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SMALL AND LARGE INTESTINE ANATOMY Mahrukh Rasheed mahrukh.rasheed@student.manchester.ac.ukYour ILOs… • associated arterieshe structures of the midgut and hindgut and name their • Name and identify the 3 regions of the small intestine and describe how their anatomy differs • Name and identify the four parts of the duodenum • Name and identify the different parts of the colon • Describe the vascular supply of the small intestine and colon • Describe the innervation of the small intestine and colon • Explain the regional histological differences that exist in the small intestine and colon Foregut becomes midgut at Basics first… MAJOR DUODENAL PAPILLA Midgut becomes hindgut around 2/3 of the way along the transverse mesocolonSECTION 1 – SMALL INTESTINE Brief intro… - Consists of the duodenum, jejunum and ileum - Small intestine basically receives partially digested food from the stomach - Muscular tube which breaks food down further for nutrients to be absorbed from it - Specialized for this by presence of villi, microvilli and plicae circularis Duodenum ■ First part of the small intestine. Runs from pylorus of stomach to duodenojejunal junction Connected to liver by the ■ Divided into 4 parts: hepatoduodenal ligament Finally, this part joins the jejunum at the DUODENOJEJUNAL FLEXURE Curves inferiorly around head of pancreas. Posterior to transverse colon, anterior to right kidney Crosses over IVC and aorta Contains the MAJOR DUODENAL PAPILLA (where pancreatic secretions enter) Blood supply to the Duodenum ■ MAJOR DUODENAL PAPILLA is important landmark for blood supply to the duodenum PROXIMAL to it: - Coeliac trunk via superior pancreaticoduodenal artery DISTAL to it: - Superior mesenteric via inferior pancreaticoduodenal artery Features of the Duodenum Circular folds on the inside of the duodenum which increase the surface area for absorption of nutrients Duodenum also contains BRUNNER’S GLANDS which produce an alkaline mucin which neutralizes the stomach acidJejunum and Ileum ■ Jejunum begins at duodenojejunal junction, the ileum terminates at the ileocecal junction ■ They are both intraperitoneal structures and attached to the posterior abdominal wall by mesentery ■ Jejunum is also specialized in the same way as the duodenum (plicae circularis, villi, microvilli all for absorption) ■ Jejunum is an important site for absorption – contains CRYPTS OF LEIBURKUHN to help with this ■ It’s not easy to tell where the jejunum becomes the ileum, more of a gradual change ■ Ileum contains fewer plicae circularis ■ Ileum mainly functions to ‘mop up’ anything left that hasn’t been absorbed. Key site of absorption for Vitamin B12 andbile saltsCharacteristic features of Jejunum v Ileum Jejunum Ileum Colour Darker (increased vascularity) Caliber Greater Wall thickness Thicker Number of plicae Large, tall and closely packed Sparse, low Vasa recta Long Short Arcades Few large loops Many short loops Fat in mesentery Less More Ileum contains PEYER’S PATCHES – lymphoid tissueBlood supply of the Jejunum and Ileum ■ The main arterial supply of the J&I is from a single artery – SUPERIOR MESENTERIC ARTERY (SMA) ■ It’s multiple branches form anastomoses loops (ARTERIAL ARCADES) – The jejunum receives blood from the SMA and the jejunal arteries – The ileum receives blood from the SMA via the ileal arteriesSECTION 2 – LARGE INTESTINE ■ Functions to absorb water from digested contents ■ Hosts a lot of bacteria (gut flora) which help produce useful things such as Vitamin K and Biotin, from the digested contentRegions of the Large Intestine Aka hepatic flexure Aka splenic flexure. Features of the Large Intestine ■ TENIAE COLI – three strips of muscle which run along the surface of the large intestine. They contract to shorten the bowel wall, producing sac-like structures known as HAUSTRA ■ EPIPLOIC APPENDAGES – Small fat-filled pouches protruding from the colon. They may cushion the colon, store fat or protect against infection These features cease at the rectosigmoid junction, where the smooth muscle of the teniae coli broaden to form the rectum Caecum ■ Most proximal part of the large intestine. Located between ileum and ascending colon in the right iliac fossa ■ Intraperitoneal structure ■ Between the caecum and ileum is the ileocecal valve which prevents reflux of during peristalsists into the ileumNeurovascular supply of the Caecum ■ Derived from the midgut, so blood supply from the SUPERIOR MESENTERIC ARTERY – Arterial supply from ILEOCOLIC ARTERY (branch of SMA) ■ Innervation from the ileocolic