Lesson Explainer: The Pancreas Biology

In this explainer, we will learn how to describe the structure and the function of the pancreas in the human body.

Did you know that your pancreas has taste receptor cells very similar to those on our tongue? Unlike those in our mouth, however, the taste receptors in the pancreas do not send a message to our brain. Instead, our pancreas uses its sweet and bitter taste receptors to regulate our appetite and metabolism, as well as maintain an appropriate sugar concentration in our blood. The activation of these receptors can cause hormones to be released, which achieve these goals.

The pancreas is an elongated organ, around 15 cm long, located just behind the stomach in your abdomen. As you can see in Figure 1, from its position in the human body, it is involved in the digestive system as an accessory organ. This means that food does not pass through the pancreas as it is not part of the digestive tract, but it does assist in the digestion of food by releasing digestive enzymes. The pancreas is, therefore, also known as a gland.

Key Term: Pancreas

The pancreas is an elongated organ located behind the stomach, which secretes digestive enzymes into the duodenum via the pancreatic duct and also secretes hormones into the blood.

Endocrine glands secrete hormones directly into the blood, whereas exocrine glands secrete substances such as enzymes via ducts. The pancreas is sometimes referred to as a “mixed gland,” as it has both endocrine and exocrine functions.

Key Term: Endocrine Gland

An endocrine gland is an organ consisting of a group of specialized cells that secrete hormones directly into the blood.

Key Term: Exocrine Gland

An exocrine gland is an organ consisting of a group of specialized cells that secrete substances such as enzymes via a duct.

Most of the pancreas is made up of exocrine tissue, so it can also function as an exocrine gland. This is because it secretes digestive enzymes and pancreatic “juice” via the pancreatic duct into the duodenum of the small intestine. At this part of the small intestine, these enzymes are involved in the chemical digestion of the nutrients in food.

The digestive enzymes secreted by the exocrine tissue of the pancreas include amylases, proteases, and lipases, all of which act in the small intestine. Amylases continue to break down starch into simple sugars, a process that begins in the mouth. Proteases, such as trypsin, break down proteins into peptides and amino acids. Lipases break down lipids into fatty acids and glycerol. The pancreatic juice contains sodium bicarbonate, an alkali that neutralizes the acidic gastric juice from the stomach as it enters the duodenum.

Example 1: Describing the Function of Pancreatic Exocrine Cells

The pancreas contains endocrine and exocrine cells. What is the primary function of the exocrine cells?

  1. To secrete hormones directly into the bloodstream
  2. To act as a supply of pancreatic stem cells and replace dead or damaged cells
  3. To secrete enzymes into the duodenum of the small intestine
  4. To line the internal cavity of the pancreas and release acid to help digest food

Answer

Endocrine glands secrete hormones into the blood, whereas exocrine glands secrete substances such as enzymes via ducts. The pancreas is sometimes referred to as a “mixed gland,” as it has both endocrine and exocrine cells that carry out different functions.

The pancreas is an endocrine gland, as it secretes hormones directly into the bloodstream.

Although the pancreas does usually contain pancreatic stem cells so that dead or damaged cells can be replaced, this does not refer to its exocrine function. Exocrine glands secrete substances via ducts.

Most of the pancreas is made up of exocrine tissue, so it can also function as an exocrine gland. This is because these exocrine cells secrete digestive enzymes and pancreatic juice via the pancreatic duct into the duodenum of the small intestine. At this part of the small intestine, these enzymes are involved in the chemical digestion of the nutrients in food, such as starch, lipids, and proteins.

While the exocrine cells of the pancreas secrete pancreatic “juice,” which is an alkaline substance, it does not release any acid. In fact, this pancreatic juice helps neutralize the gastric acid from the stomach entering the duodenum. This makes the pH conditions in the duodenum more suitable for the enzymes that work there.

The primary function of the exocrine cells is, therefore, to secrete enzymes into the duodenum of the small intestine.

Example 2: Explaining Why the Pancreas Is a Mixed Gland

Why is the pancreas referred to as a mixed gland?

  1. The pancreas contains endocrine cells that secrete hormones and exocrine cells that secrete enzymes.
  2. The pancreas contains exocrine cells that secrete hormones and endocrine cells that secrete enzymes.

Answer

Endocrine glands secrete hormones into the blood, whereas exocrine glands secrete substances such as enzymes via ducts. Mixed glands, like the pancreas, have both endocrine and exocrine cells with different functions.

The endocrine cells of the pancreas secrete hormones directly into the blood.

The exocrine cells of the pancreas secrete digestive enzymes and pancreatic juice via the pancreatic duct. This enters the duodenum of the small intestine, where the enzymes are involved in the chemical digestion of food.

