Digestion, absorption and transport

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Chapter 3

• Digestion, absorption and transport

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Digestion

• Process by which food is broken down into
  absorbable units as it travels through a flexible
  muscular tube

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• Food is lubricated as it moves through the lumen
  of the system
• Sphincters ensure passage in one direction
• Food is mechanically and enzymatically broken
  down into simpler, usable chemicals
• Nutrients are absorbed along with most of the
  water necessary for the process
• Waste is eliminated as feces

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Mouth

• Teeth bite, chew and mix food with ingested
  fluids and saliva
• Salivary glands produce lubricating saliva that
  contains salivary amylase and salivary lipase to
  initiate complex carbohydrate and fat digestion
• Tongue helps to keep food positioned between
  teeth, involved in swallowing and tasting

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Taste

• Sweet
• Salt
• Sour
• Bitter
• Umami (savory, associated with MSG)
• What you perceive as taste is really mostly
  smell think about how taste diminishes if you
  have a cold.

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Pharynx

• Receives swallowed food, now called a bolus
• Directs food from mouth into esophagus
• Is a common passageway for food and air

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Epiglottis

• Flap-like structure that closes airway during
  swallowing to prevent choking

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Esophagus

• Muscular tube that extends from pharynx to
  stomach
• Passes through opening in diaphragm

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Esophageal sphincters

• Upper sphincter opens during swallowing.
• Lower sphincter opens to allow bolus into
  stomach. It then closes to prevent food from
  passing back into the esophagus.
• Abnormal opening occurs during
• Vomiting
• Acid reflux

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Stomach

• Three layers of muscle
• Churns chyme
• Mixes food with enzymes and hydrochloric acid
• Begins protein digestion
• Absorption of water, asprin
• Releases chyme into beginning of small intestine
  through small spurts through pyloric sphincter,
  which opens and closes.

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Small intestine

• Duodenum digestion continues with addition of
  digestive juices and peristalsis
• Receives chyme from stomach
• Receives bile from gallbladder liver and (for fat
  digestion) and digestive juices from pancreas
  (for emulsification of fat, protein, carbohydrate
  and nucleic acids)
• Contributes its own digestive enzymes
• Jejunum more intestinal enzymes and peristalsis
• Ilium more intestinal enzymes and peristalsis

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• The small intestine is about twenty feet long
• Products of digestion and water are absorbed
  throughout the tract
• Different areas are specialized to absorb
  specific nutrients
• Chyme passes through the ileocecal valve
  (sphincter) into the beginning of the large
  intestine (colon)

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Large intestine (colon)

• Begins with a blind pouch called the cecum on
  lower right of abdomen
• Veriform appendix is attached to cecum
• Ascending colon
• Transverse colon
• Descending colon
• Sigmoid colon
• Rectum
• Anus

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• Water absorption
• Bacterial action
• Compaction of waste into feces
• Elimination through anus, which is guarded by 2
  sets of sphincters.

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Muscular action of digestion mouth

• Muscles of mastication move lower jaw
• Tongue of muscles of face keep food between teeth
• Tongue and other muscles voluntarily initiate
  swallowing

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Muscular action of digestion beyond the mouth

• The gastrointestinal tract is lined with layers
  of smooth muscle
• An inner circular layer
• An outer longitudinal layer
• The stomach has an extra innermost oblique layer

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Stomach muscles

• Thickest wall of tract with 3 muscle layers
• Churns and grinds food
• Mixes food with enzymes and hydrochloric acid
• The pyloric sphincter opens briefly
  3times/minute only when chyme is completely
  liquefied to allow small amounts to pass into the
  small intestine

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Peristalsis

• Movement that occurs in GI tract when circular
  rings of muscle contract and long muscles relax,
  then the long muscles tighten and the circular
  ones relax

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• Ripples occur at varying rates of speed and
  intensity in GI tract
• Quiet with periodic bursts when tract is empty
• Slow continuous waves up when food is present
• Works with sphincter muscles to keep chyme going
  in the right direction

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Segmentation

• Periodic squeezing or partitioning of the
  intestine along its length by circular muscle.
• Helps promote good mixing of chyme with digestive
  juices

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Sphincters

• Periodically open
• Upper esophageal
• Lower esophageal-prevents reflux
• Pyloric sphincter
• Ileocecal valve
• Internal and external anal sphincters work with
  tight rectal muscles to control defecation

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Secretions of digestion

• Digestive enzymes names end is ase
• Are catalysts
• Facilitate hydrolysis of food
• Produced by salivary gland, stomach, pancreas,
  small intestine

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• Hydrochloric acid
• Aids in protein digestion
• Produced only in stomach
• Low pH
• Bicarbonate ion, a base
• Produced by pancreas and released along with its
  enzymes
• Neutralizes HCL that enters the small intestine
• Higher pH

Look at the pH scale on p.77
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• Bile
• Produced by liver and stored by the gallbladder
• Emulsifies fat so droplets can be suspended in
  water component of chyme

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• Mucous
• released in all parts of tract
• Protects lining of tract
• Hormones
• Chemical messengers released by stomach and small
  intestine
• Slow or speed up activity as needed in many parts
  of the system

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The final stages of digestion

• Products of digestion, vitamins and minerals are
  absorbed by cells lining the lumen of the small
  intestine
• The large intestine receives undigested
  residue,including fiber, water

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• Bacteria ferment fibers and small fats (also
  lactose and a few other substances) and produce
  gas
• Friendly bacteria manufacture vitamins that are
  absorbed and discourage growth of pathogens

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• The large intestine absorbs most of the water
  present in the chyme and compacts the residue
  into stool
• Stool contains water, fiber, bile acids, some
  minerals, some fat food additives and
  contaminants

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Role of fiber

• Some fibers of seeds, whole wheat, peanuts and
  bananas are partially digested by bacteria of the
  colon and these products are absorbed by the
  large intestine

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• Most fibers are excreted as part of feces
• These fibers help to retain water and prevent
  constipation
• They also bind to fat, cholesterol from bile and
  minerals and thus cause their excretion

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Absorption

• Within 3-4 hours after eating the small intestine
  must finish the process of digestion and absorb
  the molecular products of digestion into the
  blood of lymph.

