Digestion Flashcards
Exocrine glands
secrete to the surface of the body or the lumen of the gut
the surface of the body (e.g. sweat glands, sebaceous glands)
The lumen of the digestive tract / gut (e.g. digestive glands)
digestive glands
Salivary glands, Gastric glands , Pancreatic glands, Intestinal glands
Salivary glands
secrete saliva which contains amylase (breaks down starch)
Gastric glands
secretes gastric juices which includes hydrochloric acid and proteases (breaks down protein)
Pancreatic glands
– secretes pancreatic juices which include lipase, protease and amylase
Intestinal glands
secretes intestinal juices via crypts of Lieberkuhn in the intestinal wall
composition of exocrine glands
a cluster of secretory cells which collectively form an acinus
The acini are surrounded by a basement membrane and are held together by tight junctions between secretory cells
The secretory cells possess a highly developed ER and golgi network for material secretion and are rich in mitochondria
Exocrine products
released (via secretory vesicles) into a duct, which connects to an epithelial surface
mechanisms control the secretion of digestive juices
Mechanical, nervous and hormonal mechanisms control the secretion of digestive juices.
The nervous control
mainly provided by the autonomic nervous system
The autonomic system is divided into the sympathetic and parasympathetic systems.
sympathetic and parasympathetic systems
The sympathetic system controls processes involved in responses to danger, while the parasympathetic system controls homeostasis and processes related to rest and digestion.The parasympathetic system speeds up digestion when food is ingested, while the sympathetic slows it down when there is no food available.
The hormones involved in the chemical control of digestion
gastrin, secretin and cholecystokinin (CCK)
secreted in the digestive system and travel through blood to the target organs
Gastrin
produced by special cells (called G cells) in the stomach, duodenum and pancreas
stimulates the production of gastric juice by the parietal cells in the gastric glands.
contains a mixture of water, hydrochloric acid and other inorganic ions, enzymes (pepsin, rennin), mucus, various polypeptides, and intrinsic factor.
Intrinsic factor is necessary for absorbing vitamin B12.
secretin
produced by special cells in the small intestine in response to the presence of acid fluid.
presence of gastric acid activates prosecretin into secretin.
When released, it stimulates the production of alkali by the pancreas, neutralising the intestinal components.
pH of the intestine reaches 8, the production of pancreatic juice is stopped by negative feedback
plays a role in osmoregulation as it regulates water homeostasis throughout the body by acting on the kidney, hypothalamus and pituitary gland.
Cholecystokinin (CCK)
produced by the small intestine when food is present in this organ.
It controls the release of pancreatic juice from the pancreas and bile from the gall bladder (bile is produced in the liver and stored in the gall bladder).
It also acts as a hunger suppressor.
Acid in the stomach
produced by the combination of hydrogen ions (H+) and chloride ions (Cl-) produced by the epithelial cells (known as parietal cells) of the stomach lining. The release of hydrogen ions through protein carriers is coupled to the intake of potassium ions (K+) from the lumen of the stomach. This process requires energy in the form of ATP
Pepsin
a protease that hydrolyses proteins to peptides.
found in the stomach in an inactive form as the pro-enzyme pepsinogen.
Pepsinogen is released by the chief cells in the lining of the stomach and is activated into pepsin by the presence of acid.
The hydrochloric acid released by the parietal cells of the stomach removes 44 amino acids from pepsinogen transforming it into pepsin.
optimum pH for this enzyme is 2
The mucus cover of the stomach
avoids self-damage
gastric ulcer
the lining of the stomach is disrupted
symptoms of an ulcer
stomach pain, heartburn, nausea and in some cases presence of blood in the stools
Helicobacter pylori
produces toxins that cause continuous gastric inflammation. The inflammatory response by the immune system damages the stomach lining.
combination of medications to treat Helicobacter pylori
Amoxicillin: antibiotic that directly inhibits the synthesis of bacterial cell walls.
Clarithromycin: antibiotic that prevents bacteria from growing by inhibiting the translation of peptides in the ribosome, thus inhibiting their protein synthesis.
Proton Pump Inhibitor (PPI): inhibits acidification of stomach.
Proton Pump Inhibitors
block the proton pump (hydrogen-potassium-ATPase pump),
responsible for the secretion of hydrogen ions (H+) in the stomach.
The decrease of H+ into the lumen of the stomach reduces the amount of hydrochloric acid (HCl) produced, which makes the stomach less acidic
wall of the small intestine
lined with villi that increase the surface area for the absorption of substances. Each villus has a central lacteal whose function is mainly the absorption of fats, and capillaries that carry the rest of the absorbed molecules to other organs. Most capillaries join to form the hepatic portal vein that carries blood to the liver. The structure of cells in the epithelium of the villus is adapted to the absorption of food
adaptions of the villus
Increased surface area due to presence of villi and microvilli.
Many mitochondria to provide energy for active transport.
Presence of capillaries in villi for absorption of digested foods.
Presence of lacteal for absorption of absorbed lipids.
Presence of pinocytotic vesicles aid the uptake of fluids.
Presence of tight junctions to ensure molecules do not escape through the membrane, and to maintain a concentration gradient.
Enzymes (peptidases and disaccharidases) bound to epithelial membranes to complete hydrolysis/digestion.
cross section of the wall of the small intestine
Mucosa: contains the epithelium formed by enterocytes, goblet cells and endocrine cells. Enterocytes are cells that have microvilli; they digest and absorb substances. Goblet cells produce mucus and endocrine cells secrete hormones.
Submucosa: contains blood vessels and connective tissue.
Circular muscle layer: smooth muscle arranged in a circular manner.
Longitudinal muscle layer: smooth muscle arranged in a longitudinal manner.
Serosa: single layer of epithelial cells with connective tissue.
Polysaccharides
mainly hydrolysed to glucose and absorbed by epithelial cells of the small intestine
Glucose
co-transported with sodium (Na+) by facilitated diffusion.
Na+ must first be transported out of the cell by active transport (along the basal membrane
proteins
digested by proteases into amino acids, which are transported by co-transport with Na+ into the epithelial cells.
Amino acid carrier proteins then carry them by facilitated diffusion into the capillaries.
Fats (or lipids)
emulsified in the intestines by bile, transforms them into smaller droplets, allowing lipases to digest them into fatty acids and glycerol.
The glycerol will be absorbed and used in carbohydrate metabolism and the fatty acids will diffuse into the small intestine epithelial cells.
Long chain fatty acids will combine with proteins to form chylomicrons. The chylomicrons travel to the border of the cell in the smooth endoplasmic reticulum and are expelled by exocytosis to later enter the lacteal, thus entering the lymphatic system
Dietary fibre
the non-digestible carbohydrates, especially cellulose, and lignin that are intrinsic and intact in plants.
Cholera
disease caused by the ingestion of food or water contaminated with the bacterium Vibrio cholerae.
toxin is released from the bacteria in the infected intestine, it binds to the intestinal epithelial cells (enterocytes), triggering endocytosis of the toxin.The toxin then becomes an active enzyme which activates ions and water to leave the infected enterocytes, leading to watery diarrhea.
The signs and symptoms of dehydration
increased thirst Dry mouth Swelling of brain Weakness Dizziness Palpitations Seizures Drop in blood pressure Fainting Decreased urine output Kidney failure Coma Death
