Digestive Fluids and Enzymes

The breakdown of swallowed/ingested food into its constituent parts requires the presence of a number of different digestive fluids and enzymes.

In fish with a stomach, hydrochloric acid is secreted to reduce gut pH and to allow pepsinogens to work, the latter acting as pepsin to break down the protein component of the diet. Other enzymes secreted by the stomach work on lipids, carbohydrates and one specifically breaks down chitin, the main component of crustacean and molluscan shells. Where there is no stomach, neither hydrochloric acid or pepsin is formed in the gut.

The pancreas secretes a large number of digestive enzymes which are stored in inactive forms, known as zymogens, to prevent self-digestion. Activation results from the presence of food in the gut which causes proteases to be produced by the intestine. These convert trypsinogen into trypsin which in turn activates other enzymes specific to protein, lipid, carbohydrate and chitin breakdown. The levels of trypsin in the pancreas before and after a meal is a key indicator of the efficiency of digestion.

Bile from the gall bladder serves to maintain the alkalinity of the gut and aids the emulsification of lipids. The majority of bile salts are re-absorbed from the intestine and returned to the liver. Several enzymes are secreted by the intestine itself and aid in the final breakdown of food into molecules small enough to be absorbed across the intestinal wall and into the bloodstream.