Introduction The human digestive system consists of long coiled tube called alimentary canal or digestive canal. The alimentary canal is a long coiled tube which extends between mouth and anus. This alimentary canal measures around 9 meters in length. The alimentary canal is also associated with some digestive glands which help in the process of digestion.
Structure of the human digestive systemThe alimentary canal of the human digestive system can be divisible into various distinct parts which include a buccal cavity, pharynx, oesophagus, stomach, small intestine, and large intestine. The associate glands of the digestive system include three pairs of salivary glands in the mouth, gastric glands in the stomach, pancreas gland beneath the stomach, liver in the abdomen, intestinal glands present on the inner surface of the small intestine. These associated glands of the digestive system secrete digestive juices and chemical enzymes for the proper digestion of food.
Structure of Human Digestive System (Courtesy:- www.wikipedia.org)
Buccal cavityBuccal cavity (oral cavity) is the space present between upper jaw and lower jaw. It consists of mouth, teeth, tongue and the two jaws. The muscular tongue in the mouth helps in manipulating food (rotating food up and down) and also help in speaking. The upper and lower jaw of the mouth is provided with four types of teeth- incisors, canines, premolars, and molars. The incisors help in biting and cutting the food. The canines are meant for tearing food. The premolars are meant for crushing the food. The molar teeth are meant for grinding and chewing food. All the teeth together help to grind the food into small bits. These small bits of food material are aggregated together with the help of salivary juice secreted by salivary glands into a ball-like structure called 'bolus'. As the bolus is slimy in nature it can easily be swallowed through the pharynx.
PharynxIt is a short narrow tube which measures around 12-14 cm. which connects the mouth to the oesophagus. The pharynx can be divisible into nasopharynx, oropharynx, and laryngopharynx. It is a common passage for both air and food. The food from the mouth is pushed into the oesophagus through pharynx only. So pharynx is a part of the digestive system and respiratory system.
OesophagusIt is a muscular narrow tube of 25 cm. the length which connects the pharynx to the stomach. It lies up to the middle of the thorax, behind the trachea and heart. The oesophagus is lined by smooth muscles and they help in causing peristaltic movements of the oesophagal tube. These peristaltic movements help to push the food little by little into the stomach. The inner mucosa of the oesophagal tube is raised as longitudinal folds called rugae. These prevent the entry of air into the stomach.
StomachIt is a large muscular elastic bag-like structure present on the left side of the abdomen. It is a J-shaped large organ that lies between the oesophagus and the small intestine. It measures around 30 cm. in length and 10 cm. in width. The stomach has 3 parts-Cardiac, fundus, and pylorus. A circular cardiac sphincter located at the junction of the oesophagus and stomach. This sphincter prevents the regurgitation of food. At the junction of the stomach and small intestine, another muscular sphincter is located called pyloric sphincter. It controls the movement of food little by little into the small intestine. On the inner surface of the stomach, a large number of tiny glands present called gastric glands. These glands secrete gastric juice. This gastric juice is composed of dilute HCl, pepsin, and renin enzymes. The inner surface of the stomach is lined by mucous which protects the wall of the stomach from the action of acid and enzymes secreted by itself. The stomach wall is provided with smooth muscles which enable it to show peristaltic movements. These peristaltic movements make the stomach to churn the food into a fine paste called 'chyme'. The food stays 4-6 hours in the stomach for its part of digestion before it enters into the small intestine.
Small intestineIt is the longest part of the alimentary canal measuring 6 meters long. The small intestine can be divisible into three parts- Duodenum, Jejunum, and Ileum
Duodenum is the shortest and widest part of the small intestine measuring about 25 cm. long. It is a C-shaped organ that receives bile duct from the liver and pancreatic duct from the pancreas whose secretions are poured through a common duct called hepatopancreatic duct.
The jejunum is the next part of the duodenum which is a J-shaped short tube measuring about 3meters in length. The diameter of this tube is about 4 cm. and is highly vascularized. No chemical digestion occurs in this region.
The ileum is the longest part of the small intestine which is highly coiled measuring about 3.6 meters long. The inner lining of the small intestine is folded into finger-like projections called villi and microvilli. Each villus is covered with epithelium, contains lymph vessels (lacteals) and blood capillaries. The inner surface of the small intestine is provided with a large number of intestinal glands that secrete 2-3 litres of intestinal juice. The intestinal juice is also known as succus entericus. The final part of digestion and absorption of digested food occurs in this region.
