FUNCTIONAL ANATOMY OF GASTROINTESTINAL TRACT

FUNCTIONAL ANATOMY OF GASTROINTESTINAL TRACT. DR. PRITAM KAYAL.

LEARNING OBJECTIVES. Basic overview GIT wall structure Functional anatomy Innervation Gut-Brain Axis.

BASIC OVERVIEW. Gastrointestinal System: a.k.a. Alimentary canal, Digestive System. A muscular tube from mouth to anus. Length: ~ 10 m (30 ft). Overall function: Absorb nutrients & water into circulation and eliminate waste..

Functions: Digestive: Transfer external nutrients, minerals and water to internal environment by following processes: Ingestion: Food in mouth → Mastication → lubrication by saliva → Deglutition (motility & secretion). Digestion: by acid and enzymes Absorption: from lumen to blood/lymph. Egestion: defecation Non-digestive: Immunity against micro-organisms invading the alimentary canal..

Main organs: Mouth Oropharynx Oesophagus Stomach Small Intestine Large Intestine Accessory organs: Salivary glands Liver Gallbladder Pancreas (Exocrine).

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Mouth (Oral Cavity). Lined with mucus-secreting, stratified squamous epithelial cells - provide protection against abrasion Lips, Cheek & Teeth Help to move and hold food in the mouth while food is torn and ground [Mastication]. involved in speech and facial expression..

Tongue A muscular organ in the mouth, covered with taste buds [Gustation]. Holding & moving food around, chewing & swallowing food, and speech. Palate: Forms the roof of the mouth; Consists of 2 parts: the hard palate and the soft palate. Plays a part in swallowing. Salivary glands: Saliva is produced in and secreted from salivary glands..

Oesophagus. Food exits the oropharynx & enters the oesophagus. ~ 25 cm in length; stratified squamous epithelium Lies in the thoracic cavity, posterior to the trachea. Function: To transport substances (food bolus) from mouth to stomach [Deglutition] Thick mucus secreted by the mucosa → aids food bolus passage and protects the oesophagus from abrasion. Upper oesophageal sphincter regulates the movement into the oesophagus, prevents air entry. Lower oesophageal sphincter (a.k.a. cardiac sphincter) regulates movement between oesophagus and stomach..

lae. [image] Oesophag Relaxed muscularis Circular muscles contract Longitudinal muscles contract Relaxed muscularis Lower oesophageal sphincter Bolus Stomach.

Stomach. A hollow muscular bag-like organ located on the left side of the upper abdomen; connected to oesophagus on upper end and duodenum at lower end. Simple columnar epithelium; Mucosa has mucus glands. Function: Temporarily holding, then churning, mixing and mechanical breakdown of food into chyme. Secretes acid (parietal cells) and enzymes (pepsin from chief cells) → protein digestion. Also secrete other GI hormones. Castle’s Intrinsic factor (parietal cells) helps in vitamin B12 absorption..

Stomach body (low power): A section of the stomach body demonstrating the layers of the gastric wall. Lumen Mucosa Muscularis mucosae Submucosa Muscularis propria Serosa.

Small intestine. Tubular structure extending from pylorus to ileocecal valve, can be divided into 3 parts: Duodenum: 1st part, C-shaped, ~25 cm length. Mucosa made of mucus glands, villi & microvilli; simple columnar epithelium. Submucosal layer contains Brunner’s glands. Function: Preparation of chyme for assimilation ► Receive mixed & churned food from stomach (chyme); Neutralize the acidity of chyme; Digestion with bile (from liver), pancreatic enzymes & intestinal juices; Fat emulsification; Absorption of folate, Fe & Vit. D; Release of GI hormones (Secretin, CCK, GIP)..

Small intestine. Jejunum: 2nd part, ~25 m long. Mucosa made of plica circulares, villi, microvilli, crypts of Lieberkuhn; simple columnar epithelium. Function: Absorption of minerals, electrolytes, carbohydrates, proteins, fats (passive: fructose; active: amino acids, small peptides, vitamins, glucose). Ileum: Last part, ~3.5 m long. Mucosa made of plica circulares, villi, microvilli; Function: Maximal absorption of nutrients (vitamin B12, bile salts, etc.).

