ALS 2304 Week 10: Digestive Physiology and Immunology
ALS 2304 Week 10: Digestive Physiology and Immunology ALS 2304
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This 15 page Class Notes was uploaded by Mara DePena on Sunday April 3, 2016. The Class Notes belongs to ALS 2304 at Virginia Polytechnic Institute and State University taught by Dr. Cline in Spring 2016. Since its upload, it has received 12 views. For similar materials see Animal Physiology and Anatomy in Agricultural & Resource Econ at Virginia Polytechnic Institute and State University.
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ALS 2304 DIGESTIVE PHYSIOLOGY DIGESTIVE SYSTEM Alimentary canal/GI Tract- Essentially a tube from your mouth to your butt. o Intrinsic controls Nerve plexuses near GI tract Short reflexes mediated by local enteric plexuses (gut brain) o Extrinsic controls- Central nervous system regulates what the gut brain is doing. It will either accelerate the process of digestion or turn it off. Long reflexes arising within or outside of the GI tract. Involve CNS centers and extrinsic autonomic nerves. Ingestion- Swallowing food. Propulsion- Moving food through the canal. Mechanical digestion- Grinding of teeth on food, kneading around in stomach. Chemical digestion- Breakdown of food. Absorption- Nutrients are pulled from the food and dumped into the bloodstream. o Takes place along the walls. Defecation- Eliminating unused nutrients and waste products. ENTERIC NERVOUS SYSTEM Endocrine cells release hormones. Distension/stretching of the canal- Activates mechanoreceptors. Chemoreceptors respond to the amount of acid in the canal. They bind to food and look at the degree of digestion. Secretory cells found all throughout the alimentary canal secrete digestive enzymes, acids, and mucus into the lumen. Sensory cells branch out from the gut brain. Digestive process is automatic. If you put something in the oral cavity, it will automatically be digested if you cut away the autonomic nervous system. Parasympathetic nervous system enhances the digestive process. The sympathetic nervous system inhibits the digestive process. An automatic mechanism starts the digestive process. PERISTALSIS AND SEGMENTATION Peristaltic waves are beltlike contractions along that canal that squeeze food down the canal. Once triggered, they go all the way down the canal. Segmentation- Bands of alimentary canal that all spontaneously contrict at one point. o Maximum contact with walls of the canal to aid with process of absorption. HUNGER AND SATIETY Stretch receptors respond to distention. Pathway from small intestine to nucleus of solitary tract, from NTS to arcuate nucleus at the base of the hypothalamus. This nucleus is a collection of soma which acts as a relay center and doesn’t affect appetite on its own. In the arcuate nucleus are two important neuron types. o Neurons that secrete neuropeptide Y Activated by NTS when there is a lack of distension Causes hunger Activates PVN and lateral hypothalamus o Neurons that release POMC When duodenum stretches, sends a signal to the NTS which communicates with the arcuate nucleus and turns on POMC release. Causes satiety Activates DMN, VMN, and PVN If you eat a bunch of food really fast it takes time for it to go to the duodenum, which will fill slowly and turn off your appetite eventually. This is how you could gain weight. Eating too fast, you’ll feel way too full. Arcuate nucleus neurons: ventral medial nucleus, dorsal medial nucleus, paraventricular nucleus, lateral hypothalamic area. o Lateral hypothalamus- Activated, makes you feel hungry. o DMN, VMN- Makes you feel full. o PVN- Sometimes when it fires, the animal is hungry, other times the animal is satiated. ORAL CAVITY Where ingestion occurs. Digestion begins when food mixes with saliva, which has the very first digestive enzyme, salivary amaylase. It breaks down carbohydrates. Breaks glycogen down. Salivary reflex causes saliva to be secreted from the salivary gland. Flows from taste bud to NTS to superior salivatory nucleus, through nerve, to salivary gland to release saliva. If you smell food, you will salivate. Swallowing Buccal phase- When food is in the oral cavity and the tongue is moving it around. Tongue motion/phase is under conscious control. Pharyngeal-espohageal phase- Unconscious control. Tongue mashes food down through the esophagus. Peristaltic