Digestive System and Gastrointestinal Tract Flashcards
Digestion
The process of breaking down polymers into substrates that can be used for catabolic reactions or anabolic reactions.
Though food can also contain vitamins.
Result of enzymatic hydrolysis.
A secondary function is defence against disease.
Digestive Reactions
Review Pg. 379, Figure 8.
Different terms for the digestive tract
Gastrointestinal tract
Alimentary canal
Gut
Tube of the digestive tract
Result of tube formed by gastrulation during embryogenesis.
Anus is derived from blastopore.
It is continuous with the outside.
Inside of the digestive tract
GI lumen
Tissue layers of the GI tract
From inside to out.
Mucosa, showing villi (epithelial tissue)
Submucosa (connective tissue)
Circular Muscularis (smooth muscle)
Longitudinal muscularis (smooth muscle)
Serosa (connective tissue)
GI Epithelium
Because it is exposed to substances from outside world, it is composed of same cells that form skin and lung tissue. By definition they are attached to a BASAL MEMBRANE.
Lumen faces the apical surface. Apical surface is coated in microvilli, and separated from the rest of the cell by tight junctions.
Basolateral surface, opposite to the apical surface.
GI smooth muscle
Smooth muscle - has smooth microscopic appearance.
Note that there are two layers in the GI tract. Circular and longitudinal.
GI motility
The rhythmic contraction of the GI tract. Depends on 5 factors:
1. Automaticity - contracts without external stimulation due to spontaneous depolarization.
2. Functional syncitium - just like heart cells, means that a depolarization in one cell will spread to others via gap junctions.
3. Enteric Nervous System - a GI tract specific nervous system
4. Hormone input - can alter GI motility
5. Parasympathetic NS - causes sphincters to relax allowing food to pass; sympathetic does the opposite.
Functions of GI motility
- Movement down the GI tract. Result of peristalsis. Note that peristalsis, first contracts behind the bolus, then around the bolus to prevent backward sliding.
- Mixing of food in the GI tract. Result of disordered contraction of smooth muscle causing churning motions.
Bolus
Food moving down the GI tract.
Enteric Nervous System
A branch of the autonomic nervous system.
Innervates the GI tract; pancreas; and gall bladder.
Helps to regulate local blood flow, gut movement, and the exchange of fluid from the gut to and from the lumen.
Can operate separately from the other two branches of the autonomic nervous system (parasympathetic/sympathetic), but both can regulate.
ENS - networks of neurons
Composed of two networks of neurons: myenteric plexus and the submucosal plexus.
Myenteric plexus - between circular and longitudinal muscle layers and regulates gut motility.
Submucosal plexus - in submucosa and helps regulate enzyme secretion, gut blood flow, and ion/water balance in the lumen. Because this is less relevant in the esophagus or anus, this plexus is not as prevalent.
GI secretions
Release of enzymes, bile, acid, is generally stimulated by food and the parasympathetic system, and inhibited by the sympathetic system.
Two types of GI secretion
Exocrine - glands that secrete into ducts that drain into the GI lumen. These glands are composed of specialized epithelial cells organized into sacs called ACINI. Primary organs include the pancreas, liver, and gallbladder, although some is performed by special cells in the gut lining itself (ex. Gastric glands; goblet cells)
Endocrine - glands that are ductless and secretions (hormones) enter capillaries –> bloodstream. Performed by specialized organs (pancreas) and specialized cells in the gut lining.
Gastric Glands
Secrete acid and pepsinogen
Goblet cells
Found along the GI tract. Secrete mucus protective layer.
Water in the GI tract
Each day gallons of water are secreted into the GI tract to facilitate digestion. It is later reabsorbed in the small intestine and the colon.
To remember regarding organs that have exocrine and endocrine function.
Exocrine and endocrine cells will tend to group such that they are microscopically distinguishable.
Ex. Pancreas
Exocrine - pancreatic acinar cells
Endocrine - islets of langerhans
Organs of the digestive system
Nasal cavity
Oral cavity
Salivary glands
Epiglottis
Trachea
Esophagus
Liver
Stomach
Pancreas
Duodenum (behind liver)
Gallbladder
Transverse colon
Ascending Colon
Jejunum
Descending Colon
Ileum
Rectum
External anal sphincter
Mouth
Three role:
Fragmentation (mastication): Incisors cut; cuspids tear; molars grind
Lubrication. Saliva.
Enzymatic digestion. Saliva.
Saliva
Contains SALIVARY AMYLASE (ptyalin).
Hydrolyzes starch. Can yield disaccharides. Note though that only monosaacharides can be absorbed by the body.
Also contains LINGUAL LIPASE. Fat digestion.
No protein digestion occurs in the mouth.
Also contains LYSOZYME, attacks bacterial walls.
Pharynx
Commonly known as the throat.
