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Function of the digestive system
- take in food
- break it down to nutrient molecules
- absorb nutrient molecules into the bloodstream
- rid the body of any indigestible remains
divisions of the digestive system
- alimentary canal organs-hollow organs
- accessory digestive organs-solid organs
alimentary canal organs
- organs through which food and food waste will actually pass
- runs from the mouth to the anus
- includes: mouth, pharyns, esophagus, stomach, small intestine, and large intestine
accessory digestive system
- contributes to the process of digestion and absorption: but no food or food waste actually passes thru them
- include: teeth, tongue, salivary glands, liver, gallbladder, and pancreas
food is enclosed within the alimentary canal
- process of moving fod thru the alimentary canal. Includes deglutition (voluntary)-swallowing, and peristalsis (involuntary)
- peristalsis-primary means by which food is propelled thru the GI tract. It involves waves of alternating contraction and relaxation of the smooth muscle in the organ walls
initial breakdown that physically prepares food for further chemical digestion. Includes chewing, mixing of food and saliva by the tongue as well as churning of food in the stomach
- hydrolytic breakdown of food molecules into their chemical building blocks by enzymes secreted into the alimentary canal.
- Small amounts occur in the mouth and stomach-majority in the small intestine
- passage of nutrients (along with vitamins, minerals, and water) from the lumen of the GI tract across the mucosa and into either blood or lymph
- Primary occurs in the small intestine
elimination of indigestible substances from the body via the anus in the form of feces
- produce 1-1.5 L of saliva per day
- -moistens and cleanses themouth
- -dissolves food particles. Allows them to stimulate taste buds
- -moistens food facilitating its compaction into a bolus
- -mucus lubricates the bolus facilitating swallowing
- -contains enzymes that begin chemical digestion of starch
- 97-99% water
- salivary amylase-an enzyme that chemically digests starch
- immunoglobulin (IgA) and lysozyme-which provide immune defense
- mucin-protein that when dissolved in water forms mucus
summary of the digestive processes that occur within the mouth
- -presence of food activates teh salivatory nuclei of hte pons and medulla nad salivation results
- -teeth and tongue mechanically digest food increasing the surface area available for digestive enzymes
- -food is mixed with saliva and compacted into a bolus
- -tongue pushes the bolus into the oropharynx as swallowing is voluntarily initiated
- food passes from the oral cavity into the oropharynx and then the laryngopharynx and onward to the esophagus
- epiglottis closes off the larynx during swallowing, preventing food from entering the respiratory tract
- pharyngeal constrictor muscles propel the bolus down the pharynx and into the esophagus
- muscular 10 inch tube that propels food from the laryngopharynx to the stomach
- no digestive processes are initiated within the esophagus
- it's collapsed when not propelling food
- the submucosa contains mucus-secreting glands for lubrication
- the presence of food in the esophagus triggers reflexes result in waves of peristalsis that force food down to the stomach
- an enlarged segment of the tract that functions mainly in storing food and mixing it with gastric juice (creating a past called chyme)
- chemical digestion of proteins
- secretion of intrinsic factor-a chemical that is necessary for vitamin B12 absorption. B12 is necessary for RBC synthesis
- destruction of ingested bacteria via secreted hydrochloric acid
- the stomach's diameter and volume vary with it's contents. The stomach expands as it fills with food
- The gastric mucosa is a simple columnar epithelium with millions of tubelike invagtinations known as gastric pits
- the surface epithelial cells are also known as surface mucous cells because they secrete a basic mucus approximately 1 mm thick
- the gastric pits lead into gastric glands, which secrete the gastric juice (2-3 L/day)
Basic cell types of the stomach
- mucous neck cells-found in the upper portion of the gland. Secrete acidic mucus and funtion as stem cells for surface mucous cells
- Chief cells-primary function is the secretion of pepsinogen, an inactive form of protease, pepsin. Pepsinogen is activated by HCl and by pepsin itself
- Parietal cells-found in the midportion of the glands. Secrete HCl as well as intrinsic factor
- Enteroendocrine cells-secrete multiple hormones inot the plasma-ex. gastrin: regulates the stomach's motility and secretory activity
the gastric muscularis externa contains 3 layers rather than the normal 2. Deep to the circular layer of muscle is the?