wave. Anything from here to the rectum is unconscious. o If you choke, that means food went into your trachea. The epiglottis covers your trachea to prevent this. o Wave reaches stomach. Cardiac/gastroesophageal sphincter is closed when food arrives. There is a circuit that runs forward that causes dilation. The food can then move down into the stomach. Distal pharyngeal esophagus phase o Acid reflux- Cardiac sphincter doesn’t close up, acid goes up into the esophagus. Stand up and gravity pulls the acid down. STOMACH Has three layers of muscle that help with contraction. When food is digested it is considered chyme. It is food mixed with HCl. Stomach lining o Gastric pit gives way to the gastric gland. Inside the gastric gland is the G-cell. Below that are mucus neck cells that secrete mucus. Below that are parietal cells that produce HCl and are found in the wall’s mucosa. Below that are chief cells that release pepsinogen. Pepsinogen- A zymogen. It is an inactive enzyme. It is the inactive form of pepsin. Pepsin breaks bonds between amino acids/breaks down proteins. Pepsin is activated from pepsinogen when it encounters HCl. The most important aspect of HCl is the activation of digestive enzymes. It is released as pepsinogen so the pepsin doesn’t digest the chief cell that secreted it. Stages of the digestive process: o Cephalic phase Cephalic means head. Brain is the major regulator during this phase. Animal sees food, tastes food, smells food, hear food- thinks about food. Those are initiators of the cephalic phase. They cause activation of the dorsal motor nucleus of the vagus nerve. Two pathways. First releases Ach onto parietal cell. Second releases GRP onto the G cell. The G cell then secretes gastrin which feeds back and causes the parietal cell to be further activated. The cell excretes protons which helps to make HCl. They do not always both occur at the same time. This is inhibited by increased sympathetic outflow, including stressful events and depression. o Gastric phase The stomach is the major regulator of the digestive process. Something in the stomach causes distension of the stomach wall. Everything in the cephalic phase continues to happen. Stretch receptors send information back up to the central system. They send them backwards through the vagus nerve to the vagal nucleus. The vagal nucleus then increases vagal outflow, increasing the frequency of Ach and GRP. This is known as a vagal-vagal reflex (information goes up and then back down.) If we cut the vagus nerve, info can’t go up or down, but the stomach can still release HCl and gastrin, because stretch receptors (local reflexes) can also release Ach. Much more HCl is secreted. o Intestinal phase The small intestine is the regulator of the digestive process. Most complicated phase. Satiety mechanism is a part- duodenum is distended, you feel full. Pyloric sphincter opens and injects the chyme into the duodenal lumen. This causes distension. Many hormones are released into local circulation. They feed back onto the stomach and onto the G cell and parietal cell. Some hormones stimulate and some inhibit. They either enhance or stop contractility of the stomach. HCls major function is to activate the zymogens. If the nutrients aren’t digested enough, hormones will be released that augment digestion. If stuff coming out is too digested, hormones will cause inhibition to stop the production of HCl. Too much HCl can burn the mucus layers. Need the right amount- enough to turn on digestive enzymes but to keep you from digesting yourself. o If you cut the vagus nerve, you will delay the digestive process slightly; the first stage wouldn’t occur. o These can all occur simultaneously. Parietal cell o Produces hydrochloric acid. o Chlorine comes through a chlorine channel while proton comes through a proton pump. Once these get into the nucleus they form HCl. o The cell cannot produce HCl in itself and exocytosis. It does not secrete HCl itself, it secretes Cl and H which are combined in the mucus. o As a hydrogen ion goes out, K goes in. Proton pump burns ATP. Potassium cycles. If it is turned on, everything else in the parietal cell happens (Downhill biochemical process). o Chlorine comes from capillary bed and enters cell through another antiporter. As soon as Chlorine goes in, HCO3 (bicarbonate) goes out and is picked up by the circulatory system. This buffers the