- Small intestine extends from Pyloric sphincter to Ileocecal valve.
- Its total length is about 20 feet or 6 meters.
- The most important function of small intestine is the absorption of food molecules.
Small intestine is divided into 3 parts
- The proximal Duodenum
- Middle Jejunum
- Distal Ileum
The walls of small intestine has typically 4 layers as seen
elsewhere in the GI tract
- Mucosa
- Submucosa
- Muscular layer
- Serosa
Villi & Microvilli in Small Intestine
- The mucosa of small intestine has numerous villi which are infoldings of mucus membrane. The villi is lined by columnar cell called enterocytes
- These enterocytes also have hair like projections which are called microvilli
- Villi and microvilli increase the surface area of the small intestine which aids in absorption of food molecules
- The villi also contain lymphatics and blood vessels
Intestinal glands
- Glands in the small intestine are in the form of Crypts of Liberkuhn.
- They are lined by columnar cells and open into the lumen of small intestine
- They undergo rapid mitotic division because of which the older cells are shed off and newer cells are formed continuously
- The columnar cells secrete enzymes
Other cells of intestinal glands
- Argentaffin cells / enterochromaffin cells - secrete Intrinsic factor of Castle
- Goblet cells - secrete mucus
- Paneth cells - secrete the cytokines called defensins
Brunner glands
- They are present in the first part of the duodenum.
- They secrete mucus
"Secretions of Small Intestine is known as Succus entericus"
Properties & Composition of Succus entericus
- Volume : 1800 mL/day
- Reaction : Alkaline
- pH : 8.3
Functions of Succus Entericus
A. Digestive function
- Proteolytic Enzymes: peptidases convert peptides into amino acids.
- Amylolytic Enzymes: Lactase, sucrase and maltase convert disaccharides into monosaccharides, Dextrinase converts dextrin, maltose and maltriose into glucose
- Lipolytic Enzyme: Lipase acts on triglycerides and converts them into fatty acids
B. Protective function
- Mucus protects the intestinal wall from the acid chime - prevents intestinal ulcer
- Defensins are antimicrobial peptides (natural antibiotics)
C. Activator function
- Enterokinase activates trypsinogen into trypsin, trypsin in turn activates other enzymes
D. Hemopoietic function
- Intrinsic factor of Castle of intestine plays an important role in erythropoiesis
E. Hydrolytic process
- Helpful in all enzymatic reactions of digestion
Functions of Small Intestine
Apart from the functions of Succus entericus, small
intestine has following additive function
F. Mechanical function
- Mixing of chyme with the digestive juices
G. Secretory function
- Secretes succus entericus, enterokinase
H. Hormonal function
- Secretes GI hormones: secretin, cholecystokinin which regulate movement of GI tract and secretory activities of Small Intestine and Pancreas
I. Absroptive function (Very important function of Small intestine)
- Villi, microvilli increase mucosal surface area which facilitates the absorptive function of small intestine
- Most of the absorption proteins, fats, carbohydrates, vitamins and minerals takes place in small intestine
Food → Villi/Microvilli → Blood vessels
Absorption of Minerals, Vitamins and Water and Minerals
- Sodium absorption takes place by active transport mechanism along with Glucose and amino acids along with a transport protein. Eg: SGLT2
- Chloride absorption take place by active transport mechanism by exchange with bicarbonate ion in the Ileum.
- Calcium absorption takes place by active transport mechanism in Duodenum.
- Most of the vitamins are absorbed in Duodenum except Vitamin B12 which gets absorbed in Ileum with the help of Intrinsic factor of Castle.
- Absorption of water-soluble vitamins is faster than fat soluble vitamins.
- Movement of water in and out of the lumen of small intestine occurs due to the osmotic pressure.
