Anatomy and physiology

• The colon extends from the cecum to the anal canal, it can be divided into 4 parts:

  1. Ascending
  2. Transverse
  3. Descending
  4. Sigmoid
• The transverse colon, sigmoid colon and the appendix are intraperitoneal structures on a mesentery

  The cecum is intraperitoneal but not on a mesentery
• The ascending colon is retroperitoneal structure that extends from the cecum. When it meets the right lobe of the liver, it turns 90 degrees to move horizontally- this is known as the right colic flexure or hepatic flexure
• The transverse colon extends from the right colic flexure to the spleen, where it turns another 90 degrees to mark the start of the descending colon- this is known as the left colic flexure of the splenic flexure. Here the colon is attached to the diaphragm.
• The sigmoid colon is located in the left lower quadrant of the abdomen. Has a characteristic S shape. The sigmoid colon is attached to the posterior pelvic wall by a mesentery, making the sigmoid colon particularly mobile.
• The large intestine has a number of characteristic features which allow it be to be distinguished from the small intestine:

  • Omental appendices - small pouches of fatty peritoneum
  • Teniae coli - longitudinal muscle
  • Haustra - give the colon its segmented appearance
  • Has a much wider appearance than the small intestine
• The characteristic features of the colon stop at the rectum
• The ascending colon and the proximal 2/3 of the transverse colon are part of the midgut and therefore supplied by the superior mesenteric artery. Innervation is provided by the superior mesenteric plexus
• The distal 1/3 of the transverse colon, descending colon and sigmoid colon are the hindgut and therefore are supplied by the inferior mesenteric artery
• The marginal artery (of drummond) provides collateral supply to the colon. Maintains arterial supply in the case of occlusion or stenosis of one of the major vessels.
• The lymphatic drainage of the ascending and transverse colon is into the superior mesenteric nodes. The descending colon and sigmoid drain into the inferior mesenteric nodes.
• Unlike the small bowel, the colon does not have plicae circulares or villi
• The terminal ileum is the most distal segment of the small bowel - it is the site of B12 absorption
• Ascending and descending colon are surrounded by peritoneum but not free moving = secondary retroperitoneal
• A mesentery is a double fold of peritoneal tissue that suspends organs from the abdominal wall
• An omentum is a double (or quadruple in the case of the greater omentum) fold of peritoneal tissue that suspends organs from organs
• The rectum is mostly retroperitoneal

  • Formation of rectovesical pouch in males
  • Formation of rectouterine pouch in females
• The longitudinal muscles of the colon become the internal sphincters of the anal canal

  Internal sphincters are innervated by parasympathetic nerves - involuntary control
• Deep, superficial and subcutaneous external sphincters of the anal canal are under voluntary control
• The anal canal is typically divided into superior and inferior segments by the dentate/pectinate line. Cancer above the line would be an adenocarcinoma, cancer below would be a squamous cell carcinoma
• The transpyloric plane is at the level of L1 :

  • End of the spinal cord
  • pylorus of the stomach
  • Fundus of the gallbladder
  • Formation of the portal vein
  • Superior mesenteric artery
  • Neck of the pancreas
  • Hila of the kidneys
• Boundaries of the abdomen:

  • Superior = xiphoid process
  • Inferior = pubic crest/inguinal ligaments
• The cricoid cartilage is at the level of C6 and marks the start of the trachea and oesophagus
• The 3 muscles of the pharynx - superior, middle and inferior constrictors

  The inferior muscle ends at the level of C6
• The tonsils are found in the oropharynx between the palatoglossal (anterior) and palatopharyngeal (posterior) arches
• The anterior 2/3 and posterior 1/3 of the tongue are separated by the terminal sulcus

  The motor control to the whole tongue is via the hypoglossal nerve (CN VII)
• The anterior 2/3 of the tongue is the oral section of the tongue

  • Sensation = trigeminal nerve via the mandibular nerve
  • Taste = facial nerve via the chordae tympani
• The posterior 1/3 of the tongue is the pharyngeal portion of the tongue

  Sensation and taste is provided by the glossopharyngeal nerve
• The salivary glands include parotid, submandibular and sublingual glands
• The parotid salivary glands:

  • Bilateral
  • Serous saliva containing amylase
  • Common site of tumours - usually benign
  • Facial nerve passes though
• Submandibular and sublingual salivary glands:

  • Mixed serous and mucus secretions
  • Thinner duct and thicker secretions than the parotid glands - more susceptible to blockage
• The oesophagus blood supply is via the left gastric artery which is a branch of the celiac trunk
• The oesophagus:

  • 25cm long fibromuscular tube
  • Extends from C6 to the cardiac orifice of the stomach at T11
  • Outer longitudinal muscle, inner circle and striated squamous mucosa
  • Bottom 1/3 of oesophagus has smooth mucosa
  • The recurrent laryngeal nerve runs along the oesophagus and trachea
• The oesophagus passes into the abdomen via at diaphragm at the level of T10 - with the right and left vagus nerve

  Enters at an angle through a sling of muscle = physiological lower sphincter
• Lower oesophageal sphincter open = GORD

  Lower oesophageal sphincter failure to relax = achalasia
• A sliding hiatus hernia is a hernia that occurs when the stomach slides up into the diaphragm. The gastro-oesophageal junction moves into the chest. An air fluid level will be seen in the chest cavity on a chest X-ray. Cause of GORD.
• The stomach:

  • Attached to the under aspect of the liver
  • Greater omentum extends from the greater curvature of the stomach
  • The gastro-oesophageal sphincter stops reflux into the oesophagus
  • The pyloric sphincter prevents stomach contents free-flowing into the duodenum
  • Rugae = folds in the mucosal lining of the stomach
• Goblet cells of the stomach produce mucus and bicarbonate
• Parietal cells of the stomach secrete hydrochloric acid and intrinsic factor (B12 absorption in the terminal ilium)
• Chief cells of the stomach secrete pepsinogen which converts to pepsin in the presence of HCl