blood against pH changes. Bicarbonate is formed when water diffuses into the cell and breaks into a hydroxyl group, liberating a proton. This proton goes through the pump. The hydroxyl group is not stable, so carbonic anhydrase takes the hydroxyl group and mashes it together with carbon dioxide. This forms the bicarbonate ion. Without this ion, the pump would not function. Sources of CO2 = Kreb’s cycle and air o Regulating HCl Go: Ach binds to muscarinic receptor, which also opens a calcium channel Gastrin binds to CCK receptor These are activated, liberating a Gaq and activating phospholipase C in the membrane. The phospholipase C takes PIP2 and liberates DAG and IP3. IP3 goes into the ER and causes the release of calcium which turns on the hydrogen pump. DAG activates PKC which also turns on the pump. Histamine- Binds receptor, liberates Gas which turns on AC, which activates cAMP, which activates PKA and turns on the proton pump . PKA is the major mechanism which turns on the pump. It turns it on to a much higher magnitude. Stop: Somatostatin comes in and binds to receptor, liberates Gai which turns down AC. At the same time, prostaglandin binds to its receptor and does the same. Inhibiting AC turns down PKA which turns down the proton pump. o Relation of parietal and enterochromaffin like (ECL) cells Vagus nerve arrives through gut brain, releases Ach on parietal cell. Gastrin binds to CCK receptor on ECL and parietal cell, encourages release of histamine on ECL cell. ECL cell hangs out near parietal cell. Branch of vagus nerve dumps Ach onto this ECL cell. ECL cell responds by producing and secreting histamine. Histamine is dumped into the extracellular space, which diffuses over and binds to the histamine receptor. This is the origin of the histamine that was mentioned previously. G cell, just south is the D cell. There is also a D cell to the right of the ECL cell. In different parts of the stomach. Both have access to what’s in the lumen of the stomach. Pick up chemistry of content of stomach. D cell picks up pH, G cell looks at chemistry of the food. GRP receptor on G cell causes it to secrete gastrin when bound. Gastrin goes into local circulation, feeds back and binds on CCK receptor which turns on the proton pump. Some of the gastrin also binds to the CCK receptor on the ECL cell and causes the release of more histamine. If we have undigested proteins in the lumen of the stomach, the G cell picks up on it, and in the absence of GRP this is a trigger for the G cell to secrete gastrin. D cell secretes somatostatin. This turns off the parietal cell. Low pH causes the release of somatostatin, because if pH is too low you can burn through your stomach. D cell is essentially the safety mechanism. Somatostatin inhibits the G cell, which slows down the release of gastrin, which slows the proton pump. Mucus cells o Bicarbonate in the mucus acts as a buffer against low pH environment of stomach o Mucus neck cells secrete bicarbonate in addition to the mucus Gastric contractile activity o Anytime something is in your stomach causing distension, this causes peristaltic waves in the stomach o Think of the stomach as a large pastry bag full of vomit… You are going to decorate your enemy’s birthday cake with vomit. This opening is tiny, so all of the vomit is not going to go out the hole. Most of it bubbles backwards. This is what happens with each peristaltic wave. Most of the chyme is injected backward as you go down. o With each wave that approaches the pyloris sphincter, the pyloris will dilate. ½-2 mils of chyme is injected into the duodenum with each wave. If it is not digested to the correct extent, the duodenum will increase the release of HCl and send a circuit back that will cause the pyloric sphincter to ignore the signal to open in the next peristaltic wave. The intestinal phase is critical in regulating the amount of HCl and the emptying rate of the stomach. Summary: Duodenum is in charge of the emptying rate of the stomach. o A stomach full of fat empties faster than a stomach full of protein, because pepsinogen -> pepsin is released onto protein to destroy the amino acids. o Diet the animal is on influences the rate at which it is released from the stomach. It affects gastric emptying rate by means of the duodenum. o What is going on when your stomach growls? The thought of food turned on the cephalic phase of digestion, which