Gastrointestinal System: An Overview

Overview of the Gastrointestinal System

• The gastrointestinal (GI) system is also known as the alimentary canal.
• The GI tract digests and absorbs food, making nutrients more available to the body step by step along its disassembly line.
• The GI tract is involved in ingestion, propulsion, mechanical digestion, chemical digestion, absorption and defecation.
• Primary digestive organs that encompass the GI tract include include: mouth, pharynx, oesophagus, stomach, small intestine, and large intestine.
• The GI tract also includes accessory digestive organs such as teeth, tongue, salivary glands, gallbladder, liver, and pancreas.
• The main function of the GI tract is to process food, and requires there be six classes of nutrients in an adequate diet to do so.
• These nutrients include carbohydrates (50-60% of diet), lipids (25-35%), proteins (15-25%), vitamins, minerals, and water (0.5-2%).

Saliva and Digestion in the Mouth

• The digestive process starts in the mouth.
• Saliva, which amounts to 1-1.5 L/day, has a basic composition of 98-99% water, salivary amylase, mucins and ions, buffers, metabolites, and antibodies.
• Saliva moistens foodstuff and converts it to a bolus, making it easy to swallow.

Histology of the GI tract

• The GI tract walls from the oesophagus to the anal canal have the same four layers (tunics).
• From the lumen outwards, the layers are mucosa, submucosa, muscularis externa and serosa.

The Stomach and its Modifications

• Mechanical breakdown of foodstuff into smaller pieces occurs.
• Muscularis layer is modified based on stomach functions which include perstalsis.
• It is composed of three layers: circular, longitudinal and an additional innermost oblique layer.
• The stomach surface is lined with invaginations known as gastric pits.
• Bicarbonate generates a pH gradient to protect the stomach wall from digestion from gastric acid and enzymes.
• Glycoprotein products acts as a primary lubricant to other specialized functions.

Different Cells on the Stomach

• Chief cells produce zymogen granules for stimulation release
• Pepsinogen is produced from chief cells and is converted to pepsin
• Pepsin breaks down proteins into smaller polypeptide and amino acid fragments.
• Gastric lipase is a fat-digesting enzyme accounting for approximately 40% of preduodenal lipolysis.
• Parietal cells secrete H+ and Cl- secretion, which increases acidity, denatures food, helps activate pepsinogen, dissolves bone, and acts as a bactericidal.
• Parietal cells also secrete intrinsic factor for vitamin B12 absorption in the small intestine.
• Only Intrinsic factor stomach function is essential for life
• G cells secrete gastrin which stimulates acid release
• D cells secrete somatostatin to inhibit gastrin secretion
• Enterochromaffin-like cells secrete histamine which stimulates acid release from parietal cells.

Why the Stomach Doesn't Digest Itself

• The stomach doesn't digest itself due to a mucous barrier, alkaline lining, epithelial cells, and tight junctions.
• Mucous barrier neutralizes acid on the stomach lining.
• Epithelial cells prevent acid from leaking to underlying tissue.

Gastric Emptying

• The rate depends on the type of ingested food.
• Fluids pass through quickly in 90 mins approximately
• Solids, remain in stomach until they are reduced to very small particles.
• Solids, dissolved in gastric juices will be broken down in 3-4 hours approximately.
• Carbohydrates emptied from the stomach first, followed by proteins, then fats which takes the longest.
• The anatomical modifications of the stomach enable its function and includes its large surface area for absorption and its three muscle layers.
• Pepsinogen is produced by chief cells.

Small Intestine and Nutrient Absorption

• Chyme enters the small intestine after leaving the stomach.
• The small intestine's subdivisions include the duodenum, jejunum, and ileum.
• The small intestine has a large surface area to increases absorption.
• Finger-like projections called villi are there to increase absorption
• There are invaginations called crypts, on the small intestine.
• Carbohydrates and proteins are partially digested upon entering the small intestine.
• Fats are undigested upon entering the small intestine
• All nutrient absorption occurs in the small intestine
• Bile from the liver and enzymes from the pancreas are added to chyme in duodenum.
• The liver aids digestion by producing bile (yellow-green alkaline solution) from cholesterol/
• Bile emulsifies fats by breaking down fat globules into smaller droplets so lipases can act more effectively.
• The gallbladder concentrates and stores bile, not synthesizes it.
• When the sphincter of Oddi is closed, bile cannot enter the duodenum and instead, bile is stored in the gallbladder.
• Bile increases the lipid droplets’ solubility and digestibility through emulsification.

Pancreas and Pancreatic Juice

• Acinar cells secrete pancreatic juice into the pancreatic duct.
• The pancreas produces a watery alkaline fluid of pH 8.
• It contains HCO3- and various digestive enzymes that are enzymes for digestion.
• Pancreatic juice also contains proteolyptic and lipolytic enzymes
• Trypsin, chymotrypsin, carboxypeptidases, nuclease, elastase and collagenase
• Lipolytic enzymes include: pancreatic lipase, cholesterol ester hydrolase, phospholipase A and B, colipase, and bile salt-activated lipase.
• Pancreatic amylase is involved in breaking down carbohydrates.
• High levels of Bicarbonate are found in pancreatic juice.
• Bile increases the lipid droplets’ solubility and digestibility through emulsification.
• alpha-Amylase does not require any activation in the pancreas.

