DIGESTIVE SYSTEM
The human digestive system passes from the mouth through the oesophagus to the
stomach where acids facilitate some chemical changes. From the stomach, food passes to
the small intestine, where some nutrients are further broken down to very simple
molecules to be absorbed into the blood stream, and then to the large intestine or colon,
where water is reabsorbed and the residual material is compacted. Exit is through the
rectum and out the anus. Above the stomach is the liver, which plays many important
roles in digestion; under the liver it is the pancreas, which secretes many of the digestive
enzymes into the small intestine.
The process of changing food into its simple components is called digestion, and the
changes occur primarily in the digestive tract. The digestive tract is essentially a tube
about 30 feet (9m) long that passes through the centre of the body from the mouth to the
anus. The GI tract comprises of four layers. From innermost to outermost there is:
1)
Mucous membrane
2)
Sub mucous layer
3)
Muscular layer
4)
Serous layer
The muscular layer of the stomach is different from other regions of the digestive tract in
that it consists of three layers: an outer longitudinal layer, a middle circular layer, and an
inner oblique layer. Such an arrangement gives the muscles elasticity, enabling the
stomach to expand considerably after meals in order to hold all the food eaten. It also
produces strong contractions, which churn the food about so that it mixes with the gastric
juices.
Foods are digested by a series of mechanical and chemical processes that break them
down into particles that can be absorbed.
In the mouth: food is physically separated into small pieces by chewing and the
movement of the tongue, cheeks, lips and lower jaw. Saliva is mixed with food during
the chewing process, and the food becomes a moist, soft mass called bolus.
Mucus - a thick, slippery secretion of the mucous membranes that line the mouth – also
helps moisten and soften food and to lubricate its passage from the mouth.
In the oesophagus: through coordinated contractions and relaxations, the 9 to 10 inch
long oesophagus delivers the bolus of swallowed food to the stomach. When the stomach
contents spurt up into the oesophagus due to vomiting or indigestion, the walls of
oesophagus are irritated by stomach acid and oesophagus signals pain. Because
oesophagus is close to the heart, this pain is commonly called “heartburn”
In the stomach: after food is fully mixed and liquefied the pyloric sphincter at the end of
the stomach relaxes for an instant and allows about 1 to 2 teaspoons of food, now called
chyme (semi fluid mass of partly digested food that is expelled by the stomach into the
duodenum) to enter the duodenum of the small intestine. This amount of chyme leaves
1
the stomach about every 30 seconds when there is food in the stomach. How does the
pyloric sphincter know when to open and close? When the pyloric sphincter relaxes,
acidic chyme slips through. The cells of the pyloric muscle on the intestinal side sense
the acid, causing the pyloric sphincter to close tightly. Only after the chyme has been
neutralized by pancreatic bicarbonate and the medium surrounding the pyloric sphincter
has become alkaline can the muscle relax again. This process ensures that the chyme will
be released slowly enough to be neutralized as it flows through the small intestine. This
is important, because the small intestine has less of a mucous coating than the stomach
does and so is not as well protected from acid.
The digestive juices secreted in the stomach contain hydrochloric acid, bicarbonate (a
base) and enzymes. Hydrochloric acid deactivates the salivary amylase that was
combined with food in the mouth and kills many of the bacteria that were consumed
along with the food. Hydrochloric acid also activates pepsinogen, the precursor of the
protein – splitting enzyme pepsin. Bicarbonate secreted by the stomach partially
neutralises the acidity of the food mixture before it is delivered to the small intestine.
The fat – digestion enzyme secreted by the stomach is gastric lipase. Fats leave the
stomach still requiring a good deal of digestion by enzymes and other substances in the
small intestine.
Mucus secreted by the stomach protects its lining from its other secretions. Without this
mucus, the stomach lining, too, would be digested.
A small amount of simple sugars, alcohol, and water are absorbed by the lining of the
stomach and enter the blood stream.
Small intestine: the small intestine is 15 to 30 feet in length and consists of the
duodenum, jejunum and ileum.
