Gross Anatomy: Intestines & Portal System (QUIZ1HBF-II LEC 48) PDF

Gross Anatomy: Intestine & Portal System Page 4 of 29 Dr. Dennis Goebel Introduction: This enrichment, will describe the anatomy of of the small intestine that is distal to the duodenum (e.g. the jejunum (2nd part of the small intestine) and the ileum (...

Gross Anatomy: Intestine & Portal System Page 4 of 29 Dr. Dennis Goebel Introduction: This enrichment, will describe the anatomy of of the small intestine that is distal to the duodenum (e.g. the jejunum (2nd part of the small intestine) and the ileum (3rd part of the small intestine)), as well as describe the anatomical divisions of the large intestine. In addition, the venous return of the small and large intestines into the hepatic portal system will also be described. Note, that the gross anatomy of the duodenum (the first part of the small intestine), pancreas and spleen will follow in a separate presentation. I. Small Intestine A. General Information: The small intestine is comprised of three divisions, the duodenum (which will be described in a separate presentation), the jejunum and the ileum. 1. The small intestine is responsible for the majority of absorption (uptake) of digested food/minerals/liquids. 2. The jejunum and ileum, along with the large intestine, occupy the majority of the infracolic compartment of the abdominal cavity (Figure 1). Figure 1 Gross Anatomy: Intestine & Portal System Page 5 of 29 Dr. Dennis Goebel a. The jejunum and the ileum are peritoneal structures (measuring ~6-7 meters in length), which are suspended off of the posterior abdominal wall by “the mesentery”. i. The mesentery is a double layered peritoneum that is centered about the superior mesenteric artery and vein. It attaches to the posterior abdominal wall at the level of LV2, and invests the jejunum and ileum (and the terminal part of the ascending duodenum). See Figure 2. b. Both the jejunum and ileum contain continuous inner circular and outer longitudinal smooth muscle bands, and a well-defined mucosa lining their respective lumens (See Figure 3). Figure 2 B. Jejunum 1. The jejunum (Latin for “empty”) begins at the duodenojejunal flexure and accounts for ~2/5’s (~2.8 m in length) of the small intestine (See Figure 2). 2. Most of the jejunum is positioned in the upper left quadrant of the abdominal cavity. Gross Anatomy: Intestine & Portal System Page 6 of 29 Dr. Dennis Goebel 3. Distinct features of the jejunum: a. The jejunum has a larger diameter (~ 3 cm) and much thicker walls than the ileum. The thickness of the walls is attributed to the mucosa’s densely packed permanent circular folds (histologically identified as plicae circulares: See Figure 3A). The circular folds are thick (2-3 mm) and greatly increase the mucosal surface area within the lumen. This increased luminal surface area is clearly observed in radiological images when barium-contrast is given to the patient to drink prior to X-Ray scans (See Figure 3B). Here, the circular folds display a dense follicular appearance. Note also, that the high-density of these luminal folds obscures resolving a continuous lumen in the jejunum (Figure 3B). Figure 3 4. Arterial Supply of Jejunum a. Arterial supply is provided by jejunal branches of the superior mesenteric artery (SMA). Jejunal branches generally arise from the left side of the SMA and pass between the two layers of mesentery to form simple (singular) arcades (arterial loops). See Figure 3 above & Figure 4 on the next page. i. Arterial arcades supplying the jejunum give rise to long (~2-3 cm) vasa recta (or straight arteries) that pass to the jejunum (See Figures 3 & 4). The vasa recta supplying the jejunum do not anastomose with each other within the mesentery; however, once they penetrate the muscular wall of the jejunum, anastomosis occurs between neighboring vasa recta. 5. Mesentery of Jejunum: The mesentery, connecting the jejunum to the posterior abdominal wall, is relatively fat free and appears transparent in the living body. Both Gross Anatomy: Intestine & Portal System Page 7 of 29 Dr. Dennis Goebel the simple arterial arcades and the long vasa recta are easily visualized in the live patient (in the OR), and in your cadaver (see Figures 3 & 4). Figure 4 C. The Ileum 1. The ileum accounts for approximately 3/5’s (~4.2 m in length) of the small intestine and is positioned between the jejunum and empties in to the cecum of the large intestine at the ileocecal valve (Figure 4). a. Note, the morphologic transition between the jejunum and ileum is gradual (i.e., not distinct). Gross Anatomy: Intestine & Portal System Page 8 of 29 Dr. Dennis Goebel b. The ileum occupies mostly the lower right quadrant of the abdominal cavity and extends into the right inguinal region (Figures 1 and 4). 