FC #3 B Digestive System

What is Secretin and Enterogastrone, and CCK?

"Secretin is a peptide hormone that causes pancreatic enzymes to be released into the duodenum. It also regulates the pH of the digestive tract by reducing HCl secretion from parietal cells and Secretin is also an enterogastrone, a hormone that slows motility through the digestive tract. Slowing of motility allows increased time for digestive enzymes to act on chyme—especially fats. Finally, cholecystokinin (CCK) is secreted in response to the entry of chyme (specifically, amino acids and fat in the chyme) into the duodenum. This peptide hormone stimulates the release of both bile and pancreatic juices and also acts in the brain, where it promotes satiety." CCK also promotes the secretion of pancreatic juices into the duodenum.

Discuss how the liver has a role in blood glucose concentration regulation

"The liver also receives all blood draining from the abdominal portion of the digestive tract through the hepatic portal vein. This nutrient-rich blood can be processed by the liver before draining into the inferior vena cava on its way to the right side of the heart. For example, the liver takes up excess sugar to create glycogen, the storage form of glucose, and stores fats as triacylglycerols. The liver can also reverse these processes, producing glucose for the rest of the body through glycogenolysis and gluconeogenesis and mobilizing fats in lipoproteins."

Discuss how glands help in the stomach to digest food

- The pyloric glands contain G cells that secrete gastrin, a peptide hormone. - Gastrin induces the parietal cells in the stomach to secrete more HCl and signals the stomach to contract, mixing its contents. - The digestion of solid food in the stomach results in an acidic, semifluid mixture known as chyme. The combined mechanical and chemical digestive activities of the stomach result in a significant increase in the surface area of the now unrecognizable food particles, so when the chyme reaches the intestines, the absorption of nutrients from it can be maximized.

Discuss how Bile is stored in the Gall Bladder

Bile is produced in the liver and travels down these bile (common Hepatic) ducts where it may be stored in the gallbladder or secreted into the duodenum. Then it goes into the cystic duct, and then into the gall bladder to be stored. This is the only purpose of the gall bladder - and release bile when it's needed by a hormone called Colasitickina (CCK). CCK causes the gallbladder to contract and release the bile. Then bile goes through the Common Bile Duct until it reaches the GI tract.

Discuss how the liver is a part of the production of bile

Bile production is one of the most significant jobs of the liver vis-à-vis the digestive system. Bile is composed of bile salts, pigments, and cholesterol. Bile salts are amphipathic molecules that can emulsify fat in the digestive system. The major pigment in bile is bilirubin, which is a byproduct of the breakdown of hemoglobin. Bilirubin travels to the liver, where it is conjugated (attached to a protein) and secreted into the bile for excretion. If the liver is unable to process or excrete bilirubin (from liver damage, excessive red blood cell destruction, or blockage of the bile ducts), jaundice or yellowing of the skin may occur.

Describe the function of Bile

Bile salts are amphipathic molecules that can emulsify fat in the digestive system. Bile emulsifies fats, making them soluble and increasing their surface area. The salt is what helps them emulsify (absorb fat). Turns fat into Mycells which is then able to be absorbed in the illium. Bile salts are derived from cholesterol. They are not enzymes and therefore do not directly perform chemical digestion (the enzymatic cleavage of chemical bonds). However, bile salts serve an important role in the mechanical digestion of fats and ultimately facilitate the chemical digestion of lipids. Bile salts have hydrophobic and hydrophilic regions, allowing them to serve as a bridge between aqueous and lipid environments. In fact, bile salts are much like the common soaps and detergents we use to wash our hands, clothes, and dishes. In the small intestine, bile salts emulsify fats and cholesterol into micelles. Without bile, fats would spontaneously separate out of the aqueous mixture in the duodenum and would not be accessible to pancreatic lipase, which is water-soluble. In addition, these micelles increase the surface area of the fats, increasing the rate at which lipase can act. Ultimately, proper fat digestion depends on both bile and lipase. Bile gets the fats into the solution and increases their surface area by placing them in micelles (mechanical digestion). Then, lipase can come in to hydrolyze the ester bonds holding the lipids together (chemical digestion)."

Discuss how the small intestine produces and uses enzymes as it acts as a site of digestion.

