Explanation Human beings have a heterotrophic mode of nutrition, which means they are dependent on others for food. Man cannot manufacture his own food like green plants. As man takes in solid food that has to be broken down into simpler forms - the mode of heterotrophic nutrition is referred to as holozoic nutrition. Most of the food that is eaten is in the form of large molecules. The body cannot use them. The food molecules have to be broken down into simple, soluble and small molecules that can be absorbed into the blood and utilized. Digestion is the process by which complex food is broken down into simple and soluble forms. Food is digested inside a tube in the body called the alimentary canal. The alimentary canal is a long, muscular tube that is highly coiled at places. The alimentary canal begins at the mouth and ends at the anus. The digestive system is made up of the alimentary canal and the associated digestive glands. The digestive glands secrete juices called enzymes into the alimentary canal, which help to digest food. Enzymes are biological catalysts that speed up digestion without itself undergoing any change. The food that is eaten is taken in at the mouth. The process of taking in food is called ingestion. What happens to the food in the mouth? The food is broken up into small bits by teeth present in the mouth. This process of chewing is called mastication. Digestion begins in the mouth. First there is mechanical digestion followed by chemical digestion. When food is physically broken down, it is called as mechanical digestion. When food is digested with the help of enzymes, it is called as chemical digestion. While chewing, the food is broken down into small pieces- a form of mechanical digestion and is ground into a pulp. At the same time it is mixed with saliva, with the help of the tongue. The saliva is secreted by the three pairs of salivary glands called parotid glands, submaxillary glands and the sublingual glands. Saliva is a slightly alkaline fluid and has the enzyme salivary amylase or ptyalin. Salivary amylase works on starch and converts it to maltose - a form of sugar. When bread is chewed for a long time, it tastes sweet. This happens because the starch has been converted to maltose, which is sweet in taste. Saliva helps the food particles to form a small ball called bolus that can be easily swallowed. Saliva lubricates the food and helps in swallowing. What happens during swallowing? During swallowing the tongue presses upward against the palate of the mouth. This forces the bolus of food into the throat (pharynx). The soft palate is the back part of the roof of the mouth cavity. The bolus pushes the soft palate upwards. This stops the food into getting into the space behind the nose. The windpipe (trachea) is present just in front of the food pipe (oesophagus). A flap of skin called the epiglottis closes the windpipe so that food does not enter it during swallowing. The bolus is squeezed past the epiglottis into the oesophagus. Incase, the food swallowed accidentally enters the windpipe, there will be immediate coughing to forcibly throw out the food substance. How does the food move along the alimentary canal? The oesophagus and the rest of the alimentary canal have circular muscles. The muscles contract behind the bolus and relax in front of it. This wave of contraction and relaxation of muscles is called peristalsis and pushes the bolus along. Peristalsis takes place all the way along the alimentary canal. Oesophagus Oesophagus is the food tube that carries food from the throat to the stomach. No digestive enzyme is produced by the oesophagus. Stomach The stomach follows the oesophagus and is expanded to form a pouch-like structure where food undergoes mechanical digestion by churning. The openings of both the ends of the stomach have a ring of muscles called sphincter muscles that help to keep the openings closed. The wall of the stomach is thick and muscular. The stomach wall contains many small digestive glands called gastric glands. The gastric glands produce the gastric juice. The gastric juice contains hydrochloric acid and an enzyme called pepsin. Pepsin digests proteins into proteoses and peptones. The hydrochloric acid kills any germs that may have entered with the food. Pepsin is first secreted as pepsinogen that is converted to pepsin by hydrochloric acid. The food remains in the stomach for about three hours and it is continuously churned. The contents in the stomach are digested and finally forms a pulp-like substance called the chyme. The stomach leads into the small intestine (duodenum). The opening of the stomach into the small intestine is called the pylorus. The pylorus opens intermittently to allow the chyme to move to the intestine in a series of jets. The stomach of an adult human being can hold about two to three litres of food. The small intestine The small intestine is a long, highly coiled tube made up of duodenum, jejunum and ileum. The first part of the small intestine is called the duodenum. It receives the common bile duct, which brings in the two digestive juices, the bile juice and the pancreatic juice. The second part of the intestine is called the jejunum. The last part of the small intestine is the ileum. The inner lining of the ileum has numerous finger like projections called villi. Bile juice Bile is an amber coloured liquid produced by the liver and stored in the gall bladder. It contains no digestive enzymes but has bile salts only. The bile emulsifies