15.8: Endocrine Pancreas

By the end of this section, you will be able to:

• Describe the location and structure of the pancreas, and the morphology and function of the pancreatic islets
• Compare and contrast the functions of insulin and glucagon

The pancreas is a long, slender retroperitoneal organ, most of which is located posterior to the bottom half of the stomach (Figure \(\PageIndex{1}\)). It is bordered on its right by the retroperitoneal section of the duodenum and on its left by the spleen. The acinar cells of the pancreas line many short cul-de-sac-shaped areas connected directly to the pancreatic duct that drains transversely through the core of the pancreas to the duodenum nestled against its right side. Although it is primarily an exocrine gland, with its acinar cells secreting a variety of digestive enzymes that comprise pancreatic juice that will function outside the epithelial lining of the small intestine, the pancreas also has an endocrine function. Scattered around the cul-de-sacs of acinar cells throughout the pancreas are about a million pancreatic islets—clusters of cells formerly known as the islets of Langerhans— that secrete the hormones glucagon, insulin, somatostatin, and pancreatic polypeptide (PP).

Figure \(\PageIndex{1}\): The Pancreas. The endocrine portion of the pancreas is small clusters of cells called pancreatic islets.  Multiple hormones are secreted by the islets, with the two regulating blood glucose levels coming from the beta cells (insulin) and alpha cells (glucagon). (Image credit: "Endocrine Pancreas" by Jennifer Lange is licensed under CC BY-NC-SA 4.0, derivatives of original works "Leiden - Drawing pancreas and neighbouring organs - no labels" by Ron Slagter and "Endocrine Pancreas" by Cenveo.)

Cells and Secretions of the Pancreatic Islets

Each pancreatic islet (Figure \(\PageIndex{1}\) and Figure \(\PageIndex{2}\)) contains four types of cells whose secretions move into the bloodstream to travel to their target cells:

• The alpha cell produces the hormone glucagon and makes up approximately 20 percent of each islet. Glucagon plays an important role in blood glucose regulation; low blood glucose levels stimulate its release.
• The beta cell produces the hormone insulin and makes up approximately 75 percent of each islet. Elevated blood glucose levels stimulate the release of insulin.
• The delta cell accounts for four percent of the islet cells and secretes the peptide hormone somatostatin. Recall that somatostatin is also released by the hypothalamus (as GHIH), and the stomach and intestines also secrete it. An inhibiting hormone, pancreatic somatostatin inhibits the release of both glucagon and insulin.
• The PP cell accounts for about one percent of islet cells and secretes the pancreatic polypeptide hormone. It is thought to play a role in appetite, as well as in the regulation of pancreatic exocrine and endocrine secretions. Pancreatic polypeptide released following a meal may reduce further food consumption; however, it is also released in response to fasting.

Figure \(\PageIndex{2}\): Histology of the Pancreas. On slides the pancreatic islets are seen as clusters of smaller, lighter staining cells that the surrounding blocks of acini (acinar cells).  (Image credit: "Histology of the Pancreas" by Jennifer Lange is licensed under CC BY-NC-SA 4.0, slide provided by the Regents of the University of Michigan Medical School © 2022.)

Regulation of Blood Glucose Levels by Insulin and Glucagon

Glucose is required for cellular respiration and is the preferred fuel for all body cells. The body derives glucose from the breakdown of the carbohydrate-containing foods and drinks we consume. Glucose not immediately taken up by cells for fuel can be stored by the liver and muscles as glycogen, or converted to triglycerides and stored in the adipose tissue. Hormones regulate both the storage and the utilization of glucose as required. Receptors located in the pancreas sense blood glucose levels, and subsequently the pancreatic cells secrete glucagon or insulin to maintain normal levels.

Figure \(\PageIndex{3}\): Glucose Homeostasis. Blood glucose levels are regulated by two pancreatic hormones that act antagonistically. (Image credit: "Glucose Homeostasis" by Gabrielle Spurlock is licensed under CC BY-NC-SA 4.0.  Pancreas image by Ron Slagter; liver image by Ron Slagter.)

INTERACTIVE LINK

Pancreas Location and Endocrine Function

Query \(\PageIndex{1}\)

 

Table \(\PageIndex{1}\): Hormones of the Pancreas

Associated hormone (and the cells that secretes it)	Effect
Insulin (beta cells)	Reduces blood glucose levels
Glucagon (alpha cells)	Increases blood glucose levels
Somatostatin (delta cells)	Inhibits insulin and glucagon release

DISORDERS OF THE...

