Endocrine System—Pancreas
Lecture 21
Today we’re going to continue our study of the endocrine system by
looking at the endocrine function of the pancreas. It’s a very large
organ we’ve already looked at in its digestive form, or the exocrine
function. Today we’re going to look at it as purely an organ of endocrine
secretion, secretion of hormones into the circulation.
A
s an endocrine organ, the pancreas produces insulin and glucagon.
After reviewing the four cell types composing the endocrine
pancreas, the lecture examines in detail several insulin-related
disorders: diabetes mellitus in its two principal types and hyperinsulinism.
The pancreas functions in two modes: It is an exocrine digestive organ, and t
is also an endocrine organ, producing insulin and glucagon.
The pancreas is nestled in the curve of the duodenum and stomach. It is entirely
retroperitoneal. Its extensive blood supply comes through anastomoses of the
gastroduodenal artery, the posterior superior pancreaticoduodenal artery, the
anterior inferior pancreaticoduodenal artery, and the splenic artery. The head
of the pancreas joins the second portion of the duodenum. The tail meets
the spleen.
Ninety-nine percent of the pancreas is made of acini, clusters of exocrine
digestive cells connecting the ducts. One percent of the pancreas is made
up of several million scattered islets of Langerhans, cells that contain the
endocrine functioning.
Physiology of the endocrine pancreas—four cell types
Alpha cells constitute 20% of the islet cells. They secrete the hormone
glucagon, which raises blood sugar to maintain normal levels. Chemoreceptors
measure the amount of sugar in the blood; low blood sugar directly stimulates
the release of glucagon from the a-cells. Glucagon acts on hepatocytes to
convert glycogen to glucose (glycogenolysis) and to convert amino acids
into glucose (gluconeogenesis). Higher-than-normal blood sugar turns off
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, the release of glucagon. Stimulation of the sympathetic nervous system in
preparation for stress, ght, and ight also affects glucagon release.
Beta cells constitute approximately 80% of islet cells. The beta cells secrete
insulin, which lowers blood sugar. Lower-than-normal blood glucose turns
off the output of insulin. Higher-than-normal blood sugar stimulates beta
cells to release insulin. High blood sugar is bad not only for the blood but
also for cells, which do not receive the glucose they need. Genetically
engineered bacteria now produce synthetic human insulin.
Insulin acts on body cells in the following ways:
It increases the speed of diffusion of glucose into the cells (especially
skeletal muscles).
It accelerates the conversion of glucose into its storage form,
glycogen.
It increases synthesis of proteins from amino acids.
It increases synthesis of fatty acids.
It decreases the rate of glycogenolysis and gluconeogenesis.
Lecture 21: Endocrine System—Pancreas
Glucagon and insulin have a clinical application in the treatment of excess
blood potassium:
Potassium is an ion with high concentration in cells and low levels
in the blood; sodium is an ion for which the opposite is true.
Movement of these ions between cells and the blood enables
electrical conduction down nerves and in the heart.
Excess levels of potassium can fatally prevent electrical
conduction.
128
Lecture 21
Today we’re going to continue our study of the endocrine system by
looking at the endocrine function of the pancreas. It’s a very large
organ we’ve already looked at in its digestive form, or the exocrine
function. Today we’re going to look at it as purely an organ of endocrine
secretion, secretion of hormones into the circulation.
A
s an endocrine organ, the pancreas produces insulin and glucagon.
After reviewing the four cell types composing the endocrine
pancreas, the lecture examines in detail several insulin-related
disorders: diabetes mellitus in its two principal types and hyperinsulinism.
The pancreas functions in two modes: It is an exocrine digestive organ, and t
is also an endocrine organ, producing insulin and glucagon.
The pancreas is nestled in the curve of the duodenum and stomach. It is entirely
retroperitoneal. Its extensive blood supply comes through anastomoses of the
gastroduodenal artery, the posterior superior pancreaticoduodenal artery, the
anterior inferior pancreaticoduodenal artery, and the splenic artery. The head
of the pancreas joins the second portion of the duodenum. The tail meets
the spleen.
Ninety-nine percent of the pancreas is made of acini, clusters of exocrine
digestive cells connecting the ducts. One percent of the pancreas is made
up of several million scattered islets of Langerhans, cells that contain the
endocrine functioning.
Physiology of the endocrine pancreas—four cell types
Alpha cells constitute 20% of the islet cells. They secrete the hormone
glucagon, which raises blood sugar to maintain normal levels. Chemoreceptors
measure the amount of sugar in the blood; low blood sugar directly stimulates
the release of glucagon from the a-cells. Glucagon acts on hepatocytes to
convert glycogen to glucose (glycogenolysis) and to convert amino acids
into glucose (gluconeogenesis). Higher-than-normal blood sugar turns off
127
, the release of glucagon. Stimulation of the sympathetic nervous system in
preparation for stress, ght, and ight also affects glucagon release.
Beta cells constitute approximately 80% of islet cells. The beta cells secrete
insulin, which lowers blood sugar. Lower-than-normal blood glucose turns
off the output of insulin. Higher-than-normal blood sugar stimulates beta
cells to release insulin. High blood sugar is bad not only for the blood but
also for cells, which do not receive the glucose they need. Genetically
engineered bacteria now produce synthetic human insulin.
Insulin acts on body cells in the following ways:
It increases the speed of diffusion of glucose into the cells (especially
skeletal muscles).
It accelerates the conversion of glucose into its storage form,
glycogen.
It increases synthesis of proteins from amino acids.
It increases synthesis of fatty acids.
It decreases the rate of glycogenolysis and gluconeogenesis.
Lecture 21: Endocrine System—Pancreas
Glucagon and insulin have a clinical application in the treatment of excess
blood potassium:
Potassium is an ion with high concentration in cells and low levels
in the blood; sodium is an ion for which the opposite is true.
Movement of these ions between cells and the blood enables
electrical conduction down nerves and in the heart.
Excess levels of potassium can fatally prevent electrical
conduction.
128
