(56) Nutrition
How the Body Works : The Digestive System
Enzymes, Hormones, and Bile Aid Digestion
The complete digestion of chyme requires chemical secretions, including enzymes, hormones, and bile. Supportive digestive organs such as the pancreas, liver, and gall-bladder contribute or concentrate many of these fluids.
The stomach and small intestine also produce digestive enzymes. The purpose of the enzymes is to break apart food particles into small, unbound nutrients that can be efficiently absorbed. There are several hormones, such as gastrin, insulin, and glucagon, which regulate digestion. Some hormones are produced in the stomach and the small intestine. Hormones don’t digest food, but they regulate the activity of other cells, such as by controlling digestive secretions (gastric and pancreatic secretions) and regulating enzymes. They are stimulated by various dietary factors and their activity varies according to the digestive function required. Hormones influence gastrointestinal motility, stomach emptying, gallbladder contraction, insulin release, cell growth, intestinal absorption, and even hunger.
The hormones insulin and glucagon are produced in the pancreas and play important roles in your body. The hormone gastrin stimulates the secretion of HCl, increases gastric motility and emptying, and increases the tone of the LES. Gastrin also causes the release of gastric secretions that contain the enzyme gastric lipase. Gastric lipase contributes to the digestion of short- and medium-chain fatty acids, which are primarily found in human milk (consumed by infants) or in other dairy foods. These types of fatty acids are found less often in the diets of adults, as most of the dietary fats we consume contain long-chain fatty acids. Thus, gastric lipase is not a particularly important enzyme in adulthood.
Terms:
Stool (feces) - Waste products that are stored in the large intestine and then excreted from the body. Consists mostly of bacteria, sloughed-off gastrointestinal cells, inorganic matter, water, unabsorbed nutrients, food residue, undigested fibers, fatty acids, mucus, and remnants of digestive fluids.
Rectum - The lowest part of the large intestine, continuous with the sigmoid colon and the anus.
Hormones - Chemical substances that regulate, initiate, or direct cellular activity.
Process of Digestion
Digestion begins in the mouth. Chewing mixes saliva with food and begins to break it down.
The bolus travels from the mouth to the stomach through the esophagus, via peristalsis.
The stomach mixes food with enzymes to break it down further.
Most digestion and absorption occurs in the small intestine.
Water, vitamins, and some electrolytes are absorbed in the large intestine.
The remaining wastes are passed out of the body in stool through the rectum and anus.
The Liver, Gallbladder, and Pancreas are Accessory Organs
Although food doesn’t pass through the liver, gallbladder, or pancreas during digestion, these three accessory organs are still essential to the process. The liver, for example, makes bile, a greenish-yellow liquid that is important for fat digestion, and the gallbladder concentrates and stores the bile. As mentioned, the pancreas makes the hormones insulin and glucagon, as well as some important digestive enzymes.
Weighing in at about three pounds, the liver is the largest gland in the body. It is so important that you couldn’t survive without it. In addition to its key role of producing bile, the liver helps regulate the metabolism of carbohydrates, fats, and protein. The liver also stores several nutrients, including vitamins A, D, and E; the minerals iron and copper; and glycogen, the storage form of glucose. The liver is also essential for processing and detoxifying alcohol. You’ll learn about each of these functions in more depth in later chapters of this book. For our overview of digestion, we’ll focus on the liver’s role in bile production.
The liver produces about 500 to 1,000 milliliters (about 2 pints) of bile each day. Bile consists of water, bile acids (and/or salts), cholesterol, phospholipids, pigments, and several ions. Bile has two main functions:
- It breaks up large fat globules into small, suspended fat droplets. This action enhances the absorption of fats because it increases the surface area exposed to fat-digesting enzymes. The breakdown of fat also increases the rate of fat digestion.
- Bile also functions as an emulsifier, dispersing fat throughout the chyme, thus helping enzymes to make contact with it and digest it. Emulsification is the dispersion of fat or the surrounding of fat with hydrophilic (water-soluble) and hydrophobic (fat-soluble) portions. This action is similar to the detergent activity of dishwashing soap on greasy dishes.
Bile is collected, drained, and released into the gallbladder. The gallbladder is attached to the liver and stores approximately 30 to 50 milliliters (1 to 2 ounces) of concentrated bile at a time. Bile is released into the GI tract in response to the ingestion of fat. Bile aids in fat digestion but isn’t digested itself. Whereas some compounds of bile leave the body in stool, the bile acids are reabsorbed and return to the liver to be reused in new bile.
