Small Intestine Anatomy

Small Intestine Overview

• The small intestine is also known as the small bowel
• It is the primary site for digestion and absorption (~90% of nutrients)
• It extends from the pyloric sphincter to the ileocecal valve and is highly coiled

Small Intestine Gross Anatomy

• The small intestine is ~6 meters (20 ft) in living state and it is a coiled thin walled tube located withing the abdominal cavity

Small Intestine Regions

• There are three regions; the duodenum, the jejunum and the ileum
• The duodenum is ~25 cm (10 in), C-shaped around the head of the pancreas, and continuous with the jejunum at the duodenojejunal flexure

Duodenum Specifics

• The duodenum is mostly retroperitoneal
• It begins at the pyloric sphincter and receives chyme, bile (from the liver), and pancreatic juice
• The major duodenal papilla is the entry point for bile/pancreatic secretions
• The minor duodenal papilla is the entry point of an additional small amount of pancreatic juice via the accessory pancreatic duct

Jejunum Specifics

• The jejunum is the middle region of the small intestine with a length of ~2.5 m (8 ft - two fifths of the small intestines total length)
• The jejunum is intraperitoneal and begins at the duodenojejunal flexure
• It is the main site of chemical digestion and nutrient absorption

Ileum Specifics

• The ileum is the last region of the small intestine and measures ~3.6 m (12 ft - three fifths of the small intestine)
• The ileum is intraperitoneal and ends at the ileocecal valve joining the cecum
• This region completes absorption and contains Peyer patches (lymphatic tissue)

Mesentery Proper

• Both the jejunum and ileum are supported and suspended in the abdomen by the mesentery proper, a fan-shaped peritoneal fold

Tunics of the Small Intestine

• The mucosa has a simple columnar epithelium with microvilli for increased absorptive surface area
• It has intestinal glands between villi, secreting intestinal juice (enzymes, mucus)
• It also contains finger-like villi projections that contain capillaries and lacteals, which are able to absorb nutrients and lipids on the circular fold surface

Small Intestine Features

• The circular folds (Plicae Circulares) are permanent ridges that increase surface area for nutrient absorption
• These folds also act like "speed bumps" to slow chyme movement, maximizing absorption
• Cell types include absorptive cells for nutrient uptake and goblet cells which secrete mucus (increase distally)

Submucosa of the Small Intestine

• The submucosa is made of dense connective tissue

Brunner's Glands

• Brunner's glands are only in the duodenum and secrete alkaline mucus to neutralize chyme acidity

Muscularis of the Small Intestine

• The muscularis has inner circular and outer longitudinal smooth muscle layers which drive peristalsis and mixing

Serosa of the Small Intestine

• The serosa is a visceral peritoneum (intraperitoneal regions)

Adaptations of the Small Intestine

• Folds, villi, and microvilli in the small intestine increase the surface area to reach ~200-250 m² which is about the size of a tennis court
• Peyer patches are also present in the ileum for immune surveillance (MALT)

Functional Significance of the Small Intestine

• Digestion happens with pancreatic enzymes/bile (in the duodenum) and brush border enzymes (in the jejunum/ileum) that break down carbs, proteins, and fats
• Absorption is preformed when nutrients are absorbed into capillaries (blood) and lipids into lacteals (lymph)
• Motility is achieved by peristalsis and mixing (segmentation) to enhance digestion and absorption

Malabsorption Clinical Insight

• Damage to villi (e.g., celiac disease) can reduce nutrient uptake, causing diarrhea/weight loss

Large Intestine Overview

• The large intestine, also called the large bowel, frames the small intestine, forming a three-sided perimeter in the abdominal cavity around the centrally located small intestine
• It processes indigestible material into feces

Large Intestine Functions

• Primary functions include absorption of water/ions, compacting waste, and hosting gut microbiota

Large Intestine Gross Anatomy

• The large intestine is ~1.5 m (5 ft) long, has a diameter of 6.5 cm (2.5 inches), and extends from the ileocecal valve to the anus

