Small Intestine | Physiology

                              Small Intestine

Keywords:  Small intestine, Duodenum, Jejunum, Ileum, Structure Brunner’s gland, Crypt of Lieberkuhn, plica circularis, Paneth cells, Enterokinase.

Table of contents:

1.	Introduction
2.	Structure
3.	Secretion
4.	Mechanism of secretion
5.	Regulation of secretion
6.	Functions

 The small intestine is a hollow muscular tubal organ of the gastrointestinal tract which extends from the stomach above and below the large intestine. Its length is about 25 ft(6.5-7.5meter) and folds many times to accommodate the abdomen. Its diameter is 3cm. When the diameter becomes more than 3cm, it is abnormal. The small intestine develops from the midgut of the primitive gut tube.

The small intestine is divided into 

1.Duodenum  2. Jejunum and 3.Ileum.

Structure of small intestine: The small intestine structure is like other parts of G.I.T. with some modifications. It has the usual four layers –

1. Serous layer is continuous with the mesentery part of the peritoneum in the Jejunum and ileum, so it is a mobile structure. Duodenum is a relatively fixed part of the small intestine.

2. Muscular layer,,, which consists of outer longitudinal and inner circular.

3. Submucosal layer with blood vessels and lymphatics as usual.

4. Mucosal layer.

Duodenum is the shortest part of the small intestine, only 20-25 cm long and C-shaped. In the space of C head of the pancreas is present. This is recognized by Brunner’s gland, and small finger-like protrusions called villi begin to appear in this part. Brunner’s glands of the duodenum resemble gastric pyloric glands. They are long, tortuous, and penetrate the muscularis mucosa. They secrete mucin and bicarbonate. Their ducts open in the Crypt of Lieberkuhn. In the duodenum,, acidic chyme from the stomach, pancreatic juice rich in digestive enzymes, and bicarbonate and bile from the liver enters slowly. Acidic chyme is neutralized by these bicarbonates,and an alkaline environment is maintained in the small intestine. Mucin of the duodenum protects the mucosa of the small intestine.

The jejunum is 2.5 meters, and the midsection of the small intestine and connected with the duodenum on one side and on another end with the ileum. The jejunum is considered to start at the suspensory muscle of the duodenum, also called the duodenojejunal flexure. The division between the Jejunum and ileum is not anatomically clear. The Jejunum contains very few Brunner’s glands (found in the duodenum  ) or Peyer’s patches (found in the ileum). The jejunum is of a larger diameter than the ileum. The Jejunum has less fat inside its mesentery than the ileum. The number and size of villi increase, and the presence of plica circularis greatly increases the surface area of the Jejunum and provides a large surface area for absorption. The jejunum is responsible for the absorption of sugars, fatty acids, and amino acids through the lining of enterocytes in the blood.

Plicae circulares: Many permanent circular folds of the mucous membrane are present in the small intestine, especially in the lower part of the duodenum, and the jejunum is known as plicae circulars. They are large valvular flaps projecting into the lumen of the small intestine. They are covered with villi (singular, villus) which are covered with microvilli. Plicae circulares, also known as valves of kerckring, valves of kerchkring, valvulae conniventes, or small bowel folds. Plicae circulars start from the second part of the duodenum. They are large and thick in the jejunum and decrease in size distally in the ileum to disappear entirely in the distal ileum. Plicae circulars mean ‘circular folds” Latin word plica meaning fold and circulares Latin for circular. Theodorus Kerckring, a german-born anatomist, described this. Valvulae conniventes Means “converging small valves’. Plicae circulars, some complete form circles, and some have spiral directions. The larger and smaller folds alternate with each other. They are permanent and not obliterated when the intestine is distended.

Between villi intestinal glands are present. They are also known as the “Crypt of Lieberkuhn’. They are simple tubular glands, do not cross muscularis mucosa, and are lined by low columnar epithelium. These columnar epithelia are actively mitotic and replace the lining of the small intestine, including villi. Cell sloughing in the intestine lumen provides 30 gm protein/day. The Crypt of Lieberkuhn is also the site of cAMP-mediated secretion of water and electrolytes.

Cells present in the Crypt of Lieberkuhn are:

1. Goblet cells that secrete mucus which protects the intestine's mucosa, lubricate the food.

2. Paneth cells are large endocrine acidophilic cells that secrete ‘defensins and ‘guanylin.’ Guanylin’ is a polypeptide that binds to guanylyl cyclase and regulates the secretion of Cl^- into the intestine lumen.

3. Argentaffin or enterochromaffin cells secrete secretin and 5- hydroxytryptamine (5 HT),a powerful stimulant of intestinal motility.

