Digestion involves the breaking of complex organic food molecules into simpler one by hydrolysis. Carbohydrates, proteins, fats and nucleic acids are large complex organic food molecules. They are insoluble and polymeric in nature. During digestion different enzymes helps in the breakdown of these complex polymers into soluble monomers which are required for energy generation.
The different steps of digestion involve:
- Movement of food through the alimentary tract
- Secretion of digestive juices and digestion of food
- Absorption of water, various electrolytes, vitamins and digestive end products.
Human digestive system mainly consists of two parts: 1) Alimentary tract and 2) secretory glands
It provides continuous supply of nutrients, vitamins, electrolytes and after. The following steps involved to achieve this.
- Movement of food through the track
- Secretion of digestive juices
- Digestion of food components
- Absorption of digestive product and water
- Excretion of unabsorbed food.
Human mouth consists of vestibule and oral cavity. The slit-like space between cheeks and gums is known as vestibule. The cavity surrounded by palate, tongue and teeth is known as oral cavity or buccal cavity. Mouth is the first passage of food where large piece of food is fragmented into small pieces with the help of teeth and mixed with saliva. Tongue manipulates food during chewing and mixing with saliva. This mixture of food with saliva, bolus, is then moved inward through the pharynx into Esophagus. This process is known as deglutition or swallowing.
It is also known as food pipe or gullet, is about 25 cm long. It presents behind the trachea and heart. Its primary function is to conduct food from pharynx to the stomach. Food in oesophagus is pushed downward by involuntary muscle contraction of circular muscle, this movement is called peristalsis. Due to contraction of the longitudinal muscles lower part of oesophagus become short which pushes its wall outward so that it can receive the bolus. The circular muscles of oesophagus then relax. The contractions are repeated in a wave that moves down to the stomach as shown in Figure. The cardiac sphincter lies between oesophagus and stomach allows the conduction of bolus into the stomach. The sphincter closes again. Medulla oblongata controls the peristalsis.
It is the widest organ of the alimentary canal. The figure shows the anatomy of the stomach and Table give the detail of enzymes. It is divided into two major parts 1) the body and 2) the antrum. Physiologically we can divide it into 1) the orad portion (first two third of the body) 2) the caudad portion (remainder portion of body and antrum). As food enters in the orad portion of the stomach, it forms concentric circles. When food stretches the wall of the stomach, vagovagal reflex travels from stomach to brain and back to stomach. Due to which the tone of muscular wall of stomach body reduces and the wall starts bulging out so that it accommodates greater quantity of food. In the completely relaxed stomach, 0.8 to 1.5 litres food can occupy. After mixing of food with the stomach secretions, the resulting mixture is called chyme, further passes down the gut. Partial digestion of food (protein and fats) takes place here. It produces castle’s intrinsic factor which is required for the absorption of vitamin B12 to be absorbed through intestinal wall.
It comprises three parts viz. duodenum, jejunum and ileum. Due to its small diameter, it is named so. The length of small intestine is correlated with the height of person but not with the weight. It is about 6.25 meters long. Thus, it is the longest part of the alimentary tract. Chyme is conducted through small intestine by peristaltic movement. When chime stretches the intestinal wall, it elicits localized concentric contractions spaced at intervals cause segmentation of small intestine. In such a way chyme moves toward the anus at a speed of 0.5 to 2.0 cm/s. Movement in proximal parts is faster than terminal portion. During movement of chyme through small intestine complete digestion of proteins, carbohydrates, fats and nucleic acids occur.
Its diameter is always larger than small intestine but it varies from one region to another. It is about 1.5 m long. It is divided into three parts: cecum, colon and rectum. Cecum is a pouch type structure. The outgrowth of cecum is a vestigial body known as vermiform appendix. The cecum is more developed in herbivorous mammals than carnivorous. The junction of ileum with cecum is guarded by the ileocecal valve. The function of this valve is to prevent backflow of fecal contents from colon to ileum. The valve can resist back pressure of at least 50 to 60 cm of water.
The main functions of the colon are
- Absorption of electrolytes and water from chyme.
