Our digestive system is one of the primary body systems which regulates energy production from the food we ingest. Proper functioning of the digestive system organs and processes is vital to sustaining life and for our survival. Ayurveda states that the strength of the digestive system or our Agni is the key to our wellbeing and immune function.

Our digestive system is responsible for the digestion of food, absorption of the released nutrients into bloodstream and lymph, and excretion of excess undigested food. It is also known as the alimentary canal, aliment means to nourish.

What does the Digestive System consist of?

Digestive system consists of the gut and accessory organs.

The gut: It is one way tube about 7.62 meters (25 feet) in length. It starts from mouth, passes through pharynx, esophagus, stomach, small intestine, large intestine and ends with the anus.

Accessory organs: Accessory organs are very important for proper digestion and assimilation of the food as they secrete the necessary juices and enzymes that enable the digestive system to work properly. These accessory organs are salivary glands, liver and gall bladder, and pancreas.

Details and Functioning of the Digestive System Organs

Mouth

Digestion of food begins in mouth where chewing occurs by teeth. The digestion of carbohydrate begins through salivary α-amylase. Also, the digestion of lipids begins through lingual lipase. Mouth is important in lubricating and moistening of food. It is also important for passage of food to the pharynx.

Salivary Glands

These exocrine glands secrete saliva, which consists of 95.5% water. The remaining 4.5% consists of:

• Ions as sodium and phosphate have a buffer effect, to maintain saliva pH between 6.35 and 6.85.
• Enzymes as amylase which digests carbohydrates.
• Immunoglobulin A which prevents microbes to enter the epithelial cells of the mouth.
• Lysozymes which make the saliva anti-microbial.

Salivary glands are located in the mucous membrane of the mouth, so they can secrete saliva directly to the buccal (mouth) cavity. Also, three pairs of salivary glands are located outside the mouth, so they secrete saliva into the mouth through ducts. These three pairs are:

• Submandibular glands: on the floor of the mouth. Submandibular glands secrete amylase in a thick liquid.
• Sublingual glands: below the tongue. Sublingual glands secrete the thickest liquid with the least amount of amylase.
• Parotid glands: lie between the skin and the masseter muscle, near the ears. Parotid glands secrete watery saliva which contains amylase.

Teeth

Our 32 permanent teeth are important for chewing food and mixing it saliva.

• Eight Incisors: four top and four bottoms, sharp front teeth to bite food.
• Four Cuspids: flank the incisors, having sharp pointed edges to tear-up food.
• Eight Premolars: posterior to cuspids, with two rounded cusps to mash food.
• Twelve Molars: the most posterior, with several pointed cusps to crush food to be easily swallowed.

Tongue

It is very important organ. It facilitates: ingestion, mechanical digestion, chemical digestion (lingual lipase), sensation (of taste, texture, and temperature of food), swallowing, and vocalization.

Pharynx

It is a short tube covered with a mucous membrane. It lubricates the food coming from the mouth and passes it to the esophagus.

Esophagus

Secretions from the mucosa of esophagus lubricate food coming from the pharynx. Then, food passes to the stomach through lower esophageal sphincter (also known as cardiac sphincter).

Stomach

It mixes food with its gastric acid and juices to form chyme then it passes this chyme to the small intestine. The digestion of proteins takes place through the active form of pepsinogen enzyme, which is pepsin. Also, the stomach can absorb some fat-soluble compounds as aspirin. Also, the stomach secretes intrinsic factors important for the absorption of Vitamin B12 in the small intestine.

Important secretions of stomach:

• Hydrochloric Acid (HCl): to maintain gastric pH between 1.5 and 3.5.
• Pepsinogen Enzyme: This is activated into pepsin through HCl.
• Mucous: to protect the stomach from HCl which can irritate the stomach.
• Gastrin Hormone: to regulate gastric emptying into the small intestine.

Small Intestine

Then, the chyme is passed to the small intestine which has alkaline pH between 7.4 and 7.8. The small intestine consists of three regions: duodenum, jejunum, and ileum.

• Chyme passes from the stomach into the duodenum through the pyloric sphincter.
• Bile and pancreatic ducts are connected to the duodenum, where bile and the pancreatic juice are released respectively.
• In the duodenum, chyme is mixed with juices at a slower rate for maximum digestion.

