Gastrointestinal Anatomy and Function Flashcards
Name the organs (including accessory) involved in the digestive system.
- Mouth and esophagus
- Stomach
- Small intestine
- Large intestine
- Rectum and anus
Accessory Organs: pancreas, liver, gall bladder
What is the primary function of the gastrointestinal tract?
To transfer nutrients, water, and electrolytes in the food we eat into the body (internal environment) for energy
Food (organic molecules) + O2 —> Energy + CO2 + Water
Function of the Mouth
Chews food to break it down. Secretes saliva with digestive enzymes (ie. amylase) which begins the break down of carbohydrates. Only absorption of some medication occurs here.
Function of the Esophagus
Swallowing. Mucus lining the organ assists with peristalsis which directs broken down food towards the stomach. No absorption.
Function of the Stomach
Receives food from esophagus. Secretes acid and digestive enzymes to digest the food (continued digestion of carbohydrates and beginning of protein digestion). Stomach muscles contract to churn food and enhance digestion.
Only absorption of lipid soluble substances (like alcohol or aspirin) but NO FOOD!
Function of the Small Intestine
Responsible for majority of digestion. Absorbs electrolytes,water, and nutrients from food passing through the digestive system.
@lumen: Carbohydrate and protein digestion continues, lipid digestion complete
@brush border: Carbohydrate and protein digestion complete
Function of the Large Intestine
Majority of water and salt absorption occurs here. Remaining content is converted into fecal matter with secretion of mucus.
Function of the Exocrine Pancreas
Produces and secretes important enzymes to the lumen of the small intestine (via ducts) to help break down food.
Function of the Liver
Regulates blood. Secretes bile to the small intestine which facilitate lipid digestion and removal of waste from blood.
Function of the Gall Bladder
Connected to the liver by the hepatic duct. Stores bile until needed in the small intestine.
Auerbach Plexus
The “myenteric plexus” which lies between the circular and longitudinal muscle of the intestinal wall
Meissner Plexus
The “submucosal plexus” which lies in the submucosa and innervates the muscularis mucosa and the epithelial layer
Interstitial cells of Cajal
Located between the myenteric plexus and longitudinal muscles of the intestinal wall. These smooth muscle cells control the basal electrical rhythm of the intestine.
Four basic functions of the gastrointestinal tract
- Motility
- Secretion
- Digestion
- Absorption
Nervous cells
Neurons and glia
Endocrine cells
Enterochromaffin cells
Enteroendocrine cells
Muscle cells
Smooth
Striated
Interstitial cells of Cajal
Inflammatory cells
Mast cells
Macrophages
Granulocytes
5HT
Released by enterochromaffin cells due to presence of gastrin. Hormone induces peristalsis.
Motilin
Secreted by M-cells due to fasting state. Hormone stimulates gastric and intestinal motility.
Enterogastrone or Gastric inhibitory peptide
Secreted by K-cells due to increased fat (>glucose) in the duodenum. It inhibits gastric emptying and intestinal motility.
Secretin
Released by S-cells due to presence of acid in the duodenum. It inhibits gastric emptying, induces contraction of the gall bladder, and relaxes the sphincter of oddi.
Chloecystokinin (CKK)
Released from I-cells due to high fat content (>protein) in the duodenum. Inhibits gastric emptying, relaxes sphincter of oddi, contracts gall bladder.
Autocrine
Stimulus causes a cell to release chemical messengers which induces an effect on itself.
Paracrine
A stimulus causes a cell to release chemical messengers to affect an adjacent cell.
Endocrine
Stimulus cause a cell to release chemical messengers into the bloodstream to affect a cell downstream.
Neurocrine
Stimulus induces neuron to release chemicals to depolarize another cell.
Succus Entericus
Intestinal juice secreted from exocrine glands composed of aqueous salt and mucus solution (no digestive enzymes). Provides protection, lubrication, and water required for digestion.
Secretion induced by presence of chyme or due to mechanical pressure.
Haustral shuttling
Means of propulsion in both directions within the large intestine to mix contents
Cephalic phase
External stimuli (smell, taste, etc.) causes neuronal stimulation of gastric secretion to prepare the stomach for digestion.
