Overview of Gastrointestinal System (Basic Anatomy) PDF

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Summary

GASTROINTESTINAL SYSTEMGI SYSTEM – INTRODUCTION & OVERVIEWThe gastrointestinal tract (GIT) is the system of organs that allows for the consumption and digestion of food, absorption of nutrients, and excretion of waste in the form of faecal matter. It includes the oral cavity, oesophagus, stomach...

Description

GASTROINTESTINAL SYSTEM GI SYSTEM – INTRODUCTION & OVERVIEW The gastrointestinal tract (GIT) is the system of organs that allows for the consumption and digestion of food, absorption of nutrients, and excretion of waste in the form of faecal matter. It includes the oral cavity, oesophagus, stomach, small intestine, and large intestine. It is derived from the primitive gut tube and can be divided into the foregut, midgut, and hindgut, each of which is distinct in its embryological development and neurovascular supply. The veins of all three embryological segments drain directly or indirectly into the portal vein, which is connected to the caval venous system via a system of portocaval shunts. The GIT is innervated by the sympathetic nervous system, parasympathetic nervous system, and enteric nervous system, the last of which is unique to the GIT. All organs of the GIT are composed of four histological layers: mucosa, submucosa, muscularis propria, and serosa (intraperitoneal organs) or adventitia (extraperitoneal organs). The secretory and regulatory products of the gastrointestinal tract vary from segment to segment depending on the regional histological characteristics, presence of specialized cells, and function.

Organs of the GI System Organs of the GI Tract  Oral cavity and pharynx  Oesophagus  Stomach  Small intestine  Large intestine Accessory Organs of the GI Tract  Liver  Gall bladder  Pancreas

Functions of the GI System  Ingestion of food and water  Mastication  Transport of the food bolus or chyme gastrointestinal tract  Digestion (i.e., breaking food down into nutrients)  Absorption of nutrients and water  Excretion of waste as faeces  Immunity

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Vasculature Arterial Supply  Most of the GIT is supplied by the anterior (unpaired) branches of the abdominal aorta. In contrast, nonGI organs in the abdominal cavity are supplied by the posterior and/or lateral (paired) branches of the abdominal aorta.  Exceptions: o Oesophagus: supplied by the oesophageal branches of the inferior thyroid artery, the thoracic aorta, and the left gastric artery o Distal anal canal: supplied by the middle rectal artery and the inferior rectal artery

o Watershed zones  Splenic flexure: supplied by the branches of both superior and inferior mesenteric arteries  Rectosigmoid junction: supplied by the superior rectal artery and last sigmoid branch of the inferior mesenteric artery Artery Coeliac trunk

Superior mesenteric artery

Inferior mesenteric artery

Characteristics  First anterior branch of the abdominal aorta  Arises at the level of the 12th thoracic vertebra (T12)

 Second anterior branch of the abdominal aorta  Arises at the level of the 1st lumbar vertebra (L1)  Passes anterior to the third segment of the duodenum  Most commonly affected by acute mesenteric ischemia

 Third anterior branch of the abdominal aorta  Arises at the level of the 3rd lumbar vertebra (L3)

Branches  Left gastric artery: supplies abdominal oesophagus and lesser curvature of the stomach  Splenic artery o Left gastroepiploic artery: supplies greater curvature of the stomach; important anastomosis with left gastroepiploic arteries o Short gastric arteries: supply fundus of the stomach o Pancreatic branches  Common hepatic artery o Proper hepatic artery  Right gastric artery: supplies the lesser curvature of the stomach; important anastomosis with left gastric arteries  Right and left hepatic arteries  Cystic artery: branch of right hepatic artery o Gastroduodenal artery  Right gastroepiploic artery: supplies the greater curvature of the stomach  Superior pancreaticoduodenal artery: supplies the head of the pancreas and the proximal duodenum  Inferior pancreaticoduodenal artery: supplies the head of the pancreas and distal duodenum  Intestinal arteries to the jejunum and ileum  Ileocolic artery: supplies the distal ileum, cecum, appendix  Right colic artery: supplies the ascending colon  Middle colic artery: supplies the transverse colon  Left colic artery: supplies distal ⅓ of the transverse colon and descending colon  Sigmoid artery: supplies sigmoid colon  Superior rectal artery: supplies rectum and proximal anal canal

Region Supp.