Endocrine System: Diabetes Mellitus

Dysfunction of insulin production and secretion, as well as the target cells’ responsiveness to insulin, can lead to a condition called diabetes mellitus. An increasingly common disease, diabetes mellitus has been diagnosed in more than 18 million adults in the United States, and more than 200,000 children. It is estimated that up to 7 million more adults have the condition but have not been diagnosed. In addition, approximately 79 million people in the US are estimated to have pre-diabetes, a condition in which blood glucose levels are abnormally high, but not yet high enough to be classified as diabetes.

There are two main forms of diabetes mellitus. Type 1 diabetes is an autoimmune disease affecting the beta cells of the pancreas. Certain genes are recognized to increase susceptibility. The beta cells of people with type 1 diabetes do not produce insulin; thus, synthetic insulin must be administered by injection or infusion. This form of diabetes accounts for less than five percent of all diabetes cases.

Type 2 diabetes accounts for approximately 95 percent of all cases. It is acquired, and lifestyle factors such as poor diet, inactivity, and the presence of pre-diabetes greatly increase a person’s risk. About 80 to 90 percent of people with type 2 diabetes are overweight or obese. In type 2 diabetes, cells become resistant to the effects of insulin. In response, the pancreas increases its insulin secretion, but over time, the beta cells become exhausted. In many cases, type 2 diabetes can be reversed by moderate weight loss, regular physical activity, and consumption of a healthy diet; however, if blood glucose levels cannot be controlled, the diabetic will eventually require insulin.

Two of the early manifestations of diabetes are excessive urination and excessive thirst. They demonstrate how the out-of-control levels of glucose in the blood affect kidney function. The kidneys are responsible for filtering glucose from the blood. Excessive blood glucose draws water into the urine, and as a result the person eliminates an abnormally large quantity of sweet urine. The use of body water to dilute the urine leaves the body dehydrated, and so the person is unusually and continually thirsty. The person may also experience persistent hunger because the body cells are unable to access the glucose in the bloodstream.

Over time, persistently high levels of glucose in the blood injure tissues throughout the body, especially those of the blood vessels and nerves. Inflammation and injury of the lining of arteries lead to atherosclerosis and an increased risk of heart attack and stroke. Damage to the microscopic blood vessels of the kidney impairs kidney function and can lead to kidney failure. Damage to blood vessels that serve the eyes can lead to blindness. Blood vessel damage also reduces circulation to the limbs, whereas nerve damage leads to a loss of sensation, called neuropathy, particularly in the hands and feet. Together, these changes increase the risk of injury, infection, and tissue death (necrosis), contributing to a high rate of toe, foot, and lower leg amputations in people with diabetes. Uncontrolled diabetes can also lead to a dangerous form of metabolic acidosis called ketoacidosis. Deprived of glucose, cells increasingly rely on fat stores for fuel. However, in a glucose-deficient state, the liver is forced to use an alternative lipid metabolism pathway that results in the increased production of ketone bodies (or ketones), which are acidic. The build-up of ketones in the blood causes ketoacidosis, which—if left untreated—may lead to a life-threatening “diabetic coma.” Together, these complications make diabetes the seventh leading cause of death in the United States.

Diabetes is diagnosed when lab tests reveal that blood glucose levels are higher than normal, a condition called hyperglycemia. The treatment of diabetes depends on the type, the severity of the condition, and the ability of the patient to make lifestyle changes. As noted earlier, moderate weight loss, regular physical activity, and consumption of a healthful diet can reduce blood glucose levels. Some patients with type 2 diabetes may be unable to control their disease with these lifestyle changes, and will require medication. Historically, the first-line treatment of type 2 diabetes was insulin. Research advances have resulted in alternative options, including medications that enhance pancreatic function.

Concept Review

The pancreas has both exocrine and endocrine functions. The pancreatic islet cell types include alpha cells, which produce glucagon; beta cells, which produce insulin; delta cells, which produce somatostatin; and PP cells, which produce pancreatic polypeptide. Insulin and glucagon are involved in the regulation of glucose metabolism. Insulin is produced by the beta cells in response to high blood glucose levels. It enhances glucose uptake and utilization by target cells, as well as the storage of excess glucose for later use. Dysfunction of the production of insulin or target cell resistance to the effects of insulin causes diabetes mellitus, a disorder characterized by high blood glucose levels. The hormone glucagon is produced and secreted by the alpha cells of the pancreas in response to low blood glucose levels. Glucagon stimulates mechanisms that increase blood glucose levels, such as the catabolism of glycogen into glucose.

Review Questions

Query \(\PageIndex{2}\)

 

Critical Thinking Questions

Query \(\PageIndex{3}\)

 

Glossary

Query \(\PageIndex{4}\)

Contributors and Attributions

• OpenStax Anatomy & Physiology (CC BY 4.0). Access for free at https://openstax.org/books/anatomy-and-physiology