Download Digestion, Absorption, and Metabolism

Digestion, Absorption,
and Metabolism
Definitions
• Digestion:
– Mechanical and chemical process in which food is
broken down in the GI tract, releasing many
nutrients in forms the body can use
• Absorption:
– Process in which released nutrients are taken into
the cells lining the GI tract
• Metabolism:
– The sum of the body processes (catabolic and
anabolic) involved in converting food necessary for
energy, tissue building, and metabolic controls
Digestion: Basic Principles
• Principle of change
– The body cannot use food as it is eaten.
– Food must be changed into simpler substances to
be absorbed and then used by cells to sustain life.
• Principle of wholeness
– The parts of the digestive process comprise a
continuous whole.
– Food components travel through the
gastrointestinal (GI) system until they are delivered
to cells or excreted.
The Gastrointestinal System
• Tube that extends from the mouth to the anus in which
the movement of muscles and release of hormones
and enzymes digest food (also called the alimentary
canal or gastrointestinal tract)
• The major parts of the gastrointestinal system:
– Mouth and Salivary glands
– Pharynx and Esophagus
– Stomach
– Small Intestine and Large Intestine
– Rectum
– Accessory organs: liver, gallbladder, pancreas
Functions of the GI System
• Motility:
– Movement through the GI tract
• Digestion:
– Mechanical and Chemical
• Secretion and Absorption:
– Into the GI tract (secretion) or into blood
(absorption)
• Storage
• Elimination
Digestion: Mechanical and
Chemical Changes
• Through a balanced system of mechanical and
chemical digestion, food is broken down into smaller
substances and the nutrients are released for biologic
use.
• Mechanical and chemical actions make up the
digestive process.
• Food must undergo these changes to be delivered to
cells.
Mechanical Digestion
• Breaking down of food and moving it along the GI tract
• Mastication:
– Process of chewing and biting food in the mouth
– First step in digestion
• GI motility:
– Beginning in the mouth, muscles and nerves in the
GI tract coordinate their actions to provide motility,
an automatic response to the presence of food.
Mechanical Digestion
• Layers of the GI tract provide two types of movement:
tonic contraction and peristalsis
• Tonic contraction:
– Ensures continuous passage of the food mass and
valve control along the way
• Peristalsis:
– Periodic muscle contraction and relaxation which
are rhythmic waves that mix the food mass and
move it forward
Mechanical Digestion
• Nerves
– The intramural nerve plexus is the network of
nerves in the GI wall extending from the esophagus
to the anus
– These nerves do three things:
• Control muscle tone in the wall
• Regulate the rate and intensity of alternating
muscle contractions.
• Coordinate all the various movements
• Problems with the nerves in the digestive tract can
cause problems with the digestive process.
Chemical Digestion
• Substances that break down food:
– Digestive Enzymes: specific proteins that break
down nutrients
– Hydrochloric Acid (HCL): acid made by the
stomach that breaks down proteins in the foods
we eat and lowers pH and total alkalinity
• HCL activates digestive enzymes that break down
food into small particles for absorption
– Buffer Ions: produces correct pH for enzyme
activity
Chemical Digestion
• Substances that break down food:
– Mucus: A clear liquid made by the intestines.
Lubricates and protects the GI tract tissues and
helps mix the food mass
– Water and Electrolytes: Carry and circulate the
products of digestion through the tract and into the
tissues. Major electrolytes in the body include
sodium, potassium, magnesium, calcium, chloride,
and phosphate.
– Bile: A digestive fluid made by the liver and stored
in the gallbladder which divides fat into smaller
pieces to assist fat enzymes
Digestion of a Meal
• You consumed the following for lunch at McDonald’s:
– Big Mac (542 calories, 279 fat calories and 52% fat)
– Small French Fries (211calories, 45 fat calories and
47% fat calories)
– Small Chocolate Milk Shake (380 calories, 81 fat
calories and 21% fat calories )
– Cherry pie (298 calories, 162 fat calories and 54%
fat)
• That is a whopping 1,431 calories, 567 fat calories and
40% fat.
– That is an artery clogging meal. I hope you remain
awake to learn how it digests!
Digestion in the Mouth and Esophagus
• Mechanical Digestion
– Mastication (biting and chewing) breaks down food
– Tongue pushes bolus (food mass) to pharynx (throat)
– Food is swallowed and passes down esophagus (long
hollow tube from pharynx to stomach) by peristaltic waves
controlled by nerves
– Gastroesophageal sphincter at stomach entrance relaxes,
allowing food to enter, then constricts to retain food.