The pancreas is an organ about the size and shape of your hand that produces hormones, including the two blood-regulating hormones, insulin, and glucagon. It also produces digestive enzymes that are delivered into the duodenum. The pancreas has a duct that merges with the bile duct (from the gallbladder) and enters the duodenum through the common bile duct.
Sodium bicarbonate and several powerful, specific enzymes essential for the final stages of food digestion are produced in the pancreas. Sodium bicarbonate neutralizes the acidic chyme (raises the pH), creating a neutral environment. This protects certain enzymes that would otherwise become inactivated in an acidic environment. Some of the most important enzymes produced by the pancreas are amylase, which digests carbohydrate; lipase, which digests fat; and trypsin, chymotrypsin, and carboxypeptidase, which digest protein. The enzymes from the pancreas are responsible for the digestion of almost all (90 percent) of ingested fat, about half (50 percent) of all ingested protein, and half (50 percent) of all carbohydrates.
Pancreatic secretion is regulated by various hormones. When acidic chyme enters the small intestine, the hormone secretin is produced by intestinal cells. Secretin stimulates the pancreas to secrete copious amounts of sodium bicarbonate and various digestive enzymes. When partially digested protein and fat enter the small intestine, the intestinal cells secrete the hormone cholecystokinin. This powerful hormone alsostimulates the pancreas to secrete digestive enzymes, slows down gastric motility (which controls the pace of digestion), and contributes to meal satisfaction.
Functions of Digestive Secretions
Secretion -Secreted -From Function
Saliva - Glands in the mouth - Moistens food, eases swallowing, contains the enzyme salivary amylase
Mucus - Stomach, small and large intestines - Lubrication and coating of the internal mucosa to protect it from chemical or mechanical damage
Hydrochloric acid (HCl) - Stomach - Activation of enzymes that begin protein digestion
Bile - Liver (stored in the gallbladder) - Emulsifies fat in the small intestine
Bicarbonate - Pancreas - Raises pH and neutralizes stomach acid
Enzymes (amylases, proteases, and lipases) - Stomach, small intestine, pancreas - Chemicals that break down food into nutrient components that can be absorbed
Hormones (gastrin, secretin, cholecystokinin, and gastric inhibiting peptide) - Stomach, small intestine - Chemicals that regulate digestive activity, increase or decrease peristalsis, and stimulate various digestive secretions
The liver, gallbladder, and pancreas produce digestive secretions that flow into the small intestine through various ducts.
Terms:
Liver - The largest gland of the body. It aids in digestive activity and is responsible for metabolism of nutrients, detoxification of alcohol, and some nutrient storage.
Bile - A greenish-yellow fluid made in the liver and concentrated and stored in the gallbladder. It helps emulsify fat and prepare it for digestion.
Pancreas - Accessory organ of digestion that produces hormones and enzymes. It’s connected to the duodenum via the bile duct.
Organs of the GI Tract and Their Functions
Organ - Function - How They Work Together to Digest a Peanut Butter
Mouth - Begins breaking down food into components through chewing - Saliva moistens the sandwich as your teeth grind the food. Amylase begins to break down the carbohydrate in the bread.
Esophagus - Transfers food from the mouth to the stomach - Bolus of sandwich moves through the esophagus to the stomach.
Sphincters (LES, pyloric, ileocecal) - Keep swallowed food from returning to the esophagus, stomach, or small intestine - The LES makes sure that the sandwich stays in the stomach once it gets there.
Stomach - Mixes food with digestive juices; breaks down some nutrients into smaller components - The HCl activates pepsin to begin digesting the protein in the sandwich. Gastric lipase starts breaking down the triglycerides in the peanut butter.
Small intestine - Completes digestion of food and absorbs nutrients through its walls - The carbohydrates, proteins, and fat are broken down further with the help of bile and other enzymes so they can be absorbed.
Large intestine - Absorbs water and some nutrients; passes waste products out of the body - The fiber in the bread leaves the body in the stool.
Accessory organs (liver, gallbladder, pancreas) - Release hormones and enzymes, and help break down food or direct digestive activity - Hormones cause the release of gastric lipase as well as cause the gallbladder to release the stored bile into the small intestine to help emulsify the fat in the peanut butter sandwich. The liver produces the bile and also regulates the metabolism of the absorbed nutrients. The pancreas produces the two important blood-regulating hormones, insulin, and glucagon.