Structures of Human
Physical Digestion
- is the breakdown of food by physical means.
- physical digestion starts when we use our knife and fork to break
down large substances.
- our teeth take over to grind food into small particles which can be
swallowed.
- physical digestion also includes peristalsis
which helps move food down the digestive tract and the muscular churning
of food within the stomach which helps mix food substances with digestive
juices and acid.
Chemical Digestion
Why is chemical digestion
needed?
- chewing, churning, and mixing of food with digestive juices can
only separate molecules from each other. These actions cannot split up
molecules. Many nutrient molecules are too large to pass through cell membranes
(i.e. they cannot be absorbed).
- molecules of water, vitamins, and minerals are small enough to be
absorbed through the intestinal wall and into the blood stream.
- proteins, carbohydrates, and fat molecules are too large and must
be broken down further by chemical means. This is chemical digestion.
Chemical Digestion of proteins, carbohydrates, and fats is needed
for 3 reasons:
-
First, these molecules are too large to pass through a cell membrane.
-
Second, fat and some carbohydrates are insoluble in water. Before nutrients
can enter the blood, they must be dissolved in water.
-
Third, the proteins, fats, and carbohydrates that you eat are not usually
the same proteins, fats, and carbohydrates that your body uses. The body
rearranges these nutrients into usable nutrients.
(Most chemical digestion occurs by hydrolysis as seen earlier.)
Dissolving, Digestion, and Lubrication
Process and Processes of Digestion
Saliva
Swallowing
Peristalsis
Pancreatic Juice
Bile
Intestinal Juice
Absorption
Saliva
-
Your mouth produces up to 1.5L of saliva every day.
-
3 main substances of saliva are: water, amylase (ptyalin),
and the protein mucin. These substances come from
3 areas of your mouth.
1.) Mucous glands in the inner lining of your cheeks produce
mucin.
2.) Salivary ducts under the tongue lead to the salivary
glands below the mouth. These glands produce water, mucin, and the
enzyme amylase. Opposite each second upper molar tooth is the entrance
to a duct from the other salivary glands. These two glands are located
on each side of your face, in front of the ears. They produce water and
amylase. (swelling of these glands is an indication of a viral infection
called mumps)
- Water helps moisten and soften food and to dissolve any
soluble nutrients
-
Amylase is an enzyme which helps break down starch molecules
into the disaccharide maltose (sugar)
-
Mucin has a non digestive role in lubricating and protecting
all internal surfaces from drying out.
Swallowing
- The initial stages of swallowing are under voluntary control.
-
The muscular tongue helps to separate a mouthful of
food into bite-sized chunks, each one called a bolus.
The tongue also forces food towards the sides of the mouth (teeth) to assist
in the chewing process.
- Pressure between the tip of the tongue and the hard palate
(roof of the mouth) squeezes the bolus to the back of the mouth towards
the Pharynx, or throat.
- There is no hard palate in the rear of the mouth. However the tissue
forming the roof of the mouth continues. This is called the soft
palate. As the tongue presses the bolus up, the soft palate
is pushed upward by the bolus. This closes the air passageway from the
nasal cavity.
- The uvula, the fold of skin hanging down from
the soft palate, helps cover the nasal passageway. (this prevents food
from being pushed up the nasal passageway).
- Once the bolus reaches the back of the throat, you lose voluntary
control of it. A swallowing reflex has started. The food now enters the
esophagus.
Peristalsis
- is the scientific name for the coordinated contractions and relaxation's
of the esophagus. Peristalsis also occurs in the intestines.
(this helps explain why someone can drink a glass of water while standing
on his/her head)
Pancreatic Juice
- When the acidic chyme from the stomach inters the small intestine,
it stimulates cells in the intestinal lining to secrete two hormones into
the blood.
1. secretin
2. cholecystokinin
These hormones stimulate the pancreas to secrete pancreatic juice
and pancreatic enzymes which pass through the pancreatic duct and into
the upper part of the small intestine.
- Pancreatic juice contains sodium bicarbonate which neutralizes the
acid in the chyme and makes it slightly alkaline (pH 8). Pancreatic enzymes
work on all the major food components - proteins, carbohydrates, fats and
nucleic acids.
Pancreatic enzymes include: amylase, which hydrolyzes any remaining
starch to maltose; protease's (protein splitting enzymes) including trypsin
and chymotripsin, which continue the break down of large proteins; and
lipase, which breaks down fats.
Bile
- produced in the liver and passes through ducts into the gallbladder,
where it is stored. It passes into the upper part of the small intestine
through the bile duct. Release of bile is stimulated by the hormone cholecystokinin,
which also acts on the pancreas.
- Bile doesn't contain enzymes, it emulsifies fats (causes fats to form
small droplets increasing surface area for enzyme action) Bile is alkaline,
aiding in the neutralization of chyme.
Intestinal Juice
- Intestinal juice contains enzymes that complete the digestion of
carbohydrates, fats, and proteins.
Absorption
- Simple sugars, amino acids, vitamins, minerals, and other substances
are absorbed through the wall of the small intestine into the blood vessels
of the circulatory system. Fatty acids and glycerol are absorbed into tiny
vessels of the lymphatic system called lacteals.
Structural features of the small intestine which promote
absorption:
1. the small intestine is very long
2. its lining has many folds
3. the lining is covered with millions of finger like
projections called villi.
4. The epithelial cells that make up the lining of the
intestine, have brush borders which. In the brush borders, the ends of
the cells that face into the intestine have microvilli which
further increases the surface area.
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