A diagram of the human body Description automatically generated.

large intestine. Parts: Appendix, cecum, ascending colon, hepatic flexure, transverse colon, splenic flexure, descending colon, rectum and anus. Mucosa: Crypts of Lieberkuhn lined by simple columnar epithelium; Muscle coat: Muscle fibres form 3 thick bands → taenia coli, they being shorter creates sacculations / haustrations; Serosa: absent in posterior aspect of ascending and descending colon, small pockets of fat project outward (appendices epiploicae). Function: Absorption of water and salt (Na+); Drying, storage and elimination of faeces; Bicarbonate & K+ secretion..

large intestine. LARGE INTESTINE TRANSVERSE COLON DESCENDING COLON ASCENDING COLON ILEOCECAL VALVE ILEUM CECUM SIGMOID COLON RECTUM VERMIFORM APPENDIX ANAL CANAL.

Salivary glands. 3 pairs of glands Parotid glands: Largest, near angle of jaw; purely serous glands, ducts open in the inner side of both cheeks. Submandibular glands: Lies bilaterally under mandibular body; mixture of serous & mucous acini; S-shaped duct opens on the sublingual papilla Sublingual glands: Smallest, under mouth mucosa; mixed acini; 8-20 ducts open on the sublingual fold..

[image] Parotid duct Zmmatic arch parotid sublingual duct Sublingual g I aM Opening Of parotid duct (near second maxillary molar) maxillary Tongue Lingual frenulum Submandibular duct Mylohyoid muscle Subntandibular gland.

Pancreas. Elongated, accessory digestive gland; retroperitoneal Anatomically: head, neck, body, tail Physiologically: Exocrine (produces pancreatic juice), Endocrine Functional unit of pancreas: serous, compound tubuloalveolar glands (acinar cells, centroacinar cells). Pancreatic ducts: Acinar secretions → intercalated ducts → interlobular ducts → main pancreatic duct (Duct of Wirsung) and accessory pancreatic duct (Duct of Santorini). Function: Enzymes necessary for digestion; Acidic chyme neutralization..

Pancreas. GALLBLADDER CYSTIC DUCT DUODENUM ACCESSORY PANCREATIC DUCT MINOR DUODENAL PAPILLA MAJOR DUODENAL PAPILLA PANCREATIC DUCT RIGHT AND LEFT HEPATIC DUCT OF LIVER COMMON HEPATIC COMMON BILE DUCT SPLEEN TAIL PANCREAS JEJUNUM.

liver. Largest gland, ~ 1500 g. Consists of lobes (Left, caudate, Right, Quadrate) → subdivided into hepatic lobules (anatomical unit; contains hepatocytes). Portal triad: Branch of hepatic artery, portal vein and interlobular bile duct; Portal lobule: area of portal triad b/w 3 adjacent hepatic lobules. Hepatic cord: two rows of hepatocytes; blood spaces around hepatocytes on each sides are lined by sinusoids. Functional unit: Acinus – A line joining two portal triads extending towards the two adjoining central veins in outwardly direction. Three zones: Zone 1: Well oxygenated but susceptible to infections. Zone 2: Middle zone, moderately oxygenated. Zone 3: Centrilobular zone; least blood supply & oxygenation..

liver. Portal triad Central vein Hepatic lobule (classical lobule) Portal lobule Liver acinus.

liver. Hepatocyte •s Portal triad Sinusoidal cell Hepatic stellate cell Myofibroblast Portal fibroblast Space Of Disse Ductular epithelial cell Collagen Endothelial cell Smooth muscle cell.

liver. Dual blood Supply of liver: 1500 mL/min. Hepatic artery: 20-25% of blood supply to liver, provides oxygenated blood, 95% O2 saturated. Porto-hepatic venous system: 75% of blood supply to liver, 85% O2 saturated. Portal system – bypasses the heart, nutrient-rich blood from GIT enters the liver first..

