Digestive System—
Anatomy of the Small Intestine, Colon, and Rectum
Lecture 18
Today we’re going to continue our way through the gastrointestinal
tract, this time beginning at the pylorus, which is the guardian of the
stomach, and work our work our way distally, towards the far end,
through the small intestine, colon, and nally end up at the anus.
W
e turn now to the anatomy of the organs in which nutrients
and water are extracted from the chyme for use by the body
and by which the resulting waste material is excreted from the
body. The small intestine (also known as the small bowel) is the organ in
which most of the absorption of nutrients and water occurs. We review its
anatomical divisions (the duodenum, jejunum, and ileum), blood supply, and
microstructure, especially the cells of the microscopic villi where absorption
occurs. We turn next to the large intestine, or colon, which absorbs remaining
water and transfers the feces to the rectum for excretion. The lecture
concludes by examining the complicated anal sphincter muscle that controls
passage through the anus.
Small intestine
The small intestine is responsible for the major absorption of nutrients and
water. It is approximately 24 feet long, though the length depends on how
and when it is measured (i.e., ante- or postmortem). It begins at the pylorus
and ends at the ileocecal valve.
The rst part is the duodenum (“twelve” inches). The duodenum runs from
the pylorus to the ligament of Treitz, where it pierces the peritoneum. It is a
retroperitoneal organ.
The second part is the jejunum (“empty”). The jejunum runs from the
ligament of Treitz to the mid–small bowel. It has numerous muscular folds
called plicae circulares. It is entirely an intraperitoneal organ.
105
, The third division is the ileum (“twisted”). The ileum runs from the mid–
small bowel to the ileocecal valve at the colon. It is an intraperitoneal organ.
Plicae circulares also prominent in the upper to mid-ileum. They increase
surface area and cause a corkscrew motion of chyme.
Blood supply of the intestine
The duodenum is supplied by the gastroduodenal artery and by branches of
the superior mesenteric artery. The jejunum is supplied by jejunal branches
Lecture 18: Digestive System—Anatomy of the Small Intestine, Colon, and Rectum
of the superior mesenteric artery. The ileum is supplied by the ileal, right
colic, ileocolic, and appendiceal branches of the superior mesenteric artery.
Superior mesenteric syndrome is a rare disease in which unfortunate
placement of the superior mesenteric artery causes it to block the duodenum
under heavy food loads.
Microstructure of the intestine
The small intestine has the same four layers as the rest of the GI tract, but
they are modi ed for maximal absorptive power.
The villi (singular: villus, “tuft of hair”) are ngerlike projections of
the mucosa.
The single villus is the functional unit for absorption.
There are 40 villi/mm2.
Each villus contains an arterial and venous capillary and a lacteal
(lymphatic equivalent of a capillary). The lymphatic system is a
circulatory system that moves between cells, drains into veins in the
neck, and can also absorb fat.
Absorptive cells have microvilli on their surfaces for increased surface area.
These microvilli form the brush border (200 million microvilli/mm2.). They
are different from the microvilli on respiratory cells, which are used for
moving foreign material out of the system.
106
Anatomy of the Small Intestine, Colon, and Rectum
Lecture 18
Today we’re going to continue our way through the gastrointestinal
tract, this time beginning at the pylorus, which is the guardian of the
stomach, and work our work our way distally, towards the far end,
through the small intestine, colon, and nally end up at the anus.
W
e turn now to the anatomy of the organs in which nutrients
and water are extracted from the chyme for use by the body
and by which the resulting waste material is excreted from the
body. The small intestine (also known as the small bowel) is the organ in
which most of the absorption of nutrients and water occurs. We review its
anatomical divisions (the duodenum, jejunum, and ileum), blood supply, and
microstructure, especially the cells of the microscopic villi where absorption
occurs. We turn next to the large intestine, or colon, which absorbs remaining
water and transfers the feces to the rectum for excretion. The lecture
concludes by examining the complicated anal sphincter muscle that controls
passage through the anus.
Small intestine
The small intestine is responsible for the major absorption of nutrients and
water. It is approximately 24 feet long, though the length depends on how
and when it is measured (i.e., ante- or postmortem). It begins at the pylorus
and ends at the ileocecal valve.
The rst part is the duodenum (“twelve” inches). The duodenum runs from
the pylorus to the ligament of Treitz, where it pierces the peritoneum. It is a
retroperitoneal organ.
The second part is the jejunum (“empty”). The jejunum runs from the
ligament of Treitz to the mid–small bowel. It has numerous muscular folds
called plicae circulares. It is entirely an intraperitoneal organ.
105
, The third division is the ileum (“twisted”). The ileum runs from the mid–
small bowel to the ileocecal valve at the colon. It is an intraperitoneal organ.
Plicae circulares also prominent in the upper to mid-ileum. They increase
surface area and cause a corkscrew motion of chyme.
Blood supply of the intestine
The duodenum is supplied by the gastroduodenal artery and by branches of
the superior mesenteric artery. The jejunum is supplied by jejunal branches
Lecture 18: Digestive System—Anatomy of the Small Intestine, Colon, and Rectum
of the superior mesenteric artery. The ileum is supplied by the ileal, right
colic, ileocolic, and appendiceal branches of the superior mesenteric artery.
Superior mesenteric syndrome is a rare disease in which unfortunate
placement of the superior mesenteric artery causes it to block the duodenum
under heavy food loads.
Microstructure of the intestine
The small intestine has the same four layers as the rest of the GI tract, but
they are modi ed for maximal absorptive power.
The villi (singular: villus, “tuft of hair”) are ngerlike projections of
the mucosa.
The single villus is the functional unit for absorption.
There are 40 villi/mm2.
Each villus contains an arterial and venous capillary and a lacteal
(lymphatic equivalent of a capillary). The lymphatic system is a
circulatory system that moves between cells, drains into veins in the
neck, and can also absorb fat.
Absorptive cells have microvilli on their surfaces for increased surface area.
These microvilli form the brush border (200 million microvilli/mm2.). They
are different from the microvilli on respiratory cells, which are used for
moving foreign material out of the system.
106
