Nutrition in Human Beings
The digestive system of man consists of the mouth, oesophagus, stomach, and small and large intestines. The digestive tube made up by these organs is known as the alimentary canal. The alimentary canal is a long tube like structure which begins from the mouth cavity and ends in the anus.
Several glands— liver, gall bladder, and pancreas—also play a part in digestion. These glands secrete digestive juices containing enzymes that break down the food chemically into smaller molecules that are more easily absorbed by the body. The digestive system also separates and disposes of waste products ingested with the food.
At first food is taken into the mouth cavity by the joint actions of lip, tongue and teeth. The tongue helps the food to move the food inside the mouth cavity. Then the food is broken down and crushed into smaller pieces by the teeth. Have you noticed that eating something we like, or sometimes even seeing that food our mouth ‘water’? This is not actually water, but a fluid called saliva, secreted from the salivary glands.
The sensations of sight, taste, and smell of the food, together cause the salivary glands, located in the mouth, to produce saliva. An enzyme in the saliva called amylase begins the breakdown of carbohydrates (starch) into simple sugars. After the food is swallowed, it is moved to the pharynx (throat) at the back of the mouth. In the pharynx, rings of muscles force the food into the oesophagus, the first part of the upper digestive tube. The oesophagus extends from the bottom part of the throat to the upper part of the stomach. The oesophagus does not take part in digestion. Its job is to move the food into the stomach. Food is moved through the oesophagus by a wavelike muscular motion known as peristalsis. This motion consists of the alternate contraction and relaxation of the smooth muscles lining the tract.
From the stomach the food is moved to the small intestine. It is a very long tube (7 mt. long and 2.5 cm. diameter) which originates from the distal end of the stomach and extends to the large intestine. The small intestine is the longest part of the alimentary canal and is greatly coiled and twisted. The small intestine is subdivided into three sections: the duodenum, the jejunum, and the ileum.
The duodenum (about 10 inches ) part of the small intestine is the main seat of digestion in the gut. The acidic chyme in the duodenum receives the bile secreted from the liver, the pancreatic juice secreted from the pancreas and the intestinal juice from the glands of the intestinal wall. Bile is a yellowish-green in colour, bitter in taste, slightly alkaline fluid secreted from the liver. Bile being alkaline in nature neutralises the acidic chyme. Bile emulsify fat into microscopic droplets and thus helps in the digestion and absorption of fat. So bile is called digestive juice though it does not contain enzyme. Again the pancreas, a large gland located below the stomach, secretes pancreatic juice into the duodenum through the pancreatic duct. There are three enzymes namely amaylase, pancreatic lipase and trypsin, in pancreatic juice that break down carbohydrates, fats, and proteins respectively. Glands of intestine are present in the mucous layer of the intestinal wall. These glands secrete intestinal juice, which contains various enzymes. The enzymes present in it finally convert the carbohydrates into glucose, proteins to amino acids and fats into fatty acids and glycerol.
The jejunum is about 8.2 feet (2.5 meters) long. The digested carbohydrates, fats, proteins, and most of the vitamins, minerals, and iron are absorbed in this section. The inner lining of the small intestine is composed of up to five million tiny, fingerlike projections called villi. The villi increase the rate of absorption of nutrients into the bloodstream by greatly increasing the surface area of the small intestine.
The ileum, the last section of the small intestine, is the longest, measuring 11 feet (3.4 meters). Certain vitamins and other nutrients are absorbed here.
The unabsorbed food is sent to the large intestine. It is a wide muscular tube, about 1.5 mt. long and 6 cm. diameter. It consists of caecum, vermifom appendix and rectum. It rises up on the right side of the body (the ascending colon), crosses over to the other side underneath the stomach (the transverse colon), descends on the left side, (the descending colon), then forms an s-shape (the sigmoid colon) before reaching the rectum and anus. The muscular rectum, about 6 inches long, expels feces through the anus, which has a large muscular sphincter that controls the passage of waste matter. The large intestine removes water from the waste products of digestion and returns some of it to the bloodstream. Fecal matter contains undigested food, bacteria, and cells from the walls of the digestive tract. Millions of bacteria in the large intestine help to produce certain B vitamins and vitamin K. These vitamins are absorbed into the bloodstream along with the water.