Intestinal Juice

• Glands normally excretes 1-2 L/day to digest food.
• It contains digestive enzymes
• The body cannot absorb disaccharides, so carbohydrate digestion breaks down disaccharides to monosaccharides.
• Protein digestion breaks polypeptides down into amino acids, while fat digestion converts lipids to monoglyceride and fatty acids.

Carbohydrate Digestion

• Consist of a continuation of pancreatic amylase hydrolysis
• A simple sugar is absorbed across the intestianl mucosa
• Carbohydrate digestion also involves cellulase conversion
• The carbohydrate cellulose, cannot be digested by humans, is passed to the large intestine.
• Nondigestible carbohydrate is called fibre.

Protein Digestion

• Protein digestion starts in the stomach with pepsin and continues in the small intestine with pancreatic fluid.
• Pancreatic fluids contains Trypsin & chymotrypsin
• Peptide fragments further dismantle into tripeptides, dipeptides, and single amino acids.
• This is later joined by amino acids and Na+ for transport

Lipid Digestion

• Triacylglycerides (TG) are the most abundant dietary fat.
• There is a major lipid breakdown which occurs by emulsifying action of bile
• Pancreatic lipase works to break down lipins
• Long-chain fatty acids, absorbed by the intestinal mucosa, reform into triacylglycerols (triglycerides).
• Chylomicrons slowly move through the lymphatic system and empty into the venous blood of the systemic circulation.
• Both Fat-soluble vitamins are absorbed with dietary lipids, which can transport those vitamins to the liver and fatty tissues.

Vitamin Absorption

• Vitamin absorption mostly occurs by diffusion in the jejunum and ileum
• Fat-soluble vitamins (A, D, E, K) binds to chylomicrons & lipoproteins
• This occurs via lipids
• Water-soluble vitamins are mostly absorbed into water
• It must also bind with intrinsic factor with the help of parietal acids

Absorption of Electrolytes

• Electrolytes, Na+ and Cl-, are absorbed via active transport.
• K+ is absorbed via passive diffusion.
• H2O is absorbed via osmosis and moves freely across intestinal mucosa.

The Large Intestine

• Colon is the largest region and extends across the ileocecal
• The Rectrum includes the last terminal
• The colon has: internal/external spincters

Anatomical Modificiations of the Large Intestine

• There is a large quantity of goblets in its cell
• Mucosa contains many goblet cells but no villi
• Alkaline mucosa to hold faeces
• Also include haustration on the lateral layer, as well as a band for circulation and longitudinal contractions

Large Intestine Function

• It serves to remove the excess water in the tubule, and dehydratres faeces to remove waste.
• It produces propulsion and involved in the defacation reflex by removing excess waste to the body.
• It is also involved in absorbing vitamin B
• And in converting the gut flora to produce vitamins
• The lumen itself contains bacteria and the process of digestion.

GI Secretion and Absorption

• Total input is 9.0L vs Total absorbed which is 8.9L. Remaining .1L leaves the body
• Total secretion for intestinal fluid in gut-1.5 L/day, total absorption as 8.2L/day
• For Colon absorption is 1l/Per Day
• Total for Per day fluid and electrolyte abosprtion is set for approximately 9.5/2 L/day
• Faecal loss is 0.3 L/day.

Defecation

• This is a process where Faeces is forced into the process via mass
• A distention of the SI wall helps to process
• And a trigger that leads to the deification reflex and process
• The anus rectum in the body sends to cns to brain, to spinal cord to trigger the Sphinter
• Abdomical muscles aids in pressing, or to loosen the waste by having the anoretic angle and helps to create less pressure, and an opening for the anus to remove material.

Motility

• Divided in 2 ways Peristalsis and Segmentation
• Peristalsis moves material down the gut
• Segmentation helps to mix various nutrient for digestion

Small Intestinal Motility

• Peristaltic contractions are responsible for forward movement of materials.
• There are (3 contractions/min)
• As Circular muscle contraction pulls food, the longitudinal muscles loosen to release the food
• Circular muscle, inner intestine will contract to push material
• The other to help hold the tissue to loosen the material, outer layer to make it circular

Segmentation.

• Functions to help the mix nutrition with digestive

Normal Movement of the Large Intestine

• Transit though occurs in 36 – 48 h aids the unstimulate portions of the circular muscles
• Aids the and longitudinal muscle contractions to slowly digest material.

Motility

• This movement occurs from. Low animal and circular and lingutinal contractions
• This moves in the S I to help process material.
• If deification does not occur, the event process will push though. 1- half days approximately, otherwise there are sympathetic reactions.

Motility Control of Colonic

• Aids to send direct intramnal
• And indirect direct reflux

Desecretion

• Deceison making central system will either signal to push or to not push. A type of volnutaty reflex
• Will either signal or remove with the spine spinal colons by applying pressure and abs
• This helps with pushing or to aid and relive waste