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Digestion and Absorbtion



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  1)Physiology of digestion
      A)Major organs of digestion 
           a)Mouth
           b)Stomach
               -What is an Enzyme
                   =Gastric Lipase
               -What is a Hormone
                   =Hormone Gastrin
           c)Small intestine
           d)Duodenum
           e)Large intestine
              -Peristalsis & Segmentation
      B)Auxilary organs of digestion
      C)Digestion-Protein digestion
                 -Fat digestion
                 -CHO digestion
  2)Absorbtion
      -Absorption of nutrients
      -Passive
      -Facilitated
      -Active

  3)Portal, Lymphatic & Entero Hepatic circulation


For more detailed information on pathophysiology of digestion click here





















 

            Physiology of digestion

     Cooking food can be viewed as first step 
in digestion. It denatures-unfolds protein and
softens tough connective tissue in meat and 
fibers in plants such as broccoli.


Click here to see an animated movie on digestion and take quick quiz. An excellent
tool for better comprehension especially for those of you, who do not know anything about 
digestion. 
 


    
   

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Major organs of digestion Mouth Esophagus Pharyngs Stomach Laryngs Small intestine Large intestine To view and learn more about these organs of digestive system click here

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Auxiliary organs- help digestion Tongue Liver Teeth Pancreas Salivary glands

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Mouth

Chevying food is the initial part of digestive process. This mechanical process mixes food with saliva and forms bolus. Saliva consists of-enzymes-salivary amylase -Lysozyme-breaks down bacteria -Mucous-lubricates food In the mouth we have beginning of starch digestion . Salivary amylase breaks down starch into dextrins and maltose. No other chemical digestion takes place in the mouth. From mouth bolus continues through PHARYNGS. In the Pharyngs bolus passes over EPIGLOTTIS -valve like flab of tissue that covers the entrance to the trachea-the windpipe. When food is swallowed it travels through Esophagus-structure that connect mouth and stomach. Before food enters stomach, it passes through first of the five SPHINCTERS (structure that controls flow of food throughout GI tract). We call this particular sphincter LOWER ESOPHAGEAL SPHINCTER or CARDIAC SPHINCTER. The Lover esophageal sphincter prevents back-flow of food from stomach back into esophagus. This sphincter, as well as all the others, are controlled by either hormonal or neural stimulation. To see organs of GI tract interactively click here

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STOMACH Stomach is a place where food is further processed to be more available for absorbtion in small intestine. In the stomach bolus is mixed with stomach secretions and becomes CHYME. Stomach has several secretions that are important for proper digestive function. They are: -HCL-hydrochloric acid -PH 2 -Pepsinogen-inactive form of enzyme Pepsin that helps to digest protein -Mucous-forms thick layer over the stomach walls to prevent auto digestion -Gastric lipase-enzyme breaking down short and medium fatty acids -Intrinsic factor-necessary for absorbtion of vitamin B12 -Gastrin-hormone that- stimulates release of HCL and psinogen. -closes Lover esophageal sphincter after food enters stomach -relaxes pyloric sphincter when food is ready to pass into small intestine. -it is activated through our visual sense, smell of food or chewing of food. We said that in mouth only CHO is partially broken down to Dextrins. During the process of MASTICATION Stomach prepares itself for upcoming food. Gastrin is released and gives signal to increase production of HCL and Pepsinogen. Once food reaches stomach PH of stomach reaches PH 2 and pepsinogen now converts into its active form PEPSIN. Pepsin will break down protein into smaller chain of amino acids that we call Peptones. Peptones are still too large for absorption, however, and have to wait for their complete digestion in small intestine. High acidity of stomach disables salivary amylase. That is why at this point there is no digestion of CHO in the stomach. It stops here. In the stomach there is only a little bit of absorbtion, mostly only water, alcohol, medication and some fats (short and medium f.a.) are absorbed here. Click here to see histology of stomach

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Small intestine From stomach food passes into small intestine. Stomach is connected with small intestine through PYLORIC SPHINCTER that allows only small amount of food to enter the small intestine at a time. We have three parts of small intestine 1) Duodenum-at this point bile and pancreatic enzyme enter small intestine through common Bile duct 2) Jejunum 3) Illeum The most digestion of food is completed in duodenum and upper portion of jejunum.

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Large intestine Mass of digested food passes from small intestine into large intestine through another sphincter, we call this sphincter ILLEO-CECAL VALVE. Large intestine consists of -cecum -ascending colon -transverse colon -descending colon -sigmoid colon Very little digestion takes place here. From here residue of food leaves body as feces. Now lets have a look on how food is actually digested. Before we do so lets identify few terms that we will be referring to throughout or journey through GI tract

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Enzyme-small protein like catalysts that increase rate of digestion - digestive enzymes are mostly made by pancreas - are PH sensitive - substrate specific - have suffix "ASE" Click here to see how enzymes work- click on play buttom when page loads

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Hormone- a substance that is produced at one place, then secreted into blood stream after which it travels to its targeted place where it performs it's action It can be- protein based -insulin - fat based -estrogen PERISTALSIS - Movement of food along the GI tract achieved by contraction and relaxation of longitudinal muscles. SEGMENTATION- Contraction of circular muscles along GI that divides mass into smaller pieces, and allows for mixing of intestinal content.

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Gastric Lipase- Acts in small intestine upon small and medium chain fatty acids
and break them down into glycerol, f.a. , and mono- glycerol. These can be absorbed directly into blood stream from the stomach wall in limited amount. Most of them are, however absorbed in the small intestine.

