Accessory Organs: Liver, Gallbladder, and Pancreas
The liver, gallbladder and pancreas are accessory organs of digestion: they are not part of the digestive tube, but they make or store substances, such as bile and digestive enzymes, that are released into the small intestine. In brief: the liver makes bile, the gallbladder stores and concentrates it, and the pancreas makes enzymes for digesting carbohydrates, proteins and fats, as well as hormones that control blood glucose. Both bile and pancreatic juice enter the duodenum, the first part of the small intestine.
Without these organs, digestion would be much less efficient, especially for fats. This guide explains the function of each organ, how their secretions reach the intestine, compares them in a table, works through a fat-digestion example and covers common problems and mistakes.
What are accessory organs?
Accessory organs help digestion, but food does not pass through them. They are connected to the digestive tube by ducts. The salivary glands are also accessory organs, but this article focuses on the three that release their secretions into the small intestine.
The common bile duct and the pancreatic duct usually join and open into the duodenum at a single point, controlled by a ring of muscle. This arrangement allows bile and pancreatic juice to mix with the acidic chyme that leaves the stomach.
What does the liver do?
The liver is the largest internal organ, weighing about 1.5 kilograms in an adult. It lies in the upper right of the abdomen, under the diaphragm, and it has an unusually large number of functions, estimated at more than 500.
Digestive role
The liver makes bile, a greenish-yellow fluid that contains bile salts, cholesterol and pigments. Bile has no enzymes. Its bile salts emulsify fats, which means they break large fat droplets into very small ones. This gives lipase enzymes a much greater surface area to work on. Bile also contains hydrogencarbonate, which helps to neutralize stomach acid and creates the alkaline conditions needed by intestinal enzymes. The liver can produce roughly half a liter to one liter of bile per day.
Other roles
- Processing nutrients: Blood from the intestine flows to the liver through the hepatic portal vein. The liver stores glucose as glycogen and releases it when needed, and it converts excess amino acids into urea.
- Detoxification: It breaks down alcohol, drugs and many poisons.
- Making proteins: It produces blood plasma proteins and clotting factors.
- Storing substances: Iron and some vitamins (A, D, B12) are stored.
- Breaking down old red blood cells: The pigment from hemoglobin is used to make bile.
The liver has a strong ability to regenerate. After part of it is removed or damaged, the remaining cells can divide and restore much of its size.
What does the gallbladder do?
The gallbladder is a small, pear-shaped sac under the liver, about 7 to 10 centimeters long. It does not make bile. It stores bile from the liver between meals and concentrates it by absorbing water.
When fatty food enters the duodenum, the intestine releases the hormone cholecystokinin (CCK). CCK makes the gallbladder contract and release bile through the bile duct. A person can live without a gallbladder because bile then flows slowly from the liver straight into the intestine, although large fatty meals may be harder to digest.
What does the pancreas do?
The pancreas is a long, flattened gland behind the stomach. It has two functions that are quite different.
Exocrine function: digestive juice
Most of the pancreas makes pancreatic juice, which flows through a duct into the duodenum. It contains:
- Amylase: digests starch to maltose.
- Proteases (trypsin and others): digest proteins to peptides. They are released in inactive form and activated in the intestine, which protects the pancreas from digesting itself.
- Lipase: digests fats to fatty acids and glycerol.
- Hydrogencarbonate ions: neutralize acid from the stomach.
Endocrine function: hormones
Clusters of cells called the islets of Langerhans release the hormones insulin and glucagon directly into the blood. These two hormones regulate the concentration of glucose in the blood by negative feedback, as explained in the article on homeostasis.
Accessory organs compared
| Organ | Makes | Stores | Role in digestion |
|---|---|---|---|
| Liver | Bile | Glycogen, iron, vitamins | Emulsifies fat, neutralizes acid |
| Gallbladder | Nothing | Bile | Releases bile when fat arrives |
| Pancreas | Enzymes, insulin, glucagon | Nothing significant | Digests starch, protein, fat |
Worked example: digesting a fatty meal
Consider a cheeseburger with fries. Follow how the accessory organs handle the fat.
- Fat and acidic chyme enter the duodenum, which signals the release of CCK and other hormones.
- The gallbladder contracts and sends stored bile through the bile duct to the duodenum.
- Bile salts emulsify the large fat droplets into tiny droplets. The total surface area increases greatly. For example, splitting a droplet into 1 000 smaller droplets of equal total volume makes the combined surface area 10 times larger.
- Pancreatic lipase acts on the surface of the droplets and breaks down triglycerides into fatty acids and glycerol.
- Hydrogencarbonate from bile and pancreatic juice raises the pH from about 2 in the stomach to about 7 to 8, which is suitable for lipase.
- The products are absorbed by the cells of the villi.
The area calculation can be checked. A droplet of radius R is divided into 1 000 droplets, each with radius R ÷ 10, because volume depends on the cube of the radius (10³ = 1 000). Each small droplet has a surface area of 4π(R/10)², which is 1/100 of the original, and 1 000 of them have a total of 1 000/100 = 10 times the original area.
What can go wrong?
Gallstones are hard deposits formed from cholesterol or bile pigments. They can block the bile duct and cause pain, and often they are removed along with the gallbladder. Hepatitis is inflammation of the liver, which may be caused by viruses or alcohol, and cirrhosis is scarring that replaces healthy tissue. Pancreatitis is inflammation of the pancreas. Diabetes involves the endocrine function of the pancreas, in which insulin is lacking or is not working properly.
How were these organs studied?
The Greek physician Galen described the liver’s role in the second century, though some of his ideas were later revised. In 1869 Paul Langerhans described the islets in the pancreas. In 1921 Frederick Banting and Charles Best, working with John Macleod and James Collip in Toronto, isolated insulin, and its first clinical use in 1922 transformed the treatment of diabetes.
Which mistakes are common?
Mistake 1: Saying that bile digests fat
Bile has no enzymes. It emulsifies fat, and lipase from the pancreas digests it chemically.
Mistake 2: Thinking the gallbladder makes bile
The liver makes bile. The gallbladder stores and concentrates it.
Mistake 3: Assuming that food passes through the liver
Only absorbed nutrients reach the liver, through the blood in the hepatic portal vein.
Mistake 4: Forgetting the pancreas has two roles
It makes digestive enzymes for the gut and hormones for the blood.