The four main stages of food processing are ingestion, digestion,
absorption,
and elimination
Ingestion, the act of eating, is the first stage.
Digestion, the process of breaking down food into small molecules the body
can absorb,
is the second stage.
- Organic food material is composed of macromolecules (proteins, fats,
carbohydrates)
that are too large to cross the membranes and enter an animal's cells.
- Digestion enzymatically cleaves these macromolecules into component
monomers
that can be used by the animal (polysaccharides and disaccharides to
simple sugars;
proteins to amino acids; fats to glycerol and fatty acids; nucleic acids to
nucleotides).
- The chemical digestion is usually preceded by mechanical
fragmentation (e.g.,
chewing) that increases the surface area exposed to digestive juices.
Absorption is the third stage, and it involves the uptake of the small molecules
resulting
from digestion.
Elimination is the fourth and final stage in which undigested material passes
out of the
digestive compartment.
IV. The Mammalian Digestive System
The digestive system in mammals includes the alimentary canal and
accessory glands
that secrete digestive juices into the canal through ducts.
- Peristalsis (rhythmic smooth muscle contractions) pushes food along the
tract.
- Sphincters (modifications of the muscle layer into ringlike valves) occur at
some
junctions between compartments and regulate passage of materials through
the
system.
- The accessory glands are: three pairs of salivary glands, the pancreas, the
liver, and
the gallbladder.
A. The oral cavity, pharynx, and esophagus initiate food processing
1. The oral cavity
Physical and chemical digestion begin in the oral cavity.
- Chewing breaks down large pieces of food into smaller pieces.
- This makes food easier to swallow and increases the surface area
available for
enzyme action.
The presence of food in the oral cavity stimulates the salivary glands to
secrete saliva
into the oral cavity.
- Saliva contains mucin (protects the mouth from abrasion and lubricates
food);
buffers that neutralize acids; antibacterial agents; and salivary amylase, an
enzyme that
hydrolyzes starch and glycogen to the disaccharide maltose or small
polysaccharides.
The tongue tastes and manipulates food during chewing and forms it into a
bolus, which
it pushes to the back of the oral cavity and into the pharynx.
2. The pharynx
The pharynx is an intersection for both the digestive and respiratory systems.
- The movement of swallowing moves the epiglottis to block the entrance of
the
windpipe (the glottis).
- This directs food through the pharynx and into the esophagus
3. The esophagus
The esophagus is a muscular tube that conducts food from the pharynx to the
stomach.
- Peristalsis moves the bolus along the esophagus to the stomach.
- The initial entrance of the bolus into the esophagus is voluntary
(swallowing); once in,
the peristalsis results from involuntary contraction of the smooth muscles.
- Salivary amylase remains active as the bolus moves through the
esophagus.
B. The stomach stores food and performs preliminary digestion
The stomach is a large, saclike structure located just below the diaphragm on
the left
side of the abdominal cavity. It functions in:
Food storage
- The stomach has an elastic wall with rugae, folds that can expand to
accommodate up
to 2 L of food.
- Storage capacity permits periodic feeding (meals).
Churning
- The stomach's longitudinal, vertical, and diagonal muscles contract in
churning
movements that mix the food.
- Stomach contents are mixed about every 20 minutes.
- Churning and enzyme action convert food to a nutrient broth called acid
chyme.
- The passage of acid chyme into the small intestine is regulated by the
pyloric sphincter at the bottom of the stomach.
- The pyloric sphincter relaxes at intervals and permits small quantities of
chyme to
pass.
Secretion
- Gastric secretion is controlled by nerve impulses and the hormone gastrin.
The
stomach epithelium contains three types of secretory cells that secrete:
1. Mucin, a thin mucus that protects the stomach lining from being
digested and
Gastrin, a hormone produced by the stomach; gastrin is released into the
bloodstream
and its action is to stimulate further secretion of gastric juice (HCl and
pepsin).