- Temporary storage of fecal matter.
The proximal region of colon concerned mainly with absorption and distal region with storage. The colon has three longitudinal bands called teniae coli and small pouches called haustra. Thus the mixing movement in the colon is called as haustration. The movement in the colon is sluggish.
The lower portion of descending colon is sigmoidal in shape and opens into rectum. It is 20 cm long and terminates in the 2 cm long anal tract. When a mass movement propels faeces into rectum, the desire for defecation occurs.
The opening of anal tract is called anus. The anus has two sphincters. Internal anal sphincter composed of smooth muscle fibre and external anal sphincter composed of striped muscle fibre (voluntary in nature). The moderate quantities of vitamin B complex and vitamin K also found by bacteria in large intestine.
|Digestion starts||Digestion complete||Final products|
|Fat||Duodenum||Duodenum||Fatty acids & glycerol|
|Nucleic acid||Duodenum||Jejunum||Nitrogenous bases + pentose sugar + inorganic phosphate|
The primary function of secretary glands is the secretion of digestive enzymes for digestion of food and mucus for lubrication and protection of tract. Most digestive secretion occurs in precise amount only in response to the presence of food in alimentary tract. We have discussed major digestive glands.
The major gland of salivation is parotid, submandibular and sublingual glands. Along with this, there are many small buccal glands. A Healthy individual secretes about 0.8 to 1.5 litres of saliva daily. Saliva mainly composed of two major type of proteins. A) Ptyalin (an α-amylase) – for digestion of starch, B) Mucin – for protection of surface. Parotid glands are larges salivary glands situated near ears. The parotid glands secrete mainly ptyalin, whereas submandibular and sublingual glands secrete both ptyalin and mucin. The small buccal glands secrete only mucus. The pH of saliva is between 6 to 7 which favours the digestive action of ptyalin.
The oesophagal glands secrete only mucous which provide lubrication for swallowing.
The entire surface of stomach lining contains mucus-secreting cells. The stomach mucosa has two types of tubular glands: Oxyntic glands (gastric glands) and pyloric glands. The oxyntic glands secrete hydrochloric acid pepsinogen, intrinsic factor and mucus. The pyloric gland secretes mainly mucus for protection from stomach acid. They also secrete gastrin hormone.
It is larges gland of the body, mainly secrete bile normally between 0.6 to 1 litre/day. Bile serves two major functions.
- Fat digestion and absorption: Along with the enzymes for fat digestion bile acids in bile help to emulsify the large fat particles of food into many small particles, the surface of which attacked by lipase enzymes secreted in pancreatic juice. Bile acids aid in absorption of end product digested fat through the intestinal mucosal membrane.
- Excretion of waste products from blood: An important waste product bilirubin, an end product of haemoglobin digestion and excesses of cholesterol are excreted out with the help of bile. A pear-shaped structure attached to the posterior surface of the liver stores 30 to 60 ml bile secreted by the liver
The pancreas is soft lobulated large compound gland whose internal structure is similar to salivary gland. It lies parallel to and posterior to stomach. Pancreatic acini secrete digestive enzymes whereas a large amount of sodium bicarbonate solution is secreted by small ductules and larger ducts. The mixture of enzymes and sodium bicarbonates passes through a long pancreatic duct. Pancreatic duct joins with hepatic duct before it empties into duodenum through the papilla of vater.
Pancreatic secretion contains enzymes for digesting all three major food component: carbohydrate, protein and fats.
|Pancreatic lipase |
These are formed by modification of surface epithelium of small intestine. The two main intestinal glands are Brunner’s gland and Crypts of Lieberkühn.
- Brunner’s glands are found only in first few centimetres of duodenum. They secrete large amount of alkaline mucus to protect the duodenal wall from highly acidic gastric juice and to neutralize hydrochloric acid.
