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Digestion
Digestion is the mechanical and chemical breakdown of foods into
smaller components that can be absorbed. Mechanical digestion breaks
down food into smaller particles. Chemical digestion consists of
hydrolysis reactions that break down dietary macromolecules into their
monomers, which allows for the absorption of nutrients. The organs of
the digestive system carry out these processes. The digestive system
consists of the digestive tract (alimentary canal), which extends from
the mouth to the anus, and several accessory organs, which release
secretions into the canal. The alimentary canal includes the mouth,
pharynx, esophagus, stomach, small intestine, large intestine, and anal
canal; the accessory organs include the teeth, tongue, salivary glands,
liver, gallbladder, and pancreas.
This table outlines the path of food through the digestive tract.
Digestion begins in the oral cavity with both mechanical and chemical breakdown of food. Chewing motion breaks down food
particles and mixes food with saliva secreted from salivary glands. There are three salivary glands that contribute secretions to
saliva. The parotid glands contribute a serous fluid rich in amylase. Amylase is a digestive enzyme that begins carbohydrate
digestion. The submandibular glands secrete a primarily serous fluid that also contains some mucus. Mucus, secreted from the
sublingual glands, helps bind food together and acts as a lubricant during swallowing. As food is swallowed, it enters the
esophagus and is moved downward by a wavelike muscular contraction called peristalsis. The contractions progress from the
superior end of the esophagus to the gastroesophageal junction, where the contents of the esophagus empty into the cardiac
region of the stomach. The lining of the stomach secretes gastric juices that contain digestive enzymes. Peristalsis churns and
mixes the food with gastric juices while moving the food toward the first part of the small intestine, the duodenum. In the
duodenum, food is digested even further. The bile duct from the liver and gallbladder, and the pancreatic duct from the
pancreas all empty their secretions in the duodenum. The bile works to break down fats, while the pancreatic enzymes aid in the
digestion of other food particles. Together, these digestive juices help prepare the food so that the small intestine can absorb
as many nutrients from the food as possible. The majority of the absorption takes place in the second portion of the small
intestine, a three foot section called the jejunum. As the food travels through the jejunum, it's gradually broken down into
smaller and smaller particles. Folds line the walls of the intestine and act to increase the absorptive surface area. The green
nutrient particles and the yellow fat particles get caught in these folds and are absorbed into the intestinal wall. Larger, less
digested particles are pushed on to the large intestine where much of the water they contain is reabsorbed before being
expelled from the body. A closer inspection of the folds reveals that the intestinal wall is covered with thousands of fingerlike
villi. A network of tiny blood vessels called capillaries runs just below the surface of each villus. These capillaries absorb the
available nutrients in the intestine and transport them to the liver. The yellow fat nutrients are absorbed into a vessel called a
lacteal that carries them to the circulatory system. Nutrients absorbed by the blood in the villi are carried to the liver by way
of the portal vein. In the liver, the blood is filtered and the nutrients are extracted and processed. After processing, the
nutrients are either stored for future use or released into the bloodstream to be used throughout the body.
Movements in the Digestive Tract
(a) Peristalsis. A wave of relaxation of the circular muscles is followed by a wave of strong contraction of the circular muscles,
which propels the bolus of food through the digestive tract.
(b) Segmental contractions. Each section of the digestive tract involved in segmental contractions alternates between
contraction and relaxation. The series of figures from top to bottom depicts a temporal sequence for one part of the small
intestine. The arrows indicate the direction material in a given part of the intestine moves with each contraction. Material
(brown) introduced at the beginning of the sequence (top figure) is spread out and becomes more diffuse (lighter color) through
time.
Histology
The digestive tract consists of four major layers: an internal mucosa and an external serosa with a submucosa and
muscularis, or muscular layer, in between. These four layers are present in all areas of the digestive tract from the
esophagus to the anus.
Mucosa
The innermost layer, the mucosa, consists of three layers: (1)
the mucous epithelium, which is moist stratified squamous
epithelium in the mouth, oropharynx, esophagus, and anal
canal and simple columnar epithelium in the remainder of the
digestive tract; (2) a loose connective tissue called the lamina
propria; and (3) a thin smooth muscle layer, the muscularis
mucosae
. Submucosa
The submucosa is a thick connective tissue layer containing nerves,
blood vessels, and small glands that lies beneath the mucosa. The
nerves of the submucosa form the submucosal plexus, a
parasympathetic ganglionic plexus.
