Small Intestine | Properties & Composition | Functions | Regulation | Method of Collection

Functional anatomy of Small Intestine .

Small Intestine .

 

[1]. Small intestine is the part of gastrointestinal (GI) tract, extending between the pyloric sphincter of stomach
and ileocecal valve, which opens into large intestine. 

[2]. It is called small intestine because of its small diameter, compared to that of the large intestine. But it is longer than large intestine. 

[3]. Its length is about 6 meter. 

[4]. Important function of small intestine is absorption. 

[5]. Maximum absorption of digested food products takes place in small intestine. 

Small intestine consists of three portions: 

1. Proximal part known as duodenum 

2. Middle part known as jejunum 

3. Distal part known as ileum. 

Wall of the small intestine has all the four layers as in stomach . 

Intestinal Villi & Glands of Small Intestine .

Intestinal gland and villus .

 

Intestinal Villi .

[1]. Mucous membrane of small intestine is covered by minute projections called villi. 

[2]. The height of villi is about 1 mm and the diameter is less than 1 mm. 

[3]. Villi are lined by columnar cells, which are called enterocytes. 

[4]. Each enterocyte gives rise to hair-like projections called microvilli. 

[5]. Villi and microvilli increase the surface area of mucous membrane by many folds. 

[6]. Within each villus, there is a central channel called lacteal, which opens into lymphatic vessels. It contains
blood vessels also. 

Crypts of Lieberkühn or intestinal glands .

[1]. Crypts of Lieberkühn or intestinal glands are simple tubular glands of intestine. 

[2]. Intestinal glands do not penetrate the muscularis mucosa of the intestinal wall, but open into the lumen of intestine between the villi. 

[3]. Intestinal glands are lined by columnar cells. Lining of each gland is continuous with epithelial lining of the villi . 

[4]. Epithelial cells lining the intestinal glands undergo division by mitosis at a faster rate. 

[5]. Newly formed cells push the older cells upward over the lining of villi. These cells which move to villi are called enterocytes. 

[6]. Enterocytes secrete the enzymes. Old enterocytes are continuously shed into lumen along with enzymes. 

Types of cells interposed between columnar cells of
intestinal glands: 

 1. Argentaffin cells or enterochromaffin cells, which secrete intrinsic factor of Castle 

 2. Goblet cells, which secrete mucus 

 3. Paneth cells, which secrete the cytokines called defensins.  

Brunner Glands .

[1]. In addition to intestinal glands, the first part of duodenum contains some mucus glands, which are called Brunner glands. 

[2]. These glands penetrate muscularis mucosa and extend up to the submucus coat of the intestinal wall. 

[3]. Brunner glands open into the lumen of intestine directly. 

[4]. Brunner gland secretes mucus and traces of enzymes. 

Properties & Composition of Succus Entericus .

Secretion from small intestine is called succus entericus. 

Properties of Succus Entericus .

Volume : 1800 mL/day 

Reaction : Alkaline 

pH :        8.3 

Composition of Succus Entericus .

Composition of succus entericus .

 

[1]. Succus entericus contains water (99.5%) and solids (0.5%). 

[2]. Solids include organic and inorganic substances . 

[3]. Bicarbonate concentration is slightly high in succus entericus. 

Functions of succus Entericus .

1. Digestive Function .

Digestive enzymes of succus entericus .

 

[1]. Enzymes of succus entericus act on the partially digested food and convert them into final digestive
products. 

[2]. Enzymes are produced and released into succus entericus by enterocytes of the villi.

Proteolytic Enzymes .

[1]. Proteolytic enzymes present in succus entericus are the peptidases . 

[2]. These peptidases convert peptides into amino acids. 

Amylolytic Enzymes .

[1]. Lactase, sucrase and maltase convert the disaccharides (lactose, sucrose and maltose) into two
molecules of monosaccharides . 

[2]. Dextrinase converts dextrin, maltose and maltriose into glucose. 

[3]. Trehalase or trehalose glucohydrolase causes hydrolysis of trehalose (carbohydrate present in
mushrooms and yeast) and converts it into glucose. 

Lipolytic Enzyme .

Intestinal lipase acts on triglycerides and converts them into fatty acids. 

2. Protective Function .

[1].  Mucus present in the succus entericus protects the intestinal wall from the acid chyme, which
enters the intestine from stomach; thereby it prevents the intestinal ulcer. 

[2]. Defensins secreted by paneth cells of intestinal glands are the antimicrobial peptides. 

[3]. These peptides are called natural peptide antibiotics because of their role in killing the phagocytosed
bacteria.  

3. Activator Function .

[1]. Enterokinase present in intestinal juice activates
trypsinogen into trypsin. 

[2]. Trypsin, in turn activates other
enzymes . 

4. Hemopoietic Function .

[1]. Intrinsic factor of Castle present in the intestine plays an important role in erythropoiesis . 

[2]. It is necessary for the absorption of vitamin B12. 

5. Hydrolytic Process .

Intestinal juice helps in all the enzymatic reactions of
digestion.

Function of Small Intestine .

1. Mechanical Function .

Mixing movements of small intestine help in the thorough mixing of chyme with the digestive juices like succus
entericus, pancreatic juice and bile. 

2. Secretory Function .

Small intestine secretes succus entericus, enterokinase and the GI hormones. 

3. Hormonal Function .

[1]. Small intestine secretes many GI hormones such as secretin, cholecystokinin, etc. 

[2]. These hormones regulate the movement of GI tract and secretory activities of small intestine and pancreas .  

