Lecture 28 Flashcards
What are endocrine and exocrine secretions and what roles do exocrine secretions play in the GI tract?
Endocrine (secreting across basal lamina and into the blood), exocrine (secreted out onto the body surface). Exocrine secretions digest food, dilute food, maintain an optimal pH and protect and lubricate of the epithelial surface of the GI tract, they mainly consist of enzymes, mucus and electrolyte (serous) solution.
How is saliva produced? What does it act to do?
Saliva is produced from 3 pairs of slaivary glanda, it is done in two stages: primary secretion is the release of iotonic, serous solution from the acinr cells and then reabsorption in duct to produce the final hypotonic saliva.It provides lubrication for chewing and swallowing, increases hygiene, dissolves food and allows for tasting, however it is not strictly necessary.
How much saliva is produced and what does it consist of? How is it regulated?
Saliva production is increased when stimulated heavily (from 0.3 ml per minute at basal rate to 1.5 ml per minute when food is introduced). It is comprised of mucus (lubrication), dilute solution of NaHCO3/NaCl to maintain an optimum pH and also the enzymes alpha amylase and lipase.It is regulated via the nervous system (thought, smell, sight of food and presence of food in the mouth or chewing, it occurs via the autonomic nervous system, the parasympathetic secretes acetylcholine for large volumes or the sympathetic for a small volume of viscous fluid via adrenaline (they are additive).
How fast do gastric secretions occur, what do they contain and what do the components do?
Gastric secretions occur at a rate of 15-30 mls per hour of mainly mucous. When there is food in the stomach or eating is occuring roughly 150 mls per hour of 150 mmol per litre of HCl, leading to a pH of one, mucus, pepsinogen and intrinsic factor is secreted. The mucus functions for protection against abrasion and acid. Intrinsic factor aids in absorption of vitamin B12 in the small intestine. Pepsin is secreted in the inactive form pepsinogen before being converted to pepsin by acid, it acts to start digestion of protein. The acid denatures the proteins, activates pepsinogen, optimises pH for pepsin, provides protection against antigens and dilutes the food.
What do each cell type in the GI tract secrete?
Chief cells secrete pepsinogen, surface cells secrete mucus, parietal cells secrete acid and intrinsic factor.
What are the three phases of gastric secretion and what is the first one in detail?
Regulation of gastric secretion has three phases (cephalic, gasrtic and intestinal).
The cephalic phase makes up 30-40% and is controlled by the brain, the stimuli are the thought, smell or sight of food as well as the act of chewing or tasting. The parasympathetic nervous system via the vagus nerve stimulates the enteric nervous system, this stimulates the parietal cells and stimulats gastrin production.
What are the key points for the gastric phase?
The gastric phase sensures sufficient secretion to handle the ingested food, the signal comes from the stomach and makes up about 60%% of secretion associated with a meal. It occurs via the local (via enteric nervous system sensing stomach wall stretch) and external reflex (signal goes back to brain stem and then back down the parasympathetic nervous system. These both stimulate the parietal cells and gastrin production.
What are the key points about the intestinal phase?
The intestinal phase is when the intestine regulaates secretion, it makes up 5-10% of secretion. It is the control of acid delivered to small intestine. it is stimulated by chyme entering the duodenum (distension of duodenum due to arrival of acid and products of digestion). It leads to Gastric inhibitory peptide release cholecystokinine and secretin to the parietal cells to inhibit secretion from parietal cells, and also to the enterogastric reflex via the vagus nerve.
What does the pancreas produce and what roles do they play?
The pancreas produces enzymes via acinar cells and alkaline fluid (bicarbonate) via duct cells to regulate pH in the intestine. The enzymes are lipase (fats), amylase (carbohydrates), trypsin and chymotrypsin (proteins). They function to chemically digest food material and are the reason the pancreas is the most important source of digestive enzymes.
The enzymes are secreted as inactive forms, in the small intestine trypsinogen is converted to trypsin by enterokinase and is then able to activate other enzymes as well as break down proteins. The bicarbonate acts to neutralise the chyme.
How is pancreatic release controlled?
Pancreatic secretion is slow during fasting, hormonal regulation occurs during the meal via cholecystokinine stimulating acinar cells and secretin stimulating duct cells.
1
What does bile contain and how is it released?
bile contains excretory products (bile pigments, cholesterol) and products associated with digestion (bile salts and lecithin (fat digestion and absorption) and bicarbonate (more neutralisation) rich fluid). It is secreted constantly and stored in the gallbladder (more secretion when food is present in the small intestine due to cholecystokinine).
What do the small and large intestine release?
The small intestine secretes mucus and sodium bicarbonate (neutralisation and dilution of food). The large intestine secretes mucus.
