Human Physiology: Digestion

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Human PhysiologyDigestion

• Topic 6.1, 11.3Option H2, H3, H4

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3 Dietary Categories

• Herbivores
• Cattle, gorillas, snails, and sea urchins,
• eat autotrophs (plants and algae)
• Carnivores
• Lions, hawks, spiders, and snakes
• Ingest other animals
• Omnivores
• Crows, cockroaches, raccoons, and humans
• Ingest both plants and animals

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How do animals obtain and ingest their food?

• Suspension feeders
• Extract food particles suspended in the
  surrounding water
• Ex. Clams and oysters
• Substrate feeders
• Live on or in their food source and eat their way
  through it.
• Ex. Caterpillars and earthworms
• Fluid feeders
• Obtain food by sucking nutrient nutrient-rich
  fluids from a living host, either a plant or an
  animal.
• Ex. Mosquitoes and ticks
• Bulk feeders
• Ingest relatively large pieces of food
• Ex. most animals

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Overview Food processing

• Four stages
• Ingestion
• The act of eating
• Digestion
• The breaking down of food into molecules small
  enough for the body to absorb.
• Two phases
• 1. Breaking food down mechanically
  (teeth?chewing) into smaller pieces
• 2. hydrolysis, chemical breakdown, catalyzed by
  enzymes
• Absorption
• Cells lining the digestive tract take up
  (absorb) the products of digestionsmall
  molecules such as amino acids and simple sugars
• Nutrients travel through blood to cells,
  where they are made into macromolecules or
  further broken down for energy
• Elimination
• undigested material passes out of the
  digestive tract

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General compartments for digestion

• Food vacuoles are the simplest digestive
  compartments.
• Phagocytosis cell engulfs food particle, which
  then fuses with a lysosome.
• Most animals have an alimentary canal, a
  digestive tube with two openings, a mouth and an
  anus.
• Allows food to move in one direction, with
  specialized regions in the digestive tube that
  carry out digestion and absorption of nutrients
  in sequence

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General compartments for digestion

• Food entering the mouth usually passes into
• A pharynx, or throat
• Then passes into the esophagus
• Passed to stomach, muscular and churns and grinds
  food
• Chemical digestion and nutrient absorption occur
  mainly in the intestine
• Undigested materials are expelled though the
  anus.

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Human Digestive Tract

• Main parts of human alimentary canal
• Mouth, oral cavity, tongue, pharynx, esophagus,
  stomach, small intestine, large intestine,
  rectum, and anus.
• Main digestive glands
• Salivary glands, pancreas, and liver
• Secrete digestive juices that enter the
  alimentary canal through ducts.
• Secretions from liver are stored in gallbladder
  before they are released into the intestine.

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Human Digestive Tract

• Peristalsis
• Rhythmic waves of contractions of smooth muscles
  in the walls of the digestive tract
• Once food is swallowed, peristalsis propels it
  through the alimentary canal.
• In only 5-10 seconds, food passes from the
  pharynx down the esophagus and into the stomach.
• Pyloric sphincter, a muscular ringlike valve,
  keeps food in the stomach by regulating the pass
  of food into the small intestive.
• Works like a drawstring, closing off the tube and
  keeping food in the stomach long enough for
  stomach acids and enzymes to begin digestion.
• http//nutrition.jbpub.com/resources/animations.cf
  m?id1debug0

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Human Digestive Tract

• Final steps of digestion and nutrient absorption
  occur in the small intestine over a period of 5-6
  hours.
• Undigested material moves slowly through the
  large intestine (taking 12-24 hours), and feces
  are expelled through the anus.

