Lecture 22: The Intestines, Pancreas and Liver
1. The Small Intestine
The stomach empties into the small intestine through the pyloric valve. The small intestine itself
consists of three main sections. The duodenum is the small section closest to the pyloric valve, about
30 cm long. Chyme from the stomach enters the duodenum as do digestive enzymes from the pancreas
and bile from the liver and gallbladder. The jejunum follows the duodenum, and is much longer - a
little over a meter. The ileum is the final section of the small intestine, it is approximately 1.5 meters
long and connects via the illeocecal valve to the large intestine. The small intestine is the predominant
site of nutrient absorption with the vast majority of nutrient absorption occurring within the first 20%.
The small intestine is anatomically designed for large rates of absorption. The inner wall of the small
intestine contains numerous protrusions called villi which face into the lumen of the intestine. Each
villus contains an artery, a vein, capillaries, and a blind-ended lymphatic lacteal. The villi enhance the
surface area available for absorption. The available surface area is increased even further as the villi
themselves are covered with smaller protrusions called microvilli. There can be thousands of microvilli
on a single intestinal epithelial cell. In between the villi are large pits, called crypts of Lieberkuhn
which also enhance the surface area available for absorption. They contain cells that produce
bicarbonate and release it into the intestinal lumen. This is necessary to decrease the acidity of the
stomach contents entering the small intestine. Many intestinal enzymes (i.e., from the pancreas) cannot
function in an acidic environment therefore the bicarbonate-induced alkalinisation of the intestinal
lumen is critical for enzymatic digestion. Surface epithelial cells in the intestine are replaced every 3-5
days, meaning that millions of intestinal cells are shed each day. 2
2. The Large Intestine
The large intestine is connected to the small intestine via the illeocecal sphincter which itself leads
into a small region of the large intestine called the cecum. The cecum connects to the colon, the area of
the intestine in which the final process of water/ion absorption occurs. Also attached to the cecum is
the appendix, a small, vestigial part of the intestine. It has no know function in digestion in humans.
The colon itself consists of the ascending colon (on the right side of the body), transverse colon, the
descending colon (on the left side) and the sigmoid colon which leads to the rectum. Finally, the rectum
empties via the anus, the opening of which is under the control of and external and an internal anal
3. Disorders of the Large Intestine
The appendix can become blocked by digested foodstuffs, which can cause inflammation and infection.
This is called appendicitis and, should the appendix rupture, the resulting bacterial infection
throughout the body cavity (peritonitis) is usually fatal. 3
Another disorder of the large intestine (colon) is the formation of a fecalith which is compacted feces.
This can be caused by a lack of fibre in the diet, and results in chronic constipation. In the worst cases,
fecaliths need to be removed surgically.
Diverticulosis and diverticulitis are primarily disorders of the sigmoid colon. Diverticulosis is due to a
weakening of the muscle surrounding the colon that causes the intestinal lining to form extruding
'pouches', called divertuculi. Diverticulitis is when these pouches become infected and inflamed, which
leads to intense abdominal pain (on the left side) and, in severe cases, perforation of the intestines and
peritonitis. Again, it is usually caused by a low fibre diet.
These diseases of the large intestine are frequently detected or confirmed using a colonoscopy. This is
a procedure in which a camera on the end of the flexible tube is inserted into the rectum and moved
through the colon. 4
4. The Pancreas
The pancreas is located underneath the liver close to the curvature of the duodenum. Secretions from
the pancreas enter the duodenum via the pancreatic duct. The pancreatic duct terminates in the
ampulla of Vater which is a common ending for the pancreatic duct and the bile duct. The opening
into the duodenum is via the Sphincter of Odi.
There are many blind-ended ducts in the pancreas that arise from the pancreatic duct. These blind-ends
are lined by unique enzyme-secreting cells called acinar cells, which produce a variety of enzymes.
Pancreatic amylase breaks down carbohydrates; pancreatic proteases break down proteins;
pancreatic lipase is produced to break down fats; and finally deoxynuclease and ribonuclease are
produced to break down nucleic acids. The proteases are secreted in an inactive form, to prevent them
from breaking down proteins within the acinar cells. These pancreatic enzymes are referred to as
exocrine secretions. The blind-ended ducts are also lined by cells which produce bicarbonate and
secrete it into the fluid containing pancreatic enzymes. The bicarbonate helps to raise the pH of the
intestinal contents thereby creating the alkaline environment which the pancreatic enzymes require to
The pancreas also produces what are referred to as endocrine secretions that come from groups of
cells within the pancreas called the islets of Langerhans. The islets of Langerhans consist of three
main cell types: alpha cells, towards the outer surface of the cell group, secrete glucagon; beta cells,
on the inside, secrete insulin; and delta cells, scattered around the border between the two, produce the
hormone somatostatin. The islets of Langerhans are supplied by numerous capillaries into which these
hormones are secreted.
5. Insulin, Glucagon and Blood Glucose Regulation
Insulin is a small dipeptide hormone whose main role in the body is to lower blood glucose levels by
signaling cells (such as muscle, kidney or fat cells) to take up glucose from the blood. It also stimulates
the conversion of glucose into glycogen in the liver. The primary stimulus for insulin release is an
increase in plasma glucose levels.
Glucagon plays the opposite function; it raises blood glucose levels when they fall too low. It does this
by stimulating the breakdown of glycogen in the liver providing glucose that can enter the blood. 6
Insulin release from the beta cells in the pancreas occurs primarily in response to an increase in blood
glucose levels. There is some release triggered by the presence of carbohydrates in the gut prior to the
elevation of blood glucose levels. Glucose molecules are taken up into the beta cells by a GLUT2
glucose transporter. Inside the cells the enzyme glucokinase (similar to hexokinase) phosphorylates
glucose and starts the process of glycolysis leading to the production of ATP. The elevation of ATP
levels causes the close of ATP-sensitive K channels on the beta cell plasma membrane. K ions are
therefore retained within the cell causing the plasma membrane to depolarise. This leads to the opening
of voltage-dependent Ca channels allowing for the influx of Ca . The rise in intracellular [Ca ] ++
triggers the release of insulin by exocytosis.