Large Intestine Regions

• Regions of the large intestine include the cecum, colon, rectum, and anal canal

Cecum Particulars

• The cecum is a blind sac that is the initial region of the large intestine
• It receives chyme via the ileocecal valve and attaches to the appendix, intraperitoneal

Vermiform Appendix Particulars

• The vermiform appendix projects inferiorly from the cecum, it is a thin, hollow, fingerlike sac and it is intraperitoneal

Colon Structure

• At the level of the ileocecal valve, the colon begins and forms an inverted U-shaped arch partitioned into four segments; the ascending colon, transverse colon, descending colon, and the sigmoid colon.

Ascending Colon Specifics

• The ascending colon is right sided, retroperitoneal, and originates at the ileocecal valve
• As it approaches the inferior surface of the liver, it makes a 90-degree turn toward the left side of the abdominal cavity called the right colic flexure, or the hepatic flexure

Transverse Colon Specifics

• The transverse colon is horizontal, intraperitoneal, and originates at the right colic flexure
• It makes a 90-degree turn inferiorly at the spleen in the left upper quadrant of the abdomen and resulting in the left colic flexure, or the splenic flexure

Descending Colon Specifics

• The descending colon is retroperitoneal, found along the left side of the abdominal cavity
• It originates at the left colic flexure and descends vertically until it terminates at the sigmoid colon

Sigmoid Colon Specifics

• The sigmoid colon is intraperitoneal, resembling the letter "S"
• It originates at the sigmoid flexure, where the descending colon curves and turns inferomedially into the pelvic cavity before terminating at the rectum

Rectum Specifics

• The rectum is straight, retroperitoneal, and stores feces before defecation
• The three thick, transverse folds of the rectum, called rectal valves, ensure fecal material is retained during the passing of gas

Anal Canal Specifics

• The anal canal is the final 2-3 cm of the GI tract, ends at the anus, and is controlled by two sphincters

Sphincters of the Anal Canal

• The internal anal sphincter is made of smooth muscle and is involuntary
• The external anal sphincter is made of skeletal muscle and is voluntary

Support Around the Anus

• Mesocolon supports the transverse and sigmoid regions

Histology of the Large Intestine

• The mucosa is made of simple columnar epithelium (absorptive cells) and abundant goblet cells (mucus secretion)
• The intestinal glands are deep crypts with no villi (unlike the small intestine)

Submucosa of the Large Intestine

• The submucosa is made of dense connective tissue, with a standard structure

Muscularis of the Large Intestine

• The inner circular layer of the muscularis is standard
• The outer longitudinal layer forms three thin, distinct, longitudinal bundles called teniae coli which are elastic and bunch up the large intestine into many sacs, collectively called haustra

Serosa/Adventitia

• Serosa in intraperitoneal regions and adventitia in retroperitoneal

Peristalsis and Haustral Churning of the Large Intestine

• Peristalsis is when weak, sluggish waves move material toward the rectum
• Haustral churning occurs when a relaxed haustrum fills with digested or fecal material until distension stimulates reflex contractions in the muscularis which causes churning

Mass Movement of the Large Intestine

• Mass movement has powerful peristaltic contractions (2–3 times/day) that propel feces to rectum
• Triggered by gastrocolic reflex, initiated when the stomach filling stimulates the colon

Gastroileal Reflex

• When the stomach is filled it increases the ileocecal valve opening, delivering chyme

Defecation Reflex

• Rectal distension signals sphincters to relax

Constipation

• Constipation typically has an impaired ability to defecate and results in compacted feces
• Constipation may result from a combination of a low fiber diet, dehydration, lack of exercise, and improper bowel habits which leads to slow movement and excessive water absorption which turns into hard feces

Diarrhea

• Diarrhea results as a disruption in normal mechanisms to absorb intestinal water, or excessive amounts of osmotically active solutes
• Rapid movement and insufficient water absorption which turns into loose stools