Bile is produced by the liver and stored in the gallbladder. It is released by the
gallbladder when the small intestine signals that fat has arrived. Bile is not an enzyme,
but an emulsifier; it makes fats partially soluble in water and thus more vulnerable to the
action of enzymes.
The pancreas produces:
pancreatic amylase for carbohydrate digestion
trypsinogen, chymotrypsinogen, and other substances for protein digestion,
and pancreatic lipase for fat digestion.
Pancreatic amylase and lipase are used directly for digestion, but enzymes present in the
small intestine must first activate trypsinogen and chymotrypsinogen. The -ogen suffix
denotes “an agent that produces” an active substance; trypsinogen, for example, is
activated by enterokinase and forms trypsin, an active enzyme. Trypsin also converts
chymotrypsinogen into the active enzyme chymotrypsin.
2
The small intestine produces enzymes that digest carbohydrates and fats. Sucrose,
lactase, and maltase break down sugars and intestinal lipase breaks down fats.
Components of foods that are incompletely digested or not digested in the small intestine
are passed through the large intestine and excreted in faeces. Little digestion occurs after
food particles leave the small intestine.
Large intestine – the large intestine, or colon, is about five feet long. It serves to collect
and transport waste products of digestion that will be excreted. The large intestine begins
with the cecum and ends with the sigmoid colon. In between the cecum and sigmoid
colons are the ascending, transverse, and descending portions of the colon.
The large intestine is home for many types of bacteria, which break down some types of
dietary fibre and produce part of the body’s supply of biotin and vitamin K. The end
products of bacterial fibre are primarily fatty acids and gases. Although some of the fatty
acids formed by bacteria from dietary fibre are absorbed, the amount appears to be too
small to make a significant contribution to caloric intake.
Summary of the sites and the nature of digestion
Site
Mouth
Oesophagus
Stomach
Type of
Action
Mechanical
Chemical
How accomplished
Mechanical
Chemical
Mechanical
Chemical
Peristalsis (delivers bolus to stomach)
N/A
Peristalsis
Action of hydrochloric acid
Bicarbonate (a base)
Gastric enzymes (pepsinogen activated to pepsin and
gastric lipase)
Mucus
Peristalsis
Bile
Pancreatic enzymes (pancreatic amylase, trypsinogen
activated to trypsin, chymotrypsinogen activated to
chymotrypsin, pancreatic lipase)
Intestinal enzymes (intestinal lipase, sucrose, lactase,
maltase)
Intestinal enzymes
Small
Intestine
Mechanical
Chemical
Intestinal
membrane
Large
intestine
Chemical
Mechanical
Chemical
Chewing
Salivary enzymes (s. amylase)
Lingual enzymes (l. lipase)
Collects and transports waste products of digestion that
will be excreted
Bacterial enzymes
3
In the duodenum the food meets pancreatic juice, which contains several proteolytic
(protein-splitting) enzymes all secreted as inactive precursors.
An overview of the participation of key enzymes, hydrochloric acid and bile in the
digestion of the energy nutrients.
Site
MOUTH
Carbohydrates
Salivary amylase
Proteins
Fats
Lingual lipase
Pepsinogen
(inactive)
STOMACH
Gastric lipase
hydrochloric acid
Pepsin
(active)
Pancreatic amylase
SMALL
INTESTINE Intestinal amylase
Sucrase
Lactase
Maltase
Tryspinogen
(inactive)
Bile
Pancreatic lipase
Intestinal lipase
Enterokinase (secreted by the intestinal
wall)
Trypsin
(active)
Chymotrypsinogen Trypsin chymotrypsin
(inactive)
(active)
Procarboxypeptidase A Trypsin Carboxypeptidases
(inactive)
Procarboxypeptidase B
(active)
Trypsin
(inactive)
Peptidases
(active)
GASTROINTESTINAL HORMONES
The stomach normally maintains a pH between 1.5 and 1.7. How does it stay that way?
One of the regulations of the stomach pH is the hormone gastrin, secreted by cells in the
stomach wall. The entrance of food into the stomach stimulates these cells to release
gastrin, which in turn, stimulates other stomach glands to secrete the components of
4
hydrochloric acid. When pH 1.5 is reached, the acid itself turns off the gastrin –
producing cells so that they stop releasing the hormone. Once the hormonal stimulus has
ceased, the glands stop producing hydrochloric acid. Thus the system adjusts itself.
