Digestive System (Organs of GI tract (Anus and Rectum
Organs of GI tract
Anus and Rectum
- The defecation reflex occurs after the mass movements force feces into the rectum. It causes the internal anal sphincter to relax, while conscious control over the external anal sphincter is maintained
- During defecation, the muscles of the rectum contract to expel the feces
- Home to much of the body’s bacterial microbiota, which conducts metabolic functions, immune functions, and more
- Harvests vitamins digested by the microbiota, reclaims water and electrolytes, but does not contribute much to digestion
- Haustral contractions are slow, segmenting movements occurring mainly in the ascending and transverse colon. Serve mainly to mix the digestive residue.
- Mass movements are long, slow, powerful waves on contraction which force the contents of the colon toward the rectum. Activated by the gastrocolic reflex
Stomach: a temporary storage tank
- Converts bolus of food to paste-like chyme
- Propulsion: peristalsis
- Mechanical breakdown: churning caused by peristalsis
- Digestion: pepsin begins enzymatic digestion of proteins
- HCl denatures (unfolds) proteins in preparation small intestine
- Milk protein (casein) is broken down by rennin in infants
- Lingual and gastric lipase begins fat digestion
- Absorption: Lipid-soluble alcohol and aspirin are absorbed (happens minorly in stomach, majorly in small intestine)
- Only stomach function essential to life is secretion of intrinsic factor for vitamin B12 absorption (pernicious anemia)
- Cells + their secretions in stomach:
- Mucous neck cells: Secrete thin, acidic mucus of unknown function
- Parietal cells: secrete HCl + Intrinsic factor
- Chief cells: secrete pepsinogen [precursor to pepsin bc its inactive] (HCl and/or pepsin activates pepsin) and gastric lipase (15% fat digestion)
- Enteroendocrine cells: secrete paracrines (histamine + serotonin) [travel locally] and hormones (somatostatin + gastrin) [hormones travel throughout whole body in blood]
- Flat muscular tube that runs from laryngopharynx to stomach. Not involved in food propulsions when collapsed.
- As food moves through the laryngopharynx, the epiglottis closes off the larynx and incoming food is routed posteriorly into the esophagus.
- Propulsion: moves food by peristalsis
- Collapsed when not involved in propulsion
- Gastroesophageal (cardiac) sphincter surrounds cardial orifice:
- Keeps orifice closed when food is not being swallowed
and mucus cells on both sides of sphincter help protect esophagus from acid reflux
- Esophageal mucosa contains stratified squamous epithelium
and has adventitia instead of serosa
- The submucosa contains mucus-secreting esophageal glands. As a bolus moves through the esophagus, it compresses these glands, causing them to secrete mucus that “greases” the esophageal walls and aids food passage.
- The buccal phase occurs in the mouth and is voluntary. It ends when a food bolus or a “bit of saliva” leaves the mouth and stimulates tactile receptors in the posterior pharynx, initiating the next phase.
- The pharyngeal-esophageal phase is involuntary and is controlled by the swallowing center in the brain stem . Various cranial nerves transmit motor impulses from the swallowing center to the muscles of the pharynx and esophagus. Once food enters the pharynx, respiration is momentarily inhibited and all routes except the desired one into the digestive tract are blocked off. Solid foods pass from the oropharynx to the stomach in about 8 seconds, and fluids, aided by gravity, pass in 1 to 2 seconds.
- From the mouth, food passes posteriorly into the oropharynx and then the laryngopharynx. Both are common passageways for food, fluids, and air.
- Contractions of skeletal muscles propel food into esophagus. No real mechanical breakdown.
- The mucosa contains a friction-resistant stratified squamous epithelium well supplied with mucus-producing glands.
- Digestive processes: site of voluntary food ingestion
- Accessary organs: tongue, salivary glands, and teeth perform functions and the mouth acts as a receptacle
- Propulsion: voluntary phase of swallowing
- Mechanical breakdown: chewing (mastication) by teeth and mixing by tongue
- Digestion (chemical - enzyme): salivary amylase begins to break down starch
Small Intestine: The Site of Digestion and Absorption
- The duodenum contains elaborate mucus-secreting duodenal glands, which produce alkaline mucus to help neutralize acidic chyme as it leaves the stomach.
Sources of enzymes for digestion
- Most are imported from the liver and pancreas, along with bicarbonate ions and bile
- Brush border enzymes come from the microvilli
- Chyme entry: too much too quickly will draw water from the blood and surrounding tissues. Feedback via the enterogastric reflex and enterogastrones is important to control the movement of chyme through the duodenum
- Motility after a meal: Segmentation is the primary form of movement, ensuring that chyme is thoroughly mixed with bile and pancreatic and digestive juices. Essentially a “massaging” motion moving the chyme backward and forward. Short bouts of peristalsis keep the food moving down. Gastroileal reflex
- Motility between meals: Peristalsis, regulated by the hormone motilin
- Gastroilial reflex (along with the hormone gastrin) causes relaxation of the ileocecal valve, allowing food to pass through it and into the colon
Accessory Organs: organs that contribute to the digestive process but are not part of the alimentary canal
- Produces enzymes that break down all categories of foodstuffs.