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Digestion Begins in the Oral Cavity

• Saliva is produced by the salivary glands through
  ducts to the oral cavity even before you eat
  its a response to presence of food
• Sight or smell of food causes nerve stimulation
• In a typical day, salivary glands secrete more
  than a liter of saliva

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Digestion Begins in the Oral Cavity

• Saliva contains several substances necessary for
  food processing
• Slippery glycoproteins
• Protects the soft lining of the mouth and
  lubricates food for easier swallowing
• Buffers
• Neutralize food acids, helping prevent tooth
  decay.
• Antibacterial agents
• Kill many of the bacteria that enter the mouth
  with food.
• Salivary amylase
• Digestive enzyme that begins hydrolyzing food

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Digestion Begins in the Oral Cavity

• Oral Cavity
• Mechanical and chemical digestion begins in the
  oral cavity.
• Chewing cuts, smashes, and grinds food, making it
  easier to swallow and exposing more food surface
  to digestive enzymes
• Teeth and tongue are prominent
• Teeth grind and crush food
• Tongue, muscular organ with taste buds, allows
  you to taste your meal and manipulates food and
  helps shape it into a ball called a bolus.
• In swallowing, it pushed the bolus to the back of
  the oral cavity and into the pharynx.

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Pharynx- Swallowing

• Pharynx has openings for both the esophagus and
  the trachea (wind-pipe)
• Most of the time, esophageal opening is closed
  and air enters the trachea and proceeds to the
  lungs.
• When you eat
• a bolus of food enters the pharynx, triggering
  the swallowing reflex
• The esophageal sphincter relaxes and allows the
  bolus to enter the esophagus
• Larynx (voice box) moves upwards and tips the
  epliglottis over the tracheal opening.
• Epiglottis prevents food from passing into the
  trachea.
• After the bolus enters the esophagus, the larynx
  moves downward, the epiglottis moves up again,
  and breathing passage reopens
• Esphogas sphincter contracts above the bolus.

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Esophagus Peristalsis

• Esophagus is a muscular tube that conveys food
  boluses from the pharynx to the stomach.
• Muscles at the very top of esophagus are under
  voluntary control thus, the act of swallowing
  begins voluntarily.
• Then, Involuntary waves of contraction by smooth
  muscles in the rest of the esophagus take over.

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Esophagus Peristalsis

• As food is swallowed, muscles above the bolus
  contract, pushing the bolus downward
• Simultaneously, muscles around the bolus relax,
  allowing the passageway to open.
• Muscle contractions continue in waves until the
  bolus enters the stomach.
• Waves of smooth muscle contraction also move
  materials through small and large intestine

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Stomach stores and breaks down food

• Stomachs are the main reason we do not have to
  eat constantly
• Highly elastic and can stretch to accommodate
  about 2 Liters of food and drink, usually enough
  to satisfy our bodys needs for many hours.
• Some digestion occurs in the stomach.
• The stomach secrete gastric juice
• made up of mucus, enzymes, and strong acid.
• Functions to break apart the cells in food
• Also kills most bacteria and other microbes that
  are swallowed with food.

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Stomach stores and breaks down food

• Stomach wall is highly folded, and has pits that
  lead to tubular gastric glands.
• Three types of cells that secrete different
  components of the gastric juice
• Mucous cells secrete mucous, which lubricates
  and protects the cells lining the stomach
• Parietal cells secrete HCl acid
• Chief cells secrete pepsinogen, an inactive form
  of the enzyme pepsin

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Stomach stores and breaks down food

• Pepsinogen, HCl, and pepsin
• 1. Pepsinogen and HCl are secreted into the lumen
  (cavity) of the stomach.
• 2. HCl converts pepsinogen to pepsin.
• 3.Pepsin then activates more pepsinogen, starting
  a chain reaction. Pepsin begins the chemical
  digestion of proteins. Proteins will be further
  digested in small intestine.

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Stomach stores and breaks down food

• Prevention of gastric juice from digesting away
  stomach lining
• Secreting pepsin in the inactive form of
  pepsinogen helps protect the cells of the gastric
  glands
• mucus helps protect the stomach lining from both
  pepsin and acid.
• Still, epithelium is constantly eroded enough
  new cells are generated by mitosis to replace the
  stomach lining completely about every three days.

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Stomach stores and breaks down food

• Gastric glands are regulated by a combination of
  nerve signals and hormones
• When you see, smell, or taste food, a signal from
  your brain to your stomach stimulates your
  gastric glands to secrete gastric juice.
• Once food is in your stomach, substances in the
  food stimulate cells in the stomach wall to
  release the hormone gastrin in the circulatory
  system.
• Gastrin circulates in the blood stream, returning
  to the stomach wall, stimulating further
  secretion of gastric juice.
• As much as 3L of gastric juice may be secreted a
  day.
• A negative-feedback mechanism inhibits secretion
  of gastric juice when the stomach contents become
  too acidic.
• Acid inhibits the release of gastrin, and with
  less gastrin in the blood, the gastric glands
  secrete less gastric juice.

