A healthy diet is not the only determinant of good nutrition. Equally important is a healthy digestive system – the collection of organs that together turn food into a form the body can use. Another name for the digestive system is the alimentary canal, the tube that begins in the mouth and includes the pharynx, esophagus, stomach, small and large intestines, and anus.
This is a vital mechanism because, if an individual cannot properly process even the most nutritious foods, then he or she will not be well nourished. Digestion prepares food to be absorbed, used and stored by breaking it down into nutrients particles small enough to pass through the intestinal walls and into the bloodstream.
During digestion, all nutrients are converted in stages into their simplest components: carbohydrates are converted into glucose or other simple sugars. proteins break down into their constituent amino acids and fats are converted into fatty acids. Digestion takes place in three consecutive phases that alternate between alkalinity and acidity, first in the mouth, then in the stomach and finally in the intestines.
Acid and alkaline are relative terms that indicate the concentration of hydrogen ions (hydrogen atoms containing a positive electrical charge) in a solution. The acidity or alkalinity of a solution is measured on the pH scale. A reading of 7 indicates that a solution is neutral, neither acidic nor alkaline. A reading of less than 7 means that the solution is acidic. A reading above 7 indicates that a solution is alkaline.
Stage One: The mouth
The first stage of digestion begins in the mouth, where food is chewed and broken down into smaller particles. The mouth contains the tongue, hard and soft palates, and three pairs of salivary-sublingual, submaxillary and parotid glands. The parotid glands are the largest and sit on either side of the face, underneath and in front of each ear. The submaxillary glands are located under the jaw on either side of the face. The two sublingual glands are located under the tongue, one on each side.
Saliva, the liquid in the mouth, is secreted by the salivary glands when an individual sees, smells, tastes, or thinks about food. Saliva is alkaline and contains an enzyme called piyalin, which helps soften and dissolve foods by breaking them down into simpler components. Saliva also lubricates chewed food, which makes swallowing easier. The muscular tube called the pharynx links the mouth to the esophagus, another muscular tube that leads to the upper part of the stomach. Once ingested, the now semi-solid food mass is pushed through the esophagus.
Stage Two: The Stomach
The gastric stage of digestion begins when the peristaltic or muscular movements of the esophagus propel the dietary mass into the stomach. The stomach is a muscular, pouch-shaped organ that flattens when empty and relaxes when full. Gastric juice – essentially hydrochloric acid (HCL) – secreted by cells in the stomach wall saturates the food mass. Gastric juice works with an enzyme called pepsin to break down proteins into their constituent amino acids.
Another enzyme, lipase, begins the breakdown of fats into fatty acids. In addition, the stomach kneads food in a thick semi liquid substance called chemo. Most bacteria in food are destroyed during the gastric stage. How long a meal stays in the stomach depends on what has been consumed, for example, a meal high in protein and fat takes longer to digest than a meal high in carbohydrates. The chyme passes through an opening in the stomach, the pylorus, which connects the stomach to the small intestine. A subset of the food channel, stomach and intestines of humans and other mammals are collectively referred to as the gastrointestinal tract.
Stage Three: The Intestines
Most digestion and all nutrient absorption takes place in the small intestine. This 20 foot long tube is extremely twisted and fits snugly into the abdominal cavity. It has three distinct sections: the duodenum, jejunum and ileum. Jejunum is the largest section of the small intestine; it also has thicker walls, is the richest in blood vessels, and contains most folds and creases. Intestinal secretions contain many enzymes, including those that act on sugars to make them usable components. These enzymes include maliasa, which converts maltose into glucose; lactase, which breaks down lactose into glucose and galactose; and sucrose, which transforms sucrose into glucose and fructose.
A secretion from the pancreas, called pancreatic amylase, first converts starch into dextrin and then maltose. Another intestinal enzyme, polypeptidase, breaks down proteins into their individual amino acids. Many other organs work together to provide a variety of substances that help carry out this stage of digestion. These include the liver, gallbladder, pancreas, kidneys, and spleen. The liver – located on the upper right side of the abdomen, near the outside of the body – is a large organ, which weighs about four pounds. The liver converts sugar into glycogen, which can be stored in the body until it becomes necessary and then can be broken down again and used for energy.
In addition, this organ makes bile, a combination of cholesterol and red blood cells that is passed along the gallbladder for storage and used in the digestive process. The gallbladder is a membranous muscle sac that attaches in its upper part to the back of the liver. Four inches long and pear-shaped, the gallbladder squeezes bile into the duodenum during the digestive process. The pancreas secretes lipase enzymes that, with the help of bile, break down fats into fatty acids and glycerol. Other pancreatic enzymes, called proteases, convert proteins into individual amino acids, as does the intestinal enzyme polypeptidase. In addition, the pancreas produces a hormone called insulin, which helps tissues absorb glucose.
Located on the back of the abdomen, the six-inch long pancreas weighs less than six ounces and is shaped like a tongue. Its right side forms a “head” abroad that is tuned to a narrower “tail”. The two kidneys are located toward the spine at approximately waist level. They also weigh less than six ounces each, remove waste and excess water from the blood, dissolving waste in the water to produce urine. The spleen is a ductless gland about six inches long and is shaped like a kidney. It is located near the outer edge of the body above the left kidney.
In addition to destroying old blood cells, storing blood and producing white blood cells, the spleen also stores iron and copper. Usually weighs less than half a pound, the size and weight of the spleen varies with the age and health status of the individual. Nutrients reach the bloodstream in one of two ways. The wall of the small intestine is carpeted with millions of tiny projections of less than one-fifth of an inch long called villi. These, in turn, are covered with a smaller roll, which absorbs amino acids and glucose and transports them into the bloodstream.
In addition, nutrients can also bypass hairiness that spreads directly through the intestinal wall. Through the bag-shaped cecum, the ileum connects the small intestine to the large intestine, which is about five feet long and surrounds the upper and sides of the small intestine. Undigested foods go to the colon, the main section of the large intestine, for temporary storage, waste moves through the colon and out of the body through the rectum and anus, the lower opening of the alimentary canal. A diet high in fiber (which is not digestible) and water produces bulky wastes that move easily through the colon.
Metabolism, the Greek word for “change”, refers to all the chemical reactions that occur within the body and its billions of cells. These include maintaining body temperature, breathing, blood circulation, urinary secretion, digestion, and formation of body cells and tissues. Any metabolic process requires fuel derived from nutrients. Basal metabolism is the amount of energy needed to maintain basic functions when the body is at rest, but not asleep, and on an empty stomach. Basal metabolism varies from person to person, depending on weight, age, gender, and activity level.