Muscularis Externa, or Muscular Layer
The next layer is the muscularis, which consists of an
inner layer of circular smooth muscle and an outer layer
of longitudinal smooth muscle. Two exceptions are the
upper esophagus, where the muscles are striated, and
the stomach, where there are three layers of smooth
muscle. Another nerve plexus, the myenteric plexus,
which consists of nerve fibers and parasympathetic cell
bodies, is between these two muscle layers. Together,
the submucosal and myenteric plexuses constitute the
intramural (meaning, “within the walls”) plexus. The
intramural plexus is extremely important in the control
of movement and secretion.
Serosa or Adventitia
The fourth layer of the digestive tract is a connective tissue layer called either the serosa or the adventitia, depending on
the structure of the layer. Parts of the digestive tract that protrude into the peritoneal cavity have a serosa as the
outermost layer. This serosa is called the visceral peritoneum. It consists of a thin layer of connective tissue and a simple
squamous epithelium. When the outer layer of the digestive tract is derived from adjacent connective tissue, the layer is
called the adventitia and consists of a connective tissue covering that blends with the surrounding connective tissue. These
areas include the esophagus and the retroperitoneal organs.
Mouth & Pharynx
Digestion begins in the mouth with the mechanical action of teeth and the
chemical action of digestive enzymes present in saliva. Salivary amylase
breaks starch into the disaccharide maltose. Saliva lubricates the contents of
the mouth. The tongue manipulates food into a bolus as it is chewed and
initiates the swallowing process. The tongue also secretes lingual lipase, an
enzyme that is activated by the acid in the stomach. It chemically digests fat
after the food has been swallowed.
Teeth are an important feature of almost all vertebrates. Mammals are characterized by differentiation of teeth for various
functions. From front to back of the mouth, the types are incisors, canines, premolars, and molars.
Teeth grind food during mastication. The exposed part of a tooth, or the crown, is covered with enamel, which is the hardest
substance in the human body. The crown consists of rounded projections or cusps that facilitate the grinding process. Over
time, food particles can cling and accumulate in the grooves between the cusps and between the teeth, forming a sticky plaque.
Bacteria, such as streptococcus, will flourish within the plaque, eating and digesting the sugars and carbohydrates. As a result,
bacteria produce acidic by-products that are harmful to the teeth. Normally, saliva will neutralize these acids but plaque
provides a barrier that prevents saliva from reaching acid that is concentrated around the tooth's enamel surface.
From a cross-sectional view, the tooth consists of the exposed crown supported on a neck with roots that project into the
mandible or maxillae. There are three tissue layers that make up the tooth: the hard enamel, the dentin, and a central region
known as the pulp. Bacterial acid from plaque begins to erode the surface of the tooth, dissolving the enamel and forming a
cavity or dental caries. This begins the process of tooth decay. The enamel contains no cells, so no pain is experienced at this
point. As the cavity progresses, bacteria can penetrate the enamel and dissolve a crater in the dentin layer. Eventually, the
cavity is extended into the soft pulp, where the destruction of pulp tissues leads to inflammation and infection. As extensive
destruction continues, a pulp abscess made up of pus and dead tissue may form. Stimulation of the nerves causes the sensation
of a toothache and eventually severe pain. The abscess can spread into the surrounding bone and could cause serious problems if
left untreated.
The salivary glands secrete saliva. This fluid moistens food
particles, helps bind them, and begins the chemical digestion of
carbohydrates. Saliva is also a solvent, dissolving foods so that they
can be tasted, and it helps cleanse the mouth and teeth. Within a
salivary gland are two types of secretory cells - serous cells and
mucous cells. These cells are present in varying proportions within
different salivary glands. Serous cells produce a watery fluid that
contains the digestive enzyme amylase. This enzyme splits starch
and glycogen molecules into the disaccharide maltose - the first
step in the chemical digestion of carbohydrates. Mucous cells
secrete a thick liquid called mucus, which binds food particles and
lubricates during swallowing.