fats and increases the surface area. The enzymes of the intestine require an alkaline medium. Bile salts react with the acid food from the stomach and provide an alkaline medium for enzymes to work on. The alkaline food mixture is called chyle. Pancreatic Juice The pancreas gland produces the pancreatic juice. The pancreatic duct carries the pancreatic juice into the duodenum. The pancreatic juice has the three enzymes-- amylopsin, trypsin and steapsin. Amylopsin or pancreatic amylase converts all undigested starch into maltose. Trypsin converts the undigested proteins and peptones to peptides and amino acids. Trypsin is secreted first as inactive trypsinogen. Trypsinogen is activated to trypsin by the enzyme enterokinase. Enterokinase is secreted by the inner lining of the duodenum. Steapsin acts on emulsified fats and converts them to fatty acids and glycerol. Digestion is completed in the ileum. Intestinal glands secrete the intestinal juice called succus entericus. Succus entericus has the following enzymes--proteolytic enzyme erepsin, amylolytic enzymes sucrase, maltase and lactase and the lipolytic enzyme lipase Protein digestion Erepsin completes the protein digestion. Erepsin converts the remaining peptides into amino acids. Carbohydrate digestion Maltase digests maltose into glucose. Lactase digests lactose into glucose and galactose. Invertase or sucrase digests sucrose into glucose and fructose. Fat digestion Lipase digests fats into fatty acids and glycerol. Absorption of food Food is absorbed in the small intestine. The inner lining of the ileum produces numerous finger-like projections called villi. The villi increase the inner surface area of absorption. Each villus is lined by single layer of epithelial cells and encloses an artery, a vein, interconnecting capillaries and a lymph vessel called lacteal. The blood capillaries absorb the simple sugars and amino acids. The fatty acids and glycerol are absorbed into the lacteal to enter the lymphatic system. The Large Intestine The undigested food enters the large intestines. It is called the large intestine, as its diameter is wider than the small intestine. It has three parts- the caecum, colon and rectum Caecum is a small pouch that has a small narrow projection called the vermiform appendix. The vermiform appendix is a functionless organ (vestigial organ). When the appendix is inflammed it causes appendicitis. The food then passes into the colon. The colon part of the large intestine looks like an inverted 'U'. The first section is called ascending colon, located at the right side of the body, it passes across the abdomen - the transverse colon and goes down the left side - the descending colon. The colon receives all the excess undigested food mixed with water. The large intestine secretes no enzyme - therefore no digestion occurs here. It absorbs much of the water, so that large quantities of valuable fluid are retained back by the body. The undigested food is left in a semi - solid form that is coloured by the excreted bile pigments. This is called the faeces. The faeces pass into the rectum. The colon and the rectum secrete mucus for lubrication. Egestion The faecal matter is stored in the rectum. The rectum leads into a small anal canal that leads to an aperture called the anus. Anus marks the end of the alimentary canal. The sphincter muscles guard the opening of the anus. The faecal matter is periodically expelled through the anus. This discharge of the undigested remains of the food from the alimentary canal (anus) is called defecation or egestion. Digestive Disorders The two most common digestive disorders are diarrhoea and constipation Diarrhoea When faeces are egested, without the water being absorbed by the large intestine, the condition is called diarrhoea. The symptom of diarrhoea is loose motions. A person suffers from loose motions due to the malfunction of large intestines. The large intestine fails to absorb the excess water. Constipation Sometimes the large intestine does not discharge the contents (faeces) regularly, leading to their retentively. This condition is called as constipation. Constipation may be due to presence of large quantities of gas in the large intestines, slowing down of peristaltic movements and lack of water. A person suffering from constipation should take large quantities of water, bulky food material with adequate roughage and practice regular bowel movements. Assimilation of food Assimilation is the build up of absorbed food and its incorporation into cell components. Glucose provides energy and excess glucose is stored as glycogen in the liver. As and when needed, the liver reconverts it to glucose. Amino acids are the building blocks of proteins. Excess amino acids cannot be stored and are deaminated by the liver in which the nitrogen containing amino group is removed and converted into urea for excretion and the remaining compound of carbon, hydrogen and oxygen is stored as glycogen or fat. Fats provide energy and excess fats are stored as subcutaneous fat under the skin or around visceral organs. Liver The liver is the largest gland of the body. It is a five lobed organ located just below the diaphragm. Its functions include production of bile, deaminaton of amino acids and the production of urea, regulation of blood sugar, production of fibrinogen and heparin, storage of iron, copper and some vitamins, production of heat, detoxification, production of red blood cells in the embryo and destruction of dead red blood cells.