2. Pepsinogen, a precursor to pepsin
3. HCl
- Protein digestion. Both components of gastric juice, HCl and pepsin, are
involved with
protein digestion:
- HCl provides acidity (pH 1 - 4) which:
- Kills bacteria
- Denatures protein
- Starts the conversion of pepsinogen to pepsin
- Pepsin splits peptide bonds next to some amino acids.
- Does not hydrolyze protein completely
C. The small intestine is the major organ of digestion and absorption
The human small intestine is about 6 m in length and is the site of most
enzymatic
hydrolysis of food and absorption of nutrients.
- Remember, only limited digestion of carbohydrates occurs in the oral
cavity and
esophagus (by salivary amylase) and of proteins in the stomach (by pepsin).
The pancreas, liver, gall bladder, and small intestine all contribute to the
digestion that
occurs in the small intestine. Their products are released into the duodenum,
the first
25 cm of the small intestine.
- The (exocrine) pancreas produces:
- Hydrolytic enzymes that break down all major classes of
macromolecules
carbohydrates, lipids, proteins, and nucleic acids.
- Bicarbonate buffer that helps neutralize the acid chyme coming from the
stomach.
NOTE: The typical vertebrate pancreas is a compound gland, having both an
exocrine,
ducted component as noted above and an endocrine, ductless component that
produces and secretes hormones, such as insulin and glucagon.
Exocrine = gland that secretes substances into ducts first.
Endocrine = gland that secretes substances directly into surrounding cells or
into the
blood. (ductless gland)
- The liver performs many functions including the production of bile, which:
- Is stored in the gallbladder
- Does not contain digestive enzymes
- Contains bile salts which emulsify fat
- Contains pigments that are byproducts of destroyed red blood cells
1. Absorption of nutrients
Nutrients resulting from digestion must cross the digestive tract lining to enter
the body.
While a small number of nutrients are absorbed by the stomach and large
intestine,
most absorption occurs in the small intestine.
- Large folds in the walls are covered with projections called villi, which in
turn have
many microscopic microvilli; this results in a surface area for absorption of
about 300
m2.
- Penetrating the hollow core of each villus are capillaries and a tiny lymph
vessels.
- Nutrients are absorbed by diffusion or active transport across the
epithelium and into
the capillaries or lymph vesses.
- Amino acids and sugars enter the capillaries and are transported by the
blood.
- Absorbed glycerol and fatty acids are recombined in epithelial cells to
form fats and
enter into the lymph vessels.
- Capillaries and veins draining nutrients away from the villi converge into
the hepatic
portal vessel, which leads directly to the liver.
- Here various organic molecules are used, stored, or converted to
different forms.
D. Hormones help regulate digestion
Hormonal control of digestion involves many different factors; chief among
them are the
four following regulatory hormones:
1. Gastrin. Released from the stomach in response to presence of food;
stimulates the
stomach to release gastric juice (HCl and pepsin); stimulates mitosis and
development
of new mucosa cells (cells that line the stomach).
2. Secretin. Released from the duodenum in response to acid chyme entering
from the
stomach; signals the pancreas to release bicarbonate buffer to neutralize acid
chyme.
3. Cholecystokinin (CCK). Released from the duodenum in response to chyme
entering
from the stomach; signals the gall bladder to release bile and the pancreas to
release
pancreatic enzymes into the duodenum; also may be involved with the satiety
reflex of
the brain.
4. Enterogastrone. Released from the duodenum in response to the presence
of fat in
the chyme; inhibits peristalsis in the stomach, and slows digestion. (fats take
longer to
digest)
E. Reclaiming water is a major function of the large intestine
The large intestine, or colon, connects to the small intestine at a T-shaped
junction
containing a sphincter; the blind end of the T is called the cecum.
- The appendix is a fingerlike extension of the cecum and is composed of
lymphoid
tissue.
- The colon is about 1.5 m long and is in the shape of an inverted "U". Its
major function
is water reabsorption. (NOTE: most absorption of water occurs in the small
intestine.)
