Download Digestive System

Lec.12
Medical Physiology
Z.H.Al-Zubaydi
Digestive System
The organs of digestive system can be separated into two main group;
gastrointestinal tract (GIT) and the accessory digestive organs.
Gastrointestinal Tract (GIT)
Gastrointestinal tract is a continuous coiled hollow muscular tube
through the ventral body cavity and is open at both ends. Its organs are
the mouth, pharynx, esophagus, stomach, small intestine, and large
intestine. The large intestine leads to the terminal opening, or anus.
The GIT is approximately less than 9 m long.
Mouth
Food enters the digestive tract through the mouth, or oral cavity, a
mucous membrane-lined cavity. The lips (labia) protect its anterior
opening, The muscular tongue occupies the floor of the mouth.
As food enters the mouth, it is mixed with saliva and masticated
(chewed). The cheeks and closed lips hold the food between the teeth
during chewing. The nimble tongue continually mixes food with
saliva during chewing and initiates swallowing. Thus, the breakdown
of food begins before the food has even left the mouth. Papillae
containing taste buds, or taste receptors, are found on the tongue
surface.
Pharynx
From the mouth, food passes posteriorly into the pharynx is subdivided
into the nasopharynx, part of the respiratoiy passageway; the
oropharynx, posterior to the oral cavity; and the laryngopharynx,
which is continuous with the esophagus below.
The walls of the pharynx contain two skeletal muscle layers. The
cells of the inner layer run longitudinally; those of the outer layer run
around the wall in a circular fashion. Alternating contractions of these
two muscle layers propel food through the pharynx into the esophagus
below.
Esophagus
The esophagus or gullet, runs from the pharynx through the
diaphragm to the stomach, it is essentially a passageway that conducts
food (by peristalsis) to the stomach.
1
The walls of GIT organs from the esophagus to the large intestine
are made up of the same four basic tissue layers, or tunics:
1. The mucosa is the innermost layer, a moist membrane that lines
the cavity, or lumen, of the organ. It consists primarily of a surface
epithelium, plus a small amount of connective tissue (lamina
propria) and a scanty smooth muscle layer.
2. The submucosa is found just beneath the mucosa. It is a soft
connective tissue layer containing blood vessels, nerve endings,
lymph nodules, and lymphatic vessels.
3. The muscularis externa is a muscle layer typically made up of
an inner circular layer and an outer longitudinal layer of smooth
muscle cells.
4. The serosa is the outermost layer of the wall. It consists of a
single layer of flat serous fluid-producing cells
Stomach
The C-shaped stomach is on the left side of the abdominal cavity.
Different regions of the stomach have been named. The cardiac region
(named for its position near the heart), through which food enters the
stomach from the esophagus. The fundus is the expanded part of the
stomach lateral to the cardiac region. The body is the midportion, and
as it narrows inferiorly, it becomes the pyloric antrum, and then the
funnel-shaped pylorus, the terminal part of the stomach. The stomach
is approximately 25 cm long, but its diameter depends on how much
food it contains. When it is full, it can hold about 4 liters of food.
When it is empty, it collapses inward on itself.
The stomach acts as a temporary "storage tank" for food as well as a site
for food breakdown. Besides the usual longitudinal and circular muscle
layers, its wall contains a third obliquely arranged layer in the
muscularis externa. This arrangement allows the stomach not only to
move food along the tract, but also to churn, mix, and pummel the food,
physically breaking it down to smaller fragments. In addition, chemical
breakdown of proteins begins in the stomach.
The mucosa of the stomach is a simple columnar epithelium that
produces large amounts of mucus. This otherwise smooth lining is
dotted with millions of deep gastric pits, which lead into gastric glands
that secrete the gastric juice. For example, some stomach cells
produce intrinsic factor, a substance needed for the absorption of
vitamin B12 from the small intestine. The chief cells produce proteindigesting enzymes, mostly pepsinogens, and the parietal cells
produce corrosive hydrochloric acid, which makes the stomach
contents acidic and activates the enzymes. The mucous neck cells
2
produce a sticky alkaline mucus, which clings to the stomach mucosa
and protects the stomach wall itself from being damaged by the acid
and digested by the enzymes. Still other cells, the enteroendocrine
cells produce local hormones, such as gastrin, that are important to
the digestive activities of the stomach.
Most digestive activity occurs in the pyloric region of the stomach.
After food has been processed in the stomach, it resembles heavy cream
and is called chyme. The chyme enters the small intestine through the
pyloric sphincter.
Small Intestine
The small intestine is the body's major digestive organ. Within its
twisted passageways, usable food is finally prepared for its journey
into the cells of the body. It is the longest section of the GIT, with an
average length of 2.5 to 7 m in a living person.
