ILEUM.The ileum is the last and longest part of
the small intestine. It is approximately 3.5 meters (11.5
feet) long. Most of the absorption of food occurs in the
ileum, where fingerlike projections (villi) provide a
large absorption surface. After ingestion, it takes 20
minutes to 2 hours for the first portion of the food to
pass through the small intestine to the beginning of the
The large intestine is so called because it is larger
in diameter than the small intestine (fig. 1-53). It is
considerably shorter, however, being about 1.5 meters
(5 feet) long. It is divided into three distinct parts: the
cecum, colon, and rectum.
CECUM AND COLON.The unabsorbed food
or waste material passes through the cecum into the
ascending colon, across the transverse colon, and
down the descending colon through the sigmoid
colon to the rectum. Twelve hours after the meal, the
waste material passes slowly through the colon,
building in mass and reaching the rectum 24 hours after
the food is ingested.
The appendix, a long narrow tube with a blind
end, is a pouchlike structure of the cecum located near
the junction of the ileum and the cecum (fig. 1-53).
There is no known function of this structure.
Occasionally, the appendix becomes infected, causing
inflammation to develop. This inflammation of the
appendix is known as appendicitis.
RECTUM.The rectum is approximately 12.5
cm (5 inches) long and follows the contour of the
sacrum and coccyx until it curves back into the short
(2.5 to 4 cm) anal canal. The anus is the external
opening at the lower end of the digestive system.
Except during bowel movement (defecation), it is kept
closed by a strong muscular ring, the anal sphincter.
ACCESSORY ORGANS OF DIGESTION
The accessory organs of digestion include the
salivary glands, pancreas, liver, and gallbladder. As
stated earlier, during the digestive process, the
accessory organs produce secretions that assist the
organs of the alimentary canal.
The salivary glands are located in the mouth (fig.
1-53). Within the salivary glands are two types of
secretory cells, serous cells and mucous cells. The
serous cells produce a watery fluid that contains a
digestive juice called amylase. Amylase splits starch
and glycerol into complex sugars. The mucous cells
secrete a thick, sticky liquid called mucus. Mucus
binds food particles together and acts to lubricate
during swallowing. The fluids produced by the serous
a n d m u c o u s c e l l s c o m b i n e t o f o r m s a l i v a .
Approximately 1 liter of saliva is secreted daily.
The pancreas is a large, elongated gland lying
posteriorly to the stomach (fig. 1-53). As discussed
earlier in The Endocrine System, the pancreas has
two functions: It serves both the endocrine system and
the digestive system. The digestive portion of the
pancreas produces digestive juices (amylase,
proteinase, and lipase) that are secreted through the
pancreatic duct to the duodenum. These digestive
juices break down carbohydrates (amylase), proteins
(proteinase), and fats (lipase) into simpler compounds.
The liver is the largest gland in the body. It is
located in the upper abdomen on the right side, just
under the diaphragm and superior to the duodenum and
pylorus (fig. 1-53).
Of the liver's many functions, the following are
important to remember:
It metabolizes carbohydrates, fats, and proteins
preparatory to their use or excretion.
It forms and excretes bile salts and pigment from
bilirubin, a waste product of red blood cell
It stores blood; glycogen; vitamins A, D, and
; and iron.
It detoxifies the end products of protein
digestion and drugs.
It produces antibodies and essential elements of
the blood-clotting mechanism.
The gallbladder is a pear-shaped sac, usually
stained dark green by the bile it contains. It is located in
the hollow underside of the liver (fig. 1-53). Its duct,
the cystic duct, joins the hepatic duct from the liver to
form the common bile duct, which enters the
duodenum. The gallbladder receives bile from the liver
and then concentrates and stores it. It secretes bile