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Methods of absorption

• 1. Simple diffusion freely cross membrane of
  intestinal cells
• Water
• Small lipids
• 2. Facilitated diffusion require a carrier
  protein located in membrane or changes in cell
  membrane of intestinal cells
• Water soluble vitamins

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• 3. Active transport require energy in the form
  of ATP, because substances travel against their
  gradient
• Glucose
• Amino acids

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Anatomy of small intestine

• 10 feet or more
• Surface area of a tennis court
• Plicae visible folds of intestinal lining

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Microscopic Anatomy villi

• Villi made of many cells have layer of muscle
  for constant motion
• Microvilli ( brush border located on membrane of
  cells on the outer surface of the villi enzymes
  and absorption
• Crypts intestinal glands that produce new cells
  of the villi
• Goblet cells scattered among cells of villi
  mucus
• Capillaries and lacteals
• Brunners glands below villi add more mucus

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Specialization

• Products of digestion are ready for at different
  times
• Regions of the small intestine are specialized to
  absorb certain nutrients

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Food combining myth

• Because of specialization, foods can be eaten
  together
• Sometimes food combinations actually enhance, not
  inhibit, absorption e.g., vitamin C and iron

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Preparation of nutrients for transport

• Water soluble products- carbohydrates, amino
  acids, nucleic acids- and small products of fat
  digestion enter capillaries in the villus
• The blood then goes to the liver

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• Larger products of fat digestion and fat soluble
  vitamins are coated with cholesterol and proteins
  and are called chylomicrons
• Chylomicrons enter lacteals, lymphatic vessels
• Lymph enters the blood stream near the heart

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Transportation Vascular system

• Water soluble products of digestion and small
  products of fat digestion enter blood contained
  in the vascular system
• Nutrients are collected from the small intestine,
  and the blood is carried through the hepatic
  portal vein first to the liver, where some are
  processed for use by the rest of the body
• Nutrients are then released into the general
  blood stream

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The vascular system

• Arteries carry blood away from the heart
• Tiny branches give rise to microscopic
  capillaries
• Capillaries are very thin and allow exchange of
  chemicals,including nutrients, gases, and fluids
• Capillaries merge together and the blood flows
  into veins
• Veins carry blood back to the heart

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The heart

• Blood that enters the right side of the heart is
  pumped through arteries to the lungs for oxygen
• The oxygenated blood from the lungs is carried
  through veins to the left side of the heart
• The left side of the heart pumps blood through
  arteries to the rest of body

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Nutrient routes

• Capillaries in villi
• Hepatic portal vein Capillary beds in liver
• Prepares absorbed nutrients for use in the body
• Removes impurites and toxins and is subject to
  damage
• Hepatic vein
• Inferior vena cava
• Right heart
• Arteries to lungs

• Veins to left heart
• Arteries to body
• Capillaries deliver oxygen and nutrient rich
  blood to tissues
• Veins carry blood back to right heart

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Transportation lymphatics

• Lymph vessels carry lymph, which is basically
  blood plasma minus some proteins

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• Larger coated products of fat digestion enter
  lacteals located in the center of the villi
• The many small vessels merge into larger and
  larger ones and eventually flow into the thoracic
  duct
• The thoracic duct empties into the left
  subclavian vein, which is near the heart
• Lymph then becomes part of blood plasma and
  general circulation
• These products of digestion bypass the liver

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GI bacteria

• 10 trillion
• Found mainly in small and large intestines where
  conditions are not acidic
• Most not harmful
• Often beneficial
• Normal microflora helps prevent establishment of
  harmful bacteria

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Probiotics

• Helpful bacteria found in food, like yogurt, are
  called probiotics
• Change conditions and native bacterial colonies
  in a beneficial way
• Help alleviate diarrhea, constipation,
  inflammatory bowel disease, ulcers, lactose
  intolerance
• Enhance immune function
• Protect against colon cancer

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Helpful bacterial products

• Digest fibers and complex carbohydrates for their
  own use, which results in production of short
  fragments of fat that colon cells can use for
  energy
• Produce 1/2 needed vitamin K-clotting factor
• Produce biotin, folate, B6, B12

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Homeostasis

• Maintenance of the bodys internal conditions by
  the bodys control systems
• Hormones
• Nerves

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Homeostasis and GI tract

• The activity of the GI tract is always changing
  to meet current needs
• Signals provided by hormones (chemicals) and
  nerves keep function within homeostasisnormal
  limits
• Hormones and nerves work through feedback systems

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Hormones

• Gastrin-produced by stomach
• stimulates stomach to produce hydrochloric acid
• Secretin-produced by small intestine
• causes pancreas to release bicarbonate to
  neutralize hydrochloric acid in small intestine

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• Cholecystekinin (CCK)-produced by small intestine
• targets gall bladder, which releases bile for
  fat digestion
• Slows motility beneficial because fat takes
  longer time to digest
• Targets pancreas, which releases juices rich in
  enzymes and bicarbonate

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Common digestive disorders

• Choking
• Vomiting
• Diarrhea
• Constipation
• Belching and gas
• Heartburn and acid indigestion
• Ulcers

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• http//east.instructor.ilrn.com/ilrn/books/wrun11l
  /animations/03/0308.html
• Follow the fate of a sandwich

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The end