As the chyme enters the intestine, the pancreas adds bicarbonate to it, so that the
intestinal contents always remain at a slightly alkaline pH. How does the pancreas know
how much to add? The presence of chyme stimulates the cells of the duodenum wall to
release the hormone secretin into the blood. As this hormone circulates through the
pancreas, it stimulates the pancreas to release its bicarbonate – rich juices. Thus,
whenever the duodenum signals that acidic chyme is present, the pancreas responds by
sending bicarbonate to neutralise it. When the need has been met, the secretin cells of the
duodenum wall are no longer stimulated to release the hormone, the hormone no longer
flows through the blood, the pancreas no longer receives the message, and it stops
sending pancreatic juice.
When fat is present in the intestine, the gall bladder contracts to squirt bile into the
intestine to emulsify the fat. How does the gall bladder get the message that fat is
present? Fat in the intestine stimulates cells of the intestine wall to release the hormone
Cholecystokinin (CCK). This hormone, travelling by way of the blood to the gall
bladder, stimulates it to contract, releasing bile into the small intestine. Once the fat in
the intestine is emulsified and enzymes have begun to work on it, the fat no longer
provokes release of the hormone, and the message to contract is cancelled.
Fat takes longer to digest than carbohydrate does. When fat is present, intestinal motility
slows to allow time for its digestion. How does the intestine know when to slow down?
Cholecystokinin and gastric – inhibitory peptide slow GI tract motility, thus keeping food
in the stomach longer.
By slowing the digestive process, fat helps to maintain a pace that will allow all reactions
to reach completion. Gastric – inhibitory peptide also inhibits gastric acid secretion.
Hormonal mechanics like these account for much of the body’s ability to adapt to
changing conditions.
Summary of Gastrointestinal hormones
HORMONE
GASTRIN
SECRETIN
CHOLECYSTOKININ
(CCK)
GASTRIC INHIBITORY
PEPTIDE
SITE OF
PRODUCTION
Cells in the
stomach wall
Cells in the
duodenum wall
TARGET
ORGAN
Stomach
Cells of the
intestinal wall
Small intestine
Gall bladder
Pancreas
Stomach
RESPONSE
Secretion of gastric
juice
Secretion of
bicarbonate – rich
pancreatic juice
- Release of bile
- Slowing of GI motility
- Inhibits gastric acid
secretion
- Slowing of GI motility
5
SYSTEMS OF THE BODY
A system is a group of organs, which are closely allied to one another and are concerned
with carrying out a particular bodily function.
THE CIRCULATORY SYSTEMS
Once a nutrient has entered the bloodstream, it may be transported to any of the cells in
the body, from the tips of the toes to the roots of the hair. The circulatory systems deliver
nutrients wherever they are needed.
THE VASCULAR SYSTEM
The circulatory or vascular system consists of the heart, the blood vessels and the blood.
It is a closed system of vessels through which blood flows continuously in a figure eight,
with the heart serving as a pump at the crossover point. As the blood circulates through
this system, it picks up and delivers materials as needed.
All the body tissues derive oxygen and nutrients from the blood and deposit carbon
dioxide and other wastes into it. The lungs exchange carbon dioxide (which leaves the
blood to be exhaled) and oxygen (which enters the blood to be delivered to all cells).
The digestive system supplies the nutrients to be picked up. In the kidneys, wastes other
than carbon dioxide are filtered out of the blood to be excreted in the urine.
Blood leaving the right side of the heart circulates by way of arteries into the lung
capillaries and then back through veins to the left side of the heart. The left side of the
heart then pumps the blood out through arteries to all systems of the body. The blood
circulates in the capillaries, where it exchanges material with the cells, and then collects
into veins, which return it again to the right side of the heart. In short, blood travels this
simple route: heart to arteries to capillaries to veins to heart.