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Stomach stores and breaks down food

• About every 20 seconds, the stomach contents are
  mixed by the churning action of muscle in the
  stomach wall and result in acid chyme.
• Opening between the esophagus and the stomach is
  usually closed until a bolus arrives.
• Backflow of acid chyme causes heartburn (which
  should be called esophagus-burn)
• Can also cause acid-reflux (gastroesophageal
  reflux disease GERD)

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Stomach stores and breaks down food

• Pyloric sphincter helps regulate the passage of
  acid chyme from the stomach into the small
  intestine.
• The stomach takes about 2-6 hours to empty after
  a meal acid chyme leaves stomach only a squirt
  at a time.
• Acid chyme rich in fats stimulates the small
  intestine to release a hormone that slows the
  emptying of the stomach, providing more time for
  digestion.
• Other hormones secreted by the small intestine
  influence the release of digestive juices from
  the pancreas and gall bladder.

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Gastric Ulcers

• Gastric Ulcers
• Open sores that form when mucus, which normally
  protects the stomach wall from the corrosive
  effect of digestive juice, fails to protect it.
• Small intestine and esophagus are also
  susceptible to ulcers
• Symptoms are usually gnawing pain in the upper
  abdomen, which may occur a few hours after
  eating.
• Were formerly thought to result from the
  production of too much pepsin/and or acid or too
  little mucus
• For years, the blame was put on factors that
  cause these effects, such as aspirin, ibuprofen,
  smoking, alcohol, coffee, and stress
• However, strong evidence now points to

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Gastric Ulcers

• H. pylori
• A spiral-shaped bacteria
• Low pH of the stomach kills most microbes, but
  not this one!
• Burrows beneath mucus and releases harmful
  chemicals
• Growth seems to result in a localized loss of
  protective mucus and damage to the cells lining
  the stomach
• WBC fight infection, causing mild inflammation of
  the stomach, called gastritis.
• Gastric ulcers form when pepsin and HCl destroy
  cells faster than the cell can regenerate from
  the H. pylori attack.
• Eventually, stomach will erode to the pint where
  it actually has a hole in it, which can lead to a
  life threatening infection in abdomen or internal
  bleeding.
• 70-90 of ulcer and gastritis sufferers have this
  bacterial infection
• Also found in 30 of healthy people.? linked to
  certain kinds of stomach cancer

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Gastric Ulcers

• Treatment
• Usually respond to a combination of anti-biotics
  and bismuth (the active ingredients of
  Pepto-Bismol) which eliminates bacteria and
  promotes healing.
• Drugs that reduce stomach acidity may also help,
  and researchers are working on preventitive
  treatment for H.Pylori.

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Small Intestine

• Once at the S.I., food has been mechanically
  reduced to smaller pieces and mixed with juices
  it now resembles a thick soup.
• Starch digestion began in the mouth, and protein
  breakdown began in the stomach.
• All other chemical digestion occurs in the s.i.
• Nutrients are also absorbed into the blood from
  the s.i.
• Length of over 6m, making it the longest organ of
  the alimentary canal. Diameter is only about 2cm,
  which is why its called the small intestine.

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Small Intestine

• Contributing to digestion in s.i. are two large
  glandular organs pancreas and liver.
• Pancreas
• Produces pancreatic juice
• a mixture of digestive enzymes and an alkaline
  solution rich in bicarbonate
• Alkaline solution neutralizes acid chyme as it
  enters the small intestine

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Small Intestine

• Liver
• Performs a wide variety of functions, including
  the production of bile
• Contains bile salts that emulsify fats, making
  them more susceptible to attack by digestive
  enzymes.
• Gall bladder stores bile until it is needed in
  the small intestine.

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Small Intestine

• First 25 cm or so of the s.i. is called the
  duodenum.
• Where acid chyme squirted from the stomach mixes
  with bile from the gall bladder, pancreatic juice
  from the pancreas, and digestive enzymes from
  gland cells in the in the intestinal wall.