The small intestine has three subdivisions: the duodenum, the
jejunum, and the ileum, which contribute 5 percent, nearly 40
percent, and almost 60 percent of the length of the small intestine
respectively.
Chemical digestion of foods begins in earnest in the small intestine.
The small intestine is able to process only a small amount of food at
one time. The pyloric sphincter controls food movement into the small
intestine from the stomach and prevents the small intestine from being
overwhelmed. Though the C-shaped duodenum is the shortest
subdivision of the small intestine, it has the most interesting features.
Some enzymes are produced by the intestinal cells. More important are
enzymes produced by the pancreas through the pancreatic ducts,
where they complete the chemical breakdown of foods in the small
intestine. Bile (formed by the liver) also enters the duodenum through
the bile duct in the same area. Nearly all food absorption occurs in the
small intestine. The small intestine wall has three structures that
increase the absorptive surface tremendously; microvilli, villi, and
circular folds.
· Microvilli are tiny projections of the plasma membrane of the
mucosa cells that give the cell surface a fuzzy appearance,
sometimes referred to as the brush border.
· Villi are fingerlike projections of the mucosa. Within each villus
is a rich capillary bed and a modified lymphatic capillary called a
lacteal. The digested foodstuffs are absorbed through the
mucosal cells into both the capillaries and the lacteal.
· Circular folds are deep folds of both mucosa and submucosa
layers.
3
Large Intestine
The large intestine is much larger in diameter than the small intestine
but shorter in length, about 1.5 m long. It extends from the ileocecal
valve to the anus. Its major functions are to dry out the indigestible
food residue by absorbing water and to eliminate these residues from
the body as feces. It has the following subdivisions: cecum, appendix,
colon, rectum, and anal canal.
The saclike cecum is the first part of the large intestine. Hanging from
the cecum is the wormlike appendix. Since it is usually twisted, it is an
ideal location for bacteria to accumulate and multiply. Inflammation of
the appendix, appendicitis, is the usual result.
The colon is divided into several distinct regions. The ascending
colon travels up the right side of the abdominal cavity and makes a
turn to travel across the abdominal cavity as the transverse colon.
It then turns again and continues down the left side as the
descending colon to enter the pelvis.
where it becomes the S-shaped sigmoid colon. The sigmoid colon,
rectum, and anal canal lie in the pelvis. The anal canal ends at the
anus, which opens to the exterior.
Because most nutrient absorption has occurred before the large
intestine is reached, no villi are seen in the large intestine, but there
are tremedous numbers of goblet cells in its mucosa that produce an
alkaline (HCO3~-rich) mucus. The mucus acts as a lubricant to ease
the passage of feces to the end of the digestive tract.
Accessory Digestive Organs
Salivary Glands
Three pairs of salivary glands empty their secretions into the mouth.
The large parotid glands lie anterior to the ears, the submandibular
glands and the small sublingual glands empty their secretions into
4
the floor of the mouth through tiny ducts.
The product of the salivary glands, saliva, is a mixture of mucus and
serous fluids. The mucus moistens and helps to bind food together
into a mass called a bolus, which makes chewing and swallowing
easier. The clear serous portion contains an enzyme, salivary amylase,
that begins the process of starch digestion in the mouth. Saliva also
contains substances such as lysozyme and antibodies (IgA) that
inhibit bacteria; therefore, it has a protective function as well. Also
saliva dissolves food chemicals so they can be tasted.
Pancreas
The pancreas is a soft, pink, triangular gland that extends across the
abdomen from the spleen to the duodenum. The pancreas produces
enzymes that break down all categories of digestible foods. The
pancreatic enzymes are secreted into the duodenum in an alkaline fluid,
which neutralizes the acidic chyme coming in from the stomach. The
pancreas also has an endocrine function; it produces the hormones
insulin and glucagon.
Liver and Gallbladder
The liver is the largest gland in the body. It is located under the
diaphragm, more to the right side of the body.
There is no question that the liver is one of the body's most
important organs. It has many metabolic and regulatory roles;
however, its digestive function is to produce bile that enters the
duodenum through the bile duct.
Bile is a yellow-to-green, watery solution containing bile salts, bile
pigments (chiefly bilirubin, a breakdown product of hemoglobin),
cholesterol, phospholipids, and a variety of electrolytes. Bile does
not contain enzymes, but its bile salts emulsify fats by physically
breaking large fat globules into smaller ones, thus providing more
surface area for the fat-digesting enzymes to work on.
The gallbladder is a small, thin-walled green sac in the inferior
surface of the liver. When food digestion is not occurring, bile backs
up the cystic duct and enters the gallbladder to be stored. While being
stored in the gallbladder, bile is concentrated by the removal of
water. Later, when fatty food enters the duodenum, a hormonal
stimulus prompts the gallbladder to contract and spurt out stored
bile, making it available to the duodenum.
5
6