Every day, seven liters of fluid are secreted by the digestive system. This fluid is composed of four primary components: ions, digestive enzymes, mucus, and bile. About half of these fluids are secreted by the salivary glands, pancreas, and liver, which compose the
accessory digestive organs of the digestive system. The rest of the fluid is secreted by the GI epithelial cells.
Ions The largest component of secreted fluids is
ions and
water, which are first secreted and then reabsorbed along the tract. The ions secreted primarily consist of , , , and . Water follows the movement of these ions. The GI tract accomplishes this ion pumping using a system of proteins that are capable of
active transport,
facilitated diffusion and open channel ion movement. The arrangement of these proteins on the
apical and
basolateral sides of the epithelium determines the net movement of ions and water in the tract. H+ and Cl− are secreted by the
parietal cells into the
lumen of the stomach creating acidic conditions with a low pH of 1. H+ is pumped into the stomach by exchanging it with K+. This process also requires ATP as a source of energy; however, Cl− then follows the positive charge in the H+ through an open apical channel protein. secretion occurs to neutralize the acid secretions that make their way into the
duodenum of the small intestine. Most of the comes from pancreatic
acinar cells in the form of
NaHCO3 in an aqueous solution. This is the result of the high concentration of both and Na+ present in the duct creating an
osmotic gradient to which the water follows.
Digestive enzymes The second vital secretion of the GI tract is that of digestive enzymes that are secreted in the mouth, stomach and intestines. Some of these enzymes are secreted by accessory digestive organs, while others are secreted by the epithelial cells of the stomach and intestine. While some of these enzymes remain embedded in the wall of the GI tract, others are secreted in an inactive
proenzyme form. When these proenzymes reach the lumen of the tract, a factor specific to a particular proenzyme will activate it. A prime example of this is
pepsin, which is secreted in the stomach by
chief cells. Pepsin in its secreted form is inactive (
pepsinogen). However, once it reaches the gastric lumen it becomes activated into
pepsin by the high H+ concentration, becoming an enzyme vital to digestion. The release of the enzymes is regulated by neural, hormonal, or paracrine signals. However, in general, parasympathetic stimulation increases secretion of all digestive enzymes.
Mucus Mucus is released in the stomach and intestine, and serves to lubricate and protect the inner mucosa of the tract. It is composed of a specific family of
glycoproteins termed
mucins and is generally very viscous. Mucus is made by two types of specialized cells termed mucous cells in the stomach and
goblet cells in the intestines. Signals for increased mucus release include parasympathetic innervations, immune system response and enteric nervous system messengers.
Bile Bile is secreted into the duodenum of the small intestine via the
common bile duct. It is produced in liver cells and stored in the gall bladder until release during a meal. Bile is formed of three elements:
bile salts,
bilirubin and cholesterol. Bilirubin is a waste product of the breakdown of hemoglobin. The cholesterol present is secreted with the feces. The bile salt component is an active non-enzymatic substance that facilitates fat absorption by helping it to form an emulsion with water due to its
amphoteric nature. These salts are formed in the
hepatocytes from bile acids combined with an
amino acid. Other compounds such as the waste products of drug degradation are also present in the bile. == Regulation ==