Brush-border enzymes are present on the luminal surface of cells lining the duodenum and break down dimers and trimers of biomolecules into absorbable monomers. The duodenum also secretes enteropeptidase, which is involved in the activation of other digestive enzymes from the accessory organs of digestion. Finally, it secretes hormones like secretin and cholecystokinin (CCK) into the bloodstream. The disaccharidases digest disaccharides. Maltase digests maltose, isomaltase digests isomaltose, lactase digests lactose, and sucrase digests sucrose. Lack of a particular disaccharidase causes an inability to break down the corresponding disaccharide. Then bacteria in the intestines are able to hydrolyze that disaccharide, producing methane gas as a byproduct. In addition, undigested disaccharides can have an osmotic effect, pulling water into the stool and causing diarrhea. This is why people who are lactose intolerant have symptoms of bloating, flatulence, and possibly diarrhea after ingesting dairy products. Peptidases break down proteins (or peptides, as the name implies). Aminopeptidase is a peptidase secreted by glands in the duodenum that removes the N-terminal amino acid from a peptide. Dipeptidases cleave the peptide bonds of dipeptides to release free amino acids. Unlike carbohydrates, which must be broken down into monosaccharides for absorption, di- and even tripeptides can be absorbed across the small intestine wall. Enteropeptidase (formerly called enterokinase) is an enzyme critical for the activation of trypsinogen, a pancreatic protease, to trypsin. Trypsin then initiates an activation cascade. Enteropeptidase can also activate procarboxypeptidases A and B to their active forms.

Describe Cirrhosis of the liver

Cirrhosis of the liver can occur from many different processes, including chronic alcohol consumption, hepatitis C infection, autoimmune hepatitis, and fatty liver disease. However, the outcome is the same. Cirrhosis is scarring of the liver, and this scar tissue builds up, creating increased resistance within the portal vein, resulting in portal hypertension. This causes a backup of fluid within the portal system, resulting in swollen veins in the digestive system, especially the esophagus, which may rupture and cause life-threatening bleeding. This often manifests as hematemesis, or vomiting of blood. Cirrhosis also causes bleeding disorders because production of clotting factors is disrupted. The inability to properly dispose of ammonia results in increased ammonia in the blood, which affects mentation. Finally, cirrhosis may also cause hepatocellular carcinoma, or cancer of the liver.

Discuss how the small intestine absorbs food molecules and water.

Food leaves the stomach through the pyloric sphincter and enters the duodenum. The presence of chyme in the duodenum causes the release of brush-border enzymes like disaccharidases (maltase, isomaltase, lactase, and sucrase) and peptidases (including dipeptidase) (more in other notecard). The jejunum and ileum are involved in the absorption of nutrients. Simple sugars, such as glucose, fructose, and galactose, and amino acids are absorbed by secondary active transport and facilitated diffusion into the epithelial cells lining the small intestine, as shown in Figure 9.7. Then, these substances move across the epithelial cells into the intestinal capillaries. Blood is constantly passing by the epithelial cells in the capillaries, carrying the carbohydrate and amino acid molecules away from the epithelial cells. This creates a concentration gradient such that the blood always has a lower concentration of monosaccharides and amino acids than inside the epithelial cells. Thus, simple carbohydrates and amino acids diffuse from the epithelial cells into the capillaries. The absorbed molecules then go to the liver via the hepatic portal circulation. Small fatty acids will follow the same process as carbohydrates and amino acids by diffusing directly into the intestinal capillaries. These fatty acids do not require transporters because they are nonpolar, so they can easily traverse the cellular membrane. Larger fats, glycerol, and cholesterol move separately into the intestinal cells but then reform into triglycerides, as shown in Figure 9.8. The triglycerides and esterified cholesterol molecules are packaged into insoluble chylomicrons. Rather than entering the bloodstream, they enter the lymphatic circulation through lacteals, small vessels that form the beginning of the lymphatic system. These lacteals converge and enter the venous circulation through the thoracic duct in the base of the neck, which empties into the left subclavian vein. Vitamins are also absorbed in the small intestine. Vitamins can be categorized as either fat-soluble or water-soluble. Because there are only four fat-soluble vitamins (A, D, E, and K), these can be easily memorized. All other vitamins (B complex and C) are water-soluble. Fat-soluble vitamins dissolve directly into chylomicrons to enter the body. Failure to digest and absorb fat "properly, which can be due to pathologies in the liver, gallbladder, pancreas, or small intestine, may lead to deficiencies of fat-soluble vitamins. The water-soluble vitamins are absorbed, along with water, amino acids, and carbohydrates, across the endothelial cells of the small intestine, passing directly into the plasma. In addition to fats, carbohydrates, amino acids, and vitamins, the small intestine also absorbs water. Much of the water in chyme is actually the result of secretions. The average person may consume up to two liters of fluid per day, but secretions into the upper gastrointestinal tract may total up to seven liters of fluid per day. In order to maintain proper fluid levels within the body, much of this fluid must be reabsorbed by osmosis. As solutes are absorbed into the bloodstream, water is drawn with it, eventually reaching the capillaries. Water passes not only transcellularly (across the cell membrane), but also paracellularly (squeezing between cells) to reach the blood."

