Digestive Physiology - the

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Digestive Physiology - the How component

• By Karla Nix and Donna Mook

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What is Digestive Physiology?

• The Physiology of digestion is how the anatomical
  components of the digestive system actually
  digest intaken food. The anatomical components
  complete digestion by breaking down complex,
  large food substances into smaller, simpler
  substances. These simpler substances are then
  absorbed by the system and any wastes are
  excreted from the system.

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Digestion

• Digestion is accomplished by
• Ingestion taking in food substances
• Propulsion the movement of the food bolus
• Mechanical/Chemical Digestion Absorption of
  nutrients usually in small intestine
• Defecation how solid wastes are excreted
  accomplished by the large intestine

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Mechanical Digestion

• Occurs in area from mouth to small intestine and
  involves
• Mastication chewing
• Degultition both voluntary and involuntary
• Churning in stomach segmentation in small
  intestine

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Chemical Digestion

• Occurs in area from mouth to small intestine and
  involves
• Major portion in stomach and upper small
  intestine
• Saliva components and many gastric juices, which
  are acidic (low pH), and break down food particles

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Digestive Activity Control

• By hormone producting cells and local nerve
  plexuses
• By CNS autonomic nerves

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Physiological Travel through the Digestive
System

• Mouth
• Stomach
• Small Intestine
• Large Intestine

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The Mouth (Oral Cavity)

• Digestion begins here, both mechanical and
  chemical
• Salivary glands found here produce saliva, an
  aqueous substance

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Saliva

• Consists mainly of water
• Slightly acidic
• Salivation controlled by PNS through 7th and 9th
  cranial nerves

• Contains
• Defensin produced when trauma occurs
• Salivary Amylase enzyme polysaccharides
  monosaccharides
• Elecrolytes
• Protein mucin to lubricate oral cavity
• IgA antibodies
• Nitric Oxide bacteriostatic enzyme
• Lysozyme bacteriostatic enzyme

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Stomach

• The stomach is a major site of chemical
  digestion, as one would imagine. The digestion of
  proteins here, leads to Chyme production.
  Parietal cells of the stomach produce Intrinsic
  factor, which is the stomachs most important
  function. Without intrinsic factor, Vitamin B12
  would not be absorbed and this would lead to
  immature RBCs resulting in a form of anemia
  known as Pernicious anemia. The PNS is chiefly
  involved here, and stretch receptors of the
  stomach act on the PNS, causing gastrin
  production from chief and parietal cells, once
  they are activated by a full stomach. The PNS
  is also responsible for churning in the stomach,
  or what we refer to as growling.

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The Stomach (continued)

• Basic(high pH) foods, caffeine, and partially
  digested proteins lead to gastrin secretion.
  Gastrin leads to HCL secretion from parietal
  cells and pepsinogen secretion from chief cells.
  HCL creates an acidic environment necessary for
  protein digestion and pepsinogen activation.
  About 3mL of chime is ejected into duodenum by
  each peristaltic wave, the remaining chyme is
  moved back and churned again by the stomach.
  This achieves a thorough mixing and also a
  breaking down of food particles. Stomach emptying
  usually takes 4 hours , with large amount of
  liquids making emptying faster, and fatty meals
  making emptying slower!

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Gastric Secretion regulation in Stomach

• Cephalic----an example is seeing cakes at the
  bakery---the act of seeing or thinking of food
  stimulates vagus nerve of PNS and causes gastric
  secretion.
• Gastric phase----gastrin and vagus nerve
  stimulate secretion after food has already
  entered stomach
• Intestinal----increased secretion (food entering
  duodenum) followed by inhibition of gastric
  secretions and pyloric sphincter
  closure---allowing food to enter intestine in
  small portions and at intervals
• HCL secretion controlled by PNS nerve fibers,
  gastrin, and histamine

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Small Intestine

• Main function is absorption
• Food remains here 4-6 hours
• Intestinal juice (water and mucusalkaline)
  produced by Brunners glands in duodenum and
  intestinal goblet cells, when acidic food enters
  duodenum
• Segmentation ----mixing of foods accompanied by
  enzymatic action
• Bile delivered to duodenum by way of bile
  duct----hormone CCK is stimulus for bile entering
  duodenum, rather than its normal storage in the
  gall bladder

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Bile

• Consists of bile salts, phospholipids, bile
  pigments and cholesterol
• Functions to emulsify (mix) fats causing their
  breakdown
• Recyclable bile salts act to facilitate
  cholesterol and fat absorption, and also keep
  cholesterol dissolved in bile

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Pancreatic Juice

• Aqueous in nature
• Contains protein digesting proteases along with
  enzymes amylases , lipases, and nucleases
• Contains bicarbonate ions which provide a basic,
  high pH, environment for optimal pancreatic
  enzyme function, and also neutralize the acidic
  chyme

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Absorption in Small Intestine

