Colon Anatomy and Physiology

Colon

• The colon is suspended by the sigmoid mesocolon.
• It’s length and disposition are highly variable, ending at the rectosigmoid junction.
• At the rectosigmoid junction, the teniae, haustra, and omental appendices cease.
• This junction is located anterior to the third sacral segment.
• The part of the large intestine proximal to the left colic flexure including the cecum, appendix, ascending, and transverse colons receives blood from the superior mesenteric vessels.
• Most of the large intestine distal to the left colic flexure or aborad including the descending and sigmoid colons and superior rectum is served by the inferior mesenteric vessels.
• The left colic flexure marks the division between cranial (vagal) and sacral (pelvic splanchnic) parasympathetic innervation of the alimentary tract.
• Sympathetic fibers reach the large intestine via abdominopelvic (lesser and lumbar) splanchnic nerves through the prevertebral (superior and inferior mesenteric) ganglia and peri-arterial plexuses.
• The middle of the sigmoid colon constitutes a divide in sensory innervation of the abdominal alimentary tract.
• Orad, visceral afferents for pain travel retrogradely with sympathetic fibers to spinal sensory ganglia
• Those conveying reflex information travel with parasympathetic fibers to vagal sensory ganglia.
• Aborad, both types of visceral afferent fibers travel with parasympathetic fibers to spinal sensory ganglia.

Spleen

• The spleen is an ovoid, purplish, pulpy mass, similar in size and shape to one’s fist.
• As the most vulnerable abdominal organ, it is relatively delicate.
• The spleen is located in the superolateral part of the left upper quadrant (LUQ)/ hypochondrium of the abdomen.
• Protection is afforded by the inferior thoracic cage.
• Being the largest of lymphatic organs, it participates in body defenses.
• The spleen is a site of lymphocyte (white blood cell) proliferation.
• It provides immune surveillance and response.
• Prenatally hematopoietic (blood-forming), after birth it identifies, removes, and destroys expended red blood cells (RBCs) and broken-down platelets
• It also recycles iron and globin.
• The spleen stores RBCs and platelets, acting as a blood reservoir.
• The spleen can provide a “self-transfusion” in response to hemorrhage to a limited degree.
• Despite its many useful and important functions and size, it is non-vital.
• It is a soft, vascular (sinusoidal) mass with a relatively delicate fibroelastic capsule.
• Visceral peritoneum covers the thin capsule entirely, except at the splenic hilum.
• At the splenic hilum, the splenic artery and vein branches enter and leave.
• The spleen’s structure allows for marked expansion and some rapid contraction.
• Its a mobile organ that normally rests on the left colic flexure and does not descend inferior to the costal (rib) region.
• Located posteriorly with the left 9th–11th ribs, its long axis is roughly parallel to the 10th rib
• The diaphragm and the costodiaphragmatic recess- a cleft-like extension of the pleural cavity between the diaphragm and the lower part of the thoracic cage separates the spleen and the ribs.
• Organs and structures related to the Spleen:
  • Anteriorly, the stomach
  • Posteriorly, the left part of the diaphragm separating it from the pleura, lung and ribs 9-11
  • Inferiorly, the left colic flexure
  • Medially, the left kidney
• Spleen size, weight, and shape varies greatly
• The spleen is usually approximately 12 cm long and 7 cm wide.
• A useful, nonmetric memory device exploits odd numbers: The spleen is 1 inch thick, 3 inches wide, and 5 inches long and weighs 7 ounces.
• The diaphragmatic surface of the spleen has a convex curve to fit the concavity of the diaphragm and curved bodies of the adjacent ribs.
• Rib fractures can be detrimental to the close proximity between ribs and the spleen.
• Sharp and often notched anterior and superior borders are typical.
• A rounded posterior (medial) end and inferior border are seen.
• The spleen normally does not extend inferior to the left costal margin.
• Seldom palpable through the anterolateral abdominal wall unless it is enlarged.
• It moves inferior to the left costal margin if hardened and enlarged to approximately three times its normal size, with its superior (notched) border lying inferomedially.
• A notched border is helpful when palpating an enlarged spleen.
• The notches can often be palpated when the person takes a deep breath.
• A smooth muscle in its capsule and trabeculae enable it to expel blood periodically into the circulation.
• The splenic artery's large size (or vein) indicates the volume of blood that passes through the spleen’s capillaries and sinuses.
• Dense, irregular fibroelastic connective tissue composes fibrous capsule of spleen, thickened at the splenic hilum.
• Small fibrous bands, the trabeculae arise from the capsule’s deep aspect, carrying blood vessels to and from the parenchyma/splenic pulp (the substance of the spleen).
• The spleen contacts the posterior wall of the stomach.
• It is connected by the gastrosplenic ligament to the greater curvature of the stomach and connected by the splenorenal ligament to the left kidney.
• Splenic vessels are contained in these ligaments, and are attached to the hilum of the spleen on its medial aspect.
• The splenic hilum often contacts with the tail of the pancreas, and constitutes the left boundary of the omental bursa.
• The splenic artery is the arterial supply of the spleen, the largest branch of the celiac trunk.
• It takes a winding path posterior to the omental bursa, anterior to the left kidney, and along the superior border of the pancreas.
• The splenic artery divides into five or more branches lying within the splenorenal ligament layers that enter the hilum.
• Resultant vascular segments of the spleen form upon the lack of arterial anastomosis within the spleen, two in 84% of spleens and three in others.
• These have relatively avascular planes between them, creating subtotal splenectomy. (See Clinical Box “Splenectomy and Splenomegaly”).
• Tributaries emerging from the hilum create venous drainage towards the splenic vein.
• The IMV joins the splenic vein and runs posterior to the body and tail of pancreas throughout most of its course.
• The splenic vein combines with the SMV posterior to the neck of the pancreas to create the hepatic portal vein.
• The splenic lymphatic vessels go from the nodes in the splenic hilum and run along the splenic vessels to the pancreaticosplenic lymph nodes, which then continue to the celiac nodes.
• The pancreaticosplenic nodes are related to the pancreas’ posterior surface and superior border.
• The nerves of the spleen come from the celiac plexus and are distributed primarily along the splenic artery's branches and are vasomotor in function