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Hormone gastrin besides stimulating release of HCL and also contracts Lower esophageal sphincter to prevent back flow of food into esophagus, but also relaxes pyloric sphincter to release small amount of CHYME into duodenum.

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Duodenum. Once chyme enters duodenum, hormone secretin is stimulated by its presence, and is released into blood stream. It reaches pancreas and stimulates release of BICARBONATE that will neutralize the highly acidic Chyme bringing its PH to 5-7. At the same time hormone GIP-gastric inhibitory peptide is released and it tells Pyloric sphincter that enough Chyme is present for the moment. It closes the pyloric sphincter and also tells stomach to reduce production of HCL and Pepsinogen. GIP regulates the interval of relaxing and contracting of pyloric sphincter based on the representation of protein, fat and carbohydrates in the chyme. Presence of lot of -fat causes pyloric sphincter to stay closed for a longer period of time. -Protein has intermediate effect -CHO causes pyloric sphincter to be opening in fairly quick intervals From the wall of small intestine hormone CCK-cholecystokinin is released. It reaches gall bladder and causes it's contraction to release bile, but also pancreas to release enzymes for Charbohydrate digestion- Pancreatic amylase

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Protein digestion When protein splitting enzymes enter duodenum, they are activated by enterokinase and turn into their active form. They will act upon peptones and break them down to peptides and a.a. These can be absorbed into the wall of small intestine where they will be finally all broken into single a.a for absorption by amino peptidase. - Enzymes for protein digestion - trypsin -chymotrypsin -carboxytripsin

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Fat digestion But CCK also relaxes another sphincter we didn't mentioned yet-SPHINCTER OF ODI(located in common bile duct) that allows mixed bile and pancreatic enzymes to enter duodenum. When bile enters duodenum it will together with lecithin- produced also in the small intestine- emulsify (break down) large globules of TG (long chain f.a).into smaller globules of TG, to make them more available for break down by Pancreatic Lipase- an enzyme made in the pancreas. When that happens, we end up with glycerol, mono-glycerol, and f.a.. This is packaged into micelle, and shifted into the wall of small intestine. Here it is repackaged into lipoprotein called chilomicrone for distribution throughout our body. Enzyme needed for fat digestion in the small intestine - pancreatic lipase

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Carbohydrate Digestion When pancreatic amylase enters duodenum it will resume CHO digestion. Dextrins will be broken down into disacharides and monosacharides From the wall of small intestine enzymes - maltase -lactase -sucrase are released to break down these disacharides into monosacharides. At this point they can be absorbed.

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ABSORPTIONS Most absorption takes place in the small intestine. There is only little bit of absorption in the stomach and large intestine. The wall of small intestine is folded to increase surface area. Each fold has finger like projections that we call VILLI. Each villi ( you can imagine them like fingers sticking out of the fold) is made up of numerous absorptive cells-cells that do the actual absorption. We call these absorptive cells also ENTEROCYTES. Each of these cells contain a microvili that is covered with hair like projections that we call GLYCOKALYX. Through them we absorb our nutrients. All the absorptive cells are produced in the crypts deep in mucous lining of small intestine. They travel on the surface of Villi where they are eventually slaughed off and replaced by new ones every 2-5 days Click here to see histology of small intestine We distinguish between four types of absorption a) Passive b) Facilitated d) Active e) Endocytosis-phagocytosis-cell "eating" -pinocytosis-cell" drinking"

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Passive-no energy or carrier for transport of the nutrient into the absorptive cell is needed. - We need high concentration of the nutrient in the GI(difference in concentration makes this transport possible) - nutrients transported-water most fats, some minerals.

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Facilitated-carrier needed, no energy needed - made possible because of difference in concentration of nutrient in GI -nutrient absorbed-Fructose

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Active-Carrier and energy is needed (ATP) -nutrients in low concentration can be absorbed -against concentration gradient -nutrients absorbed-glucose, galactose, amino acids, and other nutrients Endocytosis- Infants' absorption of antibodies from mothers milk.

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Portal and lymphatic circulation We can divide the villi in the small intestine into 2 different circulatory systems a) Portal b) Lymphatic The choice of circulatory systems that will be used for nutrient absorption depends on nutrient's solubility in water or organic solvent. Portal Nutrients soluble in water will enter portal circulatory system(protein, CHO, short and medium chain f.a.-butter) they will enter portal vein that collects nutrients from the little capillaries running through the intestinal villi, and will go to liver for processing before they enter general circulation. Lymphatic system Carries either fat soluble vitamins A, D, K, E, or long chain fatty acids - chylomicrones. They are too big to pass through the small capillaries. They enter thoracic duct, and travel all the way up close to the heart until they enter general circulation at the subclavian vein. Enterohepatic circulation This is a path that is used for recycling of bile . 98% of bile is recycled ,and returns back to the liver to be reused .

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ABSORPTION OF NUTRIENTS a)Protein - Peptides and amino acids are actively absorbed - I the liver we can make from amino acids-protein, glucose, energy, fat. B)Fat - fatty acids and monoglycerides -passively absorbed -short and medium chained -less than 12 carbons-they are water soluble =go to portal vein. - long chained amino acids-12 or more carbons-reform TG and transport via lymphatic system. C) Carbohydrates -Fructose=facilitated -Glucose and galactose=active -portal vein-liver-glycogen, fat , energy, release to blood.
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