How we know 2 - Digestion

How do we know the functions of the liver and gall bladder?
The functions of the liver were identified by comparing the materials in the blood before and after the blood enters the liver. What scientists noticed was that the contents of the blood leaving the liver were much different from when the blood entered.

Scientists also discovered that even between meals, when the concentrations of carbohydrates, amino acids, and fats in the vessels entering the liver were low, their concentrations in the blood that left the liver was always the same. Thus, it seems that the liver is a nutrient storage organ that releases some of its stores as they are needed.

We know that the liver is crucial for life. People die when the liver is destroyed, which often occurs in people who drink alcohol (wine, beer, whiskey) for a long time. There is a virus that also destroys the liver ("infectious hepatitis") and that also can cause death. Click here for more information about alcohol. See also our Cells Are Us unit for information on alcohol effects.

We have learned that the gall bladder's function of separating fat molecules from each other is not crucial. Often a person's gall bladder can become enlarged and inflamed, and the only treatment for this is to remove the gall bladder by surgery. People who have their gall bladders removed surgically seem to do just fine.

How do we know what the pancreas does?
Similar to the process used to study the liver, the functions of the pancreas were discovered by comparing the contents of blood that enters the pancreas with that which leaves the pancreas.

Diagram of the pancreas and its relationships with the bile duct and with blood vessels. The digestive juice duct empties into the small intestine.

Scientists found that the pancreas secretes two hormones into blood:

insulin - triggers cells to absorb sugar from the blood, and
glucagon - triggers cells to secrete sugar into the blood

Also, careful dissection reveals that two ducts connect the pancreas with the small intestine. One of the ducts comes from the gall bladder and contains a green fluid called bile. Collecting secretions from the other duct allowed scientists to discover that digestive enzymes were present to break down proteins, fats, and carbohydrates into smaller molecules.