Digested food passes through jejunum to ileum where absorption occurs. It absorbs sugars, amino acids, and fatty acids along with mineral and vitamins into the bloodstream.

Large intestine

It is twice a small intestine in diameter, about 3 inches. It absorbs water, electrolytes, and minerals and concentrates them to be excreted in feces.

The large intestine consists of four parts: caecum, colon, rectum, and anus.

• Caecum: is the first part of the large intestine where undigested food passes from the ileum to it. A winding tube called the appendix is attached to the caecum. Although the appendix consists of lymphoid tissue suggesting to have immunological activity, it is vestigial.
• Colon: It is sigmoid in shape and consists of ascending colon, transverse colon and descending colon.
• Rectum: It contains internal transverse folds called rectal valves that separate gas from feces to prevent their simultaneous passage.
• Anus: It consists of internal and external anal sphincters to control the excretion of feces.

Liver and Gall Bladder

It is the largest organ in human body. It is about three pounds. It secretes bile which is stored in the gall bladder then this bile is released in small intestine for digestion and emulsification of fats.

Bile is the yellow-brown or yellow-green solution with pH 7.6-8.6 secreted from hepatocytes. Bile consists of water, bile salts, bile pigments, electrolytes, phospholipids, cholesterol and triglycerides. Bile salts and phospholipids are most important for emulsification.

Bile is then stored, concentrated, and propelled by gall bladder which locates in the posterior aspect of the right lobe of liver.

Pancreas

The pancreas has both exocrine and endocrine functions. As an exocrine gland, it releases pancreatic juice from acinar cells for digestion of proteins, fats, and carbohydrates in the duodenum. Pancreatic juice consists of water, bicarbonate (which maintains pH between 7.1 and 8.2), enzymes and salts.

The endocrine function of pancreas represents in the secretion of insulin and glucagon hormones from islets of Langerhans for the regulation of blood glucose levels.

Human Digestive System – The process of Digestion

Digestion of food involves six steps: ingestion, propulsion, physical digestion, chemical digestion, absorption, and defecation.

Ingestion

Food enters the mouth and mixed with saliva which contains amylase enzyme for digestion of carbohydrates and lingual lipase enzymes which digest lipids. Chewing of food increases surface area and allows appropriate sized bolus to be produced.

Propulsion

It consists of swallowing and peristalsis. Swallowing of food is a voluntary movement which occurs when the tongue and pharynx propel food to the esophagus. Peristalsis is involuntary contractions and relaxations of all smooth muscles of the alimentary canal to propel food along.

Mechanical Digestion

Mechanical or Physical digestion is aimed to cut food into smaller pieces to be easily digested and absorbed. It starts in mouth through mastication, or chewing of food, and mixing it with saliva to be easily cut into smaller pieces.

In the stomach, the gastric movements allow food pieces to be mixed with juice and cut into much smaller pieces forming soup liquid called chyme. In the small intestine, segmentation occurs where smooth muscles of the small intestine contract to bring food forth and back to mix it with juice and make it easy to be absorbed.

Chemical Digestion

The chemical digestion process of the food involves multiple enzymes, acid, and juices to break down food into simpler units such that it can be utilized by our body cells to produce energy.

Carbohydrates such as sugars, fibers, and starch are broken down by the salivary amylase into disaccharides inside the stomach. Then the process is completed in the small intestine through pancreatic amylase to produce monosaccharide sugars.

Proteins breakdown occurs in stomach through pepsin enzyme. Then, it is completed in small intestine through pancreatic enzymes: chymotrypsin and trypsin to give amino acids.

Fats are digested through lingual, gastric and pancreatic lipase. Pancreatic lipase breakdowns emulsified fats in small intestine into fatty acids and monoglycereids.

Absorption

It is a very important step as, without it, there is no value of the food we eat. Almost all food, 80% of electrolytes and 90% of water are absorbed in the small intestine. Food enters the bloodstream through the epithelial cells of the lumen of the small intestine. 

Amino acids and sugars are mainly absorbed from the jejunum. Bile salts and Vitamin B12 are absorbed from the ileum. Lipids are absorbed by lacteals and transported into lymphatic vessels to enter the bloodstream.

Defecation

It is the process of removing excess undigested food, like fibers and cellulose, and some water outside the body through the anus.

References

Open Text BC Campus – The Digestive System