liver. Functions: Exocrine function: Largest exocrine gland of body; Bile salts (Na+ & K+ salts of taurocholate and glycocholate) from sinusoids is released in the canaliculi. Metabolic function: Carbohydrate, lipid, protein metabolism; Vitamin D hydroxylation. Synthetic function: Clotting factors (II, VII, IX, X), albumin, angiotensinogen, urea, cholesterol, enzymes. Detoxification: Site - Kupffer cells; Xenobiotic metabolism. Storage: Glucose, Vitamin B12, A, D, K, ferritin and hemosiderin. Immunity: Kupffer cells (macrophage); filter for antigens. Reservoir for blood (650 mL) Foetal erythropoiesis site. Destruction of old RBCs by Kupffer cells..

Gall bladder. Pear-shaped sac, undersurface of liver. Fundus, body, neck, cystic duct Function: Stores 30-50 mL of bile, Concentrates bile by absorbing water. Right & left hepatic ducts → Common hepatic duct (4 cm) + cystic duct (4 cm) → Common bile duct (8 cm) + Pancreatic duct → Common hepatopancreatic duct / Ampulla of Vater, guarded by Sphincter of Oddi..

Innervation of git. GIT is regulated by extrinsic (sympathetic and parasympathetic) and intrinsic/enteric nervous system. Enteric nervous system (ENS): Wholly contained within submucosal and myenteric plexuses in GIT wall. ‘Mini brain’. Sensory neurons → responds to mechanical, thermal, chemical and osmotic stimuli; Motor neurons → regulate motility, secretion and absorption. NTs : Ach, NE, VIP, NO, substance P, GABA, glycine, glutamate, histamine, endogenous opioid, neurotensin, CCK, somatostatin, vasopressin, oxytocin, neuropeptide Y..

Innervation of git. Plexus present in ENS: Myenteric plexus (Auerbach’s plexus): B/w circular & longitudinal muscle coat. Function: Gut motility regulation (↑s rate, intensity and velocity of contractions, Peristalsis), Inhibition (relaxation) of pyloric sphincter and ileocecal valve. Meissner’s plexus (submucosal plexus): In submucosa. Function: Regulation of intestinal secretions, Sensory function, Regulation of GI blood flow..

Innervation of git. Sympathetic: T8-L2; Preganglionic fibres – lateral sympathetic chain → splanchnic nerve → celiac, sup. & inf. mesenteric, and hypogastric ganglia; Postganglionic fibres → ENS. NE released → Smooth muscle relaxation, Gut motility inhibition, Sphincter contraction, Inhibits digestion & absorption, Splanchnic vasoconstriction. Parasympathetic: Cranial (from medulla) & sacral (from sacral SC) fibres. ACh released → Smooth muscle contraction, Sphincter relaxation, Gut motility & secretion stimulation..

Innervation of git. Medulla Cranial parasympathetic fibers Esophagus, stomach, SI, 1st half of LI Vagus.

Gut brain axis. The two-way biochemical signaling that takes place b/w GIT and CNS. Includes the CNS, neuroendocrine system, neuroimmune systems, the hypothalamic–pituitary–adrenal axis, sympathetic and parasympathetic arms of the ANS, the ENS, vagus nerve, and the gut microbiota. Cytokines, NTs, neuropeptides, chemokines, hormones, and gut microbiota metabolites (short-chain FAs, branched-chain AAs, and peptidoglycans) influence brain development, starting from birth. The intestinal microbiome divert these products to the brain via the blood, nerves, endocrine cells, and more. Studies have confirmed communication between the hippocampus, the prefrontal cortex, and the amygdala (responsible for emotions and motivation) - a key node in the gut-brain behavioral axis..

Gut brain axis. Healthy status Healthy CNS function Normal gut physiology Physiological levels of inflammatory cells/mediators Normal gut microbiota Stressldisease Alterations in behaviour, cognition, emotion, nociception Abnormal gut function Increased levels of inflammatory cells/mediators Intestinal dysbiosis.