turned on gastric contraction. The gases bubbling back causes rumbling. Peptic Ulcer Disease o Too much hydrochloric acid. Burns holes in the lining of the stomach. o Caffeine goes in and binds muscarinic receptor in absence of Ach. This tricks the system to release HCl, which starts working on the stomach lining itself. o You start to bleed into the stomach and digest your own blood. This triggers the G cell to release gastrin which increases the production of HCl. o This is a positive feedback system, and if you don’t break this the animal will destroy itself. o Our physiology did not evolve to drink coffee! If you eat something when you drink caffeine, you should be okay. RUMINANT DIGESTION As food travels down the esophagus, there is a junction between the reticulum and the rumen. The more solid stuff goes into the reticulum which holds the more solid food so regurgitation can occur. It will be remasticated many many times over until the food is fairly ground up. Next is the omasum, a filter that holds more particulate matter. It opens into the abomasum, which is similar to our stomach. After that, the digestive tract is very similar to other species. COMPLEX CARBOHYDRATES IN FEEDSTUFFS We break down starch using amylase. We don’t have enzymes that break down cellulose, but ruminants do. They convert it into glucose. There’s a large population of microflora in the ruminant. In ruminant physiology there’s a high concentration of volatile fatty acids from the food. These go into the Kreb’s cycle, and the ruminant relies on them. Meanwhile the ruminant uses glucose. AVIAN DIGESTIVE SYSTEM Esophagus -> crop -> proventriculus -> gizzard -> small intestine -> large intestine -> cloaca -> vent Proventriculus acts like our stomach, and releases HCL and pepsin. In the gizzard the chyme is shifted back and forth between it and the proventriculus. It is filled with little stones and is responsible for mechanical digestion. It does the same shift back and forth with the small intestine. VOMITING Reflex used to get the toxin out of the animal. As the stomach absorbs the toxin, it goes into circulation and becomes bloodborne. As the blood travels through the body and passes a particular part of the brain known as the area postrema. You go downtown and drink too much vodka, the stomach absorbs it and it gets into your blood. The area postrema of the brain, right beside the area NTS. It picks up on this toxic level and initiates the vomit reflex. It stimulates the DMN of the vagus nerve and causes very strict contraction of the pyloric sphincter. It induces peristaltic waves and causes the cardiac sphincter to dilate fully. The chyme is propelled up towards the oral cavity, and the soft palate elevates itself to keep the vomit from entering the nasal cavity. Hypersalivation occurs to keep the vomit from burning your mouth, and then you vomit. Horses cannot vomit, hence why they have problems with colic. SMALL INTESTINE Major function is primarily breakdown of nutrients/chemical digestion and also the absorption of those nutrients. Most proximal is concerned with chemical digestion (jejunum), while more distal (ilium) is more concerned with the absorption of the nutrients. Show this on the exam. Plicae circulares- Deep circular folds of the mucosa and submucosa. Villi- Fingerlike extensions of the mucosa. Microvilli- Tiny projections of absorptive mucosal cells’ plasma membranes. Form Follows Functions o In the duodenum are high concentrations of duodenal glands that secrete mucus to protect it from the low pH coming out of the stomach. Activation of enzymes. Major function is digestion. o Jejunum- Surface area is much greater. Major function is absorption. o Ileum- Very high concentration of immune cells. About 70% of immune system is linked up. Just south of it is the large intestine, which is full of bacteria. Some of the bacteria is used to produce bacteria, but it can creep backward into the ileum (one reason why frequent defecation is important, to flush out bacteria.) Brush border is the epithelial lining of the small intestine, where absorption takes place. Villi- A lot of absorption. Extremely delicate. There are tears in the ileum all the time, and you bleed into your ileum all the time. Immune cells deal with bacteria in the small intestine. LYMPHATIC SYSTEM Runs through the body. Once fluid leaves the capillary bed, it is called