Major Salivary Glands
Three pairs of major salivary glands - the parotid, submandibular, and sublingual glands - and many minor glands are associated
with the mucous membranes of the tongue, palate, and cheeks.
Esophagus
The esophagus is that part of the digestive tube that extends
between the pharynx and the stomach. It is about 25 cm long and lies
in the mediastinum, anterior to the vertebrae and posterior to the
trachea. It passes through the esophageal hiatus (opening) of the
diaphragm and ends at the stomach. The esophagus transports food
from the pharynx to the stomach through peristalsis.
Stomach
The muscular stomach can store up to 1 liter of food. It mixes food with the secretions of the folded stomach lining
and steadily releases the liquefied contents (chyme) through the pyloric sphincter into the small intestine. Reflux
(backflow) of stomach contents into the esophagus is prevented in part by the tonus of the gastroesophageal
sphincter, located at the interior end of the esophagus
The stomach is a J-shaped, pouchlike organ that
hangs under the diaphragm in the upper left portion
of the abdominal cavity and has a capacity of about 1
liter or more. Thick folds (rugae) of mucosal and
submucosal layers mark the stomach's inner lining
and disappear when the stomach wall distends. The
stomach receives food from the esophagus, mixes
the food with gastric juice, which initiates protein
digestion, does a limited amount of absorption, and
moves food into the small intestine.
.
Microanatomy of the Stomach
(a) Gastric glands include mucous cells, parietal cells, and chief cells. (b) Gastric pits are the openings of the gastric
glands and stud the stomach mucosa.
The chief cells secrete pepsinogen, and the parietal cells secrete hydrochloric acid (HCl). The HCl activates pepsinogen
by removing some of the amino acids from pepsinogen, thus converting it to pepsin. Pepsin’s function is to partially digest
dietary proteins, but it also catalyzes pepsinogen, thus producing more pepsin.
Parasympathetic Regulation of Gastric Secretions
Parasympathetic nerve impulses that stimulate the release of gastric juice and gastrin. Gastrin stimulates increased
secretion of gastric juices. Gastrin also stimulates intestinal motility.
Upon closer examination of the stomach, the muscular folds called rugae that make up the stomach lining are clearly visible. The
rugae gradually smooth out as the stomach fills, permitting stomach distension. A cross section of the stomach lining reveals
that in between the rugae are gastric pits, which are the openings of the gastric glands. The gastric glands are lined with
different types of cells that contribute various components to gastric juice. Chief cells and parietal cells make up the gastric
gland. Pepsinogen is secreted by the chief cells and the parietal cells secrete hydrochloric acid. Hydrochloric acid converts
pepsinogen into pepsin. Pepsin is the key component to gastric juice and is a potent digestive enzyme. By splitting protein
molecules into even smaller parts, pepsin begins the digestion of proteins.
Normally, gastric juices are secreted in the presence of food. Stress and smoking can also cause the stomach lining to secrete
gastric juices. In the absence of food, the acidic juices erode the lining of the stomach, leaving an open sore that may bleed.
This open sore is called an ulcer. Recent research has shown, however, that a bacterium, Helicobacter pylori, may be a major
factor in ulcer formation. The use of antibiotics to kill the bacteria has proved quite helpful in treating many patients with
ulcers.
Liver & Pancreas
The liver is the largest internal
organ of the body, weighing about
1.36 kg (3 pounds). It is located in
the right upper quadrant of the
abdomen, tucked against the inferior
surface of the diaphragm.
The liver parenchyma consists
primarily of hepatocytes arranged in
cylinders called hepatic lobules. In
the center of each lobule is a central
vein, which ultimately flows into the
the right and left hepatic veins. The
hepatocytes form plates that radiate
from the cental vein. In between the
plates are blood capillaries called
hepatic sinusoids, which collect blood from branches of the hepatic artery and hepatic portal vein and drain into the
central canal. As the blood flows through the sinusoids, the hepatocytes extract substances such as nutrients and add
others such as albumin, while macrophage remove bacteria and debris. The lobules are separated by a stroma, which
frequently contains a hepatic triad of two blood vessels and a bile ductule.
One of the many functions of the liver is the production of bile. Bile contains cholesterol, pigments from the breakdown
of red blood cells, and salts. Bile is stored in the gallbladder and is released into the small intestine through the bile duct.
