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Bile is produced by the liver and stored in the gallbladder.

In this lesson, you will learn about bile and its role in the breakdown of dietary fats. You will also learn how the hormones secretin and cholecystokinin help regulate these organs.

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Digestion

We previously learned that the first section of the small intestine, called the duodenum, receives enzyme-rich secretions that help digest food. Your pancreas is one of the organs that secretes enzymes into the duodenum. The enzymes found in pancreatic juice break down all of the major nutrients, including carbohydrates, proteins and fats. However, the pancreas needs a little help from the liver and gallbladder when it comes to breaking down fats.

In this lesson, you will learn about the secretions of the liver and gallbladder and how these accessory digestive organs work together to break down the foods you eat.

Liver

The liver is essentially a very large gland. It’s located under the diaphragm and lies mostly in the upper right side of your abdominal cavity. It’s a big organ with many big jobs, and even though we will only be looking at the liver in this lesson as it relates to the digestive process, it should not be overlooked that the liver also carries out many metabolic and regulatory functions for your body. With that said, we see that the liver is an important digestive organ because it produces bile.

Structure of the liver and gallbladder
Gall Bladder Liver Diagram

Bile is a yellowish-green fluid that aids in the emulsification of fats. Bile is not an enzyme, but it does contain bile salts that emulsify large fat droplets. What do we mean by emulsify fats? Well, we see that bile salts coat large fat droplets and then break large fat droplets down into smaller fat droplets. It is as if bile comes into your digestive track with a sledge hammer and smashes large droplets of fat into little ones, which are more manageable. This act of emulsification increases the overall surface area of the fat and makes it easier for the pancreatic enzyme, called pancreatic lipase, to do its job.

We previously learned that pancreatic lipase is an enzyme that breaks down dietary fat molecules.

Gallbladder

The gallbladder can be thought of as a storage sac that helps the liver. It is defined as a small sac-shaped organ that stores and concentrates bile. Your gallbladder lies beneath your liver. When there is no food in the small intestine, bile – that was initially made by the liver – backs up into the gallbladder to be stored. It is almost like your body doesn’t want to waste the bile that the liver worked so hard to produce; so, your body conserves unused bile by storing it in your gallbladder until the next time you eat.

While it’s in storage within the gallbladder, bile is concentrated by the removal of water. Then, when you eat a fatty meal and that fat reaches your duodenum, a hormonal stimulus tells the gallbladder to contract, pushing the stored bile into your digestive tract. We will learn more about this hormonal stimulus later in this lesson.

Common Bile Duct

The liver and gallbladder each have their own ducts that lead directly from the organs. However, there is a common duct that carries bile from the liver and the gallbladder to the duodenum, known as the common bile duct. This is an efficient transportation system in a normal healthy body, but if this common duct becomes blocked due to a gallstone or some other disorder, bile will not be able to reach the small intestine. This can cause bile to back up into the liver.

Because bile contains a yellowish pigment, this back up of bile can enter into the bloodstream and lead to a yellowing of the skin or whites of the eyes known as jaundice.

Bile ducts extend from the liver and the gallbladder.
Bile Duct Diagram

Secretin

We mentioned earlier that we would discuss hormonal stimulus, and what we see is that hormones play an important role in many digestive processes.

As food travels through your digestive tract, its acidity and physical presence triggers the release of hormones. These hormones travel through your bloodstream to their target organ. Let’s look at an example. Chyme, which is the partially digested food mass, is very acidic when it comes out of the stomach. As this chyme moves into your small intestine, hormones produced within the walls of the intestine are released to keep the environment favorable for digestion and help digestion run smoothly.

One of these hormones produced within your small intestine wall is called secretin. Secretin stimulates the liver to produce bile and the pancreas to release bicarbonate, which neutralizes the acidity of the chyme. So, we see that secretin is a hormone that increases secretions.

Cholecystokinin

Another hormone that is produced is cholecystokinin. Cholecystokinin is a hormone produced in the duodenum that stimulates the gallbladder to release bile.

You can recall this term if you break it down into its root words. For instance, we see that the word ‘chole’ is Greek for ‘bile,’ ‘cysto’ is Greek for ‘sac,’ and ‘kinin’ means ‘move.’ Therefore, if we put the root words together, we see that cholecystokinin literally means ‘move the bile sac,’ and because we know that the gallbladder is the sac that stores bile, it makes sense that this would be the hormone (cholecystokinin) that stimulates the gallbladder to contract and release bile.Hormones help your body coordinate efforts so digestion runs smoothly. In fact, it is through the actions of hormones, such as secretin and cholecystokinin, that bile and pancreatic juice are able to enter the duodenum at the same time. This is important because, as we know, bile is not an enzyme. Instead, it acts as an emulsifier to break large fat droplets into smaller ones, providing more surface area for pancreatic enzymes to work together.

Lesson Summary

Let’s review. The liver is a large organ that produces bile. Bile plays a role in the breakdown of fats and, more specifically, is defined as a yellowish-green fluid that aids in the emulsification of fats. When a large fat droplet is emulsified, it is essentially broken down into smaller droplets.

Bile enters the duodenum when dietary fats are present. If any bile remains after the fat has been digested, it is taken back up by the gallbladder. The gallbladder is a small sac-shaped organ that stores and concentrates bile.

The common bile duct is a common duct that carries bile from the liver and the gallbladder to the duodenum. If this duct is blocked, it can cause the yellowish-colored bile to back up and enter the bloodstream. This can lead to a yellowing of the skin and the whites of the eyes, known as jaundice.Hormones play a role in the release of digestive secretions.

For example, secretin is a hormone produced in the walls of the small intestine that stimulates the liver to produce bile and the pancreas to release bicarbonate, which neutralizes the acidity of the chyme. Cholecystokinin is a hormone produced in the duodenum that stimulates the gallbladder to release bile.

Learning Outcomes

Following this lesson, you will be able to:

  • Describe the structures and functions of the liver and the gallbladder
  • Explain the importance of the common bile duct, including what happens if it is blocked
  • Summarize how bile, secretin and cholecystokinin function in the digestive process

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