oblique layer-allow the stomach to churn, mix, and pummel food
2 basic types of muscular movements in the stomach
- mixing waves-mix ingested materials with the gastric secretions
- peristaltic waves-mor powerful and force chyme towards the pyloric sphincter. EAch peristaltic wave forces a small amount of chyme thru the pylorus muscle. Food is not absorbed in the stomach-alcohol and some drugs are tho
gastric activity is stimulated by:
- sight/smell/taste/thought of food
- -cephalic phase-visual, taste, and olfactory receptors send info to the hypothalamus, which initiates parasympathetic signals to the stomach via the vagus n. Ach relased by the vagus n. stimulate gastric activity. Ach also stimulates the stomach to release the hormone gastrin. Gastrin then stimulates gastric activity
- indirectly in response to stretch or the presence of aa within the stomach. Both activate reflexes that stimulate gastric activity as well as gastrin release. Gastric phase-responsible for the greatest volume of gastric juice secretion
- indirectly by the initial filling of the duodenum with chyme. The initial presence of chyme causes duodenal endocrine cells to release intestinal gastrin, which also stimulates gastric activity-intestinal phase
gastric activity is inhibitied by:
- the accumulation of chyme within the duodenum. In response to stretch, the duodenal endocrine cells begin to release cholescystokinin and secretin. Both inhit gastric activity
- By stress, anxiety, and fear (via increased sympathetic activity)
- site of most digestion and almost all nutrient absorption
- divided into 3 unequal sections: duodenum, jejunum, and ileum
- the duodenum receives the common bile duct (delivering bile from the
- liver and gallbladder) and th emain pancreateic duct (delivering
- pancreatic juice from the pancreas)
- 2 ducts unite in the duodenal wall and the hepatopancreatic sphincter ( of Oddi) controls entry of bile and pancreatic juice into the intestinal lumen
primary site of digestion and absorption
primarily involved in absorption of water, electrolytes and vitamins
structures that maximinze surface area include:
- deep, circular, permananet folds of the mucosa and submucosa-incrase surface area and slow the movement of chyme-provides more time for absorption and digestion to occur
- Villi-fingerlike extension of the mucosa. Absorptive epithelial cells line th esurface. Within the core of each villus is the lamina propria, which contains blood capillaries (for absorption of aa and monosaccharides) and a lacteal (absorption of fatty acids)
- microvilli-tiny projections of the plasma membrane of each absorptive epithelia cell. They give the cell's luminal surface a fuzzy appearance known as the brush border. Membrane bound enzymes are embedded in the brush border and function in nutrient breakdown
intestinal glands (cypts of LIeberkuhn) secrete over 2 L/day of intestinal juice, which consists primarily of mucus, electrolytes, and water. Also contain enteroendocrine cells, which secrete hormones (intestinal gastrin, secretin, and CCK) into plasma
the proximal duodenal submucosa contains?
alkaline mucus glands that help counteract the acidic chyme
the terminal ileal submucosa contains?
small intestine is the primary site of a mixing activity known as?
segmentation-consists of alternating contractions and relaxations that mix the intestinal contents rather than propel it forward
motility and secretory acitvity of the small intestine is enhanced by and inhibited by?
- enhanced by the parasympathetic system
- inhibited by the sympathetic activity
- carbohydrate metabolism-storage and release of glucose
- removal of drugs, toxins, and foreign chemicals from the plasma
- sorage fo vitamins (A, D, E, and K) and minearls (iron and copper)
- Protein metabolism
- Lipid metabolism
- synthesis of plasma proteins
- phagocytosis of old RBCs and of pathogens
- production of bile (0.5-1 L/day)
running alongside the liver sinusoids are the ________? Hepatocytes secrete bile into _________ and ______ empty into bile ducts at the ________. The portal triad bile ducts eventually combine to yield the left and right ________ that exit the liver.