Discuss how the Large intestine absorbs Water

It's main function is to absorb water and salts (such as sodium chloride) from the undigested material left over from the small intestine. The small intestine actually absorbs much more water than the colon, so the colon primarily concentrates the remaining material to form feces. Too little or too much water absorption can cause diarrhea or constipation, respectively." Excerpt From: Kaplan. "Kaplan MCAT Biology Review: Created for MCAT 2015 (Kaplan Test Prep)." iBooks.

Discuss how the small intestine neutralizes stomach acid.

Pancreatic juices are a complex mixture of several enzymes in a bicarbonate-rich alkaline solution. This bicarbonate helps to neutralize acidic chyme, as well as provide an ideal working environment for each of the digestive enzymes, which are most active around pH 8.5. Pancreatic juices contain enzymes that can digest all three types of nutrients: carbohydrates, fats, and proteins.

Discuss how the Pancreas Transport enzymes to small intestine

Pancreatic juices are transferred to the duodenum via a duct system that runs along the middle of the pancreas. Like all exocrine cells, acinar cells secrete their products into ducts. These ducts then empty into the duodenum through the major and minor duodenal papillae."

Describe Pancreaitis

Pancreatitis, or inflammation of the pancreas, is usually caused by gallstones or excessive consumption of alcohol. Regardless of the cause, pancreatitis results from premature activation of pancreatic enzymes and autodigestion of the pancreatic tissue. This is a very painful condition that may result in a long hospital stay and long-term consequences such as diabetes and the reduced digestion of proteins and fats.

Discuss how the Stomach produces digestive enzymes and is a site of digestion

Pepsinogen --> Pepsin when activated by HCl. This only is a site of digestion for proteins - as Pepsin is a proteolase. Because pepsin is activated by the acidic environment, it follows that pepsin is most active at a low pH. This is a unique characteristic among human enzymes, as most human enzymes are most active at physiological pH. Stomach acid also kills most harmful bacteria (with the exception of Helicobacter pylori, infection with which is usually asymptomatic but can cause inflammation, ulcers, and even certain gastric cancers). The acidic environment also helps to denature proteins and can break down some intramolecular bonds that hold food together. In addition to HCl, parietal cells secrete intrinsic factor, a glycoprotein involved in the proper absorption of vitamin B12

Discuss Saliva as lubrication and source of enzymes

Saliva is secreted by 3 glands in order to lubricate the mouth and throat, and break down food via enzymes. - The goal is to make a bolus. Chewing = mastication Major enzyme: Alpha Amylase - breaks down starches & small carbohydrates (Amylose in particular). The glands primarily release Serous content (enzyme and salt rich content). They also can release mucinous things - which serves as a wetting mechanism making it easier to swallow. These glands are innervated by the parasympathetic nervous system. Glands (% of saliva released): Parotid glands - 25% Submandibular glands - 70% mainly serous Sublingual glands - sits below tongue j- 5% of saliva ^^ all release Alpha Amylase - which takes starch and breaks it down to smaller carbohydrates. Von ebners gland >5% - release an enzyme called Lingual Lipase which breaks down lipids (triglycerides into free fatty acids and diglycerides or monoglycerides). Neutral pH - but hydroxyl groups can be acidic so saliva can get acidic. But this can demineralize our teeth.