branch of the SUPERIOR MESENTERIC PLEXUSAPPENDIX ■ A small tail like organ which hangs off the caecum ■ LYMPHOID tissue. Kind of like a store for good gut bacteria ■ Commonly gets inflamed due to it being an enclosed pouch ■ McBurney’s point: 2/3 of the way between umbilicus and right ASIS. In appendicitis, this is the point of max. tenderness. Supplied by appendicular arteryColon ■ This part of the GI tract extends from the caecum to the anal canal ■ It absorbs water and electrolytes from digested food received from the small intestine, forming faeces ■ Anatomically, it is divided into 4 parts: – Ascending – Transverse – Descending – Sigmoid This is known as a WATERSHED AREA because it receives Parts of the colon blood from the distal branches of SMA and IMA, making it Left colic flexure (splenic flexure) vulnerable to Retroperitoneal. Ascends to meet the liver then turns 90 ischemia degrees at the right colic flexure (hepatic flexure) Retroperitoneal, sits anterior to the left kidney Located in the left lower quadrant. S shaped. Attached to posterior pelvic wall by a mesentery – sigmoid mesocolon Paracolic Gutters Clinically important structures. Spaces between the ascending/descending colon and the posterolateral abdominal wall They allow material released from inflamed/infected abdominal organs to accumulate elsewhere Blood supply of the Colon Transverse colon derived from BOTH midgut and hindgut, so suppled by branches of BOTH SMA and IMA Descending colon supplied by single branch of the IMA Ascending colon receives arterial supply from these 2 branches of the SMA Sigmoid colon supplied by sigmoid ■ Reminder, MIDGUT- arteries, branch of IMA derived structures supplied by SMA, HINDGUT-derived structures supplied by IMA Rectum ■ Begins at the level of S3 and is a continuation of the sigmoid colon ■ Important role in temporary storage of faeces ■ Does not contain taenia coli, haustra, epiploic appendages ■ Contains two major flexures: – Sacral flexure – Anorectal flexureNeurovascular supply of the Rectum Blood supply through 3 main arteries: – Superior rectal artery – Middle rectal artery – Inferior rectal artery Sympathetic nervous supply to the rectum is from the lumbar splanchnic nerves and superior and inferior hypogastric plexuses. Parasympathetic supply is from S2-4 via the pelvic splanchnic nerves and inferior hypogastric plexuses.Anal Canal ■ Final segment of the Gi tract and begins as a continuation of the rectum. Main function is defecation ■ Except during defecation, the anal canal is normally collapsed by the internal and external anal sphincters to prevent the passage of faecal material ■ The rectum terminates and the anal canal begins at a muscular ring known as the ANORECTAL RING Anal Sphincters Play a crucial role in faecal continence Surrounds upper 2/3 of anal canal. Formed from Surrounds involuntary smooth lower 2/3 of muscle anal canal. Formed from voluntary muscle Internal structure of the anal canal These are joined at their Mucosa of the anal inferior ends by ANAL canal is organized into VALVES longitudinal folds known Above these valves as ANAL COLUMNS are small pouches called ANAL SINUSES which contain mucous secreting glands The anal valves collectively form the pectinate lineNeurovascular supply of the anal canal PECTINATE LINE is a significant landmark… ABOVE – Superior rectal artery (branch of IMA) BELOW – Inferior rectal artery (branch of internal pudendal artery) Innervation ABOVE – Visceral innervation via INFERIOR HYPOGASTRIC PLEXUS BELOW – Somatic innervation via INFERIOR RECTAL NERVESInnervation of the Small and Large Intestine The intestines have TWO types of nerve supply: 1. SYMPATHETIC (fight or flight) : Reduces movement and secretion 2. PARASYMPATHETIC (rest and digest) : Increases movement and secretionSECTION 3 – HISTOLOGICAL DIFFERENCES BETWEEN SMALL INTESTINE AND COLON■ The primary histological difference between the small intestine and the colon is the presence of VILLI (nutrient absorption) in the small intestine, while the colon LACKS villi. – Instead, the colon has deeper CRYPTS lined with numerous GOBLET CELLS allowing for water absorption and stool formation ■ Note: both small intestine and colon have crypts, but they are just deeper in the colon and contain a higher proportion of goblet cells ■ The colon has taeniae coli, not found on the small intestine ■ The small intestine has circular folds called PLICAE CIRCULARES; the colon lacks these ■ PEYER’S PATCHES (type of lymphatic tissue) are more prominent in the small intestine (esp. ileum), compared to the colon