• If the gastroesophageal sphincter is not working properly, acid
and mixed food seep back into esophagus, resulting in
heartburn
Digestion in the Mouth and Esophagus
• Chemical Digestion
– Saliva: mixture of water, protein, and salts that
makes food easy to swallow
– Salivary glands (parotid gland, sublingual gland,
and submandibular gland) secrete material
containing salivary amylase or ptyalin (enzyme that
breaks down carbohydrates)
– Ebner’s glands at the back of the tongue secrete a
lingual lipase (enzyme that breaks down fat)
Digestion in the Mouth and Esophagus
• Chemical digestion
– Salivary glands also secrete a mucous material to
lubricate and bind food particles, facilitating the
swallowing of the food bolus.
– Secretions from the mucous glands in the
esophagus help move food toward the stomach.
The Stomach
• Regions of the stomach:
– Fundus: upper portion of the stomach
– Pyloric region (pyloric sphincter): opening or valve
– Duodenum: first part of the small intestine
• Functions:
– Store food
– Initiate digestion of proteins
– Kill bacteria with the strong acidity (low pH of the
gastric juice)
– Make chyme
The Stomach
Stomach Digestion
• Mechanical Digestion
– Under sphincter control, the food enters the upper
portion of the stomach as individual bolus lumps
– Stomach muscles knead, store, mix, and propel the
food mass forward
– By the time the food mass reaches the lower portion
of the stomach, it is a semiliquid acid/food mix
called chyme
– Chyme is released slowly into the first section of the
small intestine (duodenum) by the pyloric valve
Stomach Digestion
• Chemical Digestion: three types of gastric
secretions
– Hydrochloric Acid: Parietal cells in the
stomach lining secrete acid to promote gastric
enzyme activity
– Mucus: Secretions protect the stomach lining
from the erosive effect of the acid and also bind
and mix the food mass and help move it along.
– Enzymes: Pepsinogen is secreted by stomach
cells and activated by acid to become pepsin, a
protein-splitting enzyme
Stomach Digestion
• Various sensations and emotions
stimulate the nerve impulses that trigger
these secretions
• Anger and stress can increase secretions
• Sadness and fear can decrease
secretions and diminish blood flow
• Do you feel more or less hungry when you
are:
–Stressed, sad, or angry,
Stomach Digestion
• High-calorie foods—especially fatty
foods—empty out of the stomach at a
slower rate; thus the feeling of fullness
lasts longer.
• Caloric density influences the rate in
which the stomach empties
• Problems with valves and sphincters can
cause reflux and discomfort
Small Intestine
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About 20 feet in length
Small in diameter compared to large intestine
Fills most of the abdomen
Functions:
– Peristalsis
– Protection against infection
– Hormone secretion (secretin and cholecystokinin)
– Majority of food digestion
Small Intestine
• Three regions of the small intestine:
– Duodenum: upper region-connecting stomach to
ileum
• Absorbs carbohydrates, lipids, amino acids, Ca2+,
iron
– Jejunum: middle and largest region
• Absorbs carbohydrates, lipids, amino acids, Ca2+,
iron
– Ileum: lower region
• Absorbs bile salts, vitamin B12, water and
electrolytes
Small Intestine
Small Intestine Digestion
• Mechanical Digestion
• Under nerve control, intestinal muscles produce
several types of movements as follows:
– Peristaltic waves slowly push food mass forward
– Pendular movements sweep back and forth
– Segmentation rings chop food mass into
successive soft lumps and mix them with
secretions
– Longitudinal rotation rolls food in a spiral motion,
exposing new surfaces for absorption
Small Intestine Digestion
• The small intestine together with accessory organs
(pancreas, liver, and gallbladder) secrete enzymes
• Pancreatic enzymes
– Carbohydrate: Pancreatic amylase converts starch
to maltose and sucrose
– Protein: Trypsin and chymotrypsin split large protein
molecules into small peptide fragments and
eventually into single amino acids
– Fat: Pancreatic lipase converts fat to glycerides and
fatty acids
Small Intestine Digestion
• Intestinal enzymes
– Carbohydrate: Disaccharides (maltose,
lactose, and sucrose) converted into
monosaccharides (glucose, galactose, and
fructose)
– Protein: Enterokinase activates trypsinogen
from the pancreas to split the following:
• polypeptides
dipeptides
amino acids
– Fat: Intestinal lipase splits fat into glycerides
and fatty acids
Small Intestine Digestion
• Mucus is important because the contents entering
from the stomach are highly acidic and it protects the
mucosal lining from irritation and erosion
• Bile: emulsifying agent produced by the liver and
stored in the gallbladder which breaks fat into smaller
pieces so it can be broken down easier
• Hormone secretin keeps the environment alkaline at a
pH of 8 so pancreatic enzymes can work.
