Small Animal Digestive Tract 2

Small Animal Digestive Tract 2

1. Arterial Blood Supply

   • comes from branches of the aorta, either paired or unpaired
   • Unpaired = coeliac, cranial mesenteric and caudal mesenteric
     • coeliac to the foregut, cranaial mesenteric to the midgut and caudal mesenteric to the hindgut
     • Coeliac to stomach, liver and pancreas
     • Cranial to pancreas, small intestine, caecum and ascending colon
     • caudal to descending colon to rectum
   • Paired
     • Phrenicoabdominal
       • to diaphragm and wall
     • Renal
       • kidneys
     • Gonadal
       • testes and ovaries
     • Deep Circumflex Iliac
       • flank
     • External Iliac
       • hindlimbs
     • Internal Iliac
       • pelvic viscera

2. Venous Drainage

   • Hepatic Portal Vein that drains the GI via the liver into the caudal vena cava
   • Hepatic Portal System
     • branches are equivalent of the coeliac, cranial mesenteric and caudal mesenteric arteries
     • Gastroduodenal Vein
     • Splenic Vein
     • Cranial Mesenteric
       • draining Jejunum, Caecum and Ileum
     • Iliocolic Vein
       • draining the last part of Ileum, Colon and Rectum
   • Paired Drainage
     • all drain into caudal vena cava
     • Hepatic Veins
       • Liver
     • Phrenicoabdominal Vein
     • Renal
       • Kidneys
     • Gonadal
       • Left gonadal vein drains into the left renal vein before entering the caudal vena cava
     • Deep Circumflex Iliac Vein
     • Common Iliac
       • further divides to include the external iliac veins, internal iliac veins and the median sacral veins

3. Nerve Supply

   • Motor (autonomic) - general visceral efferent via Parasympathetic and Sympathetic Sensory - general visceral afferent via cranial and mixed spinal nerves
   • Parasympathetic Innervation
     • Vagus supplies foregut and midgut
     • Pelvic nerves to hindgut and pelvic organs
       • S1, S2, S3
   • Sympathetic Innervation
     • Paraverterbral Chain (paired)
       • paired on either side of the body to supply the smooth muscle of the abdominal wall
     • Preverterbral Chain (unpaired)
       • innervates the smooth muscle of the viscera
       • Coeliac Ganglia
         • innervates the foregut
         • Both known as the coeliacomesenteric
       • Cranial Mesenteric Ganglia
         • innervates the midgut
       • Caudal Mesenteric Ganglia
         • innervates the hindgut
         • Hypogastric Nerves
           • innervates the pelvis

4. Liver

   • Intrathoracic on RHS, 3-4% of body weight
     • caudal edge (caudate process of caudate lobe) palpable from beyond the last rib
     • Metabolism, Bile production and digestion processing
   • Blood Supply
     • Hepatic Artery and Hepatic Portal Vein
       • hepatic artery is a branch of the coeliac artery, majority of blood supply is via the portal vein
       • hepatic artery brings oxygenated blood, whereas the portal vein brings poorly oxygenated blood
     • Drainage via Hepatic Veins
       • returns to caudal vena cava
   • Structure
     • Lobulations
       • Left lateral, left medial, Quadrate, Right Medial, Right Lateral, Papillary process of caudate lobs, caudate process of caudate lobe
     • Impressions
       • Renal Impression
         • right kidney sits against caudate process of caudate lobe
       • Duodenum
         • sits against right medial lobe
       • Gastric
         • sits against left lateral lobe
     • Porta
       • hilus of the liver
       • Hepatic Arteries
       • Hepatic Portal Vein
       • Bile Duct
     • Gall Bladder
   • Embryological Remnants
     • Teres and Falciform Ligaments
       • Umbilical Vein becomes the Teres Ligament
       • Ventral Mesentery forms the Falicform Ligament
         • Pass between Quadrate Lobe and Left Lobes of the liver
     • Ductus Venosus
       • fetal bypass shunt from umbilical vein to caudal vena cava
     • Portosystemic Shunt
       • blood from hepatic portal vein (GI Tract) doesn't pass through the liver and directly enters the caudal vena cava

5. Stomach

   • Blood Supply
     • Spleen to the left of the stomach and Liver to the right
     • Coeliac Artery
       • Left Gastric Artery
         • on lesser curvature near the cardiac opening
       • Splenic Artery
         • branches into left and right gastroepiploic arteries
         • Left Gastroepiploic
       • Hepatic Artery
         • branches into right gastric and right gastroepiploic
         • Right Gastric and Right Gastroepiploic
       • Gastric Arteries supply the lesser curvature
       • Gastroepiploics supply the greater curvature
   • Development
     • Greater Curvature forms from the Dorsal Mesogastrium
       • dorsal side forms faster, expands more
     • Lesser Curvature forms from the Ventral Mesogastrium
     • Spleen NOT from gut tube
       • develops from aggregates of mesenchyme in dorsal mesogastrium
     • Ruminants
       • cranial part expands to form rumen-reticulum
         • folds dorsally to form abomasum
     • Mesenteries
       • Rotation around cranio-caudal axis
         • 90 degree rotation forms greater omentum and lesser omentum
         • Rotation around dorso-ventral axis
           • creates the curved stomach position and pulls the greater omentum around the greater curvature whilst compacting the lesser omentum
       • Dorsal Mesogastrium folds
         • folds back on itself to form the greater omentum, hanging off the greater curvature of the stomach

6. Pancreas

   • found in curve of duodenum left lobe in greater omentum right lobe in mesodudodenum
   • Ducts
     • Major pancreatic into major duodenal papilla
     • Minor pancreatic into minor duodenal papilla
   • Development
     • Liver and pancreatic buds arise
       • arise from gut tube close to the foregut/midgut boundary
     • Liver buds invade septum transversum
       • results in the close association with the diaphragm
     • Rotations cause pancreatic buds to fuse
       • forms the single pancreas with two lobes
   • Species Differences
     • Dog, Horse
       • separate openings of common bile duct and accessory pancreatic ducts
     • Cat, Sheep, Goat
       • common openings of pancreatic and bile ducts
     • Cattle, Pig
       • one pancreatic duct separate to bile duct
         • pancreatic ducts fuse together

7. Mid and Hindgut

   • Development
     • elongated midgut forms tube
       • Midgut elongates to form a loop that protrudes into the umbilicus
       • 180 rotation to return to abdomen
         • 90 rotation
   • Root of Mesentery
     • where the cranial mesenteric artery starts and from where nerves and blood vessels head off towards mid/hindgut
     • Transverse colon passes cranial
       • In ALL species, the transverse colon passes cranial to the root of the mesentery
   • Complications
     • Lumen of the Gut
       • Stenosis
         • narrowing of the lumen
       • Atresia
         • occlusion of the lumen
     • Hernia
       • midgut pushes out into the umbilicus and doesn't return to the abdomen, can result in whole intestines falling out of the dog via the umbilicus at birth
     • Duct connecting midgut and yolk sac should atrophy
       • Fistula
         • doesn't atrophy and bowel contents can leak out via the abdominal wall
       • Cyst
         • Atrophied tube still acts like bowel material and secretes substances which form a cyst within the closed tube
       • Fibrous Cord
         • doesn't detach and instead the connective tissue remains joining the bowel to the abdominal wall can allow other viscera to get wrapped around it/blocked off
       • Diverticulum
         • small protrusion within the bowel (cup-shaped) where things can get caught/stuck
     • Imperforate anus/anal atresia
       • anal membrane doesn't breakdown and foetal contents can't be released

Images and files in the map