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Ingegneria delle tecnologie per la salute Fondamenti di anatomia e istologia aa. 2019-20 Lesson 7. Digestive system and peritoneum
Ingegneria delle tecnologie per la salute
Fondamenti di anatomia e istologia
aa. 2019-20
Lesson 7. Digestive system and peritoneum
Overview of the Digestive System
After this lesson, you will be able to: • List and describe the functional anatomy of the organs and accessory organs of the digestive system • Discuss the processes and control of ingestion, propulsion, mechanical digestion, chemical digestion, absorption, and defecation • Discuss the roles of the liver, pancreas, and gallbladder in digestion • Compare and contrast the digestion of the three macronutrients
DIGESTIVE SYSTEM
Overview of the Digestive System function of DS is to break down the foods, release their nutrients, and absorb those nutrients (small
intestine is workhorse of DS)
digestive system does not work in isolation!
DIGESTIVE SYSTEM
DIGESTIVE SYSTEM
Digestive System Organs
Alimentary Canal Organs = gastrointestinal (GI) tract or gut = a one-way tube about 7.62 mts (25 feet) in length during life (10.67 mts (35 feet) after death, once smooth muscle tone is lost), beginning at mouth and terminating at anus (between those: pharynx, esophagus, stomach, small and large intestines) to fit functional needs of the body main function of alimentary canal organs =
nourish the body both mouth and anus are open to external
environment: thus, food and wastes within alimentary canal are technically considered to be outside body
only through process of absorption nutrients in food enter into and nourish body’s “inner space.”
Accessory Structures = aids in breakdown of food: within mouth, teeth and tongue begin
mechanical digestion, whereas salivary glands begin chemical digestion once food products enter small intestine, gallbladder, liver, and pancreas release secretions (bile + enzymes) essential for digestion
called accessory organs because they sprout from lining cells of developing gut (mucosa) and augment its function (= vital contributions), maintaining a connection to gut by way of ducts, even after complete development
DIGESTIVE SYSTEM
Histology of Alimentary Canal
Layers: 1. mucosa, 2. submucosa, 3. muscularis, 4. serosa ,
continuing with mesentery
Throughout its length, the alimentary tract is composed of the same 4 tissue layers, varying details of their structural arrangements to fit their specific functions.
Histology of Alimentary Canal mucosa = mucous membrane, because of mucus production of gut epithelium, consisting: a. epithelium = (in mouth, pharynx, esophagus, anal canal = non-keratinized,
stratified squamous epithelium, in stomach, intestines = simple columnar epithelium); in direct contact with lumen: interspersed among epithelial cells = goblet cells, secreting mucus and fluid, + enteroendocrine cells, secreting hormones into interstitial spaces between cells; epithelial cells = very brief lifespan, averaging from only a couple of days (mouth) to about a week (gut).
b. lamina propria = layer of loose connective tissue containing numerous blood and lymphatic vessels, transporting nutrients absorbed and also serving immune function by housing clusters of lymphocytes = mucosa-associated lymphoid tissue (MALT), particularly substantial in distal ileum (= Peyer’s patches) immune system evolved as means of defending against pathogens encountered within because alimentary canal is exposed to foodborne bacteria and foreign matter
c. muscularis mucosa (not to be confused with the muscularis layer) = thin layer of smooth muscle in constant state of tension, pulling mucosa of stomach and small intestine into undulating folds, dramatically increasing surface area available for digestion and absorption
Histology of Alimentary Canal submucosa = lying immediately beneath mucosa, broad layer of dense connective
tissue connecting mucosa to underlying muscularis and including blood and lymphatic vessels (transporting absorbed nutrients), with submucosal glands releasing digestive secretions and additional submucosal plexus (=dense branching network of nerves)
muscalaris = (also called muscularis externa), in small intestine [= double layer of smooth muscle: inner circular layer + outer longitudinal layer their contractions promote mechanical digestion, expose more of food to digestive chemicals, and move food along canal], in mouth, pharynx, anterior part of esophagus, and external anal sphincter [= made up of skeletal muscle, voluntary control over swallowing and defecation], in stomach [= addition of a third layer, oblique muscle], in colon [= 2 layers as small intestine, but longitudinal layer segregated into 3 narrow parallel bands, tenia coli, making it to look like series of pouches rather than a simple tube]
serosa = portion of alimentary canal superficial to muscularis, present only in regions of alimentary canal within abdominal cavity, consisting of a layer of visceral peritoneum overlying a layer of loose connective tissue [instead of serosa, mouth, pharynx, and esophagus = dense sheath of collagen fibers called adventitia, serving to hold alimentary canal in place near ventral surface of vertebral column].
Histology of Alimentary Canal
Histology of Alimentary Canal
Histology of Alimentary Canal
Histology of Alimentary Canal
DIGESTIVE SYSTEM: Nerve Supply
Intrinsic innervation = provided by enteric nervous system, running from esophagus to anus, containing approximately 100 million motor, sensory, and interneurons (unique to this system compared to all other parts of peripheral nervous system), grouped into 2 plexuses: myenteric plexus (plexus of Auerbach, lying in muscularis layer, responsible for motility, especially rhythm and force of contractions of muscularis), submucosal plexus (plexus of Meissner, in submucosal layer, responsible for regulating digestive secretions and reacting to presence of food)
Extrinsic innervations = provided by autonomic nervous system, including both sympathetic (fight-or-flight response, restricts activity of enteric neurons, thereby decreasing GI secretion and motility) and parasympathetic nerves (rest-and-digest response increases GI secretion and motility by stimulating neurons of enteric nervous system)
DIGESTIVE SYSTEM: Blood Supply blood vessels serving DS have 2 functions: 1. transport
protein and carbohydrate absorbed by mucosal cells after digestion (lipids absorbed via lacteals of lymphatic system), 2. supply organs with nutrients + O2 needed to drive cellular processes.