• Hormone cholecystokinin triggers release of bile from
the gallbladder
What Happened to the McDonald’s
Meal?
• Carbohydrates in bun, fries, and cherry pie reduced to
simple sugars (glucose, fructose, galactose)
• Proteins in hamburger patty and milk shake become
amino acids
• Lipids in burger, fries, milk shake and cherry pie
become fatty acids and glycerides
• Lettuce is reduced to cellulose or fiber
• Any vitamins and minerals: liberated from food
• Excess calories are stored as fat in adipose tissue
Bioavailability
• Bioavailability refers to:
– Amount of nutrient present in the GI tract
– Competition between nutrients for absorption
– Form in which the nutrient is present
• All nutrients present in a food are not absorbed
because of differing bioavailability.
– This is considered when determining dietary intake
standards
Absorption in the Small Intestine
• The inner surface of the small intestine is 600 times
larger than its outer area, facilitating the delivery of
nutrients into circulation
• Three absorbing structures
– Mucosal Folds: surface of small intestine piles into
folds
– Villi: small, finger-like projections cover the mucosal
folds, increasing the area of exposed intestinal
surface
– Microvilli: smaller projections cover each villi (look
like bristles on a brush)
Absorption in the Small Intestine
• Each villus can receive protein and carbohydrate
material as well as receive fat materials in its lymph
vessels
• These lymph vessels are called lacteal
• If the entire surface of the small intestine were spread
out, it would be half the size of a basketball court
Large Intestine
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4 to 5 feet in length
Populated by bacteria which synthesize Vitamin K
Consists of the cecum and colon
Primary function is to reabsorb water from the stool
and eliminate the remaining relatively dry waste
• Food and drink consumption and intestinal secretions
result in approximately 5 gallons
(19 L) of fluid being placed in the large intestine daily
Large Intestine
• Water is taken up by the large intestine
– Most water in chyme is absorbed in the first half of
the colon
– Only about 100 mL of water is left in the large
intestine after the digestion process to form and
eliminate feces
• Dietary fiber is not digested
– Contributes bulk to food mass and gas
– What foods high in dietary fiber is associated with
gas?
– Helps form feces
Transport
• The vascular system (blood circulation) is responsible
for supplying the body with nutrients, oxygen, and
other vital substances
– Veins and arteries
– Transports waste, such as carbon dioxide and
nitrogen, to lungs and kidneys for removal
• Water-soluble products of digestion can be absorbed
into the vascular system.
• Nutrients travel to the liver by the portal circulation,
then to cells throughout the body
Transport
• Lymphatic system
–Route for fatty materials, which are not
water soluble
–Fat molecules pass into lymph vessels
in villi
Metabolism
Energy Storage: Glycogenesis
• Anabolic process of converting
extra glucose into glycogen
• Glycogen is stored in the liver and
muscles for quick energy to be
used at a later time
Energy Storage: Lipogenesis
• When glycogen reserves are full,
additional excess energy from
carbohydrates, fat, or protein is stored
as fat in adipose tissue
• Lipogenesis: The building up of
triglycerides for storage in adipose
tissue
Energy Storage: Gluconeogenesis
• Conversion of amino acids to glucose
• Excess protein is not stored as
muscle but is further broken down
–Nitrogen unit is removed
–Remaining carbon chain can be
converted to glucose (if needed) or to fat
for storage
Metabolic Pathways of Excess Energy
Alcohol
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Alcohol is a food-7 kcal/gram
Cannot be stored in the body-toxic
Absorbed without chemical digestion
Alcohol metabolism is first because a byproduct
of alcohol is acetaldehyde-which destroys
healthy tissue
Liver detoxifies alcohol and uses remaining to
produce fatty acids-triglycerides stored in liver
Single drinking binge can accumulate fat in the
liver
Can lead to liver disease
Blood alcohol levels peak after one drink in 3045 minutes 12oz of beer, 5 oz of wine, 1.5 oz of
80 proof alcohol
www.health.org
Discussion Questions
1. What is the difference between mechanical
and chemical digestion?
2. What functions do the following perform:
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Stomach
Small Intestine
Pancreas
Gallbladder
Liver
Large Intestine