One group of digestive enzymes are amylases, which continue to break down starch into simple sugars, a process that starts in the mouth. Proteases, such as trypsin, are another group of digestive enzymes that break down proteins into peptides and amino acids. The final group, lipases, breaks down lipids into fatty acids and glycerol. The pancreatic juice contains sodium bicarbonate, an alkali that neutralizes the acidic gastric juice from the stomach as it enters the small intestine.

Therefore, the pancreas is referred to as a mixed gland, as it contains endocrine cells that secrete hormones and exocrine cells that secrete enzymes.

Let’s look at the microscopic structure of the pancreas.

The exocrine tissues of the pancreas that secrete digestive enzymes and pancreatic juice are called pancreatic acini (singular: acinus). The pancreatic duct branches throughout the acini to easily transport their exocrine secretions to the duodenum.

The endocrine tissues of the pancreas that secrete hormones are called the islets of Langerhans. They are called islets because they appear like small islands in a sea of pancreatic acini. The islets of Langerhans contain blood vessels so that hormones can be easily transported from the endocrine pancreatic cells via the bloodstream to the tissues that require them elsewhere in the body. These blood vessels may also play a role in maintaining the function of the cells within the islets of Langerhans.

Key Term: Islets of Langerhans

The islets of Langerhans are specialized cells within the pancreas that are responsible for producing hormones such as insulin and glucagon.

Micrograph taken by a light microscope of a section through the pancreas

Figure2

The image above is a micrograph taken by a light microscope of a section through the pancreas. You can see an endocrine islet of Langerhans at the center appearing as a large, round, pale, purplish-blue structure. The rest of the micrograph shows the surrounding exocrine pancreatic acini as darker pink and blue structures. You can see the nuclei of the different cells throughout the tissues as darker purple circles.

So, what is the endocrine function of the pancreas? The islets of Langerhans release hormones to control the blood glucose levels by releasing two hormones that work in a contrasting way.

Glucose is a sugar that our bodies obtain by breaking down carbohydrates in our food. Glucose is also the primary substrate that is broken down in cellular respiration, a biological process that all of our cells carry out to produce energy. The hormones released from the islets of Langerhans maintain the blood glucose concentration at normal levels.

Key Term: Glucose

Glucose is a simple monosaccharide sugar molecule made up of six carbon atoms.

Alpha cells and beta cells are two of the different types of cells found within the islets of Langerhans. In order to maintain a normal blood sugar level, alpha cells produce and secrete a hormone called glucagon, which works in various ways to increase the blood glucose levels when they are too low. For example, glucagon stimulates the breakdown of the glycogen stored in the liver into glucose and the conversion of amino acids and fats into glucose. This increases the blood glucose concentration.

Key Term: Alpha Cells

Alpha cells are cells located in the islets of Langerhans of the pancreas that release glucagon in response to low blood glucose levels.

Key Term: Glucagon

Glucagon is a hormone released from the alpha cells of the pancreas that functions to increase the blood glucose concentration to maintain a normal blood sugar level.

In order to maintain a normal blood sugar level, beta cells produce and secrete a hormone called insulin, which functions to decrease the blood glucose levels when they are too high, such as after a meal. Insulin works by stimulating glycolysis, which is the first stage of cellular respiration, and the uptake of glucose into skeletal muscle cells in particular. Insulin can also cause liver cells to store glucose in the form of glycogen. Finally, fat cells can take in more glucose to store as fats. This maintains a normal blood sugar level.

Key Term: Beta Cells

Beta cells are cells located in the islets of Langerhans of the pancreas that release insulin in response to high blood glucose levels.

Key Term: Insulin

Insulin is a hormone released from the beta cells of the pancreas that functions to lower the blood glucose concentration to maintain a normal blood sugar level.

Example 3: Describing the Primary Function of Insulin

Fill in the blank: The primary function of insulin is to the levels of blood glucose.

  1. decrease
  2. increase

Answer

The endocrine tissues of the pancreas that secrete hormones are called the islets of Langerhans.

Within the islets of Langerhans, there are two types of cells: alpha cells and beta cells. Both of these cells work to maintain a normal level of glucose in the blood. Glucose is a sugar that is broken down in the cells to release energy via cellular respiration.

In order to maintain a normal blood sugar level, alpha cells produce and secrete a hormone called glucagon, which works in various ways to increase the blood glucose level when it is too low. Beta cells produce and secrete a hormone called insulin, which functions to decrease the blood glucose level when it is too high, such as after a meal.

Therefore, the primary function of insulin is to decrease the levels of blood glucose.

Example 4: Describing the Roles of the Pancreas and Its Cells

For the following statements, state if they are true or false.

  1. Beta cells of the islets of Langerhans secrete glucagon.
  2. The pancreas is an example of an endocrine gland, not an exocrine gland.

Answer

Endocrine glands secrete hormones into the blood, whereas exocrine glands secrete substances such as enzymes via ducts. Mixed glands, like the pancreas, have both endocrine and exocrine cells with different functions.

