Three major types of secretory products are elaborated by the cells and organs of the digestive tract. These are water and electrolytes, mucus, and specialized organic molecules.
Throughout the entire length of the GI tract, epithelial cells secrete water and electrolytes. In addition, the salivary glands, pancreas, and liver produce copious secretions of fluid and ions. These secretions function in a variety of ways. Water liquifies the luminal contents, dissolves a significant portion of nutrients, and provides a medium for the chemical reactions of digestion to occur. Hydrogen kills some bacteria, takes part in the digestion of protein, and acidifies the gastric contents. Bicarbonate ion protects the mucosa from acid injury and increases the pH of the intestinal contents into the range for optimal enzyme activity.
The primary functions of mucus are derived from its glycoproteins. Although the chemical nature of these may vary slightly in different parts of the GI tract, its functions to lubricate and protect the mucosa are the same throughout. Mucus is produced and secreted by specialized cells found along the entire length of the tract. Chewing mixes food with mucus secreted by the salivary glands so that it can be swallowed easily. In the colon, mucus causes fecal particles to adhere and coats them so they can be excreted readily. Throughout the tract mucus lines the digestive cavity, preventing abrasions and keeping the cells from coming in contact with acid.
Digestive enzymes constitute the major group of specialized organic compounds secreted into the GI tract. Important steps in the digestion of all three classes of nutrients occur with the lumen of the gut. In addition, some organic compounds such as bile acids and intrinsic factor are essential for optimal digestion and/or absorption.
This chapter describes these secretions and their sources and how the various specialized cells of the mucosa and glands produce and release these secretory products. Considerable space is given to describing the sometimes involved regulatory processes which ensure that the secretions are elaborated in the quantities and sequence such that optimal digestion and absorption are achieved.
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