Gastric phase
Acid secretion is stimulated by gastrin and ach in response to distension and protein digestion products.
Intestinal phase
The increased acid content of the lumen (pH=2) induces D-cells to secrete stomatostatin which inhibits the release of gastrin from G-cells in a Paracrine manner. The decrease in gastrin levels reduce acid and pepsinogin secretion.
Mucosal Protection
The parasympathetic nerves secrete acetylcholine which induce mucus surface and neck cells to secrete mucus (mucin). This layer of mucus prevents acid from contacting the mucosa because it acts as a buffer barrier (neutralizing the acid).
It is made of a peptide core and carbohydrates side chains, embedded with trefoil factors.
Ulcer
When there is an epithelial breach in the gut tissue. This breach allows acid and bacteria to flow into the gut wall and cause inflammation.
Epithelial cells of the Antrum
Mucosal surface Mucosal neck Enterochromaffin like cells G-cells D-cells
Epithelial cells of the fundus
Mucosal surface Mucosal neck Enterochromaffin like cells Parietal Chief D-cell
Components of the biliary system
Liver, gall bladder, associated ducts.
Dual blood supply of the liver
Hepatic artery brings in oxygen rich but nutrient poor blood.
Portal vein brings in oxygen poor but nutrients rich blood (direct from digestive tract). This is important because the liver processes, stores, and detoxifies, the blood before it enters the general circulation.
When in the liver, the portal vein breaks up into smaller capillaries called…?
Sinusoids
Hepatocytes
Liver cells arranged in lobules (hexagonal arrangement) which are perfused by blood from the portal vein and hepatic artery.
Sinusoidal blood
Hepatic artery and portal vein blood mix in the sinusoids of the liver. They are the leakiest capillary beds in the body which allows the blood to bathe surrounding hepatocytes.
How does blood leave the liver?
Eventually the blood from the capillaries percolate into hepatic veins that lead to the inferior vena cava.
Rappaport zones of the Acinus
Regions by function:
- Periportal (oxyrich) closest to trio ducts
- Mid zonal
- Peri central (oxypoor) closest to central vein which drain blood to the inferior vena cava.
Kupffer cell
Macrophage in the liver that detects pathogen from incoming blood
Fenestrations of the sinusoid
Space of Disse
Stellate cells or Lipocyte
Cells in the liver which store fat and vitamin a
What types of junctions are found between hepatocytes?
Tight junctions
Gap junctions
Desmosomes
Bile Canaculi
Bile capillaries found between hepatocytes which merge to form hepatic ducts to remove bile from hepatocytes.
Function of peri portal cells in the liver
Absorption of oxygen, nutrients, and bile salts (therefore making this region oxygen and nutrient rich)
What is the function of the perivenular or pericentral zone?
Absorption of toxins, therefore it is low in oxygen and nutrient content.
Bile composition
Composed of two secretions
- From hepatocytes
- From cells that line the bile duct (changes composition of bile as it moves through the duct)
Biliary tract component and functions
- Canaculiculus: bile formation
- Ductules and ducts: modify bile via absorption and secretion of content
- Gall bladder: concentrate, store, and deliver bile
- Sphincter of oddi: regulate delivery of bile by preventing the reflux of bile from the duodenum.
Function of bile
- Enhance intestinal absorption of lipids, Ca, and Fe.
- Excrete solutes poorly filtered by the kidney
(Lipid soluble compounds, protein bound trace minerals, antigen antibody complexes, immunoglobulin a)
Primary bile acids
Cholic acid or chenodeoxycholic acid
Metabolites of cholesterol
Secondary bile acids
Deoxycholic acid or lithocholic acid
Deoxylated forms of the primary acids due to gut bacteria
Conjugated bile acids or final bile salts
Any of the primary or secondary bile acids combine with either glycine or taurine to form the final bile salt.
Ie. Glycocholic acid
Ie. Taurochenodeoxycholic acid
Nature of biles salts
Bile salts are amphipathic which means they have a hydrophobic and hydrophilic side.
This makes bile salts strong detergents with the ability to break lipids into smaller molecules.