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Small Intestine

• All four types of macromolecules (carbohydrates,
  proteins, nucleic acids, and fats) are digested.
• Refer to table 21.11 on pg. 438

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Small Intestine

• Carbohydrate digestion
• Begins in the oral cavity and is completed in the
  s.i.
• Pancreatic amylase hydrolyzes starch (a
  polysaccharide) into the disaccharide maltose
• Maltose is then hydrolyzed into glucose via
  maltase.
• Sucrase hydrolyzes table sugar and lactase
  digests milk sugar (lactose)

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Small Intestine

• Protein digestion
• S.i. completes protein digestion from the stomach
• Pancreas and duodenum secrete hydrolytic enzymes
  that completely dismantle polypeptides into amino
  acids.
• dipeptidase
• Hydrolyzes fragments only two or three amino
  acids long.
• trypsin and chymotripsin
• Break polypeptides into smaller polypeptides
• Trypsinogen (in pancreas) is converted into
  trypsin by the action of enteropeptidase (the
  enzyme that is bound to the membranes of the
  small intestine).
• aminopeptidase and carboxypeptidase
• Split off one amino acid at a time, working from
  both ends of a polypeptide.

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Small Intestine

• Nucleic acid digestion
• Nucleases hydrolyzes the nucleic acids in food.
• From the pancreas
• Split DNA and RNA (which are present in the cells
  of food items) into their component nucleotides,
  which are then broken down into nitrogenous
  bases, sugars, and phosphates by other enzymes
  produced by the duodenal cells.

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Small Intestine

• Fat digestion
• Most fat remains undigested until it reaches the
  duodenum.
• Hydrolysis of fats is problematic due to fats
  insolubility in water. Emulsification Problem
  Solved!!!
• Bile salts in bile cause fat globules to be
  physically broken up into smaller fat droplets, a
  process called emulsification.
• Many small droplets allows for a larger surface
  area of fat exposed to lipase, an enzyme that
  breaks fat molecules down into fatty acids and
  glycerol.

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Small Intestine

• Problems with lipid digestion in a hydrophillic
  medium
• Lipids tend to coalesce (lump together) and are
  only accessible to lipase at the lipid-water
  interface.
• Bile molecules have a hydrophobic end and a
  hydrophilic end which emulsifies (prevents from
  coalescing) the lipids
• Lipase must be water-soluble and has a
  hydrophobic active site (for its substrate,
  lipids)
• The increased surface area allows lipase greater
  access to its substrate

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Small Intestine

• By the time persistalsis has moved the mixture of
  chyme and digestive juices through the duodenum,
  chemical digestion of your meal is just about
  complete.
• Main function of the rest of the small intestine
  is the absorption of nutrients and water.

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Small Intestine

• Structurally, great for nutrient absorption.
• Lining has a huge surface area roughly 300 m2,
  about the size of a tennis court
• Extensive surface area results from several kinds
  of folds and projections.
• Villi large circular folds with numerous, small
  fingerlike projections around the inner wall of
  the s.i.
• Microvilli many tiny surface projections found
  on epithelial cells lining a villus.
• extend into the lumen of the intestine and
  greatly increase the surface area across which
  nutrients are absorbed.

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Small Intestine

• Some nutrients are absorbed via simple diffusion
  other nutrients are pumped against concentration
  gradients into the epithelial cells
• The core of each villus is penetrated by a small
  lymph vessel and a network of capillaries.
• After fatty acids and glycerol are absorbed by an
  epithelial cell, these building blocks are
  recombined into fats and are transported into the
  lymph vessel.
• Amino acids and sugars pass out of the intestinal
  epithelium and then across the thin walls of the
  capillaries into the blood.

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Small Intestine

• Capillaries that drain nutrients away from the
  villi converge into larger veins and eventually
  into a main vessel, the hepatic portal vein, that
  leads directly to the liver
• Liver gets first access to nutrients absorbed
  from a meal
• Converts many nutrients into new substances that
  the body needs.
• One of its main functions is to remove excess
  glucose from the blood and convert it to
  glycogen, which is stored in liver cells.
• From the liver, blood travels to the heart, which
  pumps the blood and the nutrients it contains to
  all parts of the body.