Contains the opening to two tubes: Trachea (cartilage lined tube - air to lungs) and esophagus (muscle lined tube - food to stomach).
Epiglottis
Flat cartilagionous flap that excludes food/water from the trachea.
Muscular rings that regulate movement of food through the esophagus.
Upper esophageal sphincter - top of esophagus.
Lower Esophageal sphincter (cardiac sphincter) - at the end of the esophagus.
Stomach
Large hollow organ, three functions:
- partial digestion of food
- regulated release of food into small intestine
- destruction of microorganisms
Gastric - means “related to the stomach”
Gastric Acidity
pH is about 2. Result of secretion of HCL by parietal cells located in gastric mucosa.
Facilitates:
1. microorganism destruction
2. acid-catalyzed hydrolysis of dietary proteins
3. conversion of pepsinogen to pepsin
Gastric Pepsin
Secreted by Chief Cells, and catalyzes proteolysis.
Secreted as pepsinogen, the inactive form of pepsin (zymogen). Conversion by gastric acidity.
Gastric Motility
Stomach constantly churns food. Exposes food to gastric acidity.
Chyme
Food mixed with gastric excretions.
Sphincters
Lower esophageal sphincter - prevents reflux of chime into the esophagus.
Pyloric sphincter - regulates passage of chyme into the duodenum. Activity is inhibited when the small intestine already has food in it (mediated by hormones and the nerves connecting duodenum and stomach)
Cholecystokinin
Helps to regulate activity of the pyloric sphincter.
Also causes contraction of the pancreas and gall bladder.
Secreted by epithelial cells in the wall of the duodenum.
Gastric Gastrin
Hormone secreted by G cells in the stomach. Stimulates acid and pepsin secretion.
Stimulated by food in stomach and parasympathetic system.
Histamine is secreted in response to food in stomach and to gastrin. It binds parietal cells and stimulates acid release.
Cimetidine and ranitidine (Tagamet and Zantac respectively) bind the Histamine H2 receptor in the stomach to prevent binding.
Small intestine
About an inch wide and 10 ft long. 25 ft after death due to relaxation.
Three segments: Duodenum, jejunum, and ileum.
Digestion is completed in the duodenum and jejunum.
Small Intestine Capacity for absorption
Surface Area: Length, villi, microvilli.
Villi - macroscopic (multicellular), projections on intestinal wall
Microvilli - microscopic foldings of cell membrane of individual epitheial cells.
Brush border
Refers to the lumenal surface of the small intestine as a result of the microvilli.
Structures In/On Villi
(1) Capillaries - absorb monosaccharides and amino acids. Capillaries –> Vein –> Hepatic portal vein.
(2) Lymphatic vessels, Lacteals - absorb dietary fats. Lacteals merge to form large lymphatic vessels, that transport dietary fats to the thoracic duct that empties them into the blood stream.
(3) peyers patches - a part of the immune system. Collection of lymphocytes dotting villi that monitor GI contents.
Key feature of the duodenum
Two ducts empty into it:
(1) Pancreatic Duct: delivers exocrine secretions from pancreas (digestive enzymes and bicarbonate)
(2) Common bile duct: delivers bile, which contains bile acids made from cholesterol in the liver. Can be recycled. Bile is stored in the gallbladder and is important for excretion and digestion of fats.
Sphincter of Oddi
Both the Bile Duct and the pancreatic Duct empty into the duodenum via this sphincter.
Duodenal Enzymes
Some doudenal epithelial cells secrete enzymes.
Enterokinase (enteropeptidase) activates pancreatic trypsinogen.
Some are not secreted, but are attached to the cell wall surface at the brush border = BRUSH BORDER ENZYMES. Function to hydrolyze disaccharides and dipeptides, into mono…
Duodenal Hormones
Three main:
1. Cholecystokinin (CCK) - secreted in response to fat. Causes pancreas to secrete enzymes; contraction of gall bladder (bile release); and decreases gastric motility.
- Secretin - response to acid in the duodenum. Causes pancreas to release high-pH buffer, namely HCO3-. Duodenal pH must be neutral or slightly basic for digestive enzyme function.
- Enterogastrone - decreases stomach emptying.
Jejunum and Ileum
Complete the remainder of absorption.
Vitamin B-12 is only absorbed in the ileum - and needs to be bonded by an intrinsic factor (glycoprotein) secreted by the stomach.
Ileocecal valve - Separates the small and large intestine. Cecum is the first part of the large intestine.
Colon
Large intestine.
3 - 4 ft long and a couple inches wide.
Absorbs water and minerals. Stores feces before excretion.
First part - cecum.
Second - Appendix (finger like projection)
Third - rectum.
Anal sphincter
Two portions: Inner and external (just like urinary sphincter).
Inner is under autonomic control, smooth muscle.
