Digestive system

 

Food provides energy to all living things, and organic stuff promotes growth. Food is an organic substance, typically derived from plants or animals. Carbs, proteins, and fats are the main ingredients in our diet. In trace amounts, it also includes vitamins and minerals. Digestion is the process through which our digestive system breaks down complex dietary ingredients into simple, absorbable forms. It is done mechanically and biologically. In addition, the digestive system removes waste from our bodies and absorbs water, vitamins, and minerals.

The digestive system performs six basic digestive processes :

  1. Ingestion - taking food into the mouth
  2. Secretion - Secretion of enzymes, acid and bile into the lumen of GI tract
  3. Motility - Mixing of foods through the GI tract
  4. Digestion - Mechanical and chemical breakdown of food
  5. Absorption - Assimilation of digested products from GI tract into the blood and lymph
  6. Defecation - The elimination of feces from the GI tract

The Gastrointestinal tract, often known as the digestive tract or GI tract, and the associated glands make up the human digestive system. From the mouth to the anus, the GI tract is a continuous tube. The GI tract includes the following organs :

1. Mouth


The mouth is formed by the cheeks, teeth, lips and tongue. For the purpose of swallowing, teeth chop, rip, and pulverize food to aid in the physical breakdown of solids. The tongue is made up of skeletal muscle covered with mucous membrane, and it forms the floor of the oral cavity. The tongue's movements play a crucial role in moving food through the mouth during chewing and swallowing. The tongue contains the majority of the taste buds.

2. Pharynx


Food travels from the mouth into the pharynx, often known as the throat, a tube that resembles a funnel and runs posteriorly from the internal nares to the esophagus. It serves as a shared pathway for the respiratory and digestive systems. It is lined with mucous membrane and consists of skeletal muscle.

3. Esophagus


The stomach and throat are connected via the esophagus. It is a muscular tube that is around 25 centimeters long. There is no function of the esophagus in the chemical breakdown of food. Conducting the food bolus from the mouth to the stomach is its exclusive function. The bolus is forced from the esophagus's beginning to its finish by the peristaltic wave.

4. Stomach


Situated between the small intestine and the esophagus, the stomach resembles a sac-like structure. Its duties include controlling the pace at which the contents of the stomach are discharged into the small intestine as well as storing, dissolving, and partially digesting food particles. The meal is mixed with gastric juice by the churning of the stomach's muscular walls. Based on differences in structure and function, the stomach is separated into five sections: the cardia, fundus, corpus, antrum, and pylorus.

5. Small intestine

The majority of digestion and absorption occurs in the small intestine. After the luminal contents leave the small intestine, no more digestion is completed. The small intestine is around 6 meters long on average. Sections of the small intestine comprise the jejunum, ileum, and duodenum. The duodenum is the first segment of the small intestine. This duct connects the pancreatic and liver ducts, forming the pancreatic juice and liver. The ileum is the last and longest section of the small intestine. The opening from the ileum into the large intestine is guarded by sphincter muscle valve called Ileocecal Sphincter.

The small intestine wall has structures called villi that resemble fingers and protrude into the intestinal lumen. Every villus is made up of several tiny projections known as Microvilli on its apical surfaces. The surface area of the small intestine is increased by the combination of villi and microvilli.

6. Large intestine

Although the large intestine is shorter in length than the small intestine, it has a wider diameter. Most of the cells in its mucosal epithelium are goblet and absorbent cells. It absorbs water, salts, and certain vitamins but does not create any digestive enzymes. Additionally, it holds indigestible material until the anus gets rid of it.

The colon, rectum, and cecum are the three sections of the large intestine. The confluence of the small intestine and the large intestine, which is the blind end, is where the cecum is located. It possesses a little protrusion known as the vermiform appendix. The human appendix is involved in the defense against infections. The big intestine is mostly composed of the colon. The sigmoid colon is formed when the descending colon's terminal section becomes a S shape. The anus is the final segment of the large intestine that connects to the exterior of the anal canal.

The large intestine's primary functions include storing and excreting waste products as well as absorbing water and salt. The colon contains a wide variety of bacteria. They can break down modest amounts of other materials, including cellulose. In addition, these bacteria produce riboflavin, thiamine, vitamin B12, and vitamin K.

Accessory digestive organs

Salivary glands: It is a gland that secretes saliva into the oral cavity. The function of this gland is to produce saliva which softens, moistens, and dissolves foods; cleanses mouth and teeth; and initiates the digestion of starch. Saliva begins digestion of carbohydrate in the mouth through action of salivary amylase.

Liver: The liver is the largest gland of the body weighing about 1.2 to 1.5 kg in adult human. It is situated in the abdominal cavity, just below the diaphragm. It is reddish-brown in colour and comprises of four anatomical lobes. The structural and functional unit of liver is hepatic lobules. Each lobule is covered by a thin connective tissue sheath called the Glisson's capsule. Each hepatic lobule is made up of many hepatic cells (hepatocytes). It performs several functions, which includes -

  • Detoxifies blood
  • Stores iron and fat-soluble vitamins A,D,E,K, & B12
  • Makes plasma proteins
  • Stores glucose as glycogen and converts glycogen into glucose to maintain blood glucose concentration
  • Produces urea after breaking amino acids
  • Produces bile, concentrated in a thin muscular sac called gallbladder.
  • Helps regulate blood cholesterol levels
  • Play phagocytic functions with the help of Kupffer cells.

Gallbladder: The gall bladder is a pear-shaped, muscular sac attached to the surface of the liver. It stores and concentrates bile and releases it into the duodenum. Thus, bile is directly transported from the liver to the gallbladder. Bile is subsequently stored and concentrated in the gallbladder between meals. After a meal, bile enters the duodenum as a result of the combined effects of relaxation of the sphincter of Oddi, gallbladder contraction and increased bile secretion by the liver.

Pancreas: The pancreas is an elongated gland that lies behind and below the stomach, above the first loop of the duodenum. It has both an endocrine and exocrine function. As an endocrine gland, it secretes two major hormones - insulin and glucagon. The exocrine glands secrets a pancreatic juice. It has two components - pancreatic enzymes secreted by by the acinar cells and an aqueous alkaline solution secreted by the duct cells that line the pancreatic ducts. The acinar cells secrete four types of pancreatic enzymes -

  1. Pancreatic proteolytic enzymes for protein digestion
  2. Pancreatic amylase for carbohydrate digestion
  3. Pancreatic lipase for lipid digestion
  4. Pancreatic nuclease for nucleic digestion

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