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Small Intestine

• Structural features of an epithelial cell of a
  villus
• Villi provide a huge surface area for
  absorption
• Epithelium cells single layer of small cells,
  packed with mitochondria the source of ATP
  (metabolic energy) for active uptake across the
  plasma membrane
• Pump proteins in the plasma membrane of
  epithelium cells actively transport nutrients
  across the plasma membrane into the villi
• Network of capillaries large surface area for
  uptake of amino acids, monosaccharides, and fatty
  acids and glycerol into blood circulation
• Lacteal branch of the lymphatic system into
  which triglycerides (combined with protein) pass
  for transport to body cells
• Mucus from goblet cells in epithelium
  lubricates movement of digested food among the
  villi and protects plasma membrane of epithelial
  cells
• Microvilli these tiny, finger-like infoldings
  of the cell surface facing the lumen of the gut
  greatly increase the surface area in contact with
  material to be absorbed.
• Mitochondria these organelles are present in
  large numbers, suggesting a significant demand
  for ATP in these cells.
• Pinocytotic vesicles these are the site of
  pinocytosis by which fluid is taken up or
  released in tiny vesicles, across the plasma
  membrane of a cell.
• Tight junctions these bind together the
  individual epithelial cells, so that the only way
  into the tissues of the body is through the
  epithelium.

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Small Intestine
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Small Intestine

• Mechanisms used by Ileum to absorb and transport
  food
• Facilitated diffusion. Some substances need a
  little assistance to enter and exit cells. The
  transmembrane protein helps out by changing
  shape.
• Active transport. Some substances need a lot of
  assistance to enter cells. Similar to swimming
  upstream, energy is needed for the substance to
  penetrate against an unfavorable concentration
  gradient.
• Endocytosis. Cells can use their cell membranes
  to engulf a particle and bring it inside the
  cell. The engulfing portion of the membrane
  separates from the cell wall and encases the
  particle in a vesicle.

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Small Intestine

• Material not absorbed and are egested
• cellulose and lignin from plant matter
• the remains of intestinal epithelial cells
• bile pigments
• bacteria

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Small Intestine

• Absorption vs. Assimilation
• Absorption involves the passage of digested
  nutrients into the blood from the
  gastro-intestinal tract, glucose, fructose and
  amino acids go straight to the blood capillaries,
  whereas fatty acids and monoglycerides so first
  into the lymphatic system and then the blood
  system. Assimilation involves the integration
  of these absorbed molecules into the living
  processes of the organism that ingested them,
  using them to build new molecules that are
  necessary for its normal functioning and
  survival. Or using them to produce energy

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Large Intestine

• Also known as the colon
• 1.5 m long and 5 cm in diameter
• It joins the small intestine at a T-shaped
  junction, where a sphincter controls the passage
  of unabsorbed food material out of the small
  intestine.
• One arm of the T is a blind pouch called the
  cecum.
• A small, fingerlike extension of the cecum is the
  appendix contains a mass of w.b.cs that make a
  minor contribution to immunity.

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Large Intestine

• Main function is to absorb water from the
  alimentary canal.
• About 7L of fluid enters the lumen of the
  digestive tract each day as the solvent of the
  various digestive juices.
• About 90 of this water is absorbed back into the
  blood and tissue fluids, with the small intestine
  reclaiming most of it and the colon finishing the
  job.

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Large Intestine

• As water gets absorbed, remains of the digested
  food become more solid as they are moved along
  the colon via peristalsis.
• Remains make feces, the waste products of
  digestion, which consist mainly of indigestible
  plant fibers

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Large Intestine

• Surprise, surprise!
• Bacteria, such as E.coli, live in your colon!
• Produce important vitamins (biotin, folic acid,
  B vitamins, and Vitamin K) that are absorbed into
  the bloodstream through the colon

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Large Intestine

• Rectum terminal portion of the colon where feces
  are stored until they can be eliminated.
• Strong contractions of the colon create the urge
  to defecate.
• Two rectal sphincters, one voluntary and one
  involuntary, regulate the opening of the anus.