When does the sphincter of oddi contract and relax?
The sphincter of oddi contracts during periods of fasting.
The sphincter of oddi relaxes when CCK is present: allowing bile to enter the duodenum for the emulsification of fat.
How does the gall bladder change the composition of the bile?
The gall bladder absorbs Na,Cl, and water to concentrate the bile. It releases H+ and mucin to increase the acidic content of the bile while protecting itself.
When relaxed, bile is stored.
When contracted, bile is being delivered to the small intestine.
Active transport of bile acid
Bile acid is secreted into the bile canaliculi against a gradient, therefore energy is required. Once in the canaliculi, bile salts are stored in micelles which decreases their detergent actions
Bile secretion from duct cells
Duct cells secrete alkaline components (HCO3) of the bile which neutralizes the gastric acid and allows bile salts to form micelles. This is under the control of secretin from the S-cells in the duodenum.
De novo synthesis of bile is low
Bile circulates within the digestive tract so bile synthesis is low unless bile acids are lost. Bile secrete from the liver is released into the gall bladder then the duodenum and absorbed in the ileum into the portal vein which carries content of the digestive tract to the liver.
Crypts of lieberkuhn
Secrete intestinal juice (succus entericus)
Function of enterocyte
Absorption
Function of goblet cells
Secretion of mucus to protect small intestine from acidic content of the stomach
Why do proteases require secretin of precursors?
They cannot be secreted in their active form because they could digest, degrade, and dissolve surrounding cells.
Exocrine function of the pancreas
The acini release digestive enzymes and cofactors to the duodenum (neutralizing occurs in ducts)
Endocrine function of the pancreas
Islets release hormones to the portal vein
How are digestive enzymes transported from the pancreas?
Digestive enzymes are produced in the Golgi complex of an acina cell. They condense into vacuoles then as zygomen granules before secretion at the apical surface of the cell.
Why does the pancreas not digest itself?
- Enzymes that attack membranes are secreted as inactive zygomens while enzymes that do not attack membrane are secreted in their active forms.
- Enzymes are secreted within membrane bound compartments
- Pancreas contains intracellular trypsin inhibitor
- Activator for enzymes (enterokinase) is geographically separated
Enterokinase
Enzyme secreted by the brush border which activates trypsinogen by converting it to trypsin. Trypsin activates other zygomen enzymes including itself.
Glycocalyx
Glycoproteins secreted by the brush border cells that aid in the absorption of carbohydrates and lipids
Digestion of protein
Begins in the stomach and continues in the small intestine.
Protein to oligopeptides to dipeptide/tripeptide to amino acids
Why do proteins need to be broken down into amino acids?
Amino acids can be readily absorbed in the small intestine directly though the enterocyte and passed through the blood stream.
PEPT1
Peptide transporter 1: important for protein assimilation.
It is located on the apical membrane of enterocytes and recognizes dipeptides and tripeptides.
Requires H+ generated by the Na/K exchanger which is powered by the Na/H exchanger
Exogenous and endogenous proteins
Exogenous are proteins we ingest
Endogenous are proteins that are no longer required by the body
Maximum protein digestion occurs where?
The ileum at 80%; protein digestion is slow and incomplete in the duodenum
Carbohydrate digestion
Initiated in the mouth, but majority occurs in the small intestine due to copius amounts of amylase secreted by the pancreas and brush border enzymes that break down larger polysaccharides
Amylase
Digests starch to maltose and maltotriose
Where are sucrose and lactose digested?
In the brush border and not the lumen
Sucrose: glucose and fructose
Lactose: glucose and galactose
Where does majority of carbohydrates take place?
Distal part of duodenum and in the jejunum
Lipid digestion
Initiation occurs in the proximal gut.
- Lingual acid lipases account for 10-30% of digestion
- Motor activity of stomach breaks down some fat
Small intestine
- Secretion of bile with amphipathic properties break down fat molecules to form micelles
What happens to absorbed micelles?
They are converted into triglycerides in the enterocyte and exit as chylomicrons
Majority of lipid absorption occurs where?
The jejunum and the ileum