2. Musculature of Ileum a. The diameter of the ileum is narrower (~2 cm in diameter), and its walls are thinner compare to jejunum. i. The walls consist of an outer longitudinal and inner circular smooth muscle layers. ii. The mucosa lining of the ileum displays a gradual decline in the density of the permanent circular folds (plica circulars), as it progresses proximal-to-distal, and are absent as the ileum nears the ileocecal junction (See Figure 5). As a result, upper GI-contrast X-Ray scans of the ileum typically display a clearly defined lumen (Figure 5B). Figure 5 Gross Anatomy: Intestine & Portal System Page 9 of 29 Dr. Dennis Goebel 3. The Mesentery of Ileum a. The ileum and jejunum are suspended off of the posterior body wall by the mesentery. The visceral peritoneum of the mesentery forms fan-shaped folds that wrap around the coils of the jejunum and ileum. These folds are prominently noted in the ileum, where its coils course in a zig-zag fashion (See Figures 4 and 6B). i. The mesentery enclosing the ileum normally contains an abundant accumulation of fat, and is less transparent than that of the jejunum (Figure 5). 4. Arterial Supply of Ileum Arterial supply is provided by ileal branches from the SMA (See Figure 4). The arterial branches to the ilium form complex arterial arcades within the mesentery and have shorter vasa recta, than those supplying the jejunum (See Figures 4 & 5). 2. Contrasting features between the jejunum and the ileum. There are no clear line of demarcation between the jejunum and ileum, as the morphological character of the small intestine changes gradually. However, the characteristics of the beginning of the jejunum, compared with the end of the ileum are very distinct, and easily differentiated both by their respective external and luminal characteristics (See Table below). These can be easily noted in upper GI barium-contrast XRay/CT images (See Figures 3 A&B and 6 A&B). Jejunum Ileum Arterial arcades Simple 1-2 layers Complex (2-4 layers) Vasa recti Long Short Plicae circulares Densely packed Greatly diminished in # Mesentery fat Sparse Heavy accumulation D. Venous Drainage of the Jejunum and Ileum 1. Venous drainage of the jejunum and ileum mirrors the paths of the accompanying arterial vasa recta, arcades, and jejunal arteries contained within the mesentery. These will eventually drain into the Superior mesenteric vein (SMV), which pairs to the right of the SMA within the root of the Mesentery (See figure 6 on the next page). 2. The SMV, carrying nutrient rich blood, empties directly into the hepatic portal vein, where it enters the liver for further processing. Details of the hepatic portal system will be described in Section V of these notes. Gross Anatomy: Intestine & Portal System Page 10 of 29 Dr. Dennis Goebel Figure 6 E. Lymphatics of the Jejunum and Ileum 1. Lymphatic drainage is extensive for both the jejunum and ileum. a. Lymphatics of the small intestine are called lacteals (translates to “milk-like”). Dissolved fats taken up by the jejunum (and the ileum) give the lymphatic fluid the milk-like appearance. b. Lymphatic vessels pass into the mesentery and drain into mesenteric lymph nodes (See Figure 7). i. Jejunum and Ileal lymph nodes are located within the mesentery near the surface of the intestinal wall, along the arterial arcades, arterial branches of the SMA, and the proximal part of the SMA. All lymph nodes ultimately drain into superior mesenteric lymph nodes (see Figure 7 on the next page). These then empty into an elongated sac, called the cisterna chyli, which is Gross Anatomy: Intestine & Portal System Page 11 of 29 Dr. Dennis Goebel positioned on the posterior abdominal wall between the aortal and the IVC (See Figure 7). From there, the lymph passes into the thoracic duct and eventually empties into the left brachiocephalic vein at the junction between the left external jugular and left subclavian veins (See Figure 7). Figure 7 Gross Anatomy: Intestine & Portal System Page 12 of 29 Dr. Dennis Goebel F. Innervation of the small intestine 1. Enteric system: The distal esophagus, stomach, the small and large intestines contain an endogenous nervous plexus (known as the enteric nervous system) that provides low-level peristalsis and glandular secretion. This system functions (at a reduced level) in the absence of both, parasympathetic and sympathetic innervation, and is mediated by the myenteric and submucosal plexuses (will be covered in histology) that are located in the intestinal wall. 2. Parasympathetic innervation of the small intestine is provided by the right and left vagal trunks (X). See Figure 8. a. Vagal stimulation increases smooth muscle peristalsis and glandular secretion (i.e. enhances digestive activity). Figure 8 Gross Anatomy: Intestine & Portal System Page 13 of 29 Dr. Dennis Goebel 3. Preganglionic sympathetic innervation of the small intestine is provided by the thoracic splanchnic nerves. A portion of these preganglionic fibers synapse on postganglionic neurons located in the superior mesenteric ganglia. The postganglionic SANS axons then utilize the superior mesenteric artery, and its jejunal and ileal branches, to reach the small intestines (Figure 8). a. Sympathetic stimulation reduces blood flow to the small intestine and decreases both smooth muscle motility and glandular secretion. This system is enhanced in the “fight or flight response” for the purpose of diverting oxygenated blood flow from the viscera to skeletal muscle. 4. Sensory of the intestines: The entire alimentary tract from the stomach to the rectum is insensitive to touch and most painful stimuli including cutting and burning. However, it is sensitive to distension and ischemia. In the small intestine, the later modalities are relayed back to the spinal cord via the thoracic splanchnic nerves. These pain sensations are perceived as spastic, burning or “doubling-over” abdominal pain. The afferent pain fibers (monitoring stretch/distention and oxygen levels) coming from the small intestine (including the duodenum) are “referred” via thoracic splanchnic nerves to the somatic dermatomes that they originate (T5-T12), whereby the sensation of pain/burning is then referred to the anterior abdominal wall. G. Clinical relevance of the small intestine 1. Ileal (Meckel’s) diverticulum (MD) is a congenital anomaly resulting from the persistence of the vitelline duct (yolk stalk). It typically forms a ~ 2 inch out- pocketing, (diverticulum) in the ileum. MD are found in ~2-4% of the population, they are located approximately ~2 feet (40-60 cm) from the ileocolic junction, (Referred to as “the rule of 2’s”: See Figure 9). a. In most cases the diverticulum doesn’t present any problems, however, if it becomes inflamed/infected, it can mimic symptoms produced by the appendix (e.g., high fever, intense abdominal pain and elevated white blood cell counts). Other developmental abnormalities resulting from persistence of the vitelline duct are discussed in my Embryology notes on the development of the intestines. Figure 9 Gross Anatomy: Intestine & Portal System Page 14 of 29 Dr. Dennis Goebel 2. Small bowel Ischemia (Ileus): The jejunum and ileum receive their blood supply exclusively from branches provided by the superior mesenteric artery. Thrombosis occurring within the superior mesenteric artery, or localize to a stretch of straight arteries, will signal visceral pain receptors that are sensitive to ischemia (via visceral afferents in the greater splanchnic N.), and relay these pain modalities to the spinal cord (dermatomes T8-T9). Early symptoms include mild-, to moderate-discomfort following a meal that intensifies over a period of time (as the thrombosis grows). a. Sudden onset of severe “colicky ”pain (spasmodic abdominal pain) localized to the periumbilical region (T8-9 dermatome), usually signifies an acute ischemic event localized to the small intestine. Long-term ischemia usually results in the affected region to undergo necrosis. This is hallmarked by intense pain localized to anterior abdominal wall above the umbilicus, combined with fever and vomiting. Chronic conditions can lead to infection/gangrene-sepsis/peritonitis, and result in death, if not immediately attended to. II. LARGE INTESTINE A. General Information: The large intestine measures approximately 1.5 M in length and is regionally defined. From proximal to distal, it consists of the cecum w/wo appendix, the ascending colon, transverse colon, descending colon, sigmoid colon, rectum and the anal canal (See Figure 10). The rectum and the anal canal will be discussed (in detail) in a separate enrichment presentation. Figure 10