External consists of skeletal muscle and is under voluntary control.
How quickly is food passed as feces?
Usually food is passed about a day after consumption.
Although, waste from a meal is present after just a few hours. And residue of some meals can remain days after.
Colonic Bacteria
Many are faculative or obligate anaerobes.
Produces gas as a byproduct of bacterial metabolism.
Important:
1. Presence of large numbers of bacteria impedes the proliferation of harmful bacteria.
- Supply us with Vitamin K, which is necessary for blood clotting.
GI Accessory Organs
Those that play a role in digestion but are not part of the alimentary canal.
Pancreas, liver, gall bladder, and salivary glands.
Pancreas and liver are essential for digestion.
Pancreatic Enzymes
Secreted into the duodenum via the Sphincter of Oddi and all are inactive (safety) when secreted - many are activated by trypsin.
Pancreatic Amylase - hydrolyzes polysaccharides to disaccharides.
Pancreatic lipase - hydrolyzes triglycerides at the surface of a micelle.
Nucleases - hydrolyze DNA and RNA.
Pancreatic proteases - hydrolyze polypeptides to di and tripeptides.
Pancreatic Enzymes activated by tripsin
Chymotripsinogen (Active: chymotripsin)
Procarboxypeptidase (Active: carboxypeptidase)
Procollagenase (Active: collagenase)
Enzymes that cause secretion of pancreatic enzymes
Cholecystokinin (CCK) - secreted into blood stream by duodenum.
Secretin - causes secretion of water and bicarbonate
Endocrine Pancreas
Small regions within the pancreas known as the ISLETS OF LANGERHANS.
(a) alpha cells secrete glucagon in response to low blood sugar. Causes liver to hydrolyze glucagon to release glucose into blood and stimulate adipocytes to release fats.
(b) beta cells secrete insulin in response to blood sugar. Effects are opposite of glucagon.
(c) sigma cells - somatostatin. Inhibits digestive processes.
Why is high glucose in the blood stream a bad thing?
Directly destroys many physiological systems at the cellular level (neurons, blood vessels, and kidneys). Longer time period though.
Insulin is essential for lowering blood glucose levels.
What can raise blood glucose?
Three hormones can raise blood glucose.
(1) Glucagon
(2) Epinephrine (secreted from adrenal medulla)
(3) Cortisol (Adrenal cortex)
It is important to have redundancy here as low blood glucose is immediately fatal.
Exocrine activity of the liver?
It secretes bile. Produces about a liter a day.
Bile = bile acids (bile salts in deprotonated anionic form), cholesterol, bilirubin.
Bile emulsifies large fat particles in the duodenum and produces MICELLES. Allows for better digestion by lipases and transfer acros the intestinal mucosal membrane.
Bile Destination
Two locations:
Either secreted directly to the duodenum.
OR;
Stored in the gallbladder and released after a fatty meal.
Gallstone = a large crystal that forms in the gallbladder.
Gallbladder secretion
No secretory activity. regulated by the endocrine (CCK) and nervous system (parasympathetic)
Liver function
More important for processing than digestion of materials.
Recieves blood from hepatic arteries and the hepatic portal system (venous blood).
Hepatocytes in the liver monitor the blood condition.
Common factor between liver and skeletal muscle?
Both can store glucose as glycogen.
However, only the liver can release glucose to the bloodstream. Requires glucose-6-phosphatase, and this is only in the liver.
Liver production of urea
When proteins are broken down to amino acids and even further during starvation (krebs cycle). Nitrogens are produced (NH3) which can be toxic, so liver converst to urea.
Liver production of lipoproteins
Lipids enter lymphatic system as Chylomirons. Chylomicrons can be degraded by lipases into triglycerides, glycerol, and cholesterol-rich chylomicron remnants.
Hepatocytes can take up these remnants and produce lipoproteins (HDL, LDL, VLDL).
These can re-enter the bloodstream and serve as sources of cholesterol and triglycerides.
Liver production of blood proteins
Albumin, Fibrinogen, and other clotting factors are produced in the liver.
People with liver disease often struggle to close wounds.
Hepatocyte Smooth ER
Essential for detoxification.
Control of appetite.
Empty, gastric cells produce - Ghrelin
When the colon is full, the jujenum produces - Peptide YY, to reduce appetite.
Leptin - produced by white adipose tissue, is an appetite suppressor.
These three hormones can impact multiple tissues, but primarily impact the ARCUATE NUCLEUS of the hypothalamus.
Obesity
May lead to hypothalamus resistance to leptin, as a result of receptor downregulation.
Vitamins
Divided into fat soluble and water soluble.
See table 2 for details. pg. 395.
Gastrin
Decreases stomach pH; activates chief cells to release pepsinogen; indirectly stimulates protein degradation
Amylase
Hydrolyzes starch into maltose.
