Introduction to the Digestive System

Bile aids in the digestion of lipids, primarily triglycerides, through emulsification. Emulsification is a process in which large lipid globules are broken down into several small lipid globules. These small globules are widely distributed in the chyme rather than forming large aggregates.

Pathologies that affect the digestive organs-such as hiatal hernia, gastritis, and peptic ulcer disease-can occur at greater frequencies as you age. Problems in the small intestine may include duodenal ulcers, maldigestion, and malabsorption. Problems in the large intestine include hemorrhoids, diverticular disease, and constipation. Conditions that affect the function of accessory organs-and their abilities to deliver pancreatic enzymes and bile to the small intestine-include jaundice, acute pancreatitis, cirrhosis, and gallstones. Food that has been broken down is of no value to the body unless it enters the bloodstream and its nutrients are put to work.

The tongue and teeth are accessory structures located in the mouth. The salivary glands, liver, gallbladder, and pancreas are major accessory organs that have a role in digestion. These organs secrete fluids into the digestive tract.

The pancreas is composed of pancreatic exocrine cells, whose ducts are arranged in clusters called acini (singular acinus). The cells are filled with secretory granules containing the precursor digestive enzymes (mainly trypsinogen, chymotrypsinogen, pancreatic lipase, and amylase) that are secreted into the lumen of the acinus. These granules are termed zymogen granules (zymogen referring to the inactive precursor enzymes.) It is important to synthesize inactive enzymes in the pancreas to avoid auto degradation, which can lead to pancreatitis.

Digestion

Because of this, IBS patients experience problems with bowel movement, motility, and the sensation having more sensitive pain receptors in their GI tract. Many IBS patients suffer from depression and anxiety which can make symptoms worse. Cholestasis is the blockage in the supply of bile into the digestive tract. It can be “intrahepatic” (the obstruction is in the liver) or “extrahepatic” (outside the liver).

Absorption

  • When the stomach is empty, the walls are folded into rugae (stomach folds), which allow the stomach to expand as more food fills it.
  • Bile contains bile salts, which have hydrophobic and hydrophilic sides.
  • The digestive process is completed here by enzymes and other substances made by intestinal cells, the pancreas, and the liver.
  • For example, the sight, smell, and taste of food initiate long reflexes that begin with a sensory neuron delivering a signal to the medulla oblongata.

Before food can be used it has to be broken down into tiny little pieces so it can be absorbed and used by the body. In humans, proteins need to be broken down into amino acids, starches into sugars, and fats into fatty acids and glycerol. The first step in the digestive system can actually begin before the food is even in your mouth.

The micelles move into the brush border of the small intestine absorptive cells where the long-chain fatty acids and monoglycerides diffuse out of the micelles into the absorptive cells leaving the micelles behind in the chyme. The long-chain fatty acids and monoglycerides recombine in the absorptive cells to form triglycerides, which aggregate into globules and become coated with proteins. These large spheres are called chylomicrons. Chylomicrons contain triglycerides, cholesterol, and other lipids and have proteins on their surface.

How each of these components is digested is discussed in the following sections. Digestion starts when food enters the mouth and ends when feces are excreted.

The final step in digestion is the elimination of undigested food content and waste products. After food passes through the small intestine, the undigested food material enters the colon, where most of the water is reabsorbed.

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