Digestive Juices: Meaning and Groups

Digestive juices are released by the salivary glands, stomach, liver, pancreas, and small intestine. Food provides energy for humans by converting carbohydrates, proteins, and fats into simpler molecules through digestion. Digestive enzymes and gastrointestinal hormones play a crucial role in converting complex substances into simpler ones.

• Numerous digestive fluids are stored in the stomach, where they are later combined with the food that has been consumed and sent into the small intestine.
• The gastric juice's substantial acidity helps to create a grinding effect that further lowers the size of the food substance known as chyme.

Key Terms: Food, Digestive Juices, Small Intestine, Hormones, Enzymes, Hydrochloric Acid, Saliva

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What are Digestive Juices?

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By breaking down food into basic nutrients, digestive juices promote digestion. After that, these nutrients are used by the body for a number of processes.

• Chemical reactions in the body are sped up by enzymes contained in digestive juices. 
• While many other specialist enzymes are also involved, amylase, lipase, and protease are the primary digestive enzymes employed by the body to break down food.

Below are some digestive enzymes and fluids that are utilized during digestion.

1. Gastric Juice

Gastric juice primarily affects proteins in its activity. The two primary proteolytic enzymes found in gastric juice are pepsin and rennin. Gastric juice also contains the enzymes lipase, gelatinase, and amylase.

• Most of the time, pepsin is secreted as inactive pepsinogen. Pepsinogen is changed into pepsin by hydrochloric acid.
• Then, pepsin converts proteins into proteases, peptones, and polypeptides.
• Pepsin also causes milk's casein to digest and curdle.
• A lipolytic enzyme called gastric lipase converts butter fat or tributyrin into fatty acids and glycerols.
• Collagen, type IV, and type V gelatin are all broken down into peptides by gelatinase. 
• Ammonia is produced when urea and urease react.
• Starch is broken down by gastric amylase, although its effects are modest.
• Only newborns and animals have rennin, which causes milk to curdle.

2. Intestinal Juice

Intestinal juice is clear to light yellow, watery secretion produced by the glands and mucous membrane lining of the small and large intestines. Additionally to releasing gastrointestinal hormones into the circulation and neutralizing hydrochloric acid from the stomach, intestinal juice also includes digestive enzymes that aid in food absorption and digestion.

• The release of intestinal juice is triggered by hormonal signals, the stimulation of the vagus nerve, and mechanical factors resulting from the presence of food.
• Additionally, intestinal juice contains hormones, digestive enzymes, mucus, compounds that buffer stomach acid, and erepsin, which further breaks down polypeptides into amino acids, completing the digestion of proteins.

3. Pancreatic Juice

Pancreatic juice, a fluid secreted by the pancreas, contains a variety of digesting enzymes such as amylase, trypsin, and lipase. Because it protects the gut from chyme's detrimental effects, the pancreatic juice's neutralizing function is essential.

• Pancreatic amylase is the name of the amylolytic enzyme present in pancreatic juice. 
• Similar to salivary amylase, pancreatic amylase converts starch into dextrin and maltose.
• The two primary proteolytic enzymes found in pancreatic juice are trypsin and chymotrypsin.
• Other proteolytic enzymes include nuclease, elastase, collagenase, and carboxypeptidases.
• Among the lipolytic enzymes present in pancreatic juice are pancreatic lipase, cholesterol ester hydrolase, phospholipase A, phospholipase B, colipase, and bile salt-activated lipase.

4. Bile 

The liver cells secrete bile juice, which is then released into the bile canaliculus. Fat must first be emulsified by bile in order to be digested. Bile acts like a detergent in the stomach, assisting the body in absorbing the products of broken-down fat.

• Bile salts join with lipids to produce micelles in this context.
• Following that, the gut mucosa absorbs this. Bile juice facilitates the absorption of vitamins that are soluble in fat.
• Additionally, bilirubin, a by-product created during the decomposition of red blood cells, is excreted through the bile.

5. Saliva

The salivary glands release saliva, a digesting liquid that is found in the mouth. While the mouth is still physically chewing the meal, it starts the chemical digestion process. Saliva contains a trio of digestive enzymes:

• Lingual lipase functions to break down lipids through a process called lipolysis. It breaks down milk lipids (pre-emulsified fats).
• Lingual lipase converts triglycerides into fatty acids and diacylglycerol by hydrolysis.
• Amylase in saliva is an enzyme that digests carbohydrates.
• It functions by generating dextrin and maltose from cooked or boiled starch.
• The enzyme maltase, which is present in human saliva at extremely small levels, converts maltose into glucose.

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Things to Remember

• Digestion includes mechanical and chemical processes, with the buccal cavity mainly masticating food, lubricating it with saliva, and forming a bolus.
• Salivary glands, stomach, liver, pancreas, and small intestine release digestive juices.
• Salivary amylases and lysozymes in the oral cavity hydrolyze carbohydrates, converting 30% of starch molecules into maltose, and act as antibacterial agents against infections.
• Gastric glands in the stomach contain mucous neck cells, peptic or chief cells, and oxyntic cells. 
• Gastric secrete pepsinogen, an enzyme for protein conversion, and secrete enzymes like pepsin, rennin, and lipase for fat conversion.
• Bile juice from the liver, a yellow fluid, digests fats and activates lipase enzymes.
• Bile holds bilirubin, biliverdin, and phospholipids, assisting in fat emulsification and activating lipase enzymes.
• Pancreatic juices activate enzymes like trypsinogen, chymotrypsinogen, and procarboxypeptidase via intestinal secretions.
• The intestinal mucous epithelium contains brush border and goblet cells, secreting intestinal juice with enzymes like Maltese, dipeptidases, lipases, nucleosidases, and lactases.

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