What Is the Primary Function of the Digestive System?
Digestion is the process by which food is broken down and metabolized into nutrient molecules. These molecules are then in a form that can be readily used by the body to sustain biological processes necessary for survival. Proteins are broken down into amino acids, carbohydrates into glucose, and fats into lipids.
Amino acids are necessary for new cell growth and repair, production of enzymes, hormones, antibodies, and chemical messengers (neurotransmitters) of the nervous system. They also help maintain fluid and electrolyte balance.
Glucose is the body's most preferred energy molecule. Nerve cells, including the brain, rely heavily upon glucose for energy. Serving as the body's energy reserves, lipids or fats protect the body from extremes in temperature, cushion vital organs, carry vitamins soluble in fats, and are major components of cell membranes.
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Structure
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The human digestive system consists of several organs: the mouth, throat, esophagus, stomach, liver, pancreas, gallbladder, small and large intestine, rectum and anus. Housed within the abdominal cavity, organs of the digestive system lay in front of the spinal column, behind the abdomen. It can be viewed as a long, continuous hollow tube.
Function
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The process of digestion begins when food is ingested, chewed by the teeth and broken down into smaller particles. This process of mastication is called mechanical digestion. Forming a food bolus, salivary glands secrete saliva containing amylase, an enzyme which aids in the decomposition of complex carbohydrates or starches.
The food bolus is swallowed, travels down the throat, past the pharynx, through the esophagus. As food enters the stomach, hydrochloric acid (HCL) and enzymes are secreted, turning the bolus into a milky substance called chyme.
Pancreatic enzymes and bile (excreted by the liver and gallbladder) are added to the chyme in the duodenum of the small intestine. While pancreatic enzymes primarily break down proteins further into amino acids and carbohydrates, bile emulsifies fats. Nutrients are absorbed across the mucosa, into the circulatory and lymphatic systems . The small intestine is where most nutrients, water and electrolytes are absorbed.
In the large intestine, remaining water and electrolytes are absorbed from leftover indigestible food material, such as fiber. Material is mixed with mucus and bacteria forming a large mass, feces, which is then excreted as waste.
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Transport
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Digestion occurs through a mechanism of the smooth muscle tissue lining the hollow tubes of the entire digestive tract. This movement, called peristalsis, allows walls of the esophagus, stomach and intestines to move, pushing and propelling food and fluid along through the system. Peristalsis occurs involuntarily, without conscious control.
Between the esophagus and stomach is a ringlike muscle, called the lower esophageal sphincter, which opens and closes allowing food to pass from the esophagus into the stomach.
Nutrient Absorption
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The chief site of nutrient absorption takes place within the small intestine. Finger-like projections called villi, increase the surface area available for the transport of molecules into the blood. Depending upon the type of nutrient molecule (i.e. proteins, lipids, etc) being absorbed, it will either be stored and deposited in the body being reserved for later use or further metabolized and used.
Regulation
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The nervous and endocrine systems are responsible for the regulation and control of the gastrointestinal tract.
Three principle hormones, gastrin, secretin, and cholecystokinin (CCK) are active in digestion. Gastrin triggers gastric acid production within the stomach and is responsible for growth of the stomach's mucosa. Secretin, released in the duodenum of the small intestine, signals secretion of bile in the liver and bicarbonate ion in the pancreas, in order to neutralize acid. Aiding in the release of digestive enzymes from the pancreas into the small intestine, CCK triggers the delivery of bile into the duodenum. Other hormones, ghrelin and peptide YY, act to stimulate and reduce appetite.
There are two kinds of nerves affecting the digestive system: intrinsic and extrinsic. Intrinsic nerves are signaled by the presence of food within the walls of the esophagus, stomach, small and large intestine. These nerves may speed up or slow down the digestive process. Coming in from the brain and spinal cord, extrinsic nerves are responsible for the release of acetylcholine and adrenaline. Acetylcholine triggers muscles of the digestive tract to squeeze more forcefully, pushing food through the system. Adrenaline slows this process down, allowing the muscles to relax, decreasing the force.
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