Digestive System—
Physiology of the Pancreas, Liver, and the Biliary Tree
Lecture 17
Today we’re going to continue with the physiology of the pancreas, the
liver, and the biliary tree and see how the processes that we described
Lecture 17: Digestive System—Physiology of the Pancreas, Liver, and the Biliary Tree
anatomically come to play in what will be mostly the molecular biology
and the chemistry of digestion.
T
he pancreas and liver secrete digestive juices and enzymes that aid in
digestion and absorption. In this lecture, we review the components
of the exocrine pancreas, the main pancreatic hormones, and the
mechanisms by which pancreatic enzymes are secreted into the small
intestine. After reviewing several pancreatic disorders (pancreatitis and
pancreatic cancer), we turn to the liver and examine the various functions that
it performs. We conclude with a review of common liver disorders, notably
jaundice, trauma, hepatocellular carcinoma, cirrhosis, and cholelithiasis.
Primary processes of digestion and absorption
The primary processes of digestion and absorption occur in the small
intestine. However, these functions depend heavily on the digestive juices
and hormones secreted by the pancreas and the liver. The exocrine pancreas
has the following components and functions:
The pancreas produces 1,000–1,500 ml of pancreatic juice. per day.
It consists mainly of water, NaCl (sodium chloride, or salt), and
NaHCO3 (sodium bicarbonate).
Sodium bicarbonate buffers the juice and makes it alkaline (pH
7.1–8.2).
The alkalinity stops the action of gastric pepsin and prepares chyme
for the milieu of the small intestine.
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, Pancreatic hormones
Amylase digests the remaining carbohydrates into simple sugars. It is
released into the blood in cases of pancreatitis. Amylase acts on three main
sugars. These sugars are composed of other, simpler sugar molecules.
Maltose is acted on by maltase and is broken down into two
molecules of glucose.
Sucrose is acted on by sucrase and broken down into glucose
and fructose.
Lactose is acted on by lactase and broken down into glucose
and galactose.
Trypsin, chymotrypsin, and elastase all digest proteins.
Lipase digests triglycerides into fatty acids and monoglycerides.
Physiological considerations
Almost all pancreatic enzymes are secreted in an inactive form to prevent
autodigestion (pancreas literally means “eats all esh”). Inactive forms
end in –gen, e.g., trypsinogen. In severe pancreatitis, activated enzymes
may travel back into the pancreas and digest it. Acinar cells also contain a
trypsin inhibitor that inactivates any active trypsin accidentally released in
the pancreatic tissues. Enterokinase (from the small intestine) activates the
enzymes once they are in the safe con nes of the small intestine.
Basic pancreatic function (chemical production)
Food consists of generally complex molecules that are broken down into
simple molecules and reassembled into necessary compounds by the
pancreas, liver, and other organs. The body has evolved to favor molecules
with similar structures. For example, the cyclopentanophenanthrene ring
structure is common to cholesterol, estradiol, testosterone, and cortisol. The
only differences between these compounds are one or two groups attached to
the outside of a ring structure. However, these compounds have enormously
different functions in the body. The body can replace a particular missing
compound by modifying the creation process of a similar compound.
99
Physiology of the Pancreas, Liver, and the Biliary Tree
Lecture 17
Today we’re going to continue with the physiology of the pancreas, the
liver, and the biliary tree and see how the processes that we described
Lecture 17: Digestive System—Physiology of the Pancreas, Liver, and the Biliary Tree
anatomically come to play in what will be mostly the molecular biology
and the chemistry of digestion.
T
he pancreas and liver secrete digestive juices and enzymes that aid in
digestion and absorption. In this lecture, we review the components
of the exocrine pancreas, the main pancreatic hormones, and the
mechanisms by which pancreatic enzymes are secreted into the small
intestine. After reviewing several pancreatic disorders (pancreatitis and
pancreatic cancer), we turn to the liver and examine the various functions that
it performs. We conclude with a review of common liver disorders, notably
jaundice, trauma, hepatocellular carcinoma, cirrhosis, and cholelithiasis.
Primary processes of digestion and absorption
The primary processes of digestion and absorption occur in the small
intestine. However, these functions depend heavily on the digestive juices
and hormones secreted by the pancreas and the liver. The exocrine pancreas
has the following components and functions:
The pancreas produces 1,000–1,500 ml of pancreatic juice. per day.
It consists mainly of water, NaCl (sodium chloride, or salt), and
NaHCO3 (sodium bicarbonate).
Sodium bicarbonate buffers the juice and makes it alkaline (pH
7.1–8.2).
The alkalinity stops the action of gastric pepsin and prepares chyme
for the milieu of the small intestine.
98
, Pancreatic hormones
Amylase digests the remaining carbohydrates into simple sugars. It is
released into the blood in cases of pancreatitis. Amylase acts on three main
sugars. These sugars are composed of other, simpler sugar molecules.
Maltose is acted on by maltase and is broken down into two
molecules of glucose.
Sucrose is acted on by sucrase and broken down into glucose
and fructose.
Lactose is acted on by lactase and broken down into glucose
and galactose.
Trypsin, chymotrypsin, and elastase all digest proteins.
Lipase digests triglycerides into fatty acids and monoglycerides.
Physiological considerations
Almost all pancreatic enzymes are secreted in an inactive form to prevent
autodigestion (pancreas literally means “eats all esh”). Inactive forms
end in –gen, e.g., trypsinogen. In severe pancreatitis, activated enzymes
may travel back into the pancreas and digest it. Acinar cells also contain a
trypsin inhibitor that inactivates any active trypsin accidentally released in
the pancreatic tissues. Enterokinase (from the small intestine) activates the
enzymes once they are in the safe con nes of the small intestine.
Basic pancreatic function (chemical production)
Food consists of generally complex molecules that are broken down into
simple molecules and reassembled into necessary compounds by the
pancreas, liver, and other organs. The body has evolved to favor molecules
with similar structures. For example, the cyclopentanophenanthrene ring
structure is common to cholesterol, estradiol, testosterone, and cortisol. The
only differences between these compounds are one or two groups attached to
the outside of a ring structure. However, these compounds have enormously
different functions in the body. The body can replace a particular missing
compound by modifying the creation process of a similar compound.
99