The Large Intestine and Water

• The large Intestine absorbs ~90% of remaining water/ions from chyme creating indigestible material into feces (cellulose, bacteria, bile pigments)

Appendicitis

• Appendicitis is an inflamed appendix, that involves the risk of rupture and then peritonitis

Accessory Digestive Organs

• Accessory digestive organs produce secretions that facilitate chemical digestive activities
• The most important accessory digestive organs are the liver, the gallbladder, and the pancreas

Liver Overview

• The liver is the largest internal organ and a vital accessory digestive organ
• It supports digestion, metabolism, and detoxification

Liver Gross Anatomy

• The liver weighs 1–2 kg (2.2–4.4 lbs) and 2% of an adult's body weight
• The liver is covered by a connective tissue capsule and a layer of visceral peritoneum, except for a small region on its diaphragmatic surface called the bare area
• Located in the right upper quadrant, beneath the diaphragm, and spans epigastric/hypochondriac regions

Liver Lobes

• The liver is composed of four incompletely separated lobes: right lobe, quadrate lobe, caudate lobe and left lobe
• The right lobe is the largest, is located on the right side of the body and its subdivisions include the caudate lobe and the quadrate lobe
• The caudate lobe is posterior and adjacent to the inferior vena cava
• The quadrate lobe is inferior and adjacent to the gallbladder
• The left lobe is the smaller lobe of the liver which extends leftward

Falciform Ligament Particulars

• The falciform ligament separates the right lobe from the smaller left lobe
• Falciform is a peritoneal fold that attaches the liver to the anterior abdominal wall

Ligamentum Teres (Round Ligament of Liver)

• In the inferior free edge of the falciform ligament lies the round ligament of the liver ( or ligamentum teres)
• The ligamentum teres represents the remnant of the fetal umbilical vein

Coronary Ligament Definition

• The coronary Ligament attaches the liver to the diaphragm

Porta Hepatis Particulars

• Porta Hepatis is the hilum on inferior surface.
• The Entry to the hilum is the Hepatic artery, portal vein.
• The Exit from the hilum is the Hepatic ducts

Liver Surfaces

• The liver includes the diaphragmatic and visceral surfaces

Hepatic Lobules

• A connective tissue capsule branches through the liver and partitions the liver into thousands of small, hexagonal hepatic lobules
• These lobules are the classic structural and functional units of the liver

Hepatocytes Inside the Liver

• Within the hepatic lobules are cuboidal liver cells that radiate from a central vein performing liver functions

Portal Triads

• At each lobule periphery several portal triads are present containing a Branch of Hepatic artery (oxygenated blood), a Branch of hepatic Portal vein (nutrient-rich blood from gut), and a bile duct (collects bile)

Sinusoids Described

• In cross section, the hepatic lobule looks like a side view of a bicycle wheel
• The hub of the wheel is the central vein
• At the circumference of the wheel where the tire would be are several portal triads that are usually equidistant apart.
• The numerous spokes of the wheel are the hepatic sinusoids which are bordered by cords of hepatocytes

Hepatic Sinusoids

• Hepatic sinusoids are thin-walled, porous or "leaky" capillaries where venous and arterial blood are mixed and then flow slowly through the hepatic lobule toward the central vein
• Lined with stellate cells called reticuloendothelial cells (or Kupffer cells), which are phagocytic and have an immune function

Central Vein Definition

• The central vein drains the venous blood from the lobule into hepatic veins into the IVC (Inferior Vena Cava)

Bile Canaliculi Definition

• bile canaliculi channel that conduct bile from the hepatocytes to the bile duct in the portal triad

Liver Functions

• Produces ~600–1000 mL/day of bile which emulsifies fats for digestion/absorption in the duodenum
• Detoxifies drugs, alcohol, toxins via hepatocytes
• Stores glycogen (glucose reserve) and vitamins (A,D,B12) which it then releases when they are needed
• Synthesizes blood, plasma proteins such as albumin, globulin, and clotting factors
• Converts nutrients (e.g., glucose to glycogen), processes lipids, and amino acids