Discuss how the Pancreas produces enzymes

The bulk of the pancreas is made of exocrine cells called acinar cells that produce pancreatic juices. As mentioned earlier, pancreatic juices are bicarbonate-rich alkaline secretions containing many digestive enzymes that work on all three classes of biomolecules. Pancreatic amylase breaks down large polysaccharides into small disaccharides and is therefore responsible for carbohydrate digestion. The pancreatic peptidases (trypsinogen, chymotrypsinogen, and carboxypeptidases A and B) are released in their zymogen form, but once activated are responsible for protein digestion. Enteropeptidase, produced by the duodenum, is the master switch. It activates trypsinogen to trypsin, which can then activate the other zymogens, and also activates procarboxypeptidases A and B to their active forms. Finally, the pancreas secretes pancreatic lipase, which is capable of breaking down fats into free fatty acids and glycerol.

Describe Ingestion (esophagus, transport function)

The food first goes in through the mouth (chewing) and hydrolysis. The pharynx connects not only to the esophagus, but also to the larynx, which is a part of the respiratory tract. The pharynx can be divided into three parts: the nasopharynx (behind the nasal cavity), the oropharynx (at the back of the mouth), and the laryngopharynx (above the vocal cords). Food is prevented from entering the larynx during swallowing by the epiglottis, a cartilaginous structure that folds down to cover the laryngeal inlet. Failure of this mechanism can lead to aspiration of food and choking. Next it goes through the esophagus. Upper Sphinctor = circular localization of muscle - that's closed unless it's told to relaxed. It's composed of the skeletal muscle (it's under our control. Lower Esophageal Sphincter - not actually a sphincter because it's not a ring of muscle - instead it's a sheet that sits there, a sheet of muscle, that lines the connection between the thoracic and abdominal cavity. Diaphragm makes a ring around the sphincter, it holds the esophagus in place (if it moves upwards/downwards it gives heart burn or GRD). "lower esophageal sphincter (cardiac sphincter) relaxes and opens to allow the passage of food." Peristalsis = wavelike propulsion of food. You have the esophagus contraction upwards, and relaxes downward - so the food wants to move downwards. **The enteric nervous system is a collection of one hundred million neurons that governs the function of the gastrointestinal system. These neurons are present in the walls of the digestive tract and trigger peristalsis, or rhythmic contractions of the gut Esophagus is split into 3rds 1. Skeletal muscle (top) 2. Mix of skeletal muscle and smooth muscle (middle). 3. Smooth muscle (bottom/last 1/3).

Discuss how the Stomach stores and churns food

The human stomach can store up to 4 liters of food at any time. The stomach can take the bolus and churn it using the muscular walls. this breaks up the food more. The stomach can store food even after breaking it down; until it's time for the chyme to be squirted into the small intestine.

Describe the Structure of the Large Intestine

The large intestine has a larger diameter but shorter length than the small intestine. It is divided into three major sections: the cecum, the colon, and the rectum. The cecum is simply an outpocketing that accepts fluid exiting the small intestine through the ileocecal valve and is the site of the appendix. The appendix is a small fingerlike projection that was once thought to be vestigial, although recent evidence suggests that it may have a role in warding off certain bacterial infections and repopulating the large intestine with normal flora after episodes of diarrhea. The colon itself is divided into the ascending, transverse, descending, and sigmoid colons. Finally, the rectum serves as a storage site for feces, which consists of indigestible material, water, bacteria (E. coli and others), and certain digestive secretions that aren't reabsorbed (enzymes and some bile). The anus is the opening through which wastes are eliminated and consists of two sphincters: the internal and external anal sphincters. The external sphincter is under voluntary control (somatic), but the internal sphincter is under involuntary control (autonomic).

Discuss how the large intestine has a Flora of Bacteria

The large intestine—and even the small intestine—is home to many different species of bacteria. In fact, 30 percent of the dry matter in stool consists of bacteria. Most of these bacteria are anaerobes, but the cecum is also home to many aerobic bacteria. This relationship is symbiotic: the bacteria are provided with a steady source of food, and the byproducts produced by the bacteria are beneficial to humans. For example, bacteria in the gut produce vitamin K, which is essential for the production of clotting factors, and biotin (vitamin B7), which is a coenzyme for many metabolic enzymes.