a) more anterior parts of alimentary canal = supplied with blood by arteries branching off aortic arch and thoracic aorta; b) below this point = supplied with blood by arteries branching from the abdominal aorta [celiac trunk = liver, stomach, duodenum, sup and inf mesenteric arteries = remaining small and large intestines].
veins collecting nutrient-rich blood from small intestine (where most absorption occurs) hepatic portal system liver (nutrients are either processed or stored for later use) back to heart (during “resting and digesting,” about 1/4 of blood pumped enters arteries serving the intestines).
The Peritoneum
= a broad serous membranous sac made up of squamous epithelial tissue, surrounded by connective tissue, holding in place digestive organs within abd cavity, composed of 2 different regions: parietal peritoneum, lining abd wall, and visceral peritoneum, enveloping abd organs
peritoneal cavity = space bounded by visceral and parietal peritoneal surfaces, with few milliliters of watery fluid acting as lubricant to minimize friction between serosal surfaces of peritoneum
The Peritoneum
visceral peritoneum includes multiple large folds, enveloping various abd organs, holding them to dorsal surface of body wall and containing blood vessels, lymphatic vessels, and nerves
5 major peritoneal folds first portion of small intestine (called duodenum), pancreas, and
portions of large intestine (ascending and descending colon, and rectum) remain completely or partially posterior to peritoneum [retroperitoneal organs]
Digestive System Processes and Regulation
DS uses mechanical and chemical activities to break food down into absorbable substances
Digestive System Processes and Regulation
1. Ingestion = entry of food into the alimentary canal through the mouth, where is chewed (increasing surface area of food and allowing an appropriately sized bolus to be produced) and mixed with saliva, containing enzymes that begin breaking down carbohydrates plus some lipid digestion via lingual lipase.
2. Propulsion = movement of food through digestive tract including both voluntary process of swallowing and involuntary process of peristalsis (consisting of sequential, alternating waves of contraction and relaxation of alimentary wall smooth muscles, to propel food along and also playing a role in mixing food with digestive juices).
Digestive Processes: 6 activities
Digestive System Processes and Regulation
3. Mechanical digestion = purely physical process not changing chemical nature of food, but making it smaller to increase both surface area and mobility: includes mastication, or chewing, tongue movements, mechanical churning in stomach (creating an acidic “soup” called chyme), segmentation in small intestine (consisting of localized contractions of circular muscle of the muscularis layer, isolating small sections of intestine, moving their contents back and forth while continuously subdividing, breaking up, and mixing contents, in order to mix food with digestive juices and facilitate absorption)
4. Chemical digestion = starting in mouth, digestive secretions break down complex food molecules into their chemical building blocks, varying in composition, but typically contain water, various enzymes, acids, and salts and is completed in small intestine
5. Absorption = entering of nutrients in bloodstream, which takes place primarily within small intestine, through epithelial cells (lipids absorbed into lacteals and transported via lymphatic vessels to bloodstream, ie subclavian veins near heart)
6. Defecation = final step of undigested materials removed from body as feces
Digestive System Processes and Regulation
In some cases, a single organ is in charge of a digestive process (for example, ingestion occurs only in mouth and defecation only in anus)
However, most digestive processes involve interaction of several organs and occur gradually as food moves through alimentary canal. Some chemical digestion occurs in mouth. Some absorption can occur in mouth and stomach, for example, alcohol and aspirin.
Regulatory Mechanisms
regulatory mechanisms working to maintain optimal conditions in lumen needed for digestion and absorption, stimulating digestive activity through mechanical and chemical activity, are controlled both extrinsically and intrinsically and can be divided in: 1. Neural Controls walls of alimentary canal contain a variety of sensors helping regulate digestive functions, including mechanoreceptors, chemoreceptors, and osmoreceptors, capable of detecting mechanical, chemical, and osmotic stimuli, respectively appropriate reflex furthering process of digestion, sending a message that activates glands that secrete digestive juices into lumen, or stimulate muscles within alimentary canal, thereby activating peristalsis and segmentation walls of entire alimentary canal embedded with nerve plexuses interacting with CNS and other nerve plexuses-either within the same digestive organ or in different ones: interactions prompt several types of reflexes: extrinsic nerve plexuses orchestrate long reflexes, which involve central and autonomic nervous systems and work in response to stimuli from outside digestive system. Short reflexes orchestrated by 2 intrinsic nerve plexuses (enteric nervous system) within alimentary canal wall, regulating activities in one area of digestive tract and coordinating local peristaltic movements and stimulating digestive secretions.
Regulatory Mechanisms
2. Hormonal Controls variety of hormones involved in digestive process, secreted by specialized epithelial cells, called endocrinocytes, entering bloodstream, through which they can reach their target organs (gut and its accessory organs) Gastrin = main digestive hormone of stomach, secreted in response to presence of food, stimulating secretion of gastric acid [produced by duodenum:] Secretin = stimulates a watery secretion of bicarbonate by the pancreas Cholecystokinin (CCK) = stimulates secretion of pancreatic enzymes and bile from liver and release of bile from gallbladder Gastric inhibitory peptide (GIP) = inhibits gastric secretion and slows gastric emptying and motility.