Liver Immune Function

• Reticuloendothelial cells in the liver sinusoids phagocytize debris in the blood also break down and recycle components of aged erythrocytes/damaged or worn-out

Input Blood Supply of the Liver

• The liver receives a dual blood supply.
• The hepatic artery proper brings about 25% of the blood volume to the liver and is split into left and right hepatic arteries.
• Hepatic portal vein brings about 75% of the blood volume to the liver and carries blood from the capillary beds of the GI tract, spleen, and pancreas. This type of blood, is rich in nutrients, other absorbed substances while being poor in oxygen

Output Blood Supply of the Liver

• Central veins collect venous blood and merge throughout the liver forming hepatic veins eventually empty into the inferior vena cava

Cirrhosis Clinical Insight

• Cirrhosis which is scarring from chronic damage from hepatitis or alcohol, impairs the functions of the liver

Gallbladder Overview

• The gallbladder is small, pear-shaped sac that stores and concentrates bile from the liver and is an accessory organ
• It is critical for fat digestion in the small intestine

Gallbladder Gross Anatomy

• The location of the gallbladder is the inferior surface of the liver, in the gallbladder fossa in the right upper quadrant
• The average size of the gallbladder is ~7–10 cm long with a capacity of 40–60 mL

Gallbladder Regions

• Regions of the gallbladder include the Fundus, Body and Neck

Defining each gallbladder region

• The Fundus is the rounded, inferior end that projects below the liver edge
• The Body is the main central portion, stores bile
• The Neck is the Narrow, superior end that is continuous with the cystic duct

Cystic Duct Defintion

• The cystic duct connects the gallbladder to common bile duct

Spiral Valves

• The spiral valves (mucosal folds) present in the cystic duct regulate bile flow

Gallbladder Position particular

• The gallbladder is intraperitoneal, covered by visceral peritoneum besides where it is attached to the liver

Gallbladder Histology

• The mucosa has a simple columnar epithelium with rugae-like folds when empty
• The mucosa then expands to store bile and flattens when full and also absorbs water and ions to concentrate the bile that is present

Gallbladder Muscularis

• There is a thin layer of smooth muscle (oblique, circular, longitudinal fibers) that contracts to eject bile into cystic duct

Gallbladder Serosa

• Visceral peritoneum on free surfaces
• Adventitia where attached to liver (no serosa)
• No submucosa, mucosa directly overlies muscularis

Overall Function of the Gallbladder

• The gallbladder holds bile (~600–1000 mL/day) which is produced by the liver between meals, this is its main storage
• Furthermore it absorbs water and ions, concentrating bile 5–20 times enhancing fat emulsification and stored concentrated bile becomes greenish, viscous, and rich in bile salts/lipids

Bile Release Control

• Bile Release is triggered by cholecystokinin (CCK) from the duodenum by its response fats
• After that Smooth muscle contracts, cystic duct opens, causing bile to enter common bile duct which will then lead to the duodenum. which will ultimately emulsifies fats to increase surface area for pancreatic lipase action

Gallbladder Biliary Pathway

• Path of hepatic fluid through the Biliary Pathway requires the cystic duct
• Biliary Pathway leads to the joining of the common hepatic duct into the common bile duct and then joining at the hepatopancreatic ampulla to lastly drain at the major duodenal papilla.

