SMALL INTESTINE

ORGANISATION OF GUT WALL

INTRODUCTION Duodenum – 25 cm Jejunum and Ileum – 260 cm Duodenum I Part II Part III Part Second part of duodenum receives bile and pancreatic secretions

SMALL INTESTINE It is the major part of digestion and absorption of carbohydrates, proteins and fats. It is presented with 9 litres of fluid/day (2 litres - dietary sources and 7 litres - GI secretions). 1-2 litres passes onto the colon.

MUCOSA OF SMALL INTESTINE

STRUCTURE OF SMALL INTESTINE Finger like projections of 1mm height – Villi (20- 40 villi/mm2) Covered by columnar epithelium which has microvilli (1 M in length and 0.1 M in width). Each villus has a central lymphatic vessels – lacteal. Villus also has nerve net & capillaries.

INTESTINAL GLANDS Between villi are tubular glands - Crypts of Lieberkuhn. Duodenum in addition has coiled tubuloacinar glands- Brunner’s gland. Epithelium of crypts are mitotic, move upwards, and shed off.

STRUCTURE OF VILLI

VILLI

ENTEROCYTE

STRUCTURE OF VILLI

STRUCTURE OF VILLI

CRYPTS OF LIEBERKUHN

CRYPTS OF LIEBERKUHN

Outer border of microvilli of enterocyte has digestive enzymes. Epithelium – enterocyte. Outer border of microvilli of enterocyte has digestive enzymes. Paneth cells in the crypts secrete defensins. Ileum has aggregate of lymphatic nodules Peyer’s patches.

INTESTINAL JUICE- SUCCUS ENTERICUS COMPOSITION Daily secretion – 3 litres pH - 7.6 Water - 98.5% Solids - 1.5% i) Inorganic - 0.7% Cations - Na+, K+, Ca2+, Mg2+ Anions - Cl-, HCO3-,PO43-

INTESTINAL ENZYMES Enterokinase Proteolytic enzymes 1) Erepsin 2) Nucleases Sucrase, Maltase, Lactase, α- dextrinase Intestinal Lipase Cholesterol esterase Lecithinase Alkaline Phosphatase

Presence of food, chemical, CONTROL OF SECRETION Presence of food, chemical, mechanical stimuli - ↑ secretion Local irritation - ↑ mucus secretion. Vagal stimulation - ↑ secretion of Brunner’s gland.

MOVEMENTS OF SMALL INTESTINE

MIGRATING MOTOR COMPLEX The periodic intense electrical activity seen in the empty stomach or small intestine that last for 3-6 min and spread from stomach to ileum is called MMC. It is cyclical and repeated every 90 min.

SMALL INTESTINE Inner circular (thicker) Outer longitudinal (thinner)

TYPES OF MOVEMENTS Rhythmic segmentation contractions (Mixing contractions) Pendular movements Peristalsis Movements of villi

1.Segmentation contractions A loop of intestine is divided into a number of segments of nearly equal size. As one set of contraction relaxes, a new set begins but the contraction occur at new points between the previous contractions. It helps to chop the chyme 2 – 3 times per minute.

SEGMENTATION CONTRACTIONS

Control of segmental contractions The pacemaker for these movements - near ampulla of Vater. The interstitial cells of Cajal (present between LM and CM) initiates BER which decides the frequency of segmentation contractions.

Frequency  frequency of slow waves. Strength of contraction  frequency of spikes. This frequency is controlled by the amplitude of the slow waves. + Gastrin, CCK – PZ and motilin. - Secretin and glucagon.

12/min – duodenum and proximal jejunum. 8 - 9/min – terminal ileum. Vagus N ↑ and sympathetic nerve ↓ the movements.

FUNCTIONS OF SEGMENTATION CONTRACTIONS Agitation of intestinal contents It tends to increase the degree of subdivision of food particles. Mixing of food with intestinal secretions. Changing the layers of food in contact with mucosa, facilitating absorption.

2. Pendular movements These are side to side swaying movements accompanied by lengthening and shortening of the intestine. Function: similar to segmentation contractions.

3. Peristalsis It is defined as a wave of contraction preceded by a wave of relaxation that travel aborally.

PERISTALSIS

PERISTALSIS

Peristalsis

It is neurogenic and depends on the myenteric plexus. They move analward at a velocity of 0.5 – 2cm / sec. They are very weak and die out at a distance of 3 – 5 cm. Net movement – 1cm / min.

Law of the gut or polarity of intestine Starling and Bayliss The contractile wave is conducted only in aboral direction. This requires the integrity of the myenteric plexus.

MECHANISM OF PERISTALSIS

Factors that influence peristalsis It is increased after a meal. This is caused by gastro-enteric reflex. Gastrin, CCK, insulin, and serotonin enhance GI motility. Secretin and glucagon inhibit small intestinal motility.

Functions of peristalsis Progression of the chyme towards the ileo-caecal valve. Spreading of the chyme along the intestine .

PERISTALTIC RUSH Powerful irritation of the intestinal mucosa - relieving the SI from irritating chyme. Initiated by physical or chemical irritation. Powerful and rapid peristalsis. Sweeps the contents into colon. Cause of infective diarrhea.

4. Movements of villi They are initiated by local nervous reflexes that occur in response to chyme. The muscularis mucosa extends into each villus as a filament of the smooth muscle fiber.

Lashing movements Villus pump Function: These movements accelerate the flow of blood and lymph and increase the absorption.

Villi movements

DISORDERS OF SMALL INTESTINE CAUSES 1. Abnormal digestion of foods – Pancreatitis 2. Malabsorption Syndrome Causes Resection of small intestine Gastro-colic fistula Sprue Coeliac disease

APPLIED PHYSIOLOGY 1. Adynamic ileus or paralytic ileus Injury either due to surgery or due to infection. It reappears in the small intestine within 6 – 8 hours. 2. Mechanical obstruction of small intestine

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