Digestion and Absorption

We require food on a daily basis. Food provides us with nutrients that help in growth, repair and immunity. The major nutrients of food are Proteins, Carbohydrates, Fats, Vitamins and Minerals. We cannot use food as it is. It needs to be broken down into simpler substances to make it available for utilization. This process of obtaining food for growth and health is called nutrition. The various processes in nutrition are as follows:

1. Ingestion: It is the process of taking food into the mouth.
2. Digestion: It is the process of converting complex food substances into simpler ones.
3. Absorption: It is the process of taking nutrients from the digestive system into the blood so that they can be used in the body.
4. Assimilation: It is the process of using absorbed nutrients for growth, repair, immunity etc.
5. Egestion: It is the process of removal of unwanted undigested food from the digestive tract.

The process of digestion occurs through both mechanical and biochemical processes. The human digestive tract consists of an alimentary canal and a few associated glands. The various parts of the digestive system are as follows:

• Teeth: It helps in the mastication of food into a fine paste called a bolus. It is present in the buccal cavity whose opening is the mouth. Teeth are divided into 3 parts. The Enamel is the outermost and the hardest layer of teeth. Dentine lies inner to the enamel. The pulp cavity is the innermost and contains blood vessels and nerves. There are four types of teeth Incisors, Canine, Premolar, and Molar. An adult human has 32 teeth with a dental formula of 2123 (for 1/4th jaw). Human dentition is Diphyodont (teeth which come twice in life), Heterodont (humans have different types of teeth), Bunodont (the ends of teeth are blunt cusps), and Thecodont (teeth are well embedded in sockets).
• Tongue: It is a muscular, glandular structure present at the base of the mouth. The frenulum attaches the tongue to the floor of the mouth. It has various taste buds like foliate, fungiform, circumvallate etc which helps in gustation. The tongue has well-differentiated regions to identify various tastes like Sweet at the tips, Sour on the sides, Salty on the sides and tips, and Bitter on the base. Sulcus terminalis divides the tongue into the anterior oral part and posterior pharyngeal part.
• Pharynx: It is the common passage for food and air. The various parts of the pharynx are oropharynx, laryngopharynx, nasopharynx. The lymphoid tissue of pharynx is called tonsils which includes Nasopharyngeal tonsils, Palatine tonsils, Lingual tonsils and Tubal tonsils.
• Oesophagus: It is the pipe through which food passes into the stomach. The food pipe lies anteriorly and the windpipe lies posteriorly. The opening of the windpipe is called the glottis which is covered by a cartilaginous flap called epiglottis which prevents the entry of food into it. Gastro-oesophageal or cardiac sphincter regulates the opening of oesophagus into the stomach and prevents the regurgitation of food. Food passes down the oesophagus through the peristaltic movements of its muscular walls. There are no glands in the food pipe. 
• Stomach: It is a J-shaped organ located in the upper left portion of the abdominal cavity. The four major parts of the stomach are the Cardiac, Fundic, Body and Pyloric regions. Its major function is protein digestion. The stomach has two sphincters the cardiac sphincter at the oesophagus-stomach opening and the pyloric sphincter at the stomach-intestine opening. The wall of stomach possesses three major glands which have three major cells. Mucus/Neck cells secrete mucus. Peptic/Chief/Zymogenic cells secrete proenzyme pepsinogen. Parietal/Oxyntic cells secrete HCL and Castle’s intrinsic factor essential for the absorption of extrinsic factor i.e. Vitamin B12. HCL converts proenzyme pepsinogen into active enzyme pepsin which helps in protein digestion. HCL makes the environment acidic which is necessary for protein digestion. It also kills the harmful bacteria which have entered the stomach along with food. 
• Small intestine: It is a major site for digestion and absorption. It is divided into three parts Duodenum, Jejunum, and Ileum. The intestinal wall has longitudinal and circular muscles. They have epithelial cells with microvilli for increasing absorptive area. The intestinal glands/ Crypts of Lieberkühn secrete mucus, Paneth cells secrete digestive enzymes, and Argentaffin cells secrete some hormones. Brunner’s glands present in the submucosa of the duodenum also secrete mucus. The lymphoid tissue of SI is called Peyer’s patches.
• Large intestine: It has three parts Caecum, Colon and Rectum. Caecum is connected to ileum of SI by an ileocecal valve which prevents the backflow of digested food. Caecum has a lot of symbiotic microorganisms. Vermiform appendix is a vestigial organ that arises from the caecum. The colon has 4 parts ascending, transverse, descending and sigmoid colon. The rectum is the main site for the storage of faecal matter. It opens out through the anus.
• Salivary glands: There are 3 pairs of it Parotid, Sub-maxillary/Sub-mandibular, and Sub-lingual glands. They secrete saliva which performs various functions. Saliva contains ptyalin or salivary amylase and lysozyme. Salivary amylase converts starch into maltose. Lysozyme has antibacterial properties. 
• Liver: It is the largest gland in our body. Its structural and functional unit is the hepatic lobule which contains hepatic cells. These cells secrete bile which is stored and concentrated in the gall bladder. Bile aids in fat digestion. 
• Pancreas: It is a leaflike heterocrine gland. Its endocrine part secretes hormones like insulin, glucagon, and somatostatin. The exocrine portion secretes pancreatic juice which contains various enzymes like lipases, amylases etc necessary for food digestion.