Pancreas

• The pancreas is an elongated accentory digestive gland laying retroperitoneally.

• It overlies and transversely crosses the bodies of the L1-L2 vertebra (level of the transpyloric plane) on the posterior abdominal wall.

• The pancreas lies posterior to the stomach, between the duodenum on the right and the spleen on the left.

• The pancreas produces:

  • An exocrine secretion (pancreatic juice from acinar cells) that enters the duodenum through the main and accessory pancreatic ducts.
  • Endocrine secretions including glucagon and insulin from pancreatic islets(of Langerhans) that enter the blood.

• For descriptive purposes, the pancreas can be divided into the:

  • Head
  • Neck
  • Body
  • Tail

• The head of the pancreas is the expanded portion embraced by the C-shaped curve of the duodenum, to the right of the superior mesenteric vessels just inferior to the transpyloric plane.

• The uncinate process, is fixed to the medial aspect of the descending and horizontal parts of the duodenum, and is a projection from the inferior part of the pancreatic head that extends medially to the left, posterior to the SMA.

• The pancreatic head lies posteriorly on the IVC, right renal artery and vein, and left renal vein.

• On its way to opening into the descending duodenum part, the bile duct lies in a groove on the posterosuperior surface of the head, or is embedded in its substance.

• The neck is short (1.5–2 cm) and overlies the superior mesenteric vessels creating a groove in its posterior aspect.

• The neck’s anterior surface, covered by peritoneum, is adjacent to the pylorus of the stomach.

• The SMV joins the splenic vein found posterior to the neck where the hepatic portal vein is formed.

• The body continues from the neck, and lies to the left of the superior mesenteric vessels.

• The body passes over the aorta and L2 vertebra, continuing just above the transpyloric plane posterior to the omental bursa.

• Peritoneum covers the anterior surface of the body and lies in the floor of the omental bursa, forming part of the stomach bed.

• The posterior surface lacks peritoneum, and contacts the aorta, SMA, left suprarenal gland, left kidney, and renal vessels.

