Untitled Flashcards Set

Gastrointestinal Physiology (GI) Lecture 1 recording 1: General Introduction  Functions of the Gastrointestinal Tract (GIT):  Transfer digested organic nutrients, minerals and water, from the external environment into the internal environment: involves digestion and absorption o Digestion  Form absorbable molecules from food through GIT motility, pH changes, and biologic detergents and enzymes (enzymes are predominantly produced by the pancreas) o Absorption  Movement of digestive food from the intestine into the blood or the lymphatic system  Functions of the Gastrointestinal Tract (GIT):  Functions of the GIT: o Excretion  Non-absorbable components of food, bacteria, intestinal cells, and hydrophobic molecules (drugs), cholesterol and steroids are excreted o Host defense  Lumen of the GIT is considered to be outside the body  It is continuous with the exterior of the body  The GIT forms a barrier with the outside environment and contains a highly developed immune system  The GIT can inactivate t harmful bacteria or other microorganisms  Components of the GIT:  Components of the GIT: o Mouth, pharynx, esophagus, the stomach, the small intestine (duodenum, jejunum, ileum), large intestine  3 accessory organs: o Pancreas, liver, gallbladder Lecture 1 recording 2: Layers of the GIT  Structure of the GIT:  GIT is a long muscular tube stretching from the mouth to the anus o Composition is similar form mid-esophagus to anus  The top third of the human esophagus is made of skeletal muscle, while the rest of the GIT is composed of smooth muscle  Tube of the intestine o Lumen  Inside of the tube  Contains many folds and processes to increase the surface area  Circular fold is where the entire inner surface folds in on itself  Contains villi (singular villus)  Villus projects into the lumen of the tube, and below the surface still is a crypt or an invagination  Structure of the GIT:  Layers of the GIT o Mucosa  The mucosa contains three different subsections:  Epithelium → a very thin layer of cells  Lamina propria  Muscularis mucosa → a very thin, smooth muscle layer o Submucosa o Muscularis externa (outer muscular layer) o Serosal layer (connective tissue layer)  Layers of the Mucosa:  3 layers of the mucosa: o Epithelium, lamina propria, muscularis mucosa  Epithelium:  A layer of cells that lines all body cavities and surfaces  Epithelial cells are polarized cells: they have a basolateral surface and an apical surface  Apical surface → inserts the inside of the tube or the lumen of the tube  Basolateral surface → closest to the blood surface, facing away from the tube; basal surface and the lateral surface  The polarized epithelial layer has different transport proteins at the apical surface compared to the basolateral surface  Transport proteins are confined to the different cell surfaces due to presence of tight junctions  Layers of the Mucosa:  Epithelial layer: o Single layer of cells o Function:  Selective uptake of nutrients, electrolytes and water  Prevent the passage of harmful substances o Surface area is amplified by the presence of villi and crypts  Villus contains a single layer of epithelial cells containing microvilli  Crypt is a region which invaginates into the lamina propria o Stem cells within the crypts divide and produce daughter cells which differentiate into a variety of cells  Stem cells divide and migrate up the villus. At the top of the villus, they reach the end of their life and slough off  In the small intestine the epithelial cell layer is replaced every 5 days  As these cells are dividing rapidly, they are affected by anticancer drugs and are killed by the drugs before they can be replaced  Selective Transport of Nutrients Across the Epithelium: o Epithelial layer is selective, allowing specific nutrients across the intestinal epithelium and into the body  2 pathways that chemicals or molecules can use to get across an epithelial layer:  Paracellular pathway → chemicals move between cells across the cell junctions; limited by tight junctions between cells, so only water and small ions can actually diffuse through the tight junctions; not many substances can get through this way in a healthy intestine  Transcellular pathways → cross the cell and therefore require transport proteins  More Layers of the Mucosa: o Lamina propria  Includes everything above the muscle layer  Connective tissue, blood vessels, nerve fibers and lymphatic vessels, immune and inflammatory cells for immune protection  Lacteal or lymphatic vessel o Muscularis mucosa  Thin layer of smooth muscle  Not involved in contraction of the GIT and may function in moving the villi Lecture 1 recording 3: More Layers of the GIT  Structure of the GIT: (Shows the different layers of the GIT)  Submucosa:  Beneath the mucosa layer  Submucosa: contains blood vessels, lymphatic vessels, submucosal nerve plexus (network of nerves), connective tissue o Submucosal nerve plexus relays information to and away from the mucosa  Muscularis Externa:  Muscularis externa: circular muscle, myenteric nerve plexus, longitudinal muscle o Circular muscle: fibers are orientated in a circular pattern and contract and relax to close and open the tube o Myenteric nerve plexus: myo = “of muscle”  Regulate the muscle function of the GIT o Longitudinal muscle: lengthens and shortens to control the length of the tube (does not change the diameter)  Serosa:  Serosa layer: a connective tissue layer that encases the intestine and forms connections with the intestine and the abdominal wall Lecture 1 recording 4: Blood Supply to the GIT  Blood Supply to the GIT:  GIT blood supply carries away many nutrients as well as other components absorbed from the diet o Blood supply transports water-soluble nutrients and other molecules o Lacteals (lymphatic system; lamina propria in the mucosa layer) are important for fat absorption  Food → digested in the stomach → absorption and secretion in the small intestine → processing in the colon → elimination of feces containing ingested material that cannot be digested and absorbed o Blood is highly oxygenated entering the GIT but loses oxygen as it perfuses the intestine  Blood does not flow directly from GIT back to the heart- blood is taken to the liver before returning to the heart. o Blood that perfuses the intestine goes to the liver via the portal vein  Portal Circulation:  Portal circulation → circulation that carries the blood from the intestinal tract to the liver o Blood in the portal circulation is nutrient rich o Important for:  Liver removes harmful substances (liver acts as a filter and has many enzymes)  Processing of nutrients  Portal Circulation:  Other organs in the body are perfused by arterial blood (fully oxygenated) o Liver is an exception:  When you have not been eating, ~ 30% of the blood in the liver is from an arterial source (ie. fully oxygenated)  After eating, this is reduced to 10%  Hepatic artery → contains fully oxygenated blood that perfuses the liver  Hepatic portal vein → carries blood to the liver that has already perfused the stomach, pancreas, SI and LI o These two blood supplies mix, so that the liver is perfused with nutrient-rich blood coming from the GIT organs with a poor oxygen content  “In Series” versus “In Parallel” Circulation:  Most organs are perfused in parallel within the systemic circulation, but the liver is perfused in series as the liver is perfused predominantly by blood that has already perfused another organ; blood flows from the digestive organs to the liver