Digestive system, Metabolism

From Biol557

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Revision as of 21:40, 19 April 2010

• started here on 04/19/10.

Contents 1 The digestive system 1.1 Functions of the digestive system 1.2 Major components 1.2.1 Oral cavity 1.2.1.1 Tongue 1.2.1.2 Salivary glands 1.2.1.3 Saliva 1.2.1.4 Mucus 1.2.1.5 Fluids 1.2.1.6 Mumps 1.2.1.7 Teeth 1.2.2 Pharynx 1.2.3 Esophagus 1.2.3.1 Barrett's esophagus 1.2.3.2 Acid reflux disease 1.2.3.3 Hiatal hernia 1.2.4 Stomach 1.2.4.1 Functions 1.2.4.2 Anatomy 1.2.4.3 HCl secretion 1.2.4.4 Gastric fingering 1.2.4.5 pH of stomach 1.2.4.6 Enterooendocrine cells 1.2.4.7 Involuntary protein spills 1.2.4.8 Pyloric problems affecting infants 1.2.4.9 Hormones involved in digestion= 1.2.5 Regulation of gastric activity 1.2.5.1 Gastric secretion: Cephalic phase 1.2.5.2 Gastric secretion: Gastric phase 1.2.5.3 Gastric secretion: Intestinal phase 1.2.6 Digestion in the stomach 1.2.6.1 Absorption in the stomach

The digestive system

• Sedentary body requireds ~30 kcal / kg body weight per day.

Functions of the digestive system

• Ingestion, getting food in.
• Secretion, we'll talk about all the secretions. (pancreas, liver, gall bladder, stomach acid, salivery glands).
• Digestion proper
  • Three different parts:
    • Mechanical: physical breakdown
    • Chemical breakdoyn, different than enzymes; think HCL in the stomach, not enzymes like in the small intestine.
    • Enzymatic; more specific.
• Absorption, that is how do we get the nutrients from lumen into blood.
• Defectation; getting rid of stuff that isn't useful.

• We're gong to think about microvilli to maximize absorption, peristalsis (mixing and physical breakdown).

Major components

• The tube.
• Accessory structures that protude into it or communicate with it via ducts.

Oral cavity

• There is a surface lining called the mucosa
  • There is lots of friction and abrasion so we want something that is stratified, so we have a stratified epithelial that is non-keratinized.
  • It is moist.
  • Some areas are keratinized where abrasion is occuring: surface of the tongue, hard pallat, too.

Tongue

• Intrinsic and extrinsic muscles.
• Extrinsic anchor muscle in mouth, way back to the highway bone. These are responsible for the general movement of the tongue and for formation and movement of the bolus.
• Intrinsic muscles are responsible for shaping the tongue for things like speach.
• Taste buds on the surface of the tongue has receptor cells.
  • These occur in the papillae of the tongue.

Salivary glands

• We have three major areas of where we find salivary glands: sublingual ducts, submandibular, and paratoid.
• Not all generate the same amount.
• Minor ducts are located around the lips and throat.
• Stones in the salivary glands can be very painful.
• The internal structure is similar between types.
• Along the sides of the ducts are cells that form the acinus (plural acini).
• Serous cells are watery. Glucocells are thicker.
  • These two cell typhes generate the saliva.
  • You can determine which type they are by histology.

Saliva

• You produce 1.5L per day (up to 2L).
• Secretions from different glands aren't necessarily of the same composition.
• Also, the ion composition will change based on the flow rate (as it goes up, NaCL release increases).
• Saliva is slightly acidic; this correlates with the preferable pH of alpha amylase.
• There is buffering, too, through bicarbonate.
• Amylase is in the secretion to break down carbohydrates.
• Lipase is also released but it is only active in the stomach.
• Lysozyme is also released.
• Antibodies like IgA are released, too, which is common around many openings.
• Last point.

Mucus

• Lubrication is one function, also helps cause fecal material to adhere to form a stolid stool.
• Along the GI tract we need to protect the underlying epithelium, so it is interesting that the mucus helps to protect the epithelia.
• The mucus creates a physical barrier to the epithelia and also generates a microenvironment.
• Bicarbonate is found in the mucus, so we call this the neutralizing barrier.

Fluids

• We secrete about 10L of liquids into the GI tract but only excrete 100 ml / day b/c the intestine is good at reabsorbing.

Mumps

• Caused by RNA virus.
• Gets into salivary gland such that there is a huge swelling on the side of the neck.
• Can cause sterility in males through epididymysis.
• Unilateral paratoid gland swelling is generally caused by obstruction like a tumor.

Teeth

• Gingevitis is an infection of the gingeval tissue (the gum line), caused by a bacteria. Generally contributed to plaque which encourages the build up of bacteria because plaque is a biofilm (a place where bacteria can grow and hide from antibiotics).
  • Biofilms are also seen in bladder infections.
• Abscessess

Pharynx

• Divided into regions: nasal pharynx (connects GI and respiratory), oral pharynx (connects with bottom of pharynx), laryngopharynx (connects to esophagus).
• In swallowing, the soft pallate has to elevate to block off nasopharynx. AAnd the epiglottis must come down to block trachea.
• Then bolus moves to the back of the tongue / mouth where it is involuntarily swallowed.

Esophagus

• Connects pharynx to stomach.
• Has sphincter muscles: circular smooth muscles tha thelp open and close openings.
  • Some skeletal, some smooth.
• Air in the system causes noises.