• The tail lies anterior to the left kidney, related closely to the splenic hilum and the left colic flexure.

• The mobile tail enters the splenorenal ligament between it’s layers and contains the splenic vessels.

• The main pancreatic duct starts in the tail and runs through the pancreatic head’s parenchyma.

• It turns inferiorly, closely related to the bile duct.

• It then unites with the bile duct at the short, dilated hepatopancreatic ampulla (of Vater); this opens into the descending duodenum part at the major duodenal papilla’s summit.

• 25% of the time or more the ducts may open into the duodenum separately.

• There are three smooth muscle sphincters that prevent digestive secretion and duodenal content reflux composed of:

  • The sphincter of the pancreatic duct (around the terminal duct part)
  • The sphincter of the bile duct (choledochal sphincter, around the bile duct termination)
  • The hepatopancreatic sphincter (of Oddi, around the hepatopancreatic ampulla)

• The sphincter of the bile duct is the only of the three to significantly control digestive secretion flow (bile) into the duodenum.

• The accessory pancreatic duct Opens into the duodenum at the minor duodenal papilla summit.

• Usual communication exists between the accessory duct and the main pancreatic duct.

• In some cases the main pancreatic duct and the accessory pancreatic duct do not connect, and the accessory duct carries most of the pancreatic juice.

• Multiple pancreatic arteries form several arcades with pancreatic branches of the gastroduodenal/ superior mesenteric arteries

• The arterial supply of the pancreas is mainly the branches of the tortuous splenic artery.

• There can be 10 or more branches passing from the splenic artery to the pancreas’ body and tail.

• Branches of the gastroduodenal artery provide anterior and posterior superior pancreaticoduodenal arteries.

• Anteriorly/ posteriorly placed arcades supply the head, formed by the SMA branches of anterior/posterior inferior pancreaticoduodenal arteries.

• Venous drainage happens via corresponding pancreatic veins as tributaries of the splenic and superior parts of the hepatic portal vein with most tributaries emptying into the splenic vein.

• Pancreatic lymphatic vessels follow the blood vessels with most vessels ending in the pancreaticosplenic lymph nodes that are found along the splenic artery.

• Some vessels end in the pyloric lymph nodes.

• Efferent vessels from these nodes drain into the superior mesenteric lymph or to the celiac lymph nodes via the hepatic lymph nodes.

• Nerves of the pancreas stem from the vagus and abdominopelvic splanchnic nerves passing through the diaphragm

• Parasympathetic and sympathetic fibers reach the pancreas by passing along the arteries from the celiac or superior mesenteric plexus.

• Pancreatic acinar cells and islets are distributed sympathetically or parasympathetically .

• Parasympathetic fibers are secretomotor so pancreatic secretion is mainly mediated by secretin and cholecystokinin, which are hormones formed by epithelial duodenal cells and proximal intestinal mucosa by acid from the stomach.

