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Anatomy and physiology of GIT

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1 Anatomy and physiology of GIT

2 Foregut Midgut Hindgut Coeliac artery Superior mesenteric artery
Pharynx to duodenum Superior mesenteric artery Midgut Duodenum to first 2/3 of transverse colon Inferior mesenteric artery Hindgut Last 1/3 of transverse colon to upper half of anal canal

3

4 Accessory digestive organs
Teeth Tongue Salivary glands Liver Gallbladder Pancreas

5 Nerve: Ant + post gastric nerves (vagi) , sympathetic branches of thoracic trunk.
Esophagus Internal circular and external longitudinal layers of muscle 1/3: voluntary 1/3: mix 1/3: smooth muscle Pharynx 25cm A: L gastric artery (from celiac trunk) V: Portocaval anatomososes 3 constrictions Cervical constriction: upper oesophageal sphincter Thoracic: Diaphragmatic constriction: Internal circular and external longitudinal layers of muscle Superior 1/3: external layer consists of voluntary striated muscle Inferior 1/3: smooth muscle Middle 1/3: both Nerves: esophageal nerve plexus distally. Formed by vagal trunks (ant and post gastric branches) and thoracic sympathetic trunks Abdominal part of oesophagus: arterial supply: L gastric artery (branch of celiac trunk) and L inferior phrenic artery. Venous: portal venous system through the left gastric vein and the systemic venous system through the oesophageal veins entering the azygous vein. Portocaval anastomoses Left gastric lymph nodes – drain mainly to celiac lymph nodes. Z line- abrupt mucosal transition Type of epithelium. stratified squamous non-keratinized epithelium Lymph: Lt gastric nodes Drain mainly to celiac lymph nodes Stomach

6 Function: Oral cavity and esophagus
Mechanical: Chew  swallow  peristalsis to stomach Secretion: Saliva (lysozyme, defensins, and IgA ab), amylase, lipase Digestion: Carbohydrates and fat (minimal) Absorption: None

7 Cardiac orifice Fundus Lesser curvature Body Greater curvature Pylorus
Lt of midline, T11 Simple columnar Covered by mucous layer Cardiac orifice Fundus Lesser curvature Body Greater curvature Pylorus Rt of midline, L1 (Transpyloric plane) Antrum Can hold up to 2-3L Stomach Function: accumulation of food which it chemically and mechanically prepares for digestion. Enzymatic digestion. Gastric juice converts food into chime. Can hold 2-3L of food. Epithelium: Covered by a mucous layer Gastric rugae: when contracted Vessels: celiac trunk Lesser C: R&L gastric arteries Greater: R&L gastro-omental arteries Lymph: carry along arteries, to celiac lymph node Nerves: Parasympathetic & sympathetic form plexus Pain is poorly localised. It radiates to the dermatomal level which receives visceral afferent fibres from the organ concerned. Visceral referred pain: from a gastric ulcer is referred to the epigastric region because the stomach is supplied by pain afferents that reach the T7 and T8 spinal sensory ganglia. Endocrine/Exocrine

8 Lymph: follows arteries  celiac nodes
Celiac trunk Nerves: Celiac plexus – both sympathetic and parasympathetic Stomach Lymph: carry along arteries, to celiac lymph node Nerves: Parasympathetic & sympathetic form plexus Pain is poorly localised. It radiates to the dermatomal level which receives visceral afferent fibres from the organ concerned. Visceral referred pain: from a gastric ulcer is referred to the epigastric region because the stomach is supplied by pain afferents that reach the T7 and T8 spinal sensory ganglia. Endocrine/Exocrine Epithelium Portal vein Pain – poorly localised Referred – gastric ulcer – T7,T8 sensory ganglia

