chapter 5 digestion, absorption, and metabolism 1
TRANSCRIPT
Chapter 5
Digestion, Absorption, and Metabolism
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Ch 5Key Concepts:Whole foods must be changed to simpler
substances in order to be useful to the body. The body must free the food nutrients and re-form and re-route them to meet special life needs.
Special organ structures and functions conduct these tasks through the successive parts of the overall system.
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ObjectivesIdentify the
principles of digestion
Describe the absorption process
Identify the human water balance system
Identify human acid – base balance system
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Digestion
Basic principles:Food can not be used as eatenFood must be changed to a simpler form for absorption
Digestion is the process in which food is broken down in the gastrointestinal tract
Absorption is the process in which these nutrients are taken into the cell lining the gastrointestinal tract
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Principle of WholenessThe parts of this overall process of change do not
occur separately but make one continuous whole
Mechanical and chemical changes – Food must go through a series of mechanical and chemical changes
Together these two types of actions make up digestion:Mechanical digestion: gastrointestinal motilityChemical digestion: gastrointestinal secretions
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Gastrointestinal system
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Mechanical Digestion
Motility = the ability to move spontaneously
Starting in the mouth, muscles and nerves in the walls of the GI tract coordinate their actions to provide the needed motility for digestion to proceed
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Mechanical DigestionMuscles – Layers of muscles provide two types of movement Muscle Tone - tonic contraction ensures the continuous passage of the food mass and valve control along the way
Periodic muscle contraction and relaxation – rhythmic waves that mix the food mass and move it forward
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Mechanical Digestion
Nerve - Complex nerves in the gastrointestinal wall extend from the esophagus to the anus Control muscle tone
in the wallRegulate the rate
and intensity of muscle contractions
Coordinate all of the various movements
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Chemical DigestionA number of secretions work together to
make chemical digestion possible
5 types of substances involved:Hydrochloric acid and buffer ions –
needed to produce the correct pH which is necessary for enzyme activity
Digestive Enzymes – digestive enzymes are proteins, specific in kind and quantity for breaking down specific nutrients
Mucus- lubricate and protect the mucosal tissues; help mix food mass
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Chemical Digestion
Water and electrolytes – products of digestion are carried and circulated through tract and into tissues by water and electrolytes
Bile – made in liver and stored in gallbladder, bile breaks down fats into smaller pieces to assist fat enzymes
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Chemical Digestion Secretory Cells: in the
GI tract produce each of the preceding substances.
The action of these cells is stimulated by:Presence of foodNerve impulsesHormones specific
for certain nutrients
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Digestion in the Mouth and EsophagusMechanical Digestion:
Mastication (chewing) breaks down food into small particles
Muscles at the base of the tongue facilitates swallowing
Food is swallowed and passes down the esophagus through peristaltic waves and gravity
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Digestion in the Mouth and Esophagus
Gastroesophageal sphincter at the stomach entrance relaxes, allowing food to enter, then constricts to retain food
GERD (Gastroesphageal Reflux Disease) – Reflux – Heartburn If the gastroesophageal sphincter is
not working properly, it may allow the acid-mixed food to seep back into the esophagus a uncomfortable feeling of “heartburn”
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Digestion in the Mouth and Esophagus
GERD cont.“Heartburn” –
unrelated to cardiacCalled this
because the sensations are perceived as originating in the area of the heart
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Digestion in the Mouth and EsophagusChemical Digestion
Salivary glands secrete material containing salivary amylase or ptyalin – a “starch-splitting” enzyme in the mouth
Ebner’s serous glands at the back of the tongue secrete a lingual lipase
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Digestion in the Mouth and Esophagus
Salivary glands also secret a mucous material to lubricate and bind food particles, facilitating the swallowing of the food bolus (lump)
Secretions from the mucous glands in the esophagus help move food toward the stomach
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Digestion in the StomachMechanical digestion:
Under sphincter control, the food enters the upper portion (fundus) of the stomach as individual bolus lumps
The muscles knead, store, mix, and propel the food mass forward in a slow controlled movement
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Digestion in the StomachMechanical digestion cont.
By the time the food mass reaches the lower portion of the stomach, it is a semi-liquid acid/food mix called chyme.
Chyme is released slowly into the first section of the small intestine (duodenum) by the pyloric valve
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Digestion in the StomachChemical digestion:
Gastric secretions contain acid, mucus and enzymes which aid in digestion
Hydrochloric Acid – parietal cells secrete HCL to activate enzymes
Mucus – protect stomach lining from the erosive effect of the acid, and bind and mix the food mass and help it move along
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Digestion in the StomachChemical digestion cont.
Enzymes - inactive form (pepsinogen) is secreted by the stomach cells and activated by HCl to become pepsin, a protein-splitting enzyme
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Digestion in the Small Intestine
The structural parts, synchronized movements and array of specific enzymes of the small intestine are highly developed for the final task of mechanical and chemical digestion.