Barrett's esophagus

• Can cause serious problems.
• 1% of the population.
• Reflux is coming back from the stomach.
• Acid causes irritation.
• In severe cases, epithelum of the esophagus will take on a new, more gastric-like epithelial state.
  • Not clear why, but thought to be a protective response because gastric cells can withstand acid environment more easily.
• 1-5% of patients will get esophageal cancer.

Acid reflux disease

• The cardiac sphincter fails to remain tightly closed such that stomach contents can go back into the esophagus.

Hiatal hernia

• Can live with this if not too severe.
• A portion of the stomach protrudes back through the diaphram into the hiatus yielding a hernia.

Stomach

Functions

• Storage of food.
• Mechanical breakdown of food.
• Chemical breakdown via HCl.
• Enzymatic break down via salivary lipase and pepsin.
  • Pepsin breaks down connective tissue.
• Production of intrinsic factors
  • Stimulates absorption of B12.
  • I think he called this an endocrine function.

Anatomy

• Rugae are the pleats that allow for expansion from 50-100mL area.
• In the body of the stomach we have gastric pits.
  • Lined by different types of pits that contribute to gastric secretions.
  • Two types of cells, primarily: chief cells and parietal cells.
  • Chief cells are responsible for secreting pepsinogen (inactive).
  • Parietal cells secrete HCl and intrinsic factor.
• How does the pepsinogen get from in the pit into the lumen without getting digested by HCl?
  • Because of a process called "fingering".
• Infants secrete rennin which coagulates milk proteins and allows them to digest some fat in the stomach.

HCl secretion

• This is an active process, so we're physically moving H+ ions from the blood and putting them in the lumen.
• At the same time we're moving HCO3 into the blood.
• This process is moved by NaK atpase and requires the bicarb / Cl cotransporter.
  • Exchanged from blood into cell to maintain balance (HCO3 into blood).
• Carbonanic anyhdrase is found in the epithelial cells.
• As before, the H+ is coming from the conversion of Co2 and H20 into Hc03 and H via CA and chance.

Does K+ move?

Gastric fingering

• The acid shoots up and through the mucus layer to end up in the lumen of the stomach.
• If some of the acid lands on the mucus gel in contact with the epithelial surface, it will be neutralized by the bicarbonate that is held close to the epithelial cells. This is why we care about the chemical microenvironment.

pH of stomach

• Varies by time of day and meals.
• Most acidic during meals.
• As you eat, it changes the composition of the lumen, causing the pH to rise.
• There is also a change in the amount of somatostatin that is released. It affects gastrin secretion. Gastrin secretion affects HCl release.

Enterooendocrine cells

• G cells secrete gastrin
  • stimulates HCl secretion, pepsinogen synthesis and muscle contractions.
• D cells secrete somatostatin
  • This inhibits gastrin secretion and thus impacts the amount of HCl produced and released.
• The nervous system can affect these cells as can the consumption of food.

Involuntary protein spills

• These are severe vomiting spells.
• Tooth enamal erosion can occur.
• Can cause alkylosis and dehydration.
• Zollinger-Ellison syndrome
  • Tumor of duodenom
  • Can cause overproduction of gastrin and HCl causing a very acidic stomach.
• Several brain stem areas control vomitting.
  • Feedback to the vomitting center occurs from the intestine, the stomach, the pharynx, and esophagus.
  • The outward signals go to the abdominal muscles and diaphragm because we squeeze the stomach.
• The signal to induce this is reverse peristalsis, that is, reverse peristalsis.
  • Irritation of the stomach and duodenum can cause this reversal of peristalsis which will move food backward until the stomach is distended and stomach is squeezed.
  • Not unusual for bile to be in the vomit.
• The chemoreceptor trigger zone is found in the fourth ventrical (base of cerebellum) is also involved in vomitting.
  • Can be stimulatee by electrical or chemica signals.
  • Electrical signals come from motion sickness (sent from ears).
  • Chemicals like drugs (morphine) can directly stimulate the chemorecptor trigger zone.

Pyloric problems affecting infants

• Pylormic spasms:
  • If the pyloric sphincter doesn't relax, the stomach becomes distended and the baby has to remove food via projectile vomitting.
  • Routine projectile vomitting indicates a blockage.

Hormones involved in digestion=

• CCK = cholecystokinin
• Secretin
• One other

Regulation of gastric activity

• Cephalic phase: prepares stomach to receive food.
• Gastric phase: while food is in stomach.
• Intestinal phase: getting rid of food

Gastric secretion: Cephalic phase

• Nerves tell G cells of the stomach to release HCl.
• Negative nerves activit on D cells causes release of gastrin.

Gastric secretion: Gastric phase

• Stretching causes feedback such that....?
• Protein begins to break down which stimulates G cells to produce gastrin.
• Gastrin feeds back to increase HCl production.
• The D cell is released from inhibition such that they stimulate (via gastrin) G cells to release HCl.

Gastric secretion: Intestinal phase

• Breakdown of carbs stimulates cells of intestine to release hormones.
• This feeds back on chief and parietal cells in the stomach to turn off whatever they produce.

Digestion in the stomach

• 10-20% of the protein is broken down.
  • Most protein being broken down by pepsin is the connective tissue.
• Some carb breakdown occurs via salivary amylase until it gets deactivated by pH.

Absorption in the stomach

• Lipids can be absorbed.
• Alcohol can be converted to acetaldehyd bye alcohol dehydrogenase.
  • Acetaldehyde is poisonous!
• Alcohol is continuously absorbed in the stomach. This explains the delay of blood alcohol level and stomach alcohol level.
• Most fat absorbed in the duodenum.

• stopped here on 04/19/10.