Epithelial and Paneth cells produce a great variety of enzymes, enterokinase.

Enzymes digest carbohydrates, fats, proteins, and nucleic acids.

Enterokinase activates trypsinogen into trypsin.

Intestinal juice or Succus Entericus:

Daily secretion 1-1.5 L, Isotonic and alkaline.

Water 98.5%

Solids:

Organic: mucus and enzymes.

Inorgnic  : cations  Na⁺,K+,,Ca++,Mg++

                    Anions  Cl-, HCO3-

Mucus secreted from Brunner’s gland forms a protective coat over the mucosa of the small intestine to protect it. Brunner’s glands also secrete bicarbonate which neutralizes gastric acid chyme and maintains alkaline Ph for optimal actions of enzymes.

Enzymes are present on the luminal border, also known as the brush border of the intestinal epithelial cells. These cells are shaded into the intestinal lumen. The enzymes are :

1. Enterokinase or enteropeptidase converts trypsinogen to trypsin. Trypsin, in turn, converts pancreatic proteolytic proenzymes to active enzymes.

2. Proteolytic enzymes

Erepsin is a mixture of several different peptidases, such as aminopeptidase, and dipeptidases that act on peptones and polypeptides to form amino acids.

Caseinogen and other proteins are digested slowly by erepsin into aminoacids.

Nuclease,nucleotidase,nucleosidase,etc. Hydrolyze the nucleic acids and release purine and pyrimidine bases.

2. Invertase or sucrose converts sucrose into one molecule of glucose and one molecule of fructose.

Maltase converts maltose into two molecules  of glucose,

Lactase converts lactose into one molecule of glucose and one molecule of galactose.

α limiting dextrinase converts α- altering dextrin into glucose.

4. Intestinal lipase to differentiate it from pancreatic lipase. Intestinal lipase is concerned with the hydrolysis of the primary ester linkages. At the first terminal, the fatty acid is removed to produce an αβ -diglyceride, then another terminal fatty acid is removed to create a β- monoglyceride. This β- monoglyceride is the primary end product of fat digestion.

5. Cholesterol esterase acts on cholesterol esters to form free cholesterol.

6. Lecithinase acts on phospholipids to convert them into simple phospholipids.

7. Alkaline phosphatase acts on organic phosphate to form free phosphate.

The mechanism of secretion is not clearly known but cations  Na^{+  and} K⁺ are secreted by active transport, anions move to maintain electrical neutrality, and water moves to maintain osmotic balance.

Control of secretion:

Intestinal juice secretion is regulated by local, mechanical, and chemical stimuli on Intestinal mucosa.

For example, VIP stimulates watery secretion, and vagal stimulation increases intestinal juice secretion.

Ingestion of food increases secretion slowly.

Mechanical stimulation distension of intestinal mucosa increases secretion of volume and enzymes.

Local irritation increases the secretion of mucus-rich juice.

The transport of nutrients across epithelial cells through Jejunum and ileum includes active transport of amino acids, small peptides, vitamins, and most glucose and passive transport of fructose.

Functions of the intestine:  Secretin produced in the small intestine causes an additional effect on the pancreas and promotes the release of bicarbonate into the duodenum.

Cellulose is not digested in human beings. The cellulose is made up of beta glucose,making inter-monosaccharide binding different from the ones present in starch which consists of alpha glucose. Human beings lack the enzymes for splitting the β- glucose bonds. The enzymes for splitting the beta glucose bonds are present in herbivores.

Digestion, mixing chyme, and local defense, protect from infection.

Absorption majority of nutrients are absorbed in the jejunum except

 1. Iron in the duodenum.

2. Bile salts and vit B₁₂ in terminal ileum.

3 .Folate  B₉ in the duodenum and jejunum.

4. Water is absorbed by osmosis in the whole small intestine.

5. Lipid by passive diffusion in the whole small intestine.

6. Fructose by facilitated diffusion.

7. Sodium bicarbonate by active transport.

8. Glucose and amino acids by co-transport.

The absorbed substances are transported via the blood vessels to different body organs. Finally, the unabsorbed substances pass into the large intestine.

Small intestine gut flora appears to contribute passively to the immune system.

Johann Nathanael Liberkuhn, a German physician and physiologist, discovered it, and the Crypts of Liberkuhn is named after him.

  Hashtag: Small intestine, Duodenum, Jejunum, Ileum, Structure Brunner’s gland, Crypt of Lieberkuhn, plica circularis, Paneth cells, Enterokinase.

 

 

                             

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