58 3.41 Digestive Hormones, Accessory Organs & Secretions
Before we go into the digestive details of the small intestine, it is important that you have a basic understanding of the anatomy and physiology of the following digestion accessory organs: pancreas, liver, and gallbladder. Digestion accessory organs assist in digestion, but are not part of the gastrointestinal tract. How are these organs involved?
Upon entering the duodenum, the chyme causes the release of two hormones from the small intestine: secretin and cholecystokinin (CCK, previously known as pancreozymin) in response to acid and fat, respectively. These hormones have multiple effects on different tissues. In the pancreas, secretin stimulates the secretion of bicarbonate (HCO3), while CCK stimulates the secretion of digestive enzymes. The bicarbonate and digestive enzymes released together are collectively known as pancreatic juice, which travels to the small intestine, as shown below.
In addition, CCK also stimulates the contraction of the gallbladder causing the secretion of bile into the duodenum.
Pancreas
The pancreas is found behind the stomach and has two different portions. It has an endocrine (hormone-producing) portion that contains alpha and beta cells that secrete the hormones glucagon and insulin, respectively. However, the vast majority of the pancreas is made up of acini, or acinar cells, that are responsible for producing pancreatic juice. The following video does a nice job of showing and explaining the function of the different pancreatic cells.
| Web Link |
Bicarbonate is a base (high pH) meaning that it can help neutralize acid. You can find sodium bicarbonate (NaHCO3, baking soda) on the ruler below to get an idea of its pH.
The main digestive enzymes in pancreatic juice are listed in the table below. Their function will be discussed further in later subsections.
Table 3.411 Enzymes in pancreatic juice
| Enzyme |
| Pancreatic alpha-amylase |
| Proteases |
| Pancreatic Lipase & Procolipase* |
| Phospholipase A2 |
| Cholesterol Esterase |
*Not an enzyme
Liver
The liver is the largest internal and most metabolically active organ in the body. The figure below shows the liver and the accessory organs position relative to the stomach.
The liver is made up two major types of cells. The primary liver cells are hepatocytes, which carry out most of the liver’s functions. Hepatic is another term for liver. For example, if you are going to refer to liver concentrations of a certain nutrient, these are often reported as hepatic concentrations. The other major cell type is the hepatic stellate (also known as Ito) cells. These two cell types are depicted below.
The liver’s major role in digestion is to produce bile. This is a greenish-yellow fluid that is composed primarily of bile acids, but also contains cholesterol, phospholipids, and the pigments bilirubin and biliverdin. Bile acids are synthesized from cholesterol. The two primary bile acids are chenodeoxycholic acid and