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Large Intestine

• Diarrhea
• If the lining of the colon is irritated by a
  viral or bacterial infection, the colon is less
  effective in reclaiming water
• Constipation
• Peristalsis moves the feces along too slowly the
  colon reabsorbs too much water, and the feces
  become too compacted.
• Usually caused by a diet with not enough fiber or
  from a lack of exercise.

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Digestion Animation

• http//highered.mcgraw-hill.com/sites/0072495855/s
  tudent_view0/chapter26/animation__organs_of_digest
  ion.html

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Digestion Exocrine Gland Cells

• Exocrine glands are responsible for the release
  of digestive fluids.
• Exocrine glands secrete into ducts. Exocrine
  gland has a duct portion and a glandular portion.
  
• At the end of each branch is an acinus formed at
  secretory cells of two types serous cells (which
  secrete proteins such as enzymes), and mucous
  cells (which secrete mucus).

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Liver

• Liver performs more functions than any other
  organ in the body.
• After breaking down a.a. for energy or for
  recycling into other molecules, the liver
  prepares nitrogenous wastes (ammonia) for
  disposal synthesizing urea.
• It also converts toxins such as alcohol and other
  drugs into inactive products that can be excreted
  in urine.

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Liver

• Liver can synthesize bile for digestion of fats,
  as well as proteins essential to many of the
  bodys functions
• Proteins made in the liver include plasma
  proteins important in blood clotting and in
  maintaining osmotic balance of the blood, as well
  as lipoproteins that transport fats and
  cholesterol to body tissues.

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Liver

• Regulates the amount of glucose in blood.
• Converts glucose into glycogen, which it stores
  for later use.
• Plays a key role in body metabolism

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Liver

• Strategically located between the intestines and
  the heart
• Capillaries from the si and li converge into
  vessels that lead into the hepatic portal vein.
• Transports nutrients absorbed by the intestines
  directly to the liver.
• Gives the liver the opportunity to modify and
  detoxify substances absorbed by the digestive
  tract before the heart (and then the rest of the
  body)

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Liver
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Liver
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Liver

• Alcohol consumption
• Under heavy, frequent exposure to alcohol, damage
  may become so severe that abnormal scar tissue
  forms in the liver called cirrhosis
• Distorts the livers internal structure
• Scar tissue slowly replaces functioning liver
  tissue, gradually diminishing blood flow through
  the liver.
• Usually causes a build-up of toxins in blood,
  excessive bruising or bleeding, diabetes, and
  cancer.
• About 25,000 Americans die each year from
  alcohol-related cirrhosis.

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Liver

• Cirrhosis treatment
• Stop drinking!
• Liver transplant
• To prevent cirrhosis, U.S. guidelines advise not
  more than two alcoholic drinks for men and 2
  drink for women per day.

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Kidney

• Main processing centers of our excretory system
  are the two kidneys.
• Each is a compact organ, about the size of your
  fist, nearly filled with about 80 km of fine
  tubes (tubules) and an intricate network of blood
  capillaries
• Body contains about 5L of blood, which circulates
  repeatedly, allowing for about 1,100-2,000 L to
  pass though the capillaries in our kidneys every
  day.

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Kidney

• From this enormous (1,100-2000 L) traffic of
  blood, our kidneys extract daily about 180L of
  filtrate.
• Filtrate is made of water, urea, and a number of
  valuable solutes including glucose, amino acids,
  ions, and vitamins.
• If we excreted all the filtrate as urine, we
  would lose vital nutrients and dehydrate rapidly.
  
• But our kidneys refine the filtrate,
  concentrating the urea and returning most of the
  water and solutes to the blood.

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Kidneys
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Kidneys

• In a typical day, we excrete only about 1.5 L of
  urine.
• Plumbing plan of kidneys
• 1. Blood to be filtered enters each kidney via a
  renal artery
• 2. Blood that has been filtered leaves the kidney
  in the renal vein
• 3. urine leaves each kidney via a duct called a
  ureter and passes into the urinary bladder.
• 4. periodically, the bladder empties during
  urination via a tube called the urethra, which
  empties near the female vagina or through the
  male penis

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Kidney

• Has two main regions
• Renal cortex (outer layer)
• Renal medulla (inner region)
• Urine flows into a chamber called the renal
  pelvis, and from there into the ureter.