• Most foodstuffs, water, and electrolytes absorbed
  here
• Fat and water absorbed by passive transport---
  glucose, fructose, galactose and amino acids
  absorbed by active transport
• Fat absorbed in mucosal surface as fatty acids
  and monoglycerides---both of which have been
  broken down from larger structures
• Chylomicronslipids combined with proteins in
  intestinal cells, they enter the lacteals and are
  carried to thoracic duct
• Most vitamins diffuse easily into cells w/
  exception of vitamin B12
• Peristalsis---starts after most nutrients
  absorbed and allows the contents of the small
  intestine to enter cecum by its waves of motion

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Large Intestine

• Functions to absorb water from undigested food
  and excretion of semi-solid and solid wastes
• Bacterial flora found here ferment carbohydrates
  (partially or undigested) resulting in flatus,
  and synthesize Vitamins K and B
• Nutrient-poor food usually spends 24 hours here
  
• Very little digestion takes place
• Water and few electrolytes reabsorbed into colon
• Propulsion of fecal matter is a main function and
  it is carried out by food residue-filled
  haustrums contract and propel residue to another
  haustrumthis is a slow movement accompanied by
  further water re-absorption

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Metabolism

• Enzymatic action leads to food being absorbed
  into blood stream, then the blood carries the
  nutrients to the cells.
• At the cellular level, nutrients are broken down
  by reactions or used to further synthesize
  carbohydrates, proteins and fats
• Reactions in body
• Either---ANABOLICGENESIS------smaller substance
  put together to make larger, complex
  substances---requires energy, or
  CATABOLIC----LYSIS---larger substances broken
  down to form smaller substances---usually by
  hydrolysis----releases energy
• Glucose is the most immediate form of energy
  (ATP) found in the body
• ATP is the ONLY usable form of energy in the
  bodyevery energy form must be broken down or
  converted into ATP in order for the body to take
  advantage of it

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How Glucose (Carbohydrate) is converted to ATP
and used by the Body

• Glycolysisthe anaerobic pathway or breakdown of
  glucose occurring in cytoplasm of cell
• 2?Glycolysis Glucose 2NAD 2ATP 2P----
  pyruvic acid 2NADH 2ATP ( a net gain of 2
  ATP molecules for every one glucose)
• Glycolysis used by mitochondria-lacking RBCs
  and by skeletal muscles for short periods of
  activity

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Carbohydrate Metabolism continued

• Pyruvic acid produced by glycolysis rxn has a
  high bond-energy
• If O2 is available, pyruvic acid enters
  mitochondria, where they are broken down and used
  in rxn to make Acetyl CoA
• Acetyl CoA helps in formation of citric
  acid---also 1st step of Krebs Cycle
• Krebs Cycle--- aerobic metabolism an
  oxidative rxn, occurs in mitochondria, and
  produces carbon dioxide, ATP and NADH
• Electron Transport Chain---aerobic rxn in
  mitochondria whose products meet 90 of the
  bodys ATP needs------WOW !!!!
• 1 glucose molecule processed leaves the cell with
  36 net ATP molecules
• More ATP is produced in the mitochondria than the
  cytoplasm

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Protein Metabolism

• Proteins broken down into their amino acid
  building blocks which are then oxidized to form
  ATP. Ammonia produced is converted to urea by the
  liver and excreted in the urine
• Synthesize new proteins, enzyme synthesis,
  antibody production ----protein synthesization
  promoted by growth hormone, estrogen,
  testosterone, and thyroid hormones
• Remaining amino acids used in GLUCONEOGENESIS
  (making glucose) or LIPOGENESIS (making lipids)

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Lipid Metabolism

• Non-soluble triglycerides are combined with
  proteins and transported into blood
• LIPOPROTEINS---combination of fat and
  protein----chylomicrons, VLDL,LDL,HDL
• Chylomicrons transport lipids in diet, VLDLs
  transport lipids produced by the body to adipose
  tissue, LDLs carry the majority of blood
  cholesterol (High LDL can form plaques), and HDLs
  remove excess cholesterol from blood and cells
  which is removed after transport to the liver
• Fats can be oxidized to produce
  ATP---triglycerides broken into fatty acids and
  glycerol
• Glycerol can be used to produce ATP also, but it
  is converted to glucose if the body does not need
  ATP

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Clinical Applications

• Pernicious Anemiamentioned previously---a type
  of anemia that occurs when there is an absence of
  intrinsic factor (by parietal cells) in the
  stomach
• Gastric ulcers---condition where mucosal barrier
  of stomach broken which results in an erosion of
  the surface epithelium----can cause bleeding,
  perforation and possibly peritonitis
• Gall Stones----due to a decrease in bile salts,
  which leads to crystallization and ultimately
  gall stone formation
• Hirschsprungs disease----condition where large
  intestine lacks parasympathetic
  ganglia---especially in rectal area----leads to
  chronic constipation, and must be treated
  surgically