Gallstones Clinical Insight

• Gallstones are crystallized bile components (e.g., cholesterol) that may block cystic duct and can lead to causing pain/inflammation (cholecystitis)

Biliary Apparatus

• The biliary apparatus transports bile to the duodenum for use for fat digestion

Pancreas Overview

• The pancreas is a gland that has dual roles with exocrine to support digestion and endocrine (blood sugar regulation)

Pancreas Gross Anatomy

• The pancreas is located retroperitoneal, and Extends horizontally from the medial edge of the duodenum and towards the left side of the abdominal cavity meeting at the spleen
• It measures in at ~12–15 cm long, that is ~2–3 cm thick, weighing ~70–100 g

Pancreas Regions

• Head is the broad organ, adjacent to the curvature of the duodenum
• Body is the central, elongated portion, posterior to stomach.
• Tail is the tapered end that will extend toward the spleen

Pancreatic Ducts Overview

• There are two pancreatic ducts; the Main Pancreatic Duct (Wirsung), and the Accessory Pancreatic Duct

Main Pancreas Duct Particulars

• The Main Pancreatic Duct (Wirsung) runs length of pancreas and it joins common bile duct at hepatopancreatic ampulla into draining at the major duodenal papilla

Accessory Pancreas Duct Particulars

• The Accessory Pancreatic Duct is reduced in size and drains a small amount of pancreatic juice directly at the minor duodenal papilla in the duodenuming
• Note however that it can have variable presence between individuals

Parts of the Pancreas Consitered

• It can be either considered to be Exocrine or Endocrine depending on the type of material it is exporting

Exocrine Pancreas

• Weighing around 99% of the total mass
• Involves the Acinar Cells that form grape-like clusters (acini) will later produce pancreatic juice (secretion of digestive enzymes and bicarbonate) through a dedicated duct system draining out at at the intestines

Endocrine Pancreas

• Weighing around 1% of the total mass and it involves the Pancreatic Islets (Islets of Langerhans) that are small clusters of endocrine cells sitting across the organ
• These clusters of endocrine cells are of 4 primary groups; Alpha cells producing Glucagon, Beta cells producing Insulin, Delta cells producing Somatostatin, and lastly PP cells producing Pancreatic Polypeptide
• These hormones will get released locally into the bloodstream by the means of local capillaries as they regulate metabolic functions more broadly than just digestion

Exocrine Functions of the Pancreas

• Pancreatic Juice is released within ~1–1.5 L/day into the duodenum

Pancreatic Enzymes

• There are a number of enzymes utilized, the most of note:
• Amylases which are released to breakdown complex Carbs into basic sugars
• Lipases that break down Fats into single or shorter chain carbon molecules
• Proteases in their inactive form such as Trypsinongens that will beactivated into Trypsin that eventually turn Proteins into smaller Peptides

Pancreas Regulation

• The pH of the stomach content can become extremely acidic, and so a Bicarbonate agent (HCO₃⁻) is deployed across the pancreas so that it will reach an approximate pH level of ~8pH and protect the duodenum form being chemically digested by its own juice and enzymes

Endocrine Role of Pancreas

• Primarily, hormones are secreted and they include: Insulin (Beta Cells) involved in storing the energy from the meal by signaling the uptake of blood glucose and other nutrients inside body tissues
• Glucagon (Alpha Cells) released opposite of insulin that raise available blood glucose by signaling stored glycogen catabolism

Exocrine Regulation of the Pancreas

• Cholecystokinin (CCK) is Released by the epithelium of the duodenum as it responds to fats/proteins signals digestive enzymes

Secretin Production

• Will be Released in response to the acidic chyme traveling down the Gi tract, triggers Bicarbonate release via exocrine pathway.

Vagus Nerve Particulars

• Vagus Nerve (CN X) that signals parasympathetic output, that leads increases in digestive secretion.

Clinical Insight Particular

• Pancreatitis, that is the inflammation of all or part of the pancreas, because they will damage the actual cells doing the digestive acinar cells, therefore the digestion process is inhibited

Aging Effects on the Digestive System

• Less secretion of Mucin, decrease of enzymes such as in the duodenum, and decrease of general acidity of the tract

Aging Effects on Muscle

• Less muscle bulk affecting tone/motility, and weakens the sphincters

Aging Effects on Epithelial Replacement

• Epithelial Replacements slows thus increasing the risk for erosion damage.