomach fermentation occurs before the stomach also in other species: kangaroo, chickenlike birds birds might have a muscular gizzard following the stomach – chyme moves back and forth 5/23 Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 15-18.
Midgut I. in vertebrates it consists of the small intestine (duodenum, jejunum, ileum), it is separated from the stomach by the pylorus shorter in carnivores, longer in herbivores – dynamic changes in tadpoles longer than in frogs relative to body size duodenum: production of mucus and fluids + receives secretions from liver and pancreas – neutralization of stomach acid and digestion jejunum: secretion of fluids, digestion, absorption ileum: mainly absorption, some secretion small intestine is characterized by a large surface epithelium: gross cylindrical surface would be 0.4 m 2, but circular folds, intestinal villi, brush border - 200-300 m 2 6/23 Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 15-20.
Midgut II. circular folds also slow down the progress of food – more time for digestion each villus (approx. 1 mm long) sits in a circular depression (crypt of Lieberkühn) – inside: network of arterioles, capillaries and venules, in the middle: central lacteal (lymph vessel) longitudinal smooth muscle fibers – their contraction empties the lymph vessels epithelium is made up of enterocytes (lifespan 3-6 days) proliferating at the bottom of the crypts (chemotherapy!) and bearing brush border (~1 long, 0.1 wide, 200,000/mm 2 ); tight junctions, desmosome on the microvilli (brush border) glycocalyx: hydrolases (glycoproteins) and luminal transporters, inside actin filaments – in the basolateral membrane Na-K-pumps and different transporters among the enterocytes sporadic goblet cells (mucus) 7/23 Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 15-21a,b. Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 15-21c,d.
Hindgut stores remnants of digested food – absorption of inorganic ions and water in vertebrates it consists of the final portion of small intestine and of the large intestine (colon) the hindgut is the major site of fermentation in many herbivores –colon fermentation (plug-flow reactor) – large animals, like horses, zebras, elephants, rhinos, sirenians (sea cows), etc. –cecal fermentation (continuous-flow, stirred reactor) - smaller animals, like rabbits, many rodents, koalas, opossums, etc. hindgut terminates in the cloaca in many vertebrates (cyclostomes, sharks, amphibians, reptiles, birds and egg laying mammals), or in the rectum defecation and urination are under behavioral control the alimentary canal in invertebrates have many differences, but similarities as well 8/23 Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 15-22. The Far Side Gallery 3, G.Larson, Andrews and McMeel, Kansas City. 1994, p..21. Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 15-17.
Motility of alimentary canal I. motility is the ability of the alimentary canal to contract its roles: –propulsion of food from intake to excretion –grinding and kneading the food to mix it with digestive juices and to convert it to a soluble form –stirring the gut contents to ensure the continuous renewal of material in contact with the epithelium in arthropods and chordates it is achieved exclusively by muscular motility, in other animal groups ciliary motility might play a supplemental or exclusively role in vertebrates at the entrance (buccal cavity, pharynx, first third of the esophagus) and exit (external anal sphincter) of the alimentary canal striated muscles – providing an at least partial voluntary control, in other places smooth muscles and the enteric nervous system dominates 9/23
Motility of alimentary canal II. layers of the alimentary canal in vertebrates: serosa, longitudinal and circular muscle, submucosa, muscularis mucosa, lamina propria, epithelium there are two basic forms of motility: peristalsis (longitudinal and circular muscles) and segmentation (circular muscles) sphincters: upper and lower esophageal, cardia (functional), pylorus, ileocecal valve (between the small and large intestine), internal and external anal swallowing is a complex reflex: tongue presses the food to the palate, soft palate closes the nasal cavity, food is propelled into the pharynx, mechanoreceptors induce the reflex, swallowing is unstoppable upper esophageal sphincter relaxes, peristalsis moves the food toward the stomach, lower esophageal sphincter relaxes, cardia opens, food enters the stomach 10/23 Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 15-22. Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 15-24.