Feces (wastes of the digestive tract) are moved through the colon by
peristalsis.
- Intestinal bacteria live on organic material in the feces, and some produce
vitamin K which is absorbed by the host.
- Feces may also contain an abundance of salts.
- Feces are stored in the rectum and pass through the two sphincters (one
involuntary, one voluntary) to the anus for elimination.
Digestive System
1.
General
a.
Animals obtain energy by breaking food molecules into smaller pieces.
b.
The basic fuel molecules are amino acids, lipids and sugars
c.
Digestion is the chemical breakdown of large food molecules into smaller
molecules that can be used by cells. The processing of food has four basic
steps:
i.
Ingestion
ii.
Digestion
iii.
Absorption
iv.
Elimination
d.
Digestion occurs in two different ways
i.
Mechanical digestion - breaking food into smaller pieces.
ii.
Chemical digestion - breaking food into smaller molecules.
2.
Food enters the digestive tract through the mouth.
a.
Chewing breaks food into smaller particles so that chemical digestion can
occur faster.
b.
c.
Teeth are important to animal digestion for capturing, tearing, and
chewing food.
i.
Carnivores possess pointed teeth for capture, cutting and shearing.
ii.
Herbivores have large, flat teeth suited for grinding plant materials.
iii.
Omnivores have both types, front like carnivores, back like
herbivores.
Food is moistened and lubricated in the mouth.
i.
Tongue mixes food with saliva.
ii.
Saliva is secreted by salivary glands.
iii.
Contains the enzyme salivary amylase to begin the breakdown of
starch.
iv.
There is continuous secretion to keep the mouth moist and to
lubricate food.
v.
Secretion of saliva is stimulated by the presence of food.
3.
Food passes to the stomach through the esophagus.
a.
Mucous lubricates and helps hold the chewed food together in a clump
called a bolus.
b.
The tongue is muscular and is used to move food. It pushes food to the
back of the mouth where it is swallowed. As the tongue moves the bolus
to the back of the mouth, the swallowing reflex takes over to move the
bolus down the esophagus.
c.
The respiratory and digestive passages meet in the pharynx. They separate
posterior to the pharynx to form the esophagus (leading to the stomach)
and the trachea (leading to the lungs). When swallowing, the epiglottis
automatically moves up to block the passage to the trachea so that food
does not end up in the lungs.
d.
The bolus is moved to the stomach by peristalsis (rhythmic muscle
contractions).
e.
A muscle called the cardiac sphincter controls the movement of food from
the esophagus to the stomach.
f.
This muscle prevents food in the stomach from reentering the esophagus.
Sometimes acid does splash up into the esophagus causing what we call
heartburn.
4.
Preliminary digestion occurs in the stomach
a.
The lining of the stomach is highly folded and expands as it fills with
food.
b.
The stomach has an extra layer of muscle to churn food. The mixture of
partly digested food and gastric juice is called chyme.
c.
Cells of the stomach lining secrete hydrochloric acid (HCl) which keeps
the stomach contents at a pH of about 2, helping to digest proteins. The
low pH also kills most bacteria. The cells of the lining are protected by a
coating of mucous that is constantly replaced as the acid destroys it.
d.
e.
f.
g.
h.
i.
5.
They also secrete pepsinogen, a protein digesting enzyme. Inactive
pepsinogen is cleaved to form pepsin - the active form of the enzyme. This
is important to protect the cells that produce pepsin from being digested
themselves. Pepsin is most active in a pH of 2.
The stomach produces about 2 L of acid and gastric secretions per day.
Seeing, smelling, tasting, or thinking about food can result in the
secretion of gastric juice.
Pepsin cuts proteins into short pieces and they are not further digested until
they reach the small intestine.
There is no digestion of carbohydrates or fats in stomach.
If too much stomach acid is produced, it can dissolve a hole through the
protective mucous coating and a gastric ulcer results. Ulcers can also be
caused by certain bacteria which can survive in the acidic environment of
the stomach. The growth of the bacteria on sections of the stomach lining
prevents it from secreting mucous, making it susceptible to the digestive
action of pepsin. Duodenal (intestinal) ulcers are more common, resulting
when excessive acidic chyme is passed to the duodenum.