interstitial fluid. Bathes all the cells in normal physiology. If it does not stop releasing this fluid, the area swells. The fluid is stuck between the individual cells. Excess interstitial fluid is put back into cardiovascular system. Osteopathic physicians will put patients with excess body fluid on a table and rock them back and forth with their feet. This causes the muscles to expand and contract, which function as a lymph pump to put excess fluid back into the heart. CHYME MOVEMENT INTO THE DUODENUM Distension of duodenal wall releases gastric inhibitory peptide (GIP). This targets the pancreas and causes the release of insulin whilst inhibiting gastric contraction. Insulin drives glucose into all cells in the body. Secretin and CCK are released. They have multiple functions, two of them: o Go up to the liver and cause the liver to secrete bile. Important in fat digestion o Target pancreas and cause release of pancreatic enzymes and buffers that move into the duodenum and are primarily responsible for digestion in a monogastric animal. These zymogens are activated by the HCl being dumped into the duodenum. Distension of the duodenum releases VIP, which increases blood flow through the capillaries of the duodenum. PRIMARY LIVER FUNCTIONS Bile production and excretion Excretion of bilirubin, cholesterol, and hormones Metabolism of fats, proteins, and carbohydrates Enzyme activation Storage of glycogen, vitamins, minerals Synthesis of plasma proteins, such as albumin and clotting factors Blood detoxification and purification Alcoholics have problems with their livers. Hepatic Portal Circulation o Blood flow associated with the liver. o Large veins drain all of the blood away from the small and large intestine, and this blood all goes to the liver. These go from the liver’s capillary beds to liver cells called hepatocytes. These then go into the heart. Liver lobule o Functional unit of the liver. o Made up of thousands of individual liver cells (hepatocytes). o Each side has a vein that comes in, draining the small intestine. Blood from small intestine comes in through vein on side. All of the veins drain towards the central vein. When the blood leaves this central vein it goes to the heart. o Individual hepatocytes reach into the blood and pull toxins, and then put metabolytes back into circulation. o Hepatocytes contribute to digestion by grabbing nutrients that pass through the capillary beds. o Bile ducts run beside veins that carry blood into hepatocytes. Hepatocytes secrete bile, which is stored in these ducts and then goes into the gallbladder for storage. Drain down and empty bile into duodenum if the sphincter is open. GALLBLADDER Stores bile when bile cannot access the duodenum. BILE Sticks to fat droplets to repel fat droplets from each other. This is called the emulsification of fat. Fat clumps to fat and can become too large to get absorbed. Regulation of Bile Release o Acidic fatty chyme is the trigger. o It causes the release of CCK and secretin. These go into circulation and travel throughout the entire body. When they go back to brain, they stimulate the vagus nerve, which synapses on the gallbladder and causes it to contract to release bile. o The secretin causes individual hepatocytes to start secreting bile. The bile then drains down the bile duct into the duodenum or gallbladder. o The CCK can also bind receptors on the gallbladder and cause contraction. o If bile itself is causing the hepatocytes to secrete more bile, this implies that some bile is absorbed when secreted into the duodenum and dumped into circulation to release more bile. PANCREAS In the pancreas we have a pancreatic eyelet. o Have alpha and beta cells, which produce glucagon and insulin respectively. o Acini cells secrete zynogens, and are linked to pancreatic duct. Secrete digestive enzymes released in form of zymogens: proteases (break down proteins), amylase (breaks down carbs), lipase (breaks down fat) and nucleus (breaks down nucleic acids). Secretes bicarbonate ions and zymogens into pancreatic duct and dumps them into the duodenum. Bicarbonate brings up pH of chyme after the zymogens are all turned on. Release of zymogens Caused by VIP, secretin, CCK (all from duodenum itself), Ach (vagus nerve) all bind receptors. Cause release of zymogens. First time a phosphatase turns something on instead of off. o Glucagon causes the liver to