Motility of alimentary canal III. vomiting – complex reflex, helped by the respiratory muscles – reverse peristalsis in the small intestine, inspirational muscles contract – negative pressure in the chest, abdominal muscles contract – large pressure difference – lower esophageal sphincter relaxes, chyme enters the esophagus chyme returns to the stomach during retching during vomiting expiratory muscles contract, upper esophageal sphincter relaxes centers in the medulla: central vomiting (without retching), retching (without vomiting), chemoreceptive trigger zone stimuli: direct (meningitis, disgust), chemical (e.g. apomorphine), mechanical (back of the throat), visceral (peritoneum, uterus, renal pelvis, testis), organ of equilibrium reflux – cardia is leaking, acidic chyme reenters the esophagus – can lead to inflammation, cancer regurgitation: in ruminants – chyme reenters the buccal cavity without vomiting 11/23
Motility of alimentary canal IV. peristalsis in the stomach by partially closed contraction ring - mixing, but it is not complete – rat experiment with differently colored food small intestine – circumscribed expansion induces peristalsis obstruction of passage – very dangerous causes: mechanical (e.g. tumor), physiological (sympathetic hyperactivity – caused by peritoneal excitation) - mechanism not completely clear large intestine absorption of water and ions, excretion of feces following eating gastrocolic reflex distal movement of the chyme – might involve mass movement – frequently occurs in babies: eating leads to defecation defecation is a complex process: posture, contraction of abdominal wall, sphincters internal sphincter autonomic, external voluntary regulation 12/23
Regulation of the intestines I. intrinsic control: contraction is myogenic in the alimentary canal – smooth muscle is capable of inducing electrical activity (rhythmic hypo-, and repolarization – might lead to Ca-spike and contraction; influenced by stretching and chemical stimuli from the chyme extrinsic control: enteric nervous system, central nervous system, peptide hormones enteric nervous system –myenteric (Auerbach's) and submucosal (Meissner’s) neuronal networks –local reflexes –sensory neurons: transmit information of mechano-, chemo-, and osmoreceptors - substance-P –interneurons: n-Ach (excitatory), enkephalinergic, somatostatin releasing (inhibitory) –effector neurons: excitatory: colocalized ACh and tachykinin (e.g. substance-P); inhibitory: VIP, NO, ATP - morphine excites the latter neurons, long- lasting contraction, constipation; on glands VIP can also be excitatory 13/23 Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 15-25.
Regulation of the intestines II. central nervous system –parasympathetic innervation (preganglionic): acting mostly on interneurons of the enteric nervous system – excitatory effect to a smaller extent on efferent neurons – stomach functions, sphincter relaxation (e.g. esophagus) –sympathetic innervation (postganglionic): vasoconstriction pre-, and postsynaptic inhibition through 2 -receptors direct excitatory effect on sphincters through 1 receptors local peptide hormones –proved hormones: secretin, gastrin, CCK, GIP (glucose-dependent insulinotropic peptide (formerly: gastric inhibitory peptide) – many more candidates –hormonal role is difficult to prove: measurement of levels, administration (physiological vs. pharmacological dose), antagonists –gastrin family - five C-terminal amino acids of gastrin and CCK are the same, both are active at different lengths –secretin family – secretin, GIP, glucagon, VIP –produced by unicellular glands detecting the composition and pH of the chyme directly – neuronal regulation in some of them 14/23 Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 15-26. Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 15-34.
Gastrointestinal hormones cellhormonestimulusstomachbilepancreas Ggastrin peptides, amino acids in the stomach HCl production, motility up CC K cholecystokinin lipids, proteins in the small intestine motility, emptying inhibited emptying the gall bladder increased enzyme production Ssecretin acid in the small intestine emptying inhibited increased HCO 3 - secretion GIP glucose-dependent insulinotropic peptide carbohydrates in the small intestine HCl production, emptying inhibited glucose dependent insulin secretion 15/23
Gastrointestinal secretions three types of secretion exist: –secretion-reabsorption type – proteins, water, electrolytes secreted in the acinus, reabsorption in the secretory duct, e.g. salivary glands –sequential secretion type – protein secretion in the acinus, water and electrolytes in the secretory duct, e.g. pancreas, liver –parallel secretion type – e.g. stomach; chief cells: pepsinogen, parietal cells: HCl, intrinsic factor, goblet cells: mucin and HCO 3 – in one day about 5-6 l digestive secretions production of saliva –three pairs of large salivary glands: parotid, submandibular, sublingual + many small ones in the buccal cavity –function: lubrication (dry mouth - thirst), mucin, lysozyme, IgA, rinsing (dog-breath), amylase –serous and mucous acinus cells –saliva is hyposmotic because of the reabsorption of NaCl –mostly parasympathetic innervation, sympathetic activation results in thick, viscous saliva –unconditional and conditional reflexes – trumpet player and licking of lemon 16/23 Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 15-29. The Far Side Gallery 3, G.Larson, Andrews and McMeel, Kansas City. 1994, p..24.Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 15-28.
Secretion in the stomach secretion is parallel in the stomach; in addition, G-cells produce gastrin fluid is acidic and isosmotic functions of the low pH: optimal environment for pepsin, chemical degradation of the food (denaturation), killing of bacteria large invaginations (canaliculi) in the membrane of the parietal cells with H-K-ATPase molecules - 10 6 concentration gradient – “world” record (exit of Cl – and K + through channels) source of H + : CO 2 and water (carbonic anhydrase, HCO 3 – /Cl – exchange) facilitation: vagus nerve (m-ACh), gastrin, histamine cephalic, gastric, intestinal phase inhibition: HCl level, fatty acids longer than 10 C in the small intestine secretion of chief cells increased by n-ACh, HCl 17/23 Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 15-32. Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 15-33.