- bile canaliculi
- bile canaliculi
- portal triads
- hepatic ducts
- primary digestive function of the liver
- mixture of bile salts, pgments (bilirubin) and other chemicals
- synthesized by the liver, stored and concentrated by the gallbladder, and secreted into the duodenum
- bile salts emulsify fats. Becasue of their hypophobic nature, fats tend to clump together in the watery environs of the GI tract. Clumped fat reduces surface area exposed to fat-digesting enzymes. Emulsification is the act of separating the large fat globules into tiny separate fatty droplets. This increases the available surface area for lipases to work upon
- Note taht bile salts are reabsorbed in the ileum and travel back to the liver (via the hepatic portal circulation) where they are reused
The liver continuously produces bile, however, the hepatopancreatic sphincter is normally closed. This results in what?
bile backing up into the common bile duct, cystic duct, and ultimately into the gallbladder
when fatty chyme arrives in the small intestine, intestinal glands secrete the hormone?
CCk causes gallbladder contraction and relaxation of the hepatopancreatic sphincter allowing?
bile flow into the duodenum
- consists mainly of acini-small clusters of enzyme secreting cells-empty their secretion into small ducts
- Small ducts coalesce into larger ducts that empty into the main pancreatic duct, which runs centrally along the long axis of the pancreas
major function of the acinar and duct cells in the pancreas
secretion fo pancreatic juice (1.5 L/day)
acinar cells contribute digestive enzymes to the pancreatic juice including:
- protein-digesting enzymes such as proteases
- fat-digesting enzymes such as pancreatic lipase
- carbohydrate-digesting enzymes such as pancreatic amylase
duct cells contribute a ________________which has a slightly alkaline pH that helps neutralize the acidity of the chyme
watery bicarbonate-rich solution
when acidic, fatty chyme arrives in the duodenum, its enteroendocrine cells secrete?
secretin and CCK
CCK travels in the blood to the pancreas where it?
primarily stimulates the secretion of enzymes by the acinar cells
secretin also travels to the pancreas and stimulates the duct cells to release?
large amounts of the bicarbonate-rich fluid
CCK also causes contraction of the gallbladder as well as?
relaxation of the hepatopancreatic sphincter
during the cepahlic and gastric phases of gastric secretion, parasympathetic input to the pancrease is via?
the vagus nerve-also prompts pancreatic juice release
- functions primarily to propel indigestible food remains and then expel them as feces
- also absorbs any excess water remaining
- it's about 5 feet in length
- begins at the ileocecal valve and terminates at teh anus
the colonic mucosa is?
simple colunar epithelium with multitudes of goblet cells. Goblet cells provides fecal lubrication
the colon has no?
plicae, villi, or microvilli
the colon does have?
colonic intestinal glands-primary output is mucus
millions of bacteria colonize the large intestine. They breakdown and produce?
breakdown indigestible carb residues and produce many B vit. as well as most of the body's vit. K supply
Typically 100 g of feces are produced per day consisting of?
- 70% water and 25% solids
- solids include dead bacteria, fat, inorganic matter, protein, undigested plant fibers, bile pigments and shed epithelial cells
muscular movements within the colon include?
- migrating motor complexes
-mixing increases the contact between feces and mucosa, which facilitates water reabsorptionhaustral contracteions (Haustrations)-push food residue and fecal matter of haustrum to haustrum
migrating motor complex
-sweeping waves of peristalsis that move over large areas of the colon and force its contents towards teh rectum. they typically occur 1-3 times a day. Mass movements are also initiated by the presence of food within the stomach. Known as gastrocolic reflex-mediated by motilin
-when fecal matter enters and stretches the rectum, stretch receptors measure the degree of stretch. If stretch is above a certain level, signals are sent to the spinal cord and the defecation reflex is initiated. Parasympathetic output to the rectum and anal canal results in relaxation of the internal anal sphincter and waves of contraction of the rectal muscularis. Conscious input from the cerebral cortex maintains closure of the external anal sphincter-unless rectal stretch reaches threshold level. Defecation occurs with relaxation of the external sphincter coupled with powerful contractions of the descending colon, sigmoid colon, and rectum. It's assisted by the rise in intra-abdominal pressure created by the contraction of the diaphragm and abdominal muscles. The muscles of the pelvic floor must also relax during defecation.