Small Intestine, Large Intestine and Accesory Organs

As we move towards the small intestines, the majority of enzymatic digestion occurs here.
The surface area is increased to maximize digestion.

Simple columnar epithelium with microvilli is present.
Microvilli are smaller and shorter than cilia.
Microvilli's purpose is absorption, increasing surface area.
These cellular extensions are called microvilli and are also referred to as the brush border.
Brush border enzymes are created by the epithelial tissue and are involved in enzymatic digestion.
Enzymes from the pancreas come in at the duodenum, along with brush border enzymes.

Folds are created upwards and downwards to form villi and intestinal crypts.
Taking villi and crypts and folding them again results in plicae circularis, also referred to as circular folds.
The small intestines have a rough structure due to these plicae.
Plicae can be seen by eye, similar to rugae, while villi and microvilli are microscopic.
Mucosal folds include microvilli, villi, crypts, and plicae.

Chyme, exiting the stomach, is a nutrient-rich mixture with acids and digestive enzymes.
The stomach protects itself from acid via mucus produced by mucus neck cells in the gastric glands.
Brunner's glands, found in the submucosa of the duodenum, secrete a high pH alkaline solution to protect against the acidic chyme.
The pyloric sphincter separates the stomach from the duodenum.
The pancreas secretes a high pH bicarbonate solution into the duodenum to neutralize the chyme.

The duodenum, jejunum, and ileum all have villi and crypts.
The key difference lies in the submucosa.
The duodenum: the tiniest piece, contains Brunner's glands in the submucosa.
The jejunum has very large villi with no glands in the submucosa.
The ileum contains Peyer's patches (lymphatic tissue) in the submucosa, protecting the epithelial lining as waste begins to form.

The majority of absorption occurs in the duodenum and jejunum.
By the time the mixture reaches the ileum, it's mostly waste becoming fecal matter.
The large intestine reclaims fluid and water from saliva, gastric juices, digestive juices, and bile.
Everything fluid-like comes from blood, which carries water.
Water is reabsorbed to solidify waste before elimination.

The ileocecal valve is a one-way sphincter that prevents fecal matter from re-entering the ileum and pushes it into the cecum.

The large intestine includes the cecum, appendix, ascending colon, transverse colon, descending colon, sigmoid colon, and rectum.
The appendix harbors good bacteria for gut health and has lymphatic roles.

The colon returns to simple columnar epithelium without microvilli due to less nutrient absorption.
Crypts are maintained, and goblet cells are added for lubrication.
Goblet cells increase to reduce friction as fecal matter hardens.
Almost every other cell is a goblet cell, secreting a mucus film along the crypts.

The rectum and anus involve voluntary muscles and stratified squamous epithelium.
The internal anal sphincter is involuntary (smooth muscle) and keeps the anus closed.
The external anal sphincter is voluntary (skeletal muscle) and allows for the voluntary release of waste.

Endocrine glands secrete hormones into the bloodstream.
The pancreas secretes insulin and glucagon for blood sugar homeostasis.
Insulin lowers blood sugar, while glucagon raises it.
Type 2 diabetes results from cells becoming desensitized to insulin due to overstimulation
High concentrations of fructose leads to fatty liver disease.
Fructose is converted straight to fat.

The pancreas secretes digestive enzymes and a high pH bicarbonate buffer.
The bicarbonate buffer neutralizes chyme in the duodenum.
All secretions are released through the pancreatic duct directly into the duodenum.
Acinar cells secrete these solutions into a lumen that leads to the pancreatic duct.

The liver has over 200 known functions, including lymphatic roles, blood management, blood recycling, and blood storage.
The liver receives blood from the stomach, small and large intestines, and pancreas through the hepatic portal system.
The hepatic portal is the only system that has both a vein and artery serving it.
It makes bile, which is injected into the duodenum.

The splenic vein is associated with the liver for blood recycling.
The liver processes blood from the splenic vein and eliminates waste through the bile duct system.

The gallbladder stores bile.
After gallbladder removal, dietary changes are necessary due to the liver's limited bile production capacity.
The right and left hepatic ducts drain bile from the liver lobes.
Bile enters the cystic duct to go to the gallbladder.
When the gallbladder contracts, bile exits the cystic duct and enters the common bile duct.
The hepatopancreatic sphincter (sphincter of Oddi) controls pancreatic juices and bile secretion in the duodenum.

The functional unit of the liver is the liver lobule, a hexagonal structure.
The central vein is located in the center of the liver lobule.
Branches of the hepatic artery and hepatic portal vein are at every corner of the liver lobule.
Bile ducts also have branches at every corner of the liver lobule.
The portal triad consists of a branch of the hepatic artery, a branch of the hepatic portal vein, and a bile duct branch.
Hepatocytes within the liver lobule carry out the liver's functions.
Arterial blood serves the needs of hepatocytes.
Venous blood is poured into capillary beds for processing.

Sinusoidal capillaries in the liver allow for massive fluid and cell exchange.
Arterial blood provides nutrients to hepatocytes.
Bile ducts carry bile created by other cells.
The hepatic portal vein carries nutrient-rich blood and blood waste through the liver sinusoids.
Processed blood exits through the central vein.
Central veins collect and drain into the hepatic vein.
The hepatic vein drains into the inferior vena cava, returning blood to the heart.