Liver

• After the skin, the liver is the bodies biggest gland and single organ, weighing about 1,500 g while accounting for 2.5% of adult body weight.
• Twice as large or 5% of body weight is found in Mature fetuses because it serves as a hematopoietic organ.
• With the exception of fat, all nutrients which the gastrointestinal tract has absorbed are initially conveyed to the liver with the portal venous system.
• It carries out many other metabolic activities, such as storing glycogen as well as secreting bile.
• Bile is a yellow-brown liquid. This aids in the emulsification of fatty acids.
• Bile bypasses to the liver through bile ducts including right and left hepatic ducts which come together to generate a common hepatic duct:
  • From the bile ducts, that joins with a cystic duct to then create a common bile duct.
• The liver makes bile often.
• Bile also amasses and stores up at the gallbladder between meals which increases bile concentration from absorbing both water and salts.
• With food entering the duodenum. Concentrated bile gets transported via the bile ducts to the duodenum.
• Positioned in right sides of the upper quadrant for the abdomen, where it can get protected through the thoracic cage.
• Usually deep to ribs number 7-11 on the right part while also crossing over a midline closer to the left nipple
• Main location is the right hypochondrium as well as the upper epigastrium, all whereas expanding inside the hypochondrium on the left side.
• Usually moves alongside excursion which the diaphragm experiences. Found much more inferior at the time the body remains erect as a result of gravitational pull. The motion is important to aid liver examination.
• Features convex diaphragmatic surface, that is also both superior.
• Visceral surface found to be relative concave.
• Acute inferior border sets apart with the convex diaphragmatic, all whereas running among the best costal margin.
• The diaphragmatic surface of the liver exists to be soft and shaped similar a dome where they link under the bottom side for the diaphragm where it divides it while involving the lungs from either side.
• Subphrenic recesses that are superior extensions that comprise while between both the diaphragm including both prior with superior parts for the liver's diaphragmatic body. These recesses get set apart for the entire right including left with either which expands among the liver and that’s both prior with abdominal wall or the supracondylar compartment found to be promptly subordinate that’s named subhepatic location.
• Hepatorenal recesses (the Morison chamber/hepato renal pouch), exists that’s posterosuperior for its hypochondriac placement that connects while among the right parts to what can found that covers a visceral side on liver under the best kidney as well as suprarenal structure. While on both side there also receives liquid released across the omental bursa. All recesses that forms the peritoneal sac, where the peritoneal liquid occurs with an adjacent side to its structure is used for membrane's proper lubrication function.
• The superficial part of the liver is coated in visceral peritoneum, leaving at most a bare area where its in direct touch in addition while being divided in two with its Falciform or Coronary Ligaments. Under the side view in that scenario with other organs/ligaments can get reviewed while in side as a posterior.
• Anterior and layer of the ligament remain consistent when having the best side in layer to either left with triangle body. Liver is coated alongside visceral peritoneum other within the location to either bile under any conditions as portal vein/hepatic vessels. Distinguishable to most other outer structures these side comes a lot deeper.
• The liver’s visceral body parts connect alongside:
  • Gallbladder side
  • Superior parts from its two parts located in the right
• Ligament which can commonly expand among which either structure gets divided that includes:
• Superior parts out of its umbilical/left side. Either round organ for the liver where they linked in the liver.
• Visceral form where the umbilical formation of either vein occurs to IVC (short-circuiting under side area.
• Lesser omentum that connects among either bodies and its top curvature for both those structures. The side with either side structure covers the main veins so there can easily be portal triad
• All ligaments in area under this are what create a portal side ligament. The diaphragmatic surface is covered with visceral peritoneum.
  • Bare area located posteriorly.
  • Direct contact with diaphragm.
  • Reflection of the peritoneum demarcates to upper part and lowers the Coronary Ligament.
• Right/Left triangular Ligaments form were all reflection meets.
• The right liver is much larger in both size, but functional as both the portal lobes, hepatic vessels help both the blood supply with drainage to ensure they are equal.
  • It needs a separate major blood vessel with independent hepatic vessels.
• Caudate side: a unique portion due to its independence for either divisions or has a independent area for being cut out when a certain location needs intervention from which its cut and supplied individually too. Three can expand in 4 or all to have their vessels for segmentation/slicing for proper care for each with the segmentation part from division area.
• This division process as been helpful in situations for each to be supplied correctly. This occurs commonly among the pattern although a slice or size under all circumstances from different shapes exist all for its proper care with such division. Has been explained into clinical areas like section or any area need slice which needs explanation too.
• Most vessels get supplied from such side before either one need explain where those side may occur that includes its own hepatic veins.
• Three segment areas (VII, VIII and V) are what need a lot with different location so this is all well kept. In that situation though its not likely before which these things require care that its common that are all located with segmentation being kept and sliced.
• Blood, lymph then vessels get explained how and what functions that covers this structure of concern which need this area explained too.
• This system acts how and what connects to the body where such veins will transfer when its not in situation needed so all are kept under control either. This helps this keep both function how its structure needs.
• Lymphs with vessels may be damaged through the flow. But must converge both the portal which must do the surfaces including also lymph too.
• Such ligament and diaphragm work with many areas before all is well connected.
• Nerves or other vessels connect through the plexus where are what run alongside a path between body side and artery. Vasocontriction occurs