- It is a soft, tadpole-shaped gland that extends across the abdomen from its tail (next to the spleen) to its head, which is encircled by the C-shaped duodenum. Most of the pancreas is retroperitoneal and lies deep to the greater curvature of the stomach.
- The exocrine part of the pancreas produces pancreatic juice
- Acini: clusters of secretory cells that produce zymogen granules containing proenzymes
- Ducts: secrete to duodenum via main pancreatic duct; produce water and bicarbonate
- The endocrine part of the pancreas is a scattering of mini-endocrine glands called pancreatic islets. that release insulin and glucagon
- A thin-walled muscular sac that may appear green when filled with bile.
- The size of a kiwi fruit. It snuggles in a shallow fossa on the inferior surface of the liver
- Stores bile that is not immediately needed for digestion and concentrates it by absorbing some of its water and ions.
- Its muscular wall contracts to expel bile into the cystic duct. From there bile flows into the bile duct.
- Covered by visceral peritoneum
- Main function is to process the nutrient-rich blood delivered to it
- Largest gland in the body
- Shaped like a wedge, it occupies most of the right hypochondriac and epigastric regions extending farther to the right of the body midline than to the left. Located under the diaphragm, the liver lies almost entirely within the rib cage, which provides some protection
- Has four primary lobes
- Bile leaves the liver lobes through the right and left hepatic ducts
Tongue: occupies the floor of the mouth. Composed of bundles of skeletal muscle fibers. During chewing, it grips the food and repositions it between the teeth. Also, mixes food with saliva, forming it into bolus. Then initiates swallowing by pushing the bolus posteriorly. into the pharynx
- Lie in sockets (alveoli) in the gum-covered margins of the mandible and maxilla.
- The teeth tear and grind the food, physically breaking it down into smaller fragments.
-can act locally as paracrine in stomach, but can enter blood capillaries to diffuse and behave as a hormone
-produced in stomach mucosa and duodenal mucosa
-stimulated by food in stomach or by sympathetic nerve fibers
-inhibits gastric secretion of all products
-inhibits general sectrion
-inhibits GI blood flow
-inhibits contraction and bile release
-inhibits absorption of nutrient
-Produced by duodenal mucosa
-stimulated by acidic chyme and partially digested fats and proteins
-inhibits gastric gland secretion and gastric motility
-increases output of pancreatic juice rich in bicarbonate ions
-increases bile output
-produced by duodenal mucosa
-stimulated by fatty chyme
-inhibits stomach secretory activity
-potentiates secretions actions on liver and pancreas
-increases output of enzyme-rich pancreatic juice
-stimulates gallbladder to contract and expel stored bile
-relaxes hepatopancreatic sphincter to allow entry of bile and pancreatic juice into duodenum
-produced by stomach mucosa
-stimulated by food in stomach and acetylcholine released from nerve fibers
-increased HCL secretion
-stimulates gastric emptying
-stimulates contraction of intestinal muscle
-relaxes ileocecal valve
-stimulates mass movements
-Produced by duodenal mucosa
-stimulated by fasting
-improves peristalsis in the small intestine and clears out the gut to prepare for next meal
-high levels of motilin secreted between meals into the blood stimulates the contraction of the fundus and antrum and accelerates gastric emptying
-stimulates the contraction of gallbladder and increases the squeeze pressure of the lower esophageal sphincter
-mass movements of digestion moving forward through GI stretch rectal wall
-stretching of rectal wall initiates defacation reflex
-parasympathetic spinal reflex
-causes sigmoid colon and rectum to contract, and internal anal sphincter to relax --> feces forced into anal cancel and messages reach brain that allow body to decide to defecate or not
-increase in motility of the colon consisting primarily of giant migrating contractions, or mass movements, in response to stretch in the stomach and byproducts of digestion in the small intestine
-responsible for the urge to defecate following a meal
-a long neural reflex triggered by stomach activity, increases the force of segmentation in the ileum and relaxes the ileocecal valve
-releases acids and controls the release of stomach proteins (like gastrin)
-stimulated by the presence of acid levels in the duodenum at a pH of 3–4 or in the stomach at a pH of 1.5
-when this reflex is stimulated, the release of gastrin from G- cells of the stomach is shut off
-this inhibits gastric motility and the secretion of gastric acid (HCl)
-causes decreased motility.
-produced by stomach mucosa
-stimulated by food in stomach
-causes contraction of stomach muscle
-produced in stomach mucosa
-stimulated by food in stomach
-activates parietal cells to release HCl
-acts as paracrine locally in stomach but can diffuse into blood capillaries to influence digestive system target organs