Discuss the relationship of the liver and the gastrointestinal system

The liver contains two unique structures for communicating with the digestive system. 1. Bile ducts connect the liver with both the gallbladder and small intestine.

Discuss how the liver has a role in detoxification

The liver detoxifies both endogenous compounds (those made in the body) and exogenous compounds (those brought in from the environment). For example, the liver modifies ammonia, a toxic waste product of amino acid metabolism, to urea, which can be excreted by the kidneys. The liver also detoxifies and metabolizes alcohol and medications. Some drugs actually require activation by the enzymes of the liver. In addition, some drugs cannot be taken orally because modification of these drugs by the liver renders the drugs inactive." Excerpt From: Kaplan. "Kaplan MCAT Biology Review: Created for MCAT 2015 (Kaplan Test Prep)." iBooks.

Discuss how the Stomach has a low pH, gastric juice, and mucous protection against self destruction

The mucosa of the stomach contains gastric glands and pyloric glands. The gastric glands respond to signals from the vagus nerve of the parasympathetic nervous system, which is activated by the brain in response to the sight, taste, and smell of food. " The stomach is lined with infoldings - infoldings of the gastric wall that increase surface area. There's a layer of cells that sit around and secrete components of the gastric juice. There are 3 types of cells involved. 1. Parietal Cells: release hydrochloric acid (HCl) - very corrosive - more than battery acid. 2. Sheath Cells: secrete enzymes called Pepsinogen (which is the inactive form) (Pepsin is the active form which is activated by HCl). 3. Mucous cells: release Mucin - which is a coating that sits around the stomach to make sure pepsin and hydrochloric acid don't degrade the stomach Mucous cells produce the bicarbonate-rich mucus that protects the muscular wall from the harshly acidic (pH = 2) and proteolytic environment of the stomach.

Discuss the structure of the Small Intestine and the function and structure of the Villi

The small intestine consists of 3 segments - called the duodenum, the jejunum, and the ileum. The small intestine is very long (up to 7m), and the duodenum is responsible for most of the chemical digestion with some minor involvement in absorption. Most absorption takes place in the jejunum and ileum. The small intestine is lined with villi, which are small, fingerlike projections from the epithelial lining. Each villus has many microvilli, drastically increasing the surface area available for absorption. In addition, at the middle of each villus is both a capillary bed for the absorption of water-soluble nutrients and a lacteal, a lymphatic channel that takes up fats for transport into the lymphatic system.

Describe the structure (gross) of the Stomach

The stomach is lined with infoldings - infoldings of the gastric wall that increase surface area. There's a layer of cells that sit around and secrete components of the gastric juice. There are 3 types of cells involved. 1. Parietal Cells: release hydrochloric acid (HCl) - very corrosive - more than battery acid. 2. Sheath Cells: secrete enzymes called Pepsinogen (which is the inactive form) (Pepsin is the active form which is activated by HCl). 3. Mucous cells: release Mucin - which is a coating that sits around the stomach to make sure pepsin and hydrochloric acid don't degrade the stomach. Pepsin breaks down proteins. Degrades protein macromolecules - it's the only nutrient that's broken down in your stomach. "The stomach can be divided into four main anatomical divisions, as shown in Figure 9.2: the fundus and body, which contain mostly gastric glands, and the antrum and pylorus, which contain mostly pyloric glands. The internal curvature of the stomach is called the lesser curvature; the external curvature is called the Greater Curvature. The lining of the stomach is thrown into folds called rugae.

What are the 6 products the stomach secretes (general)?

The stomach secretes six products: HCl (kills microbes, denatures proteins, and converts pepsinogen into pepsin) Pepsinogen (cleaved to pepsin in the stomach; an enzyme that partially digests proteins) Mucus (protects mucosa) Bicarbonate (protects mucosa) Water (dissolves and dilutes ingested material) Intrinsic factor (required for normal absorption of vitamin B12)"

Describe how the mouth and the teeth aid in digestion

They provide the place for food to go from large molecules to broken down molecules. The tongue helps greatly.

Discuss the hormonal aspects of the pancreas

the endocrine functions of the pancreas include the release of insulin, glucagon, and somatostatin—peptide hormones necessary for the maintenance of proper blood sugar levels. The hormonal function of the pancreas is limited to cells residing in islets of Langerhans scattered throughout the organ.