Secretion of the pancreas indispensable for digestion sequential secretion –acinus cells: active enzymes ( -amylase, lipase, DNAase, RNAase) proenzymes (trypsinogen, chymotrypsinogen, procarboxipeptidases, prophospholipases, etc.) –secretory duct: large amount of fluid with high HCO 3 – content (alkaline) CO 2 - HCO 3 – and H + (carbonic anhydrase), Na + /H + antiporter, Na + -pump, apical HCO 3 – exit secretory duct enters the duodenum along with the bile duct at the ampulla of Vater enteropeptidase (enterokinase) secreted by the duodenum activates trypsin, which in turn activates all the other (there is also a trypsin inhibitor in the pancreas) – during inflammation early activation, necrosis and death can occur activation of acinus cells: CCK, m-ACh, VIP activation of HCO 3 – secretion - secretin 18/23
Functions of the liver secretion (bile acids) and excretion (bilirubin, cholesterol, poisons, medicines, hormones, etc.) bile is produced by the parenchyma cells (75%) and by the epithelium (25%) lining the bile ducts the latter secretes electrolytes sinusoids with large-pored endothelium, between them one-cell-thick parenchyma sheets – between neighboring hepatocytes bile canaliculi, surrounded by tight junctions – if damaged bile enters circulation bile is concentrated in the gallbladder, emptied three times a day (20-30 ml) 95% of bile acids are reabsorbed from the gut bilirubin is transformed by bacteria to stercobilin giving the brown color of the stool 19/23
Digestion and absorption I. carbohydrates completely, while lipids and proteins up to more than 90% are digested and absorbed from the gut digestion of carbohydrates and proteins is a two-step process: luminal digestion is completed by enzymes (oligosaccharidases, exopeptidases) on the surface (glycocalyx) of enterocytes absorption is mostly energized by the Na + gradient that in turn is rebuild by the K + -Na + - pump in the basolateral membrane carbohydrates: – -amylase can break the 1-4 bond, but not the 1-6 –it is unable to break the -glycosidic bond in lactose, this bond gets broken by the lactase ( -galactosidase) – the lack of this enzyme leads to lactose intolerance –uptake of glucose and galactose is accomplished by a Na + cotransporter, fructose enters the cell using GLUT-5 – it is slower, as it is a passive process –all sugars are transported through the basolateral membrane by GLUT-2 –part of the plant fibers (cellulose) are fermented by bacteria – not much usable energy is released, but huge amount of CH 4, CO 2 is produced 20/23 Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 15-35.
Digestion and absorption II. proteins: –luminal endopeptidases (pepsin, trypsin, chymotrypsin, elastase) and exopeptidases (carboxypeptidases) digest proteins to amino acids and smaller peptides –enterocyte glycocalyx: different membrane peptidases –membrane transport as amino acids (70-75%) or di-, and tripeptides (25-30%), mainly by group-specific Na + cotransporters (active transport), partly through facilitated diffusion –at the basolateral membrane: facilitated diffusion vitamin B 12 : –absorbed in protein-associated form –demand: 1-2 microgram/day – reserves in liver are sufficient for several years –B 12 binds to R-protein in the stomach, R is digested in the duodenum, B 12 binds to intrinsic factor –in the ileum, receptor induced endocytosis, in blood transported by transcobalamin II –B 12 is needed for erythropoiesis – anemia is most frequently caused by the lack of the intrinsic factor 21/23
lipids: –hydrophobic character, digestion is only possible at the lipid-water border – micelles formed with the help of bile acids –most important enzyme: pancreatic lipase; in general it cuts off 1,3 fatty acids –fatty acids, 2-monoglycerids enter the enterocytes from the micelles –micelles also contain lipid-soluble vitamins (DEKA) – lack of bile acids leads to low vitamin K levels and disturbances in hemostasis –lipids are reformed in the endoplasmic reticulum of enterocytes and form lipoproteins containing triglycerides, phospholipids, cholesterol and its esters as well as apolipoproteins –lipoproteins are classified according to their density: VLDL, LDL, HDL – the largest are the chylomicrons –lipoproteins are transported from the Golgi to the lymphatic vessels through exocytosis –lipoproteins are also produced in the liver Digestion and absorption III. 22/23 Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 15-36.
calcium: –reabsorbed partly by paracellular diffusion, but mostly by active transport –regulation: parathormone and calcitriol (1,25- dihidroxi-D 3 -vitamin) –entrance by unknown mechanism – calcium binding protein – active transport through the basolateral membrane iron: –stored in the enterocytes in the form of ferritin, transported in the blood bonded to transferrin – if the enterocyte is saturated absorption stops –demand: in men 1 mg/day, in women (because of blood loss during menses) 2-3 mg/day – iron is needed mainly because of the renewal of enterocytes water and NaCl: –Na + channels in the apical membrane (their number is regulated by aldosterone) - Na + -pump in the basolateral membrane –Cl – and water follows passively Digestion and absorption IV. 23/23 Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 15-37.