Chapter 4 Summary



Chapter 4 Summary

The digestive system, which is made up of the gastrointestinal (GI) tract and accessory organs, releases a variety of secretions needed for digestion. The three functions of the digestion system are the chemical and physical breakdown of food (digestion), the transfer of nutrients from the digestive tract into the blood or lymphatic circulatory system (absorption), and the elimination of undigested food residue from the body. The GI tract consists of four layers that carry out the functions of GI motility and secretion. Important secretions needed for digestion include water, acid, electrolytes, mucus, salts, bicarbonate, and enzymes. Muscular contractions called segmentation and peristalsis mix and move food in the digestive tract, whereas sphincters regulate the flow of food. GI motility and secretion are regulated by the enteric nervous system, the central nervous system, and the enteric endocrine system.

GI motility and the release of GI secretions are synchronized with the arrival of food into the different regions of the GI tract. For this reason, digestion is often divided into phases. The cephalic phase is initiated by the central nervous system in anticipation of food, whereas the gastric phase begins with the arrival of food in the stomach. During these two phases, GI motility and the release of GI secretions increase. The intestinal phase begins when food moves into the small intestine. During this phase of digestion, the rate of gastric motility and the release of gastric juice decrease. Digestion begins in the mouth, where food is physically and chemically broken down. After swallowing, food moves down the esophagus and enters the stomach passing through the gastroesophageal sphincter. In the stomach, the hormone gastrin stimulates the release of gastric juice, which contains mucus, hydrochloric acid, enzymes, intrinsic factor, and water. The mixing of food with gastric juice turns food into chyme. Chyme then passes from the stomach into the small intestine through the pyloric sphincter. The majority of digestion takes place in the small intestine and is aided by secretions from the pancreas and gallbladder. The final stages of digestion take place in the intestinal lumen, on the brush border surface of the small intestine, and within intestinal enterocytes. This is accomplished by enzymes produced in the pancreas and small intestine.

Nutrient absorption is the process whereby nutrients transported across enterocytes and taken up into circulation. The small intestine is the major site of nutrient absorption. The microvilli trap and pull nutrients toward the enterocytes. Nutrient absorption includes both the entry into, and the exit out of, the enterocyte. Nutrients cross cell membranes by a variety of passive and active transport mechanisms. The intestine and the liver have a unique circulatory arrangement called the hepatic portal system. Water-soluble nutrients absorbed from the digestive tract are circulated directly to the liver via the hepatic portal vein. Nutrients that are too large or insoluble, such as lipids, are taken up through lacteals and circulated by the lymphatic system.

The large intestine prepares undigested food residue for excretion as feces by removing water. Bacteria in the large intestine also help form feces by breaking down undigested food residue. Eating prebiotic and probiotic foods helps promote the growth of these bacteria. The rectum serves as a holding chamber for feces until the body eliminates it. If material in the large intestine moves too quickly, diarrhea can result, whereas if the contents move too slowly constipation can result. Other conditions that can affect these functions are irritable bowel syndrome (IBS) and inflammatory bowel diseases (IBDs).

Cells require a wide variety of nutrients and oxygen. The delivery of these substances is accomplished by the cardiovascular and lymphatic systems. The cardiovascular system consists of the systemic circulation and the pulmonary circulation. These circulatory routes deliver blood to specific regions within the body. The systemic blood circulates between the heart, the body organs, and back again. The pulmonary route circulates blood between the heart and lungs. In addition to circulating fat-soluble nutrients, the lymphatic system plays an important role in maintaining blood volume and protecting against disease.

Metabolic waste products such as carbon dioxide, urea, and water are removed by different organs, including the lungs, kidneys, and to a lesser extent, the skin. The urinary system is the primary site for excreting metabolic wastes, other than carbon dioxide. To remove metabolic waste products, the kidneys filter them as the blood flows through the nephrons. Next, water and other essential substances are returned to the blood by a process called resorption. Last, the process of secretion prepares waste products to be eliminated from the body as urine. Urine leaves the kidneys via the ureters, which empty into the bladder. Urine is excreted from the body through the urethra.

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