release glucose into the blood. This is important for ATP production. It increases blood glucose concentration. o Insulin drives glucose into cells and decreases blood glucose concentration. o Every cell in the body has an insulin receptor, which insulin binds to. This turns on a Glut-transporter which transports glucose into the cell through the capillary bed/extracellular fluid. The glucose comes through the cell and is stored as glycogen (mediated by actions of insulin). Insulin also causes a fraction of the glucose to be converted into pyruvic acid. Insulin is regulating glycolysis. Pyruvic acid can be converted into fatty acids which are used to store energy and could drive the Kreb’s cycle. Trypsinogen comes out of the pancreatic duct and enters the duodenum. Enterokinase is a membrane bound enzyme anchored to the wall of the duodenum, which activates trypsin. Trypsin recognizes particular bonds between amino acids and breaks them. Activates two other zymogens: chymotypsinogen -> chymotrypsin and precarboxypeptidase -> carboxpeptidase. All of three of these are proteases not activated by HCl. Draw these zymogens being turned into proteases. Phases of Pancreatic Secretion o In gastric and cephalic phases: Vagus nerve secretes Ach onto acini cell. G cell secretes gastrin onto it as well. Gastrin occupies the CCK receptor. Causes release of zymogens. o In intestinal phase: CCK and secretin are released because of distension. CCK comes over and binds to CCK receptor. When the receptor is occupied, the acini cell releases zymogens. Secretin binds to cell in pancreatic duct. Causes release of bicarbonate buffer. CCK drives release of zymogens, secretin drives release of buffer. The following digestion occurs in the small intestion CARBOHYDRATE CHEMICAL DIGESTION Pancreatic amylase- Comes from the pancreas. Breaks bonds between individual glucose molecules and liberates glucose from glycogen/starch. If you are eating animals you are breaking down glycogen, if you are eating plants you’re breaking down starch. Maltose, sucrose, lactose are more pancreatic enzymes that are converted into glucose. They are released in response to phases of pancreatic secretion. o The glucose is used for glycolysis (ATP production.) This is the metabolism/breakdown of carbs, which occurs in the small intestine, mostly in the duodenum. Pulling the nutrient across the brush border. In a brush border cell: o Galactose and glucose transported into epithelial cell. Sodium goes through pump and drags two sugars with it. Sodium flowing down its concentration gradient makes the pump work. There is a low concentration of sodium in this cell. There is a sodium potassium pump that pumps high concentrations of sodium out of the brush border cell. Burning of ATP on different side pulls sugar in towards the cell. (Secondary active transport.) Along the brush border are enzymes that are permanently tethered to brush border cells. When they encounter disaccharides, the disaccharides are broken into monosaccharides. These enzymes only have access to nutrients scraping the wall of the small intestine by segmentation. Meanwhile, pancreatic enzymes are just dumped into the duodenum. As you diagram this, show oral and pancreatic amylase, show three enzymes dumped into the duodenum and mixed in with the chyme. Show that some of those are tethered to the wall. Show movement of monosaccharides across brush border cells. Add that monosaccharides are dumped directly into a capillary bed. Sugars then go to liver, which modifies them or stores them as glycogen. Liver is the major storage area for glycogen. If there is a complete failure of the vagus nerve and intestinal phase, will the animal still be able to digest its food? o Yes. The enzymes are always linked to the brush border, so as food goes through the canal some of it will bump into these enzymes and automatically be metabolized. CHEMICAL DIGESTION OF PROTEINS Absorption is similar to carbohydrates. Pancreatic enzymes: Trypsin, chymotrypsin, carboxypeptidase. Free- floating. Mixed into chyme by segmentation and peristaltic waves. Na and amino acids go across brush border through secondary active transport. Sodium potassium pump. Amino acid ejected out other side of cell and ends up in capillary bed. Next stop is the liver though hepatic portal circulation. Peptidases (almost same as proteases but break down small units of amino acids) are stuck in the membrane. 