Structure and function of GIT Flashcards
What are the functions of the serosa, muscularis externa, submucosa, and mucosa in the GI tract
Serosa: The serosa is the outermost layer of the GI tract. It is a thin layer of connective tissue that covers and protects the GI tract. It also helps to anchor the GI tract to surrounding structures in the abdomen.
Muscularis externa: The muscularis externa is the layer of smooth muscle that surrounds the submucosa. It is responsible for the movement of food through the GI tract via peristalsis, which involves contraction and relaxation of smooth muscle. The muscularis externa also regulates the flow of materials through the GI tract and helps to mix and break down food.
Submucosa: The submucosa is a layer of connective tissue that contains blood vessels, lymphatic vessels, and nerves. It helps to nourish the other layers of the GI tract and carries away absorbed nutrients. The submucosa also contains nerve cells that regulate the functions of the GI tract, such as secretion and absorption.
Mucosa: The mucosa is the innermost layer of the GI tract. It is a moist layer of epithelial cells that lines the lumen, or the hollow space within the GI tract. The mucosa is responsible for the secretion of digestive enzymes, mucus, and other substances that aid in the digestion and absorption of food. It also contains specialized cells that produce hormones that regulate digestion and appetite. Additionally, the mucosa is responsible for absorbing nutrients from food and transporting them into the bloodstream.
Explain the physiological functions of salivary, gastric and pancreatic secretions
Salivary secretion: Saliva is produced by the salivary glands and contains enzymes and lubricating agents that help to break down food and facilitate its movement through the digestive tract. Salivary secretion begins the process of digestion by breaking down carbohydrates, and also contains antibodies and antimicrobial agents that help to protect the body against infection. Saliva also helps to keep the mouth moist, which is important for maintaining oral health.
Gastric secretion: Gastric secretion is produced by the gastric glands in the stomach and helps to break down food and facilitate its absorption. Gastric secretion contains hydrochloric acid, which helps to break down proteins and activate digestive enzymes. It also contains enzymes such as pepsin, which help to break down proteins into smaller peptides, and mucus, which helps to protect the stomach lining from the acidic environment.
Pancreatic secretion: Pancreatic secretion is produced by the pancreas and contains enzymes that help to break down all three macronutrients: carbohydrates, proteins, and fats. These enzymes include amylase, which breaks down carbohydrates into simple sugars, proteases, which break down proteins into amino acids, and lipases, which break down fats into fatty acids and glycerol. Pancreatic secretion also contains bicarbonate, which neutralizes the acidic chyme that enters the small intestine from the stomach, creating a more alkaline environment that is better suited for the activity of digestive enzymes.
Describe neural regulation regulating digestion and absorption of nutrients
The cephalic phase: This is the first phase of neural regulation, which is initiated by the sight, smell, and taste of food. The sensory information from the food triggers the activation of the parasympathetic nervous system, which stimulates the release of saliva and digestive enzymes from the salivary glands and pancreas, respectively.
The gastric phase: Once food enters the stomach, the distension of the stomach wall triggers the release of the hormone gastrin, which stimulates the release of gastric acid and digestive enzymes. This phase is also regulated by the vagus nerve, which activates the parasympathetic nervous system to promote gastric motility.
The intestinal phase: As food passes from the stomach into the small intestine, the release of digestive enzymes and the absorption of nutrients is regulated by a combination of neural and hormonal signals. The enteric nervous system, which is a network of neurons in the walls of the gastrointestinal tract, coordinates the contraction and relaxation of the smooth muscle in the digestive tract, facilitating the movement of food through the system. Hormones such as secretin and cholecystokinin (CCK) also stimulate the release of pancreatic juices and bile from the gallbladder, respectively.
Describe hormonal regulation involved in digestion and absorption of nutrients
Gastrin: Gastrin is a hormone that is released by the stomach in response to the presence of food. It stimulates the release of gastric acid and digestive enzymes, as well as the contraction of the stomach muscles. This hormone also promotes the growth and proliferation of the cells lining the stomach and small intestine.
Secretin: Secretin is a hormone that is released by the small intestine in response to the acidity of the chyme (the mixture of food and digestive juices) that enters the duodenum. It stimulates the release of bicarbonate from the pancreas, which neutralizes the acidity of the chyme, and it also inhibits the secretion of gastric acid and stimulates the production of bile from the liver.
Cholecystokinin (CCK): CCK is a hormone that is released by the small intestine in response to the presence of fats and proteins in the chyme. It stimulates the secretion of pancreatic enzymes, including lipase, amylase, and proteases, and it also stimulates the contraction of the gallbladder, which releases bile into the small intestine to aid in the digestion of fats.
Glucagon-like peptide-1 (GLP-1): GLP-1 is a hormone that is released by the small intestine in response to the presence of nutrients in the chyme. It stimulates the release of insulin from the pancreas, which helps to regulate blood sugar levels, and it also slows down gastric emptying, which allows for more efficient digestion and absorption of nutrients.