4.  Digestive Function .

5.  Activator Function .

6. Hemopoietic Function .

7. Hydrolytic Function .

8. Absorptive Functions .

[1]. Presence of villi and microvilli in small intestinal mucosa increases the surface area of mucosa. This facilitates the absorptive function of intestine. 

[2]. Digested products of foodstuffs, proteins, carbohydrates, fats and other nutritive substances such as
vitamins, minerals and water are absorbed mostly in small intestine. 

[3]. From the lumen of intestine, these substances pass through lacteal of villi, cross the mucosa and enter
the blood directly or through lymphatics.  

Absorption of carbohydrates .

Carbohydrates are absorbed from the small intestine mainly as monosaccharides, viz. glucose, galactose and fructose. 

Absorption of Glucose .

[1]. Glucose is transported from the lumen of small intestine into the epithelial cells in the mucus membrane of small intestine, by means of sodium cotransport. 

[2]. Energy for this is obtained by the binding process of sodium ion and glucose molecule to carrier protein. 

[3]. From the epithelial cell, glucose is absorbed into the portal vein by facilitated diffusion. However, sodium ion moves laterally into the intercellular space. 

[4]. From here, it is transported into blood by active transport, utilizing the energy liberated by breakdown of ATP. 

Absorption of Galactose .

Galactose is also absorbed from the small intestine in the same mechanism as that of glucose.

Absorption of Fructose .

Fructose is absorbed into blood by means of facilitated diffusion. Some molecules of fructose are converted into glucose .

Absorption of Protein .

[1]. Proteins are absorbed in the form of amino acids from small intestine. 

[2]. The levo-amino acids are actively absorbed by means of sodium cotransport, whereas, the dextro-amino acids are absorbed by means of facilitated diffusion. 

[3]. Absorption of amino acids is faster in duodenum and jejunum and slower in ileum.

Absorption of Fats . 

[1]. Monoglycerides, cholesterol and fatty acids from the micelles enter the cells of intestinal mucosa by simple diffusion. 

[2].  In the mucosal cells, most of the monoglycerides are converted into triglycerides. 

[3]. Triglycerides are also formed by re-esterification of fatty acids with more than 10 to 12 carbon atoms. Some of the cholesterol is also esterified. 

[4]. Triglycerides and cholesterol esters are coated with a layer of protein, cholesterol and phospholipids to form the particles called chylomicrons. 

[5]. Chylomicrons cannot pass through the membrane of the blood capillaries because of the larger size. So, these lipid particles enter the lymph vessels and then are transferred into blood from lymph. 

[6].  Fatty acids containing less than 10 to 12 carbon atoms enter the portal blood from mucosal cells and are transported as free fatty acids or unesterified fatty acids. 

[7]. Most of the fats are absorbed in the upper part of small intestine. 

[8]. Presence of bile is essential for fat absorption.

Absorption of Water and Minerals .

[1]. In small intestine, sodium is absorbed actively. It is responsible for absorption of glucose, amino acids and other substances by means of sodium cotransport. 

[2]. Water moves in or out of the intestinal lumen until the osmotic pressure of intestinal contents
becomes equal to that of plasma. 

[3]. In ileum, chloride ion is actively absorbed in exchange for bicarbonate. The significance of
this exchange is not known. 

[4]. Calcium is actively absorbed mostly in upper part of small intestine. 

Absorption of Vitamins .

[1]. Most of the vitamins are absorbed in upper part of small intestine and vitamin B12 is absorbed in ileum. 

[2]. Absorption of water-soluble vitamins is faster than fat soluble vitamins.  

Regulation of Secretion of Succus Entericus .

Secretion of succus entericus is regulated by both nervous and hormonal mechanisms. 

Nervous Regulation .

[1]. Stimulation of parasympathetic nerves causes vasodilatation and increases the secretion of succus entericus. 

[2]. Stimulation of sympathetic nerves causes vasoconstriction and decreases the secretion of succus entericus. But, the role of these nerves in the regulation of intestinal secretion in physiological conditions is uncertain. 

[3]. However, the local nervous reflexes play an important role in increasing the secretion of intestinal juice. 

[4]. When chyme enters the small intestine, the mucosa is stimulated by tactile stimuli or irritation. 

[5]. It causes the development of local nervous reflexes, which stimulate the glands of intestine. 

Hormonal regulation .

When chyme enters the small intestine, intestinal mucosa secretes enterocrinin, secretin and cholecystokinin,
which promote the secretion of succus entericus by stimulating the intestinal glands.

Methods of Collection of Succus Entericus .

In Human .

[1]. In human beings, the intestinal juice is collected by using multilumen tube. 

[2]. The multilumen tube is inserted through nose or mouth, until the tip of this tube reaches the intestine. A line is marked on the tube. 

[3]. Entrance of tip of the tube into small intestine is indicated when this line comes near the mouth. This tube has three lumens. 

[4]. To the outer two lumens, small balloons are attached. 

[5]. When these balloons are inflated, the intestine is enlarged. 

[6]. Now, the intestinal juice is collected through the middle lumen, by means of aspiration. 

In Animals .

Thiry Loop .

[1]. A portion of intestine is separated from the gut by incising at both ends. The cut ends of the main gut are
connected and the continuity is re-established. 

[2]. One end of isolated segment is closed and the other end is brought out through abdominal wall. It is called Thiry loop or Thiry fistula. 

Thiry-Vella Loop .

[1]. Thiry-Vella loop is the modified Thiry loop. In this, a long segment of intestine is cut and separated from the
main gut. 

[2]. Both the ends of this segment are brought out through the abdominal wall. The cut ends of the main
gut are joined.  

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