4 amino acids at first peptidase (thanks to pancreatic proteases, first to act on long chains and make them smaller.) As you keep going, the peptides get shorter, as each peptidase pulls off one amino acid. As soon as an amino acid is pulled off it encounters a form of transport. Entire proteins can be pulled across the brush border cell without any real metabolism of it. Through process of phagocytosis. Large protein goes into capillary bed, to liver, to heart. FATTY ACID DIGESTION AND ABSORPTION Pancreatic lipase liberates free fatty acids. First thing to do is draw the bile dumped into duodenum emulsifying fats. The release of bile was caused by the CCK and vagal stimulation. CCK and vagal stimulation was caused by distention of the duodenum. o It repels fat droplets from one another. o When they are repelled, it facilitates lipase. Second thing to do is show the lipase liberating the free fatty acids. Moved to jejunum, ilium by peristalsis and transported into brush border cells. Freely crosses membrane, goes into cytoplasm, is linked back into triglycerides. They are not ejected into the capillary bed, but are drained into a lacteal. A lacteal’s doors open when excess extracellular fluid is encountered. This is caused by pressure. Triglycerides float in, eventually drain back into heart and dump right before they can enter the heart. FUNCTIONS OF THE LARGE INTESTINE A human or pig can live completely without it. o Holds fecal material until elimination is convenient. o Water absorption, but majority occurred on the brush border. o Essential for comfort, not for life. Other than digestion of enteric bacteria, no further digestion takes place. Vitamins, water, and electrolytes are reclaimed. Major function is propulsion of fecal matter towards anus. Cecum/appendix where bacteria are located. Will constantly pull bacteria is large intestine. Immune system cannot deal with appendix bursting. Bacterial flora- Synthesizes vitamins. Anal sphincter- Consists of an external and internal sphincter. Internal is composed of smooth muscle. External is composed of skeletal muscle. Rectal valve- Just superior to anal sphincter. Sharting is a failure of the rectal valve. Moves back and forth and allows flatulence to pass over the fecal material to leave the body. Origin of the gases is the bacteria. We are always leaking flatulence, a fart is just a large volume of it. Defecation- The internal sphincter runs into the spinal cord. It is synapsed on by a sensory neuron that is part of the gut brain. When the fecal matter enters the distal portion, it causes distension. Stretch receptors fire up to spinal cord, which automatically causes dilation of the internal sphincter. If nothing occurs on the external sphincter, the animal will defecate. The external sphincter is under conscious control. As stretch receptor causes dilation of internal, a pathway goes up to the brain and makes the brain aware elimination is necessary. If it’s not convenient for elimination at that time, a circuit will fire and the sphincter will remain closed. Why do you poop your pants? o The sympathetic division predominates. Its major function is to deal with emergencies. It will shunt the ATP where it needs to go. The autonomic nervous system turned off the circuit and the internal sphincter dilated on its own. ALS 2304 IMMUNOLOGY LYMPHATIC SYSTEM Major function is to remove extra intercellular fluid. It leaks away from capillary beds and causes extracellular fluid. This extracellular fluid is carried away. Ducts drain back towards the heart and pass through lymph nodes. The lymph nodes filter the lymph. If there is an infection, it will go through the lymph node. A series of filters and a high concentration of immune cells exist there. (Highest concentration of immune cells in ileum, second highest in lymph nodes.) You are most apt to pick up bacteria in body in the small intestine because of its brush border, which is exceptionally thin. This layer of epithelial cells has capillary beds on the other side. If it was thicker it would hinder the transport of the nutrients from the lumen into the lympthatic or capillary system. The border tears and you bleed into yourself. The bacteria from the small intestine gains access to the extracellular fluid through these tears, so there is a lot of lymphatic tissue near the small intestine.