Human Digestive System (with Diagram)!
The alimentary canal and the glands associated with digestion constitute the human digestive system.
Alimentary canal:
The alimentary canal in human beings measures about 8 to 10 meters in length.
It extends from the mouth to the anus. It has the following parts.
Mouth:
The mouth consists of the oral cavity, through which food is ingested. It is bounded by lips and cheeks. It contains gums, teeth, a tongue and muscles.
The tongue tastes food and moves it into the pharynx. Teeth help in biting, cutting and chewing food. Teeth masticate the food. This makes it easier to swallow food and increases its surface area for various digestive secretions to act on. The four types of teeth are incisors, canines, premolars and molars. Our teeth are covered with a hard protective covering of enamel.
The enamel covers the dentine, which is a yellowish substance forming the bulk of the tooth. When we eat sweets, chocolates and ice creams, bacteria act on sugars and produce acids which soften the protective covering.
This causes dental caries. Bacterial cells and food particles stick to the teeth and form dental plaque. If the teeth are not brushed properly after meals, bacteria may invade deeper into the teeth. This leads to infection and toothache.
The presence of food in the mouth stimulates the three pairs of salivary glands to secrete saliva. Saliva has mucin, which lubricates the mouth and food. Saliva also has salivary amylase, a digestive enzyme that breaks down starch and glycogen to maltose (a simpler sugar).
Pharynx:
The oral cavity opens into the pharynx. The swallowing mechanism guides the masticated food through the pharynx, into a tube called oesophagus.
Oesophagus:
It is a muscular, tubular part of the alimentary canal. The muscular walls of the oesophagus move in a rhythmic wavelike manner, which carries the food down to the stomach. This muscular movement is called peristalsis. Here also salivary amylase acts on starch and glycogen in the chewed food.
Stomach:
It is located below the diaphragm (the muscular partition between the chest cavity and abdominal cavity). It is a saclike muscular structure. It serves as a storehouse of food where partial digestion takes place. The stomach has an anterior cardiac and a posterior pyloric part. As in other parts of the alimentary canal, columnar cells line the inner wall of the stomach. The inner lining has sunken pits. Each pit constitutes a gastric gland.
The cells lining a gastric pit, or gland, are of three kinds:
(1) Mucous cells (secreting mucus),
(2) Parietal, or oxyntic, cells (secreting hydrochloric acid) and
(3) Chief, or zymogen, cells (secreting the inactive enzyme propepsin). The hydrochloric acid in gastric juice converts propepsin to active pepsin and also kills bacteria ingested with food. The mucus protects the stomach lining and glands from being digested by gastric juice.
About 3 L of gastric juice is produced per day. Excess secretion of gastric juice, particularly in an empty stomach, erodes the inner lining of the stomach. This erosion causes lesions or round depressions called peptic ulcer in the stomach walls. Digestion of protein begins in the stomach. Pepsin breaks down proteins into peptones. Gastric lipase partially breaks down lipids.
Small intestine:
The small intestine is about 6 meters in length and 2.5 centimeters in thickness. There are three divisions of the small intestine: duodenum, jejunum and ileum. Duodenum is the first part. It begins from the pyloric stomach, and is C-shaped. In the middle of the duodenum two different ducts open through a common aperture.
One of the ducts is the common bile duct and the other is the pancreatic duct. Bile, a yellowish green alkaline juice, is poured into the duodenum through the common bile duct.
Liver:
It is the largest gland of the body. It performs many functions. It secretes bile, which helps indigestion. Bile juice produced by the liver is stored in the gall bladder.
There are two main functions of bile:
1. It emulsifies fats, by rendering them soluble and breaking them into small globules. In this form, fats are better exposed to the action of fat-hydrolyzing enzymes. (All digestive enzymes catalyze by breaking water molecules, and are hence called hydrolyzing enzymes.)
2. The acidic food (chyme) coming from the stomach becomes alkaline (chyle) when it is mixed with bile. This is important as the intestinal enzymes catalyze the breakdown of food only in an alkaline medium.
Pancreas:
It secretes pancreatic juice, which is carried by the pancreatic duct into the duodenum. Pancreatic juice contains a number of digestive enzymes such as amylase for the splitting of polysaccharides, lipase for the breakdown of fats, and trypsin and chymotrypsin for the breakdown of proteins. These enzymes catalyze the breakdown of their substrates in an alkaline medium. But the catalysis does not completely break all the substrates into their simplest units.
Jejunum is the middle part of the small intestine. It is found only in man. Ileum is the last and main part of the small intestine. The major part of digestion and absorption takes place here.
Intestinal glands:
The complete digestion of the remaining food material takes place in the ileum. There are numerous small glands in the walls of the small intestine. These glands secrete intestinal juice. The digestive enzymes in the intestinal juice break small peptides into amino acids, disaccharides into monosaccharide’s, lipids into fatty acids and glycerol, and nucleic acids into nucleotides.
Large intestine:
The ileum passes into the large intestine. The large intestine can be divided into two parts: anterior (colon) and posterior (rectum). At the junction of ileum and colon, there is a blind (one end closed) outgrowth called caecum. The caecum ends in the vermiform appendix (Latin vermis = worm; vermiform = worm-shaped). In man, the vermiform appendix has outlived its usefulness; it is a vestigial organ. It is an 8-cm-long blind tube, which sometimes becomes a source of trouble.
The colon has an ascending part, a transverse part and a descending part. The last part, or the descending part, opens into the rectum. The terminal part of the rectum is called anal canal. It opens through the anus, guarded by the sphincter muscles. The large intestine allows the passage of residual food mass (faucal matter), which is egested through the anus.
As the residue of the food mass passes along the large intestine, a considerable amount of water contained in the residue is absorbed into the blood through the intestinal walls. The specialized longitudinal muscles present in the colon wall regulate the passage of the faucal matter along the colon.
Take two clean test tubes. Pour 1 mL starch solution (1%) In each of them. Add 1 mL saliva in one test tube only and keep both the test tubes in a test tube holder undisturbed for half an hour. Now add a few drops of iodine solution in both the test tubes.
You will observe that the saliva-containing test tube shows no blue-black colour, while the other test tube does. What does it indicate? It shows that saliva contains some enzyme which has converted starch into some simpler compounds. In fact, salivary amylase present in saliva breaks down starch into maltose.
Absorption of Digested Food:
Absorption of completely digested food takes place in the ileum. The wall of the ileum has finger-like projections called villi that increase the surface area for absorption of digested food. The villi are richly supplied with blood vessels to carry the absorbed food (Figure 1.7).
The absorbed food is then brought into the blood capillaries. From the blood capillaries, absorbed materials are transported by veins to the liver and then to the heart for distribution to different parts of the body.
Assimilation of Digested Food:
Intake of digested food by cells of the body is called assimilation. Digested food is utilized by the body in many ways. It is used to obtain energy through the process of respiration. Excess monosaccharide’s are stored as glycogen. Amino acids are used in the synthesis of proteins.
The glycerol and fatty acids either provide energy or get reconverted into fats. These fats are accumulated in different organs below the skin. The absorbed food is also utilized for the formation of new cells and tissues, leading to the growth and development of the body.