Very little absorption occurs in the stomach, with some exceptions such as
water, aspirin and alcohol. All other absorption occurs in the intestine.
Other digestion and absorption of molecules take place in the small intestine.
a.
The duodenum is the first part of the small intestine. The passage of food
from the stomach to the small intestine is regulated by a muscle called the
pyloric sphincter, which allows chyme to enter the duodenum in small
spurts.
b.
The capacity of the small intestine is limited and digestion takes time so
only small amounts of chyme are permitted to enter at a time.
c.
The small intestine is where most digestion occurs.
i.
The length is approximately three meters. The first part, the
duodenum, is about the first 25 centimeters. The other two sections
are the jejunum and ileum.
ii.
At this point, proteins and carbohydrates are only partially digested
and lipid digestion has not begun.
iii.
In the duodenum, chyme, pancreatic enzymes, and bile from the
liver and gallbladder are mixed.
iv.
The presence of food in the duodenum also triggers the release of
various hormones
(1)
The intestine must be protected from the acid contained in
gastric juice. When chyme enters the small intestine, the
cells of the duodenum release the hormone secretin. This
hormone stimulates the pancreas to produce sodium
bicarbonate, which neutralizes the acidic chyme and shuts
off pepsin. It also stimulates the liver to secrete bile.
(2)
Another hormone (CCK, cholecystokinin), stimulates the
gallbladder to release bile and the pancreas to produce
pancreatic enzymes.
(3)
v.
Another hormone (GIP, gastrin inhibitory protein) inhibits
gastric glands in the stomach and inhibits the mixing and
churning movement of stomach muscles. This slows the
rate of stomach emptying when the duodenum contains
food.
Digestion continues and absorption occurs in all three sections of
the small intestine.
(1)
Digestive enzymes secreted by the cells of the small
intestine digest lactose, sucrose and other sugars. Some
adults lose the ability to produce lactase, resulting in the
condition called lactose intolerance.
(2)
Absorption of food in the intestine.
(a)
The walls of the small intestine are covered with
small projections called villi. These increase the
surface area of the small intestine to increase
absorption.
(b)
The villi themselves are covered with many tiny
projections called microvilli, which increase the
surface area still further.
(c)
The molecules resulting from the digestion of
proteins and carbohydrates are absorbed by cells of
the intestinal lining.
(d)
The villi contain capillaries, tiny blood vessels
which allows efficient transfer of these molecules to
the blood.
(e)
The products of fat digestion are absorbed through
the villi into the lymphatic system. They enter the
blood stream near the neck where the lymphatic
system joins the circulatory system.
6.
The pancreas secretes enzymes, bicarbonate and hormones.
a.
The pancreas is located below the stomach.
b.
Secretions of the pancreas reach the duodenum via the pancreatic duct.
The fluid contains
i.
The protein digesting enzyme trypsin.
ii.
The starch digesting enzyme pancreatic amylase.
iii.
The fat digesting enzyme lipase.
iv.
Bicarbonate to neutralize HCl from the stomach.
c.
The pancreas also produces hormones that regulate levels of sugar in the
blood. These hormones, insulin and glucagon, are produced by clusters of
cells called islets of Langerhans. See the Liver below.
7.
The liver produces bile and regulates blood composition.
a.
Role in Digestion
i.
b.
c.
Old red blood cells are destroyed by the liver and the hemoglobin
from these cells is used to make bile. The remains of the red blood
cells are eliminated with feces and give it its characteristic brown
color.
ii.
Bile is stored and concentrated in gall bladder. The presence of
fatty food in the duodenum triggers the gallbladder to release bile.
iii.
Bile travels through the common bile duct to the doudenum.
iv.
Bile salts are soluble in both lipids and water. This enables them to
break apart fat droplets in chyme to create smaller droplets. This
increases the surface area for lipase to work on and increases the
speed of their digestion.
v.