Regulation of secretion of Succus entericus
Secretion of Succus entericus is done by two mechanisms
- Neural mechanism
- Hormonal mechanism
Neural mechanism:
This is done by
- Local nervous reflexes
- Stimulation of parasympathetic nerves cause vasodilation which increases the secretion of Succus entericus
- Stimulation of sympathetic nerves cause vasoconstriction which decreases the secretion of Succus entericus
Hormonal mechanism
The hormones involved in regulation of Succus entericus are
Secretin and Cholecystokinin (CCK)
Chyme enters small intestine
▼
Stimulation of intesinal mucosa
▼
Secretion of Secretin and Cholecystokinin
▼
Stimulation of intestinal glands
▼
Secretion of Succus entericus
Movements of small intestine
Small intestine movement are essential for
- Mixing the chyme with digestive juices
- Propulsion of food molecules
- Absorption of food molecules
Movements of Small intestine
There are 4 types of small intestinal movements
I. Mixing movements
- Segmentation movements
- Pendular movements
II. Propulsive movements
- Peristaltic movements
- Peristaltic rush
III. Peristalsis in fasting – migrating motor complex
IV. Movements of villi
I. Mixing movements
They are responsible for proper mixing of chyme with the
digestive juices (pancreatic juice, bile juice and intestinal juice)
They are of two types
- Segmentation contractions
- Pendular movements
1. Segmentation contractions
- They are rhythmic contractions which occur at regularly spaced intervals in a section of intestine (1 to 5 cm)
- The sections in between are relaxed, later contracted segments relax and relaxed segments.
- These contractions chop the chyme and helps to mix the chyme with digestive juices.
2. Pendular movement
- These are Sweeping movement of small intestine resembling the movements of pendulum of clock.
- Small portions of intestine sweep forward and backward or upward and downward.
- This type movement, helps in mixing of chyme with digestive juices.
II. Propulsive movements
These movements push the chyme in the aboral direction
They are of two types
- Peristaltic movements
- Peristaltic rush
1. Peristaltic movements
- Peristalsis is a wave of contraction followed by wave of relaxation of muscle fibers in aboral direction
- Stimulation of smooth muscles of intestine initiates the peristalsis
- It travels from point of stimulation in both directions, but towards oral direction is inhibited quickly and contractions disappear
Starling’s law of intestine
The response of the intestine for a local stimulus consists
of a contraction of smooth muscle above and relaxation below the stimulated
area
- Velocity: 1 to 2 cm/sec
2. Peristaltic Rush
- They are powerful peristaltic contraction due to excessive irritation or distension of intestinal
- It begins in duodenum and reach ileocecal valve in few minutes
- It Sweeps the contents of intestine into colon there by relieving the small intestine off either irritants or excessive distention
III. Peristalsis in fasting - Migrating Motor Complex (MMC)
- It is also called as Migrating Myoelectric complex
- It occurs in stomach & small intestine during periods of fasting
- It is different from peristalsis because a large portion of stomach or intestine is involved, around 20 to 30 cm
- It occurs once in every 1½ to 2 hours
- Velocity - 6 to 12 cm/min.
- Moves from Stomach to Colon in 10 min.
Significance of Migratory Motor Complex
- It sweeps the excess digestive secretions there by prevents its accumulation in stomach and intestine
- It also sweeps residual indigested materials
IV. Movements of Villi
- It is due to the extension of smooth muscle fibers of intestinal wall into the villi.
- It is caused by local nervous reflexes initiated by chyme.
- Movements are shortening and elongation of villi occurring alternatively.
- It helps in emptying lymph from the central lacteal into the lymphatic system.
- Elongation of villi increase in surface area which helps absorption of food molecules.
- Hormone villikinin from intestinal mucosa increase the movements of villi.
Applied Physiology
Malabsorption
- Failure to absorb nutrients - proteins, carbohydrates, fats and vitamins
- It affects growth and development of body
Malabsorption syndrome
A clinical condition characterized by failure of digestion
and absorption in small intestine
Causes
- Crohn’s disease
- Tropical sprue
- Celiac disease
Crohn’s disease
- It is the Inflammatory bowel disease (IBD) inflammation of small intestine.
- It usually affects the lower part of small intestine – Ileum.
Cause:
- Abnormalities of the immune system: Immune system reacts to virus or bacterium causing inflammation Features
Features:
- Malabsorption of Vitamins
- Weight loss
- Abdominal pain
- Diarrhea
- Rectal bleeding, anemia and fever
- Delayed or stunted growth in children
Tropical sprue
- It is a malabsorption syndrome affecting residents or visitors of tropical areas
Cause:
- Not known, may be due to infectious organisms
Features:
- Indigestion
- Diarrhea
- Anorexia and weight loss
- Abdominal and muscle cramps
Celiac disease/Gluten-sensitive enteropathy/Celiac sprue/Non-tropical sprue
- It is an autoimmune disorder characterized by damage of mucosa and atrophy of villi which leads to impaired digestion and absorption.
Cause:
- Gluten, a protein present in wheat, oats, rye, barley etc.
Features:
- Diarrhea
- Steatorrhea
- Abdominal pain
- Weight loss
- Irritability
- Depression