 Foregut  Spleen

Midgut

Hindgut

Venous Drainage  The veins of most of the gastrointestinal tract drain directly or indirectly into the portal vein.  Exceptions: o Oesophagus: drains into the inferior thyroid vein, the azygos vein, hemiazygos vein, and the left gastric vein o Distal anal canal (below the pectinate line): drains into the inferior vena cava Vein Portal vein

Superior (SMV) mesenteric vein Inferior (IMV) mesenteric vein

Characteristics  Formed by union of splenic vein & superior mesenteric vein  Carries blood from GIT and accessory organs to the liver

Joins the splenic vein to form the portal vein Drains into the splenic vein, which then joins the SMV to form the portal vein

Areas drained  Foregut (directly) o Stomach  R & L gastric veins  portal vein  L gastroepiploic vein and short gastric veins  splenic vein  portal vein  R gastroepiploic vein  SMV  portal vein o Proximal duodenum  superior pancreaticoduodenal vein  portal vein  Midgut (indirectly via SMV)  Hindgut (indirectly via IMV) Midgut Hindgut (up to the pectinate line)

Innervation of GIT The gastrointestinal tract is innervated by the autonomic nervous system.  Extrinsic innervation: parasympathetic and sympathetic nervous system  Intrinsic innervation: enteric nervous system o Embryologically derived from the neural crest o Regulated by the autonomic nervous system but can function independently o Crucial for gastrointestinal motility and secretion

Drains into

Inferior Vena Cava

Portal vein Splenic vein  portal vein

Innervation

Foregut

Parasympathetic innervation

Vagus nerves

Sympathetic innervation (prevertebral ganglia)

Coeliac ganglia (input from T59 thoracic splanchnic n.)

Extrinsic Innervation

Site of referred pain

Enteric nervous system (intrinsic innervation)

Epigastric reg.

Midgut

Vagus nerves

Superior mesenteric ganglia (input from T10-12 thoracic splanchnic n.) Umbilical reg.

Hindgut Pelvic splanchnic nerves (through the inferior hypogastric plexus) Inferior mesenteric ganglia (input from L1-2 lumbar splanchnic n.) Hypogastric reg.

Submucosal plexus (Meissner plexus)

Neurons & ganglia that lie within submucosal layer of the alimentary canal

Myenteric plexus (Auerbach plexus)

 Neurons & ganglia that lie between the longitudinal and circular smooth muscle layers of the muscularis propria  Interstitial cells of Cajal (ICC) – pacemaker cells in smooth muscle of GIT that generate rhythmic phasic contractions of the gut (migratory motor complex) that occur in fasting/interdigestive state

Effect  ↑ secretion  ↑ motility  ↓ sphincter tone

 ↓ secretion  ↓ motility  ↑ sphincter tone

Regulates local gastrointestinal secretion & nutrient absorp.  Regulates inherent myogenic motility of GIT  ICC act as pacemaker cells for peristalsis

The sympathetic system has an inhibitory effect on the gastrointestinal tract. The parasympathetic system promotes gastrointestinal secretion and motility. The enteric nervous system can function independently of the central nervous system.

Motility & Contractile Activity of GIT Type of Contractions Peristalsis (mediated by ICC)

Description/Activity Inv.  Concerted waves of contraction and relaxation of the muscularis propria of the gastrointestinal tract  Propels intestinal contents caudally (antegrade peristalsis) or cranially (retrograde peristalsis)

Segmentation

Migratory motor complex (rhythmic phasic contractions)

Ultrapropulsive contractions

Tonic contractions

 Contraction of the longitudinal smooth muscle layer shortens the length of the intestine, which aids in propulsion  Relaxation of the circular smooth muscle layer causes the intestine to widen, allowing intestinal contents to pass  Waves of contraction of the muscularis propria  Aid mixing of chyme with digestive enzymes  Not propulsive  Slow basal electric rhythm with low amplitude waves  Generated by the interstitial cells of Cajal in the myenteric plexus  Stimulated by motilin, which is secreted by Mo cells of the small intestine  Predominantly occurs in the fasting (interdigestive) phase  Frequency of MMC: o Stomach: 3/min o Duodenum: 12/min o Ileum: 9/min o Colon: 3/min  Large amplitude contractions (giant complexes)  Stimulated by the parasympathetic nervous system  Occur primarily in the small intestines and colon  Responsible for mass movement of chyme/faeces  Most significant at sphincters and intestinal junctions  Regulates the passage of food/chyme  Partially responsible for faecal continence

Peristalsis (regulated by enteric nervous system) – refers to the concerted waves of involuntary contraction (in the propelling segment) and relaxation (in the receiving segment) of the muscularis propria (gastrointestinal tract) that propel luminal contents in the aboral direction The bolus of food/chyme causes distension of the intestinal lumen. This stimulates the sensory neurons, which subsequently stimulate the excitatory motor neurons proximal to the bolus and the inhibitory motor neurons distal to the bolus. The stimulation of motor neurons leads to contraction of the circular muscles in the propelling segment and relaxation of the circular muscles in the receiving segment, squeezing the luminal contents aborally. The sensory neurons also stimulate the excitatory motor neurons that supply the longitudinal muscle layer in the receiving segment of the bowel, leading to contraction of the longitudinal muscle and reducing the distance of forward propulsion.

PHYSIOLOGY: DIGESTION...