Cholesterol can bind bile together causing crystals to form. If these
crystals become large enough they can block the common bile
duct.
vi.
If the common bile becomes blocked, bile salts can accumulation in
the skin and produce a yellow color in a condition called jaundice.
Regulation of blood composition.
i.
Another role of the liver is to remove toxins from the blood.
ii.
The liver absorbs or chemically modifies toxic substances to
prevent them remaining in circulation.
iii.
Ammonia produced by the digestion of proteins is converted to a
less toxic compound (urea) by the liver. Urea is removed from the
blood by the kidneys and eliminated in urine.
iv.
Alcohol and drugs are metabolized by liver cells into less harmful
compounds. Other toxins, pesticides, and carcinogens are also
detoxified.
v.
These less harmful compounds are returned to the blood and are
removed by the kidneys.
vi.
If the liver is chronically exposed to toxins, the cells become
damaged and die. The result is cirrhosis of the liver.
Regulation of blood glucose levels.
i.
It is important to maintain a constant concentration of blood
glucose so that cells have a steady supply. This is especially
important for brain cells which store little glucose, and cannot use
fat or amino acids as an energy source.
ii.
Vertebrates eat sporadically when food is available and, often, a
period of fasting occurs between meals. Also, most food is
digested rapidly, and the resulting molecules (including glucose)
enter the blood stream. Without some control, the level of glucose
(and other compounds) in the blood would be quite variable.
iii.
The liver removes glucose from blood, converting it into glycogen.
Glycogen is stored in both the liver and in skeletal muscle.
iv.
The conversion of glucose to glycogen is stimulated by the
pancreatic hormone, insulin. If blood glucose is high (such as after
eating) the release of insulin from the pancreas causes the liver to
store glucose. If blood glucose level is low (such as between
v.
8.
meals), the opposing pancreatic hormone glucagon causes the liver
to secrete glucose into the blood.
The liver stores enough glycogen for about 10 hours of fasting. If
food is still unavailable after that, amino acids (from muscles) and
fats (from fat stored in fat cells) are used as a source of energy.
The large intestine concentrates solids by reabsorbing water.
a.
The large intestine or colon comprises last meter of the digestive tract.
b.
It has no digestive function, but functions to absorb water. If water is not
absorbed, as can happen during certain bacterial infections, diarrhea can
result, causing dehydration and salt loss.
c.
The daily total volume of food and water we take in from eating is about 2
L (including about 800 g of solids). The body adds about 7 L of its own
fluids making a total of 9 L.
i.
1.5 L saliva and salivary enzymes
ii.
2 L of gastric secretions
iii.
1.5 L of pancreatic secretions
iv.
0.5 L of bile from the liver
v.
1.5 L of intestinal secretions
d.
Nearly all fluids and solids are absorbed so that only 50 g of solids and
100 mL of liquid leave as feces. The large intestine (along with some
absorbed by the small intestine) recovers about 90% of the water that
enters the digestive system. Remember that this is very important for a
terrestrial vertebrate.
e.
Undigested material is compacted and stored until the colon is full. When
the colon is full, a signal to empty it is sent by sensors in the walls of the
colon.
f.
Bacteria (mostly E. coli) live and reproduce in the colon. Anaerobic
digestion (fermentation) of material by these bacteria produces gas in the
colon. They also produce vitamins for the host, including vitamin K some
B vitamins. Bacteria are lost when feces is eliminated, making exposure to
feces dangerous.
g.
Fiber (cellulose) tends to fill up the colon and cause it to be emptied. Low
fiber diets result in slower passage of food through colon and have been
linked to colon cancer.
h.
The rectum is the last section of the large intestine. Feces pass into the
rectum by peristaltic contractions and material exits through the anus, a
sphincter muscle.
i.
Feces is composed of approximately 75% water and 25% solids. One third
of the solids is intestinal bacteria, 2/3 is undigested materials.