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Kidney
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Kidney

• Nephron
• Tiny functional units of the kidney
• About a million
• Consists of a nephron tubule and its associated
  blood vessels.
• Extracts a tiny amount of filtrate from the blood
  and then refines the filtrate into a much smaller
  quantity of urine.
• Each nephron starts and ends in the kidneys
  cortex some extend into the medulla

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Kidney

• Nephron (continued)
• Receiving end of the nephron is a cup-shaped
  swelling called the Bowmans capsule.
• At the other end of the nephron is the collecting
  duct, which carries urine to the renal pelvis.
• Bowmans capsule envelops a ball of capillaries
  called the glomerulus which together make up the
  blood-filtering unit of the nephron.
• 1. Here, blood pressure forces water and solutes
  from the blood in the glomerular capillaries
  across the wall of Bowmans capsule and into the
  nephron tubule.
• 2. This process creates filtrate, leaving blood
  cells and large molecules such as plasma proteins
  behind in the capillaries.

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Kidney

• Nephron (continued)
• Rest of the nephron refines the filtrate. The
  tubule has three sections
• 1. proximal tubule
• in the cortex
• 2. Loop of Henle
• A hairpin loop carrying filtrate toward in some
  cases, intothe medula and then back toward the
  cortex
• 3. Distal tubule
• A. Called distal because it is the most distant
  from Bowmans capsule
• B. Drains into a collecting duct, which receives
  filtrate from many nephrons
• C. in the kidneys many collecting ducts, the
  processed filtrate, urine, passes into the renal
  pelvis and then into the ureter.

82
Nephron
83
Kidney animation

• http//www.sumanasinc.com/webcontent/animations/co
  ntent/kidney.html

84
Kidney

• Nephron (continued)
• The intricate association between blood vessels
  and tubules is key to nephron function
• Two key networks of capillaries
• 1.Glomerulus
• A finely divided portion of an arteriole that
  branches from the renal artery.
• Leaving the Glomerulus, the arteriole carries
  blood to
• 2.Proximal and distal tubules
• This second network functions with the tubules in
  refining the filtrate some of the vessels
  parallel the loop of Henle, with blood flowing
  down in one vessel and back up through another.
• Leaving the nephron, the capillaries converge to
  form a venule leading toward the renal vein.

85
Kidney
86
Kidney

• Excretory system produces and disposes of urine
  in four major processes
• Filtration
• Occurs when water and virtually all other
  molecules small enough to be forced through the
  capillary wall enter the nephron tubule from the
  glomerulus.
• Reabsorption
• Refines filtrate water and valuable solutes,
  including glucose, salt, other ions, and amino
  acids are returned to the blood from the filtrate.

87
Kidney

• Secretion
• Also refines the filtrate substances in the
  blood are transported into the filtrate.
• For example, when there is an excess of H in the
  blood, these ions are secreted into the filtrate,
  thus keeping the blood from becoming acidic
• Eliminates certain drugs and other toxic
  substances from the blood
• both reabsorption and secretion, water and
  solutes move between the tubule and capillaries
  by passing through the interstitial fluid.
• Excretion
• Urine, the product of filtration, reabsorption,
  and secretion, passes through the kidneys to the
  outside via the ureters, urinary bladder, and
  urethra.

88
Kidney

• Reabsorption and Secretion in a nephron Figure
  25.11
• pink arrows show reabsorption , which may occur
  via active transport, passive diffusion, or
  osmosis.
• Blue arrows show secretion
• Colored area shows the interstitial fluid,
  through which solutes and water move between the
  tubules and capillaries
• Intensity of color reflects solute conc. cortex
  has lowest conc. and medulla has highest conc.
• Solute conc. of the interstitial fluid exceeds
  that of the filtrate? water moves by osmosis out
  of filtrate into interstitial fluid

89
Kidney

• Reabsorption and Secretion in a nephron Figure
  25.11 (continued)
• Proximal and distal tubule function
• Proximal tubule actively transports nutrients
  such as glucose and amino acids from the filtrate
  into the interstitial fluid, to be reabsorbed
  into the capillaries.
• NaCl is reabsorbed from both proximal and distal,
  and water follows by osmosis.
• Secretion of H and reabsorption of HCO3- also
  occur in both proximal and distal, helping to
  regulate the bloods pH.
• Potassium conc. in blood is regulated by
  secretion of excess K into the distal tubule.
• Drugs and poisons processed in the liver are
  secreted into the proximal tubule.