Large intestineIt is much shorter than small intestine but a wider tube roughly measuring about 1.5 meters in length. This tube can be divisible into 3 parts- Caecum, Colon, and Rectum.
The caecum is a small pouch-like structure that is considered to be the beginning of the large intestine. At the junction of the small intestine and large intestine there present a small finger-like projection called the vermiform appendix. The vermiform appendix in humans is considered to be a vestigial or rudimentary organ. Caecum and appendix in humans have no digestive function. Caecum and appendix are much larger in herbivores where digestion of plant cellulose takes place with the aid of bacterial activity. The inflammation of the vermiform appendix is called appendicitis.
The colon is a sacculated structure that can be divisible into four parts- Ascending colon, Transverse colon, Descending colon, and Sigmoid or Pelvic colon. Colon has no digestive function. The wall of the colon absorbs excess water, salts, and minerals present in the undigested food which passes through it. Bacteria present in the colon decompose the undigested waste into the fine faecal matter.
The rectum is the final small muscular region of the alimentary canal ending in the anus. It will store undigested food for 12-24 hours before it expels through the anus. The act of expelling the faeces is called egestion or defecation.
Digestive glands of the human digestive systemSalivary glands, gastric glands, pancreas, liver, and intestinal glands are the digestive glands associated with the human digestive system.
Salivary glandsNumerous small salivary glands are present in the mucous membranes of lips, cheeks, palate, tongue, and pharynx. But three large salivary glands present outside the mouth and connect with the mouth through ducts. The three salivary glands- the largest salivary gland is the parotid gland is located behind the ear, a submaxillary gland located on the inner side of the lower jaw, and a sublingual gland lie below the tongue. The gland together secretes 0.5 to 2 litres of salivary juice daily. Salivary juice contains mucus, salts, and enzyme salivary amylase. Salivary juice helps in the moistening of food and digesting starch in mouth.
Gastric glandsGastric glands are present on the inner surface of stomach. About 2000 to 3000 cc. of gastric juice is secreted by millions of tiny gastric glands present on the inner surface of the stomach. The gastric glands present in the stomach are four kinds-Peptic or Zymogenic glands that secrete enzymes pepsinogen and prorenin, Oxyntic or parietal glands which secrete HCl and intrinsic factor, a mucous gland that secretes mucin containing mucus, and Argentaffin cells that secrete somatostatin and histamine. Gastric juice contains dilute HCl to kill germs in food, pepsin to digest proteins and renin to curdle milk.
PancreasIt is both exocrine gland (duct gland) and endocrine gland (ductless gland). As an exocrine gland, it secretes digestive enzymes that get poured into the duodenum by hepatopancreatic duct. The pancreas is a soft lobulated pinkish gland. It is 12 cm. in length and 15 cm. in width and weighs 60 gms. It secretes 500-800 cc pancreatic juice whose pH is 8.4. Pancreatic juice includes trypsin, amylase and lipase enzymes in it. Trypsin enzyme of pancreatic juice works on proteins, amylase enzyme works on starch and lipase enzyme works on fats.
LiverIt is the largest gland present on the upper right side of the abdominal cavity just below the diaphragm. It is a large reddish-brown organ that weighs about 1.5 Kg. The liver has an excellent supply of blood which receives one litre of blood per minute. The liver is the main metabolic centre of our body where the metabolism of carbohydrates, proteins, and fats takes place continuously. The reserve food of humans, the glycogen is stored in the liver. Dead RBC gets destroyed in the liver and results in the formation of bile juice. Bile juice has no digestive enzymes in it. Bile juice which enters into duodenum neutralizes acidic food into alkaline so that the enzymes of the pancreas act on it. Bile juice also helps to emulsify the fats.
Intestinal glandsOn the inner surface of small intestine two kinds of glands-Crypts of Lieberkuhn glands that are simple tubular glands present throughout the small intestine between the villi and secrete digestive enzymes and mucus. The second type of glands are Brunner's glands are convoluted and branched glands found in the duodenum that secrete alkaline mucus and enzymes. Intestinal juice includes Erepsin or Peptidase, Maltase, Sucrase, Lactase, Lipase, etc. to completely digest carbohydrates, proteins and fats.
Here is another organ system to know about:- Human excretory system
Here is a video on the structure of the human digestive system:-