Histology of human alimentary canal

The alimentary canal starting from the mouth and ending up to the rectum possesses four layers:

• Serosa: It is the outermost layer and is made of mesothelium and some connective tissues. Oesophagus lacks serosa.
• Muscularis layer: It consists of smooth muscles arranged in an inner circular and outer longitudinal layer. The stomach possesses an extra layer of oblique muscles inner to the circular muscles.
• Sub-mucosa: It is formed of loose connective tissues and also contains nerves, blood and lymph vessels. In the duodenum, Brunner’s glands are present in the sub-mucosa. 
• Mucosa: It is the innermost layer lining the lumen of the alimentary canal. It consists of irregular folds called rugae in the stomach and small finger-like foldings called villi in the small intestine.

Digestion of food

Food digestion in humans occurs by both mechanical and bio-chemical means.

Digestion in Mouth

Teeth and Tongue in the buccal cavity with the help of saliva masticate and mix up the food into a bolus. Saliva contains salivary amylase (ptyalin) and lysozyme. Ptyalin converts 30% of starch into maltose. Lysozyme kills the harmful bacteria present in ingested food. By swallowing or deglutition bolus passes down into the oesophagus. Bolus further passes down into the stomach by the muscular contraction of the oesophagus known as Peristalsis.

Digestion in Stomach

Mainly protein digestion takes place in stomach. The various gastric glands secrete HCL, proenzyme pepsinogen, mucus etc. HCL activates inactive proenzyme pepsinogen into active enzyme pepsin. It also helps in the absorption of iron. Pepsin converts proteins into proteoses and peptones. The required Ph is 1.8. A small amount of lipase is also secreted here which helps in fat digestion.

Digestion in Small Intestine

Bile, Pancreatic Juice and Intestinal Juice are the secretions released into the small intestine. Pancreatic juice contains inactive enzymes like trypsinogen, chymotrypsinogen, procarboxypeptidases, amylases, lipases and nucleases. Trypsinogen is activated by intestinal enterokinase into active Trypsin which further activates other pancreatic secretions. Trypsin, Chymotrypsin, Carboxypeptidase converts proteins, peptones, and proteoses into dipeptides. Chymotrypsin hydrolyses casein into paracaseinate. Pancreatic amylase/Amylopsin converts polysaccharides into disaccharides. Pancreatic lipase or Steapsin breaks down fats into simpler fatty acids. Nucleases convert nucleic acids into nucleosides. Bile juice helps in the emulsification of fats. The enzymes of intestinal juice also known as Succus entericus convert the above end products into simple absorbable forms. Dipeptides are converted into amino acids by dipeptidases, Maltose is converted into glucose by maltase, Lactose converts lactose into glucose and galactose, and Sucrase breaks down sucrose into glucose and fructose. Nucleotidases act on nucleotides and convert them into nucleosides. Nucleosidases further break them down into simpler constituents i.e. sugar and nitrogen bases. Lipases convert Di/monoglycerides into fatty acids and glycerol.