- Crypt of Lieberkühns are small pits located all over the entire surface of the small intestine, lies between the intestinal villi. They are covered by epithelium composed of two types of cells. 1) Goblet cells: secrete mucus. 2) Enterocytes: secrete water and electrolyte, also reabsorb the water and electrolyte along with the end product of digestion over the surface of adjacent villi. At the base of these crypts, paneth cells and argentaffin cells are present. Paneth cells found mainly in duodenum are rich in zinc and contain acidophilic granules. Argentaffin cells synthesize secretin hormone and 5-hydroxytryptamine.
|Enzyme||Substrate||Site of action|
|Ptyalin (salivary amylase)||Starch||Mouth|
Little amount of fats
|Pancreatic amylase |
Nucleic acids (DNA, RNA)
It is the process by which simpler nutrients (monosaccharide, amino acids, fatty acids etc.) pass from alimentary tract into blood and lymph. It can occur by simple diffusion, facilitated diffusion, osmosis and active transport.
Absorption starts from stomach but it is poor absorptive area because here junction between epithelial cells are tight junction and villi are absent on its inner wall. Little amount of water, salts, alcohol, few drugs and moderate amounts of sugar are absorbed through stomach. Absorption of nutrients mainly occurs in small intestine. Vitamins produced due to bacterial digestion and water absorbed in large intestine.
Intestines absorb collective amount of ingested fluid and fluid secreted in gastrointestinal secretions. Throughout the inner wall of small intestine, many folds called valvulae conniventes (also known as folds of kerckring) present, which increases the surface area for absorption. The valvulae conniventes covered by small protrusions known as villi (singular ‘Villus’). Figure shows the longitudinal section of the villus.
Absorption of Monosaccharides: Absorption of glucose and galactose occur through active transport. Sodium pump on the cell membrane helps in its active transport. Fructose is absorbed by facilitated diffusion. Glucose, galactose and fructose are absorbed into the blood capillaries. Galactose is the most rapidly transported monosaccharide
Absorption of amino acids: Amino acids are absorbed by active transport coupled with active sodium transport. They also enter the bloodstream.
Absorption of fatty acids and glycerol: Fatty acids and glycerol are insoluble in water thus they can’t enter the bloodstream directly. In intestinal lumen, bile salts and phospholipids incorporate fatty acids and glycerol into small, spherical water-soluble droplets known as micelles. Fat soluble vitamins and sterols along with fatty acids and glycerol are absorbed by diffusion by the help of micelles into intestinal cells, where they are resynthesized in the endoplasmic reticulum and are converted into small droplets known as chylomicrons. Later most of them released into lymph present in lacteals (lymphatic capillaries).
Absorption of water: Osmosis helps in the absorption of water in small intestine through epithelial cells surface and villi into the blood capillaries. In order to maintain the osmolality, electrolytes and digested food absorb along with water.
Absorption of electrolytes: Sodium can move in and out of epithelial cells by diffusion process and in mucosal cells, it moves by active transport. Many other ions such as potassium, calcium, magnesium, iron and phosphate absorbed by active transport. Whereas chloride ions can be absorbed through diffusion or active transport. Vitamin D and parathyroid hormone enhance the absorption of calcium.
Absorption of vitamins: Most water-soluble vitamins (Vitamin B complex, Vitamin C, Vitamin P) absorbed by diffusion. Castle’s intrinsic factors play an important role in reabsorption of vitamin B12.
Assimilation and egestion:
Finally, all absorbed nutrients transported by blood and lymph further transferred to blood circulation. With the help of blood, nutrients reach to target body cells, where it become integral component of protoplasm and used for energy, growth and repair. This process is known as assimilation.
The excess of monosaccharide stores in liver and muscles in the form of glycogen by the process called glycogenesis. Excess of amino acids are converted into glucose and then to fat through an irreversible reaction and then stored. Most of the fats stored in subcutaneous layers and mesenteries.
Another important step is egestion, the process by which undigested food materials eliminated through anus in the form of faeces. Egestion occurs by peristalsis movement. After absorption of water in colon, chyme converted into semisolid faeces. When faeces enter into rectum, wall of rectum feels distension which induces desire of defecation due to a defecation reflex. Due to this reflex peristalsis initiated in the sigmoidal colon and reaches to anus through rectum. Involuntary action of internal anal sphincter and voluntary action of external anal sphincter thus helps in defecation.