9 Glands The stomach is divided into three histological regions based on the nature of the glands. Cardiac region: near the opening of the oesophagus. Mucus-secreting cells. Protects the oesophagus against gastric reflux. Fundic region: long glands, narrow neck and a short, wider base. Cell types found Mucous neck cells Parietal (oxyntic) cells: HCL and intrinsic factor (B12). Chief cells: pepsinogen and a weak lipase Enteroendocrine cells: more prevalent near the base. Secrete products into lamina propria where it is taken up by blood vessels. Secretes gastrin – stimulates production of HCL. Pyloric region: mucous Mucous neck cells: Parietal (or oxynctic) cells: Found mainly in upper part of gland. HCL and intrinsic factor. HCl secretion is stimulated mainly by gastrin. Intrinsic factor needed for vit B12 binding Chief cells. Secrete pepsinogen and a weak lipase Enteroendocrine cells. These cells are more prevalent near the base but can be found anywhere in the glands. Enteroendocrine cells secrete their product into the lamina propria whence it is taken up by blood vessels. The major secretory product of the enteroendocrine cells of the stomach is gastrin, which stimulates the production of HCl. Other products are glucagon, serotonin, substance P and VIP.

10 Function: Stomach Mechanical: mixing and propulsion Secretion:
Parietal cells: HCl Chief cells: Pepsinogen and lipase Surface mucus cells: Mucus and HCO-3 G cells: Gastrin ECL cells: Histamine Digestion: Proteins and fats Absorption: Lipid soluble (alcohol, aspirin etc)

11 Lymph: Coeliac + Sup mesenteric nodes
Coeliac art Sup mesenteric art Lymph: Coeliac + Sup mesenteric nodes Through mesentry, forming arcades Major duodenal papilla – just below this is the junction of foregut and midgut Nerve: Coeliac + sup mesenteric plexus

12 Small intestine epithelium
Villi covered by simple columnar epithelium Intestinal glands Enterocytes (absorptive cell) Goblet cells: mucus secreting Paneth cells: regulate intestinal flora Enteroendocrine cells: CCK, secretin (bicarb), GIP (gastric inhibitory peptide- inhibits gastric acid) The mucosa of the small intestine is highly modified. The luminal surface is completely covered by a number of finger- or leaf-like projections called villi, mm in length. The core of a villus is an extension of the lamina propria, and its surface is covered by a simple columnar epithelium. Opening onto the luminal surface at the bases of the villi are simple tubular structures called intestinal glands or crypts of Lieberkuhn. The crypts extend downward toward the muscularis mucosae. The simple columnar epithelium lining them is continuous with that covering the villi. The predominant cell type of the epithelium is the enterocyte or absorptive cell. Each enterocyte has about 3000 microvilli at its luminal surface, which appear in the light microscope as the fuzzy striated border on the surface of the villi. [Electron microscopy: Microvilli are cylindrical protrusions, about 1 micrometer tall, of the cell membrane enclosing a core of filaments, mostly actin filaments. The actin filaments attach to the plasma membrane at the tip of the microvillus and end in the terminal web near the base of the microvillus. The terminal web consists of actin microfilaments and myosin, and is attached to the zonula adherens of the junctional complex binding epithelial cells to one another near their apical ends.] The villi and microvilli, together with folds in the submucosa called plicae circulares (below), increase the absorptive surface of the small intestine about 600 times. The epithelium of the small intestine consists of the following cell types: Enterocytes or absorptive cells. These are tall columnar cells with microvilli and a basal nucleus, specialized for the transport of substances. They are bound to one another and other cell types by junctional complexes (zonula occludens or tight junction, zonula adherens, and macula adherens). Amino acids and monosaccharides are absorbed by active transport, monoglycerides and fatty acids cross the microvilli membranes passively. Absorbed substances enter either the fenestrated capillaries in the lamina propria just below the epithelium, or the lymphatic lacteal (most lipids and lipoprotein particles). Enterocytes have a lifespan of about 5-6 days. Goblet cells. These mucus-secreting cells are the second most abundant epithelial cell. They are found interspersed among the other cell types. Their mucous is a very large glycoprotein that accumulates at the apical end of the cell, rendering it wide. The slender base of the cell hold the nucleus and organelles. Goblet cells usually appear pale or empty due to the loss of their contents upon preparation. However their glycoprotein content can be revealed with special stains (such as the PAS stain in slide #9). The abundance of goblet cells increases from the duodenum to the terminal ileum. Their lifespan is also 5-6 days. Paneth cells. Paneth cells are found only in the bases of the crypts of Lieberkuhn. These cells have an oval basal nucleus and large, refractile acidophilic granules at their apical end. The granules contain the antibacterial enzyme lysozyme, other glycoproteins, an arginine-rich protein and zinc, an essential trace metal for a number of enzymes. Paneth cells also phagocytize some bacteria and protozoa. They may have a role in regulating intestinal flora. They have a lifespan of about four weeks. Paneth cells are easy to identify with the light microscope. Enteroendocrine cells. These cells were described in the section on the stomach. In the intestine, they are most often found in the lower part of the crypts but can occur at all levels of the epithelium. Their most abundant products here are cholecystokinin or CCK, which stimulates pancreatic enzyme secretion and gall bladder contraction, secretin, which stimulates pancreatic and biliary bicarbonate secretion, and gastric inhibitory peptide or GIP, which inhibits gastric acid secretion. As in the stomach, these cells are not easily seen without special preparations.