Absorption

Absorption is the process of taking useful substances from the end products of digestion into the blood for energy, growth and repair. It is done by various processes like passive, active or facilitated transport mechanisms.

• Some amounts of monosaccharides like glucose, and amino acids and some electrolytes like chloride ions are absorbed by simple diffusion which depends upon the concentration gradient i.e. substances move from their region of high concentration to their region of low concentration.
• Some amount of glucose and a few amino acids are transported with help of carrier proteins and this process is called facilitated transport.
• Various nutrients like amino acids, monosaccharides like glucose, and electrolytes like sodium ions are absorbed by active transport which requires energy as these substances are moved from their region of low concentration to their region of high concentration i.e. against the concentration gradient.
• Fatty acids and glycerol are converted into small droplets having a protein coating and are called micelles. These are absorbed into the lymph (lacteals) and are ultimately released into the bloodstream.

Absorption of digested products takes place in various parts of the alimentary canal but the major site of absorption is the small intestine. A summary of absorption is given in the table below:

Site of absorption	Mouth	Stomach	Small Intestine	Large Intestine
Substance absorbed	certain drugs	water, simple sugars, alcohol	glucose, fructose, fatty acids,  glycerol and amino acids	water, some minerals and drugs

Disorders

1. Jaundice: Here, the amount of bilirubin increases in the blood causing yellowing in the skin and eyes. In this disorder, the liver gets affected.
2. Vomiting: It is the ejection of stomach contents from the mouth. A feeling of nausea precedes vomiting. A vomit centre is located in the medulla of the brain.
3. Diarrhoea: It is the abnormal frequency of bowel movement and increased liquidity of the faecal discharge. Food absorption gets reduced.
4. Constipation: Here, the faeces are retained within the colon and the bowel movements occur irregularly.
5. Indigestion: Food is not properly digested which results in a feeling of fullness. The reasons for it include inadequate enzyme secretion, anxiety, food poisoning, overeating and spicy food.

FAQs on Digestive System

Question 1: What are the three types of salivary glands and their ducts?

Answer:

• Parotid gland is the largest salivary gland located near the cheek. It opens through Stenson’s duct.
• Submaxillary/Submandibular gland found in the lower jaw has Wharton’s duct.
• Sublingual gland is the smallest salivary gland found below the tongue and opens through the Rivinus duct.

Question 2: Explain the duct system of the liver, gall bladder and pancreas.

Answer:

The duct of the liver is called the hepatic duct. The liver has two lobes with two ducts. The right and left hepatic ducts merge to form a common hepatic duct. The cystic duct of the gall bladder joins it to form the common bile duct. Its opening is guarded by the sphincter of Boyden. The pancreas possesses two ducts. A functional duct called the duct of Wirsung and an accessory duct known as the duct of Santorini. The pancreatic and common bile duct merge to form the hepatopancreatic duct whose opening is called Sphincter of Oddi.

Question 3: Throw some light on the histology of the alimentary canal.

Answer:

The wall of the alimentary canal possesses four layers:

• Serosa: It is an outermost mesothelial layer with some connective tissue.
• Muscularis layer: Have smooth muscles arranged into inner circular and outer longitudinal layers. An innermost oblique muscle layer is present in the stomach.
• Submucosa: It contains loose connective tissues, nerves, blood vessels, and lymph vessels.
• Mucosa: It is the innermost layer. It possesses rugae in the stomach, and villi in SI for increasing surface area. The epithelial cells of villi form microvilli. Villi possess lacteal for fat absorption. Goblet cells secrete mucus. Crypts of Lieberkühn secrete enzymes.

Question 4: What is the endocrine portion of the Pancreas known as?

Answer:

Islets of Langerhans

Question 5: Why is Small intestine (6m) called small in spite of being longer than large intestine (1.5m)?

Answer:

Small intestine (6m) is called small in spite of being longer than the large intestine (1.5m) because its width is less than that of the large intestine.