Question

Describe how the parasympathetic nervous system influences the release of bile in the body. Describe the physiology behind the release of bile. How does it happen? What occurs in the heart/blood vessels? Which scenarios or external factors would cause the parasympathetic nervous system to activate this response?

Answer 1

The parasympathetic nerves come from the cranial nerves and include the vagus nerve. The PNS nerves perform the following digestive functions:

• Stimulate the activity of the stomach
• Inhibit the release of glucose
• Stimulate the release of the gallbladder to release bile needed to digest fat
• Stimulate the activity of the intestines
• Trigger peristalsis, which helps prevent constipation
• Trigger enzyme production in the pancreas (pancreatic enzymes to break down carbs, protein and fats)
• Signal if satiated
• Signal if hungry
• Need for more stomach acid (HCL), enzymes, bile and peristalsis.

Bile secretion is stimulated by secretin, and the bile is secreted into the gallbladder where it is concentrated and stored under fasting conditions. Concentration of bile within the gallbladder is stimulated principally by cholecystokinin, with absorption of up to 90% of the water occurring within a 4-hour period.

Secretion of Bile and the Role of Bile Acids In Digestion

Bile is a complex fluid containing water, electrolytes and a battery of organic molecules including bile acids, cholesterol, phospholipids and bilirubin that flows through the biliary tract into the small intestine.  There are two fundamentally important functions of bile in all species:

• Bile contains bile acids, which are critical for digestion and absorption of fats and fat-soluble vitamins in the small intestine.
• Many waste products, including bilirubin, are eliminated from the body by secretion into bile and elimination in feces.

Adult humans produce 400 to 800 ml of bile daily, and other animals proportionately similar amounts. The secretion of bile can be considered to occur in two stages:

• Initially, hepatocytes secrete bile into canaliculi, from which it flows into bile ducts. This hepatic bile contains large quantities of bile acids, cholesterol and other organic molecules.
• As bile flows through the bile ducts it is modified by addition of a watery, bicarbonate-rich secretion from ductal epithelial cells.

In species with a gallbladder (man and most domestic animals except horses and rats), further modification of bile occurs in that organ.  The gall bladder stores and concentrates bile during the fasting state.Typically, bile is concentrated five-fold in the gall bladder by absorption of water and small electrolytes - virtually all of the organic molecules are retained.

Secretion into bile is a major route for eliminating cholesterol. Free cholesterol is virtually insoluble in aqueous solutions, but in bile, it is made soluble by bile acids and lipids like lecithin.  Gallstones, most of which are composed predominantly of cholesterol, result from processes that allow cholesterol to precipitate from solution in bile.

Pattern and Control of Bile Secretion

The flow of bile is lowest during fasting, and a majority of that is diverted into the gallbladder for concentration. When chyme from an ingested meal enters the small intestine, acid and partially digested fats and proteins stimulate secretion of cholecystokinin and secretin. As discussed previously, these enteric hormones have important effects on pancreatic exocrine secretion. They are both also important for secretion and flow of bile:

• Cholecystokinin: The name of this hormone describes its effect on the biliary system - cholecysto = gallbladder and kinin = movement. The most potent stimulus for release of cholecystokinin is the presence of fat in the duodenum. Once released, it stimulates contractions of the gallbladder and common bile duct, resulting in delivery of bile into the gut.
• Secretin: This hormone is secreted in response to acid in the duodenum. Its effect on the biliary system is very similar to what was seen in the pancreas - it simulates biliary duct cells to secrete bicarbonate and water, which expands the volume of bile and increases its flow out into the intestine.