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Digestion in the Small Intestine
Mechanical Digestion: Under the control of nerve impulses,
stomach walls stretch from the food mass and the intestinal muscles produce several types of movements:
Peristaltic waves – push food forwardPendular movements –small, local
sweeps back and forth, stirring the chyme
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Digestion in the Small Intestine Mechanical digestion cont.Segmentation rings –
alternating contractions and relaxations of circular muscles
Longitudinal rotation – long muscles roll slowly moving food in spiral motion
Surface villi motions – mix chyme at intestinal walls – exposing additional nutrients for absorption
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Digestion in the Small Intestine Chemical digestion
Pancreatic enzymesCarbohydrate Pancreatic amylase
converts starch to disaccharides, maltose and sucrose
Protein – Trypsin and chymostrypsin split large protein molecules into smaller peptide fragments and single amino acid
Fat – Pancreatic lipase converts fats to glycerides and fatty acids
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Napping: what you do after lunch!
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Digestion in the Small Intestine
Intestinal enzymes :
Carbohydrate Disaccharidases (maltase, lactase, and sucrase) convert their respective disaccharides (maltose, lactose, and sucrose) to monosaccharides (glucose, fructose, and galactose)
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Digestion in the Small Intestine
Lactose IntoleranceA large percentage of the world’s population does not produce enough lactase to digest lactose (milk sugar
As a result, these individuals cannot tolerate milk and milk products unless they are in the predigested form – e.g. yogurt, buttermilk, aged cheese, or lactase-treated milk. May also take commercially available oral lactase
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Digestion in the Small Intestine Protein
The enzyme enterokinase activates trypsinogen from the pancreas and turns it into trypsin (the protein-splitting enzyme).
Fat Intestinal lipase splits fat in to glycerides and fatty acids
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Digestion in the small IntestineMucus- large quantities of mucus
protect the mucosal lining from irritations and erosion caused by the acidic gastric contents
Bile – emulsifying agent - aids fat digestion and absorption [made in the liver, stored in the GB]
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Digestion in the small IntestineHormones
Secretin – produced by the mucosal
glands – this controls the acidity and secretion of enzymes from the pancreas.
Cholecystokinin – secreted by intestinal
mucosal glands when fat is present; triggers the release of bile from gallbladder to emulsify the fat
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Summary of digestive processes
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Factors influencing the GI tract
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Accessory Organs The “accessory
organs” (to the duodenum) make up the “biliary system”
The liver is sometimes called the “Metabolic Capital” of the human body
Metabolizes all converging nutrients
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Role of the LiverProcesses blood immediately after it
leaves the gastrointestinal tractBile productionSynthesis of protein and blood
clotting factorsRegulation of blood glucoseMetabolism of hormones and
medications
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Absorption and Transport
When digestion is complete, food has been changed into simple end-products that are ready for cell use
Transport- is the movement of nutrients through the circulatory system from one area of the body to another
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Absorption and Transport“Simple” end-products:
Carbohydrates: reduced to simple sugars (glucose, fructose, galactose)
Fats: changed into fatty acids and glycerides
Proteins: changed into single amino acids
Vitamins and minerals: liberated from food
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Absorption in the Small IntestineSpecial absorbing structures – [p. 72, Fig.5-
6]Mucosal folds - surface of the small intestine piles into folds
Villi – small finger-like projections, cover the mucosal folds, increasing the area of exposed intestinal surface
Microvilli – electron microscope reveals a covering of multiple small projections on the surface of the villi. This is the brush border because it looks like bristles on a brush
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Villi and microvilli
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Absorption ProcessesMoving vital nutrients across the inner
intestinal wall into body circulation.
Simple diffusion – the force by which particles move outward in all directions, from an area of greater concentration to lesser concentration
Facilitated diffusion – simple diffusion with the help of a protein channel for carrier assisted movement of larger items across the mucosal cell wall
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Absorption and TransportActive transport – Force by which
particles move from area of greater concentration to lesser concentration with help from a carrier or ferry
Pinocytosis – Penetration of larger materials by attaching to the thicker cell membrane and being engulfed by the cell
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Absorption in the Large IntestineMain absorptive task: to take up waterMost water in chyme is absorbed in
the first half of the colonOnly a small amount remains to form and eliminate the feces
Dietary fiberNot digestedContributes bulk to food massHelps form feces
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Water: good for digestion
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TransportNutrients must be transported to cellsVascular system (blood, circulation) –
Most of the products of digestion are water-soluble nutrients can be absorbed into the vascular systemdirectly from the intestinal cells
Composed of veins and arteries and is responsible for supplying the entire body with nutrients and oxygen necessary for life.
Transports waste, such as carbon dioxide and nitrogen, to the lungs and kidneys for removal 46
TransportLymphatic system – route for fatty
materials which are not water soluble
Fat molecules pass into the lymph vessels in the villi, flow into the larger lymph vessels of the body and eventually enter the blood stream
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MetabolismMacronutrients in food
have been broken down through digestion (-> monosaccharides, fatty acids, amino acids) and now must be absorbed into blood stream
They will be converted to energy or stored for later use.
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MetabolismEnergy for Fuel - the sum of chemical
reactions occurring within a living cell to maintain life
2 types of metabolism: Catabolism – the breaking down of larger
substances into smaller unitsEg. The process of breaking down large
carbohydrate and protein chains into their smaller building blocks – monosaccharides and amino acids- is a catabolic reaction
Anabolism – the process in which cells build large substances from smaller particles
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Help for Anabolism
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Metabolic pathways
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MetabolismEnergy for storageIf the amount of food consumed yields more energy than is needed, the “leftover’ energy is stored for later use in the body.E.g. Excess glucose can be stored as glycogen in the liver and muscles for quick energy at a later time
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Metabolism and fight/flight
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