The bile salts help break up fat droplets so they can be digested.
Duct System of the Major Abdominal Digestive Glands
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2.
3.
4.
The hepatic ducts from the liver lobes combine to form the common hepatic duct.
The common hepatic duct combines with the cystic duct from the gallbladder to form the common bile duct.
The common bile duct and the pancreatic duct combine to form the hepatopancreatic ampulla.
The hepatopancreatic ampulla empties into the duodenum at the major duodenal papilla.
Anatomical Relationship between the Pancreas and the Small Intestine
The pancreas is closely associated with the duodenum.
Bile is stored in the gallbladder. Normally, the cholesterol in the bile remains in solution. Under certain conditions, the
cholesterol may precipitate and form solid crystals. If this process continues , the crystals grow larger and become gallstones.
The stones may block the flow of bile, causing pain and jaundice. Gallstones may be removed surgically.
Small Intestine
The small intestine is the primary organ of
digestion and absorption. It consists of three
parts: the duodenum, the jejunum, and the
ileum. The entire small intestine is about 6 m
long (range: 4.6–9 m). The duodenum is about
25 cm long (the term "duodenum" means “12”,
suggesting that it is 12 inches long). The
jejunum, constituting about two-fifths of the
total length of the small intestine, is about 2.5
m long; and the ileum, constituting three-fifths
of the small intestine, is about 3.5 m long. Two
major accessory glands, the liver and the
pancreas, are associated with the duodenum.
The lining of the small intestine is characterized by numerous circular folds called plicae circulares. The plicae are lined
with fingerlike villi. From a cross-sectional view, the villus contains a network of capillaries which surround a specialized
lymphatic vessel known as a lacteal. The epithelium of an intestinal villus consists of columnar cells which are covered with
microvilli. This succession of folds and projections increases the surface of the intestinal lining for efficient absorption.
Carbohydrate digestion is completed by enzymes in the small intestine. Carbohydrates are absorbed by the villi and then
enter the capillary. Fat digestion occurs primarily in the small intestine. Fat molecules are digested and absorbed into the
epithelial cells of the villus. The fats are formed into clusters called chylomicrons which pass into the lacteal. Lymph
carries chylomicrons away from the villus. Protein digestion is completed in the small intestine. Proteins are broken down
first into peptides, then into amino acids. These are absorbed into the villi, then into the capillary.
The surface area of the small intestine is greatly increased by folding. Each large fold and furrow is covered with small
finger-like villi. They give the intestinal lining a velvety appearance. The cells lining the villi produce enzymes that complete
the digestion of peptides and sugars
Each epithelial cell on the villus has about 500 projections called microvilli. The absorptive surface of the small intestine is
increased 600-fold by its 6 million villi. Each villus contains capillaries surrounding a lymph vessel called a lacteal. The
capillaries take up monosaccharides and amino acids; the lacteal takes up glycerol and fatty acids.
Large Intestine
The large intestine is
so named because its
diameter is greater than
that of the small
intestine. This portion
of the alimentary canal
is about 1.5 meters long,
and it begins in the
lower right side of the
abdominal cavity, where
the ileum joins the
cecum. From there, the
large intestine extends
upward on the right
side, crosses obliquely
to the left, and
descends into the pelvis.
At its distal end, it opens to the outside of the body as the anus.
The large intestine reabsorbs water and electrolytes from chyme remaining in the alimentary canal,
and also forms and stores feces. In addtion, the large intestine is densely populated with bacterial
flora that ferment cellulose and other undigested carbohydrated and synthesize B vitamins and
vitamin K.
The large intestine, or colon, receives material from the small intestine after nutrients have been
absorbed. At the junction of the small and large intestines is a pouch, or cecum, with a finger-like
extension called the vermiform appendix. The colon is larger in diameter, but much shorter than
the small intestine. Much of the water used in digestion is reabsorbed here, as well as ions. Large
numbers of hundreds of bacterial species are present in the large intestine. Some of these produce
vitamins that are absorbed. Bacteria make up a large proportion of the solid matter of feces.
Before nutrients can be absorbed by
the small intestines, they must be
broken down into monomers.
Note: Enzymes do not break down
polymers, water does by hydrolysis.
Enzymes increase the rate of the
reaction.