Part 1

The pancreas is an endocrine gland, as it secretes hormones directly into the blood. The endocrine tissues of the pancreas that secrete hormones are called the islets of Langerhans.

Within the islets of Langerhans, there are two types of cells: alpha cells and beta cells. Both of these cells work to maintain a normal level of glucose in the blood. Glucose is a sugar that is broken down in the cells to release energy via cellular respiration.

In order to maintain a normal blood sugar level, alpha cells produce and secrete a hormone called glucagon, which works in various ways to increase the blood glucose level when it is too low. Beta cells produce and secrete a hormone called insulin, which functions to decrease the blood glucose level when it is too high, such as after a meal.

Therefore, the statement saying that the beta cells of the islets of Langerhans secrete glucagon is false.

Part 2

As we have just discussed, the pancreas is an endocrine gland, as it secretes hormones directly into the blood. The endocrine tissues of the pancreas that secrete hormones are called the islets of Langerhans.

Most of the pancreas is made up of exocrine tissue, so it can also function as an exocrine gland. This is because it secretes digestive enzymes and pancreatic juice via the pancreatic duct into the duodenum of the small intestine. At this part of the small intestine, these enzymes are involved in the chemical digestion of the nutrients in food.

The digestive enzymes secreted by the exocrine tissue of the pancreas include amylases, proteases, and lipases, all of which act in the small intestine. Amylases continue to break down starch into simple sugars, a process that begins in the mouth. Proteases, such as trypsin, break down proteins into peptides and amino acids. Lipases break down lipids into fatty acids and glycerol. The pancreatic juice contains sodium bicarbonate, an alkali that neutralizes the acidic gastric juice from the stomach as it enters the duodenum.

Therefore, the statement saying that the pancreas is an example of an endocrine gland and not an exocrine gland is false.

You may be wondering why we need the pancreas to control the levels of glucose in the blood. Let’s look at what can happen if the body cannot maintain a fairly normal blood glucose level.

Type 1 diabetes is a condition that arises as a result of the beta cells of the pancreas not functioning correctly. You may recall that beta cells are responsible for producing insulin, which decreases the level of blood glucose. As the beta cells would not be producing insulin in a person with type 1 diabetes, their blood glucose concentration can become very high if not controlled. This can lead to damage in many of the body’s systems, such as the nerves and blood vessels.

As insulin does not reduce the blood glucose concentration effectively in a person with diabetes, high concentrations of glucose will be present in their blood. Blood test can be used to monitor the blood glucose levels in diabetics. A blood test that shows a high concentration of glucose might indicate that this person has diabetes or another underlying health condition. High concentrations of glucose also tend to increase the frequency of urination in humans. This usually results in excessive thirst for a person with diabetes, as the concentration of water in their body decreases as more water is excreted as part of their urine.

People with type 1 diabetes must measure their blood glucose levels carefully and regularly and can manage their symptoms with daily insulin injections. Currently, however, there is no cure for type 1 diabetes beyond the methods that can be used to control its symptoms.

The graph in Figure 3 shows that a person with diabetes has a higher blood glucose level than that of a person without diabetes. Following a meal, this blood sugar concentration increases much more in diabetics, and it takes much longer to decrease again. This is because this person cannot secrete insulin from their pancreatic beta cells to regain a normal blood glucose concentration.

In type 2 diabetes, high blood glucose levels are also hard to control, either because the beta cells do not produce enough insulin or because the body’s cells cannot take up the insulin that they do produce. This leaves the glucose in the blood, increasing the concentration of blood glucose, meaning that the cells cannot use glucose themselves. This usually results from insulin receptors on cells becoming less responsive to insulin. This insulin resistance is often largely a result of a poor diet and lack of exercise. Type 2 diabetes can often be controlled by limiting the carbohydrate intake and increasing the exercise levels.

Pancreas transplants can sometimes be used to help people with type 1 diabetes to replace their nonfunctioning beta cells. Such transplants can, however, be risky, and the demand for transplants is far higher than the available supply, so alternative treatments are being researched. Nevertheless, there is potential for the use of stem cells in treating diabetes. Stem cells can be triggered to differentiate into beta cells and can then injected into the person who cannot produce insulin themselves.

Let’s recap some of the key points that we have covered in this explainer.

Key Points

  • The pancreas is an organ that has both endocrine and exocrine functions.
  • The pancreatic exocrine tissues, the acini, secrete digestive enzymes and pancreatic juice via the pancreatic duct into the duodenum of the small intestine.
  • The pancreatic endocrine tissues, the islets of Langerhans, contain alpha and beta cells.
  • In order to maintain a normal blood sugar level, alpha cells secrete a hormone called glucagon into the blood, which increases the blood glucose levels.
  • In order to maintain a normal blood sugar level, beta cells secrete a hormone called insulin into the blood, which decreases the blood glucose levels.

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