13 Function: Small intestine
M: Mixing – enzymes from pancreas and liver; propulsion – segmentation. S: Goblet cells: Mucus Hormones: CCK, Secretin, GIP D: Carbohydrates, fats, protein and nucleic acids. A: Peptides by active transport; amino acids, glucose and fructose by secondary active transport; fats by simple diffusion; water by osmosis; ions, minerals and vitamins by active transport Epithelium: Most digestive enzymes that act in small intestine are secreted by the pancreas. Enter small intestine via pancreatic duct Enzymes inter in response to CCK (cholecystokinin) – produced in small intestine in response to food Secretin – causes bicarb reliease – neutralizes potentially harmful acid coming from stomach.

14 Sup mesenteric nerve plexus
sup mesenteric nodes. Sup mesenteric nerve plexus inf mesenteric nodes. Inf mesenteric plexus: Sympathetic (lumbar splanchnic nerves) Parasympathetic S2-S4 inferior mesenteric plexus. Sympathetic innervation is from the Lumbar splanchnic nerves (L1-L2), parasympathetic innervation is from S2-S4.

15 Function: Large intestine
M: Segmental mixing; propulsion – mass movement. S: mucus by goblet cells. D: None. A: Ions, water, minerals, vitamins produced by bacteria.

16 Physiology of absorption: Carbohydrate
Glucose rapidly absorbed before terminal part of ileum. Transport affected by Na+ in intestinal lumen  sodium-dependent glucose cotransporter. Secondary active transport Congenital defective – glucose/galactose malabsorption (severe diarrhoea) Fructose different mech, independent of Na+. Insulin little effect on sugar absorption in intestine  not depressed during DM.

17 Physiology of absorption: Protein
7 diff syst for amino acids: 3  Na+ dependent, 2  Na+ & Cl-dependent. Di/tripeptides  H + dependent. Hartnup disease: defect in AA absorption from intestine and tubules in the kidneys. Cystinuria: inadequate reabsorption of cystine in PCT of kidneys. Infants: undigested proteins absorbed  maternal IgA by transcytosis. Adults: causes allergies. Absorption of antigen by microfold (M) cells  transport to Peyer’s patches, lymphocytes activated.

18 Physiology of absorption: Lipid
Passive diffusion  esterified. Uptake of bile salts by jejunal mucosa low  form new micelles. Process not fully matured in infants  fail to absorb 10-15% of ingested fat. More susceptible to fat malabsorption diseases. Cholesterol: needs bile, fatty acids and pancreatic juice. Sterols of plant origin poorly absorbed  compete with cholesterol and reduce cholesterol absorption.

19 Physiology of absorption: water and electrolytes.
98% of fluid reabsorbed,~200mL excreted in stool. Mainly in small and large intestine. Na+ diffuses across small intestine through gradient; basolateral surface has Na+-K+ ATPase  actively absorbed. Cl-  enterocytes via Na+-K+ -2Cl- cotransporters  secreted via channels. Cholera bacillus: increased Cl- secretion, reduced Na+ absorption. Glucose / cereal containing carbs (tx of diarrhoea).

20 Physiology of absorption: water and electrolytes.
Jejunum – osmolality of content close to that of plasma  absorption of osmotically active particles. Saline cathartics (Mg2+ sulfates)  poorly absorbed salts, increase intestinal volume  laxatives. K+ secreted into intestinal lumen as mucus. H+-K+ ATPase in distal colon reabsorbs. Loss of ileal or colonic fluid (diarrhoea) can lead to severe hypokalaemia.


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