90
Kidney

• Reabsorption and Secretion in a nephron Figure
  25.11 (continued)
• Loop of Henle and collecting duct function
• Major function is water reabsorption
• Long loop of Henle carries filtrate deep into the
  medulla and then back to the cortex
• Presence of NaCl and some urea in interstitial
  fluid in the medulla maintains the high conc.
  gradient that increases water reabsorption by
  osmosis.
• As soon as water passes into interstitial fluid,
  it moves into nearby blood capillaries and is
  carried away. This prompt removal is essential to
  avoid diluting the interstitial fluid and
  destroying the conc. Gradient necessary for water
  reabsorption.

91
Kidney

• Reabsorption and Secretion in a nephron Figure
  25.11 (continued)
• Loop of Henle and collecting duct function
• water reabsorption stops at hairpin because the
  tubule there is impermeable to water.
• As filtrate moves back toward the cortex, NaCl
  leaves the filtrate, first passively and then
  actively as the cells of the tubule pump NaCl
  into the interstitial fluid.
• it is primarily this movement of salt that
  creates the solute gradient in the interstitial
  fluid of the medulla.

92
Kidney

• Reabsorption and Secretion in a nephron Figure
  25.11 (continued)
• Loop of Henle and collecting duct function
• Final refining of the filtrate occurs in the
  collecting duct.
• By actively reabsorbing NaCl, the collecting duct
  is important in determining how much salt is
  excreted in the urine.
• In the inner medulla, the collecting duct becomes
  permeable to urea and some leaks out, adding to
  the high conc. gradient in the interstitial
  fluid.
• As filtrate moves through medulla, more water is
  reabsorbed before the urine passes into the renal
  pelvis.

93
Kidney

• Reabsorption and Secretion in a nephron Figure
  25.11 (continued)
• In sum, the nephron returns much of the water
  that filters into it from the blood.
• Water conservation is one of the major functions
  of the kidney
• Maintain a precise and essential balance btw
  water and solutes in our body fluids.

94
Kidney

• Antidiuretic hormone (ADH)
• Increased in the blood by a control center in
  the brain when the solute concentration rises
  above a set point.
• Signals nephrons to step up water reabsorption.
• When the solute conc. Is diluted below the set
  point, as when we drink a lot of water, blood
  levels of ADH drop and water reabsorption is
  reduced, resulting in an increased discharge of
  dilute urine.
• Alcohol inhibits the release of ADH and can cause
  excessive urinary water loss and dehydration,
  which may account for the symptoms of a hangover.

95
Kidney

• Dialysis
• A person can survive with only one functioning
  kidney, but if both kidneys fail, the build up of
  toxic wastes and the lack of regulation of blood
  pressure, pH, and ion concentrations will lead to
  certain death if untreated.
• Over 60 of kidney disease cases are caused by
  hypertension and diabetes, but prolonged use of
  pain relievers, alcohol, and other drugs are also
  possible causes

96
Kidney

• Dialysis How it works
• Dialysis means separation in Greek
• Like the nephron, the machine sorts small
  molecules of blood, keeping some and discarding
  others.
• Patients blood is pumped from an artery through
  a series of tubes made of a selectively permeable
  membrane.
• The tubes are immersed in a dialyzing solution
  much like the interstitial fluid that bathes the
  nephrons.
• As the blood circulates through the tubing, urea
  and excess ions diffuse out.
• Needed substances, such as biocarbonate ions,
  diffuse from the dialyzing solution into the
  blood
• Machine continually discards the used dialyzing
  solution as wastes build up.

97
Kidney

• Dialysis
• Although life sustaining, its costly and time
  consuming (three times a week for 4-6 hours at a
  time)
• Also requires severe dietary and lifestyle
  restrictions
• Waiting list for kidney transplants ,
  unfortunately, is quite long.