The routing of the blood past the digestive system has a special feature. The blood is
carried to the digestive system (as to all organs) by way of an artery, which (as in all
organs) branches into capillaries to reach every cell. Blood leaving the digestive system,
however, goes by way of vein, not back to the heart, but to another organ – the liver.
This vein again branches into capillaries so that every cell of the liver also has access to
the blood carried by the vein. Blood leaving the liver then again collects into a vein,
which returns to the heart. The route is then like this: heart to arteries to capillaries (in
intestines) to vein to capillaries (in liver) to vein to heart.
The liver’s placement ensures that it will be first to receive the materials absorbed from
the GI tract. In fact, the liver has many jobs to do in preparing the absorbed nutrients for
use by the body. It is the body’s major metabolic organ. You might guess that, in
addition, the liver serves as a gatekeeper to defend against substances that might harm the
heart or brain. This is why, when people ingest poisons that succeed in passing the first
barrier (the intestinal cells), the liver quite often suffers the damage – from the hepatitis
6
virus, from drugs such as barbiturates or alcohol from poisons, and from contaminants
such as mercury.
The liver’s key position in nutrient transport
1. Vessels gather up nutrients and reabsorbed water and salts from all over the
digestive tract.
2. The vessels merge into the portal vein, which conducts all absorbed materials to
the liver.
3. The hepatic artery brings a supply of freshly oxygenated blood (not loaded with
nutrients) from the lungs to supply oxygen to the liver’s own cells.
4. Capillaries branch all over the liver, making nutrients and oxygen available to all
its cells and giving the cells access to blood from the digestive system.
5. The hepatic vein gathers up blood in the liver and returns it to the heart.
THE LYMPHATIC SYSTEM
The lymphatic system is also considered part of the circulatory system. The lymphatic
system provides a one-way route for fluid from the tissue spaces to enter the blood.
Lymph fluid circulates between the cells of the body and collects into tiny vessels.
Lymph is almost identical to blood except that it contains no red blood cells or platelets,
because they cannot escape through the blood vessel walls.
The lymphatic system has no pump; instead, lymph is squeezed from one portion of the
body to another like water in a sponge, as muscles contract and create pressure here and
there. Ultimately, much of the lymph collects in a large duct behind the heart. This duct
terminates in a vein that conducts the lymph toward the heart. Thus materials from the
GI tract that enter lymphatic vessels (large fats and fat-soluble vitamins) ultimately enter
the blood circulatory system, circulating through arteries, capillaries, and veins like the
other nutrients, with a notable exception – they bypass the liver at first. Thus, in contrast,
nutrients absorbed into lymph do not go to the liver first. They go to the heart, which
pumps them to all the body’s cells. The cells remove the nutrients they need, and the
liver then has to deal only with the remnants.
Once inside the vascular system, the nutrients can travel freely to any destination and can
be taken into cells and used as needed.
Lymph vessels are interrupted by lymph nodes, which remove bacteria and other foreign
matter from lymph on its return to the blood system. Lymph nodes are grouped in
clumps and can be felt as “glands” in the armpit, groin and neck. They also produce
antibodies.
Terms used in circulatory systems
o Arteries: vessels that carry blood away from the heart
o Veins: vessels that carry blood back to the heart
7
o Capillaries: small vessels that branch from an artery. Capillaries connect arteries
to veins. Exchange of oxygen, nutrients, and waste materials takes place across
capillary walls.
o Portal vein: the vein that collects blood from GI tract and conducts it to
capillaries in the liver. (Portal = gateway)
o Hepatic vein: the vein that collects blood from the liver capillaries and returns it
to the heart. (Hepatic = liver)
o Lymphatic system: a loosely organized system of vessels and ducts that convey
fluids toward the heart; the GI part of the lymphatic system carries the products of
digestion into the bloodstream.
o Lymph: a clear yellowish fluid that resembles blood without the red blood cells;
lymph from the GI tract transports fat and fat-soluble vitamins to the bloodstream
via lymphatic vessels.
o Thoracic duct: the duct that conveys lymph toward the heart
o Subclavian vein: connects thoracic duct with the right upper chamber of the
heart, providing a passageway by which lymph can be returned to the vascular
system.
8