Ghrelin: Ghrelin is a hormone that is produced by the stomach and stimulates hunger. It also stimulates the release of growth hormone and regulates energy balance.
Local factors affecting gut digestion and absorption of nutrients
Mechanical stimulation: The mechanical stimulation of the gastrointestinal tract by food triggers the release of local factors that stimulate the secretion of digestive enzymes and the contraction of the smooth muscles in the digestive tract. For example, the stretching of the stomach walls activates mechanoreceptors, which stimulate the release of gastrin, a hormone that stimulates the secretion of gastric acid and digestive enzymes.
Chemical stimulation: The chemical stimulation of the gastrointestinal tract by nutrients and other molecules in the chyme also triggers the release of local factors that stimulate the secretion of digestive enzymes and the absorption of nutrients. For example, the presence of fatty acids and amino acids in the chyme stimulates the release of cholecystokinin (CCK), a hormone that stimulates the secretion of pancreatic enzymes and the contraction of the gallbladder to release bile.
Gut microbiota: The gut microbiota, which is the community of microorganisms that live in the gastrointestinal tract, also plays a role in the regulation of digestion and absorption. The microbiota can break down complex molecules that the body cannot digest, such as fiber, and produce short-chain fatty acids that can be absorbed and used for energy. The microbiota can also stimulate the release of local factors that regulate gut motility and secretion of digestive enzymes.
Blood flow: The blood flow to the gastrointestinal tract is also an important local factor in the regulation of digestion and absorption. Increased blood flow to the digestive organs can stimulate the release of digestive enzymes and promote the absorption of nutrients.
Describe the endocrine control of the GIT
Gastrin: Gastrin is produced by G cells in the stomach and stimulates the release of gastric acid, pepsinogen, and other digestive enzymes. It also stimulates the contraction of the smooth muscles in the stomach, which helps to mix and propel food through the digestive tract.
Secretin: Secretin is produced by S cells in the duodenum and stimulates the release of bicarbonate from the pancreas and bile from the liver. Bicarbonate neutralizes the acidity of the chyme as it enters the small intestine, and bile helps to emulsify fats so that they can be more easily digested.
Cholecystokinin (CCK): CCK is produced by I cells in the small intestine and stimulates the release of pancreatic enzymes, such as lipase and amylase, and the contraction of the gallbladder to release bile. CCK also inhibits gastric emptying, which allows for more efficient digestion and absorption of nutrients.
Glucagon-like peptide-1 (GLP-1): GLP-1 is produced by L cells in the small intestine and stimulates the release of insulin from the pancreas, which helps to regulate blood sugar levels. GLP-1 also slows down gastric emptying, which allows for more efficient digestion and absorption of nutrients.
Ghrelin: Ghrelin is produced by the stomach and stimulates hunger. It also stimulates the release of growth hormone and regulates energy balance.
Explain the physiological mechanisms that regulate the secretion of bile and role of the enterohepatic circulation
Neural regulation: The secretion of bile is regulated by the autonomic nervous system, with the parasympathetic nervous system stimulating its secretion. Parasympathetic nerve fibers stimulate the release of acetylcholine, which stimulates the contraction of the gallbladder and relaxation of the sphincter of Oddi, allowing bile to flow into the duodenum.
Hormonal regulation: The secretion of bile is also regulated by hormones, including cholecystokinin (CCK) and secretin. CCK is released by the small intestine in response to the presence of fatty acids and amino acids in the chyme, and stimulates the contraction of the gallbladder to release bile. Secretin is released by the small intestine in response to the acidity of the chyme, and stimulates the liver to produce more bile.
Enterohepatic circulation: The enterohepatic circulation is a physiological mechanism that recycles bile acids from the small intestine back to the liver. Bile acids are necessary for the digestion and absorption of fats, and are synthesized from cholesterol in the liver. After they are secreted into the small intestine, they are reabsorbed in the terminal ileum and transported back to the liver via the portal vein. This process helps to maintain the concentration of bile acids in the body, and also helps to conserve cholesterol.
Explain the role of chief cells in the GIT
Chief cells are specialized cells located in the gastric glands of the stomach. They are responsible for secreting pepsinogen, the inactive form of the digestive enzyme pepsin.
Pepsinogen is secreted into the gastric gland lumen, where it is activated by the acidic environment of the stomach. Once activated, pepsin can break down proteins into smaller peptides that can be further digested by other enzymes in the gastrointestinal tract.
In addition to producing pepsinogen, chief cells also secrete lipase, an enzyme that helps break down fats in the stomach. However, the majority of fat digestion occurs in the small intestine, where pancreatic lipase and other enzymes are secreted.
Overall, chief cells play an important role in the digestion of proteins in the stomach by producing and secreting pepsinogen, which is subsequently activated to pepsin, an enzyme that is critical to the breakdown of proteins.
