SMALL INTESTINE

Small intestine Important digestive and absorptive functions Secretions and buffers provided by pancreas, liver, gall bladder Three subdivisions: Duodenum Jejunum Ileum Ileocecal sphincter Transition between small and large intestine

The Intestinal Wall

Histology of the small intestine Plicae Transverse folds of the intestinal lining Villi Fingerlike projections of the mucosa Lacteals Terminal lymphatic in villus Intestinal glands Lined by enteroendocrine, goblet and stem cells

The Structure of the Intestine

Intestinal Villi and Microvilli

GLANDS OF SMALL INTESTINE BRUNNER’S GLANDS Present in the duodenum Seen in the submucosal coat CRYPTS OF LIEBERKUHN Present between villi Seen in jejunum & ileum

BRUNNER’S GLANDS Present in duodenal mucosa In the submucosa From pyloro-duodenal junction upto Ampulla of Vater Small quantity – 50ml/day Clear, viscous fluid Specific gravity 1.007-1.009 pH 8.2 – 9.3

BRUNNER’S GLANDS Constituents Mucoprotein & Bicarbonate Thick alkaline mucus Protects duodenal mucosa from gastric juice Little amount of Na+, K+, Cl- Weak pepsinogen Secretion is spontaneous Irritation & vagal stimulation – ↑ secretion

GLANDS OF SMALL INTESTINE CRYPTS OF LIEBERKUHN Enterocytes – Digestive & absorptive Goblet cells – mucous Paneth cells – secrete glycoprotein, immunoglobulins & lysozymes Enterochromaffic/Argentaffin cells serotonin – Local hormone Intestinal motility & secretion ↑

SMALL INTESTINE Approximately 9 L of fluid/day Dietary sources - 2L GIT secretions – 7 L But only 1 – 2 L passes into the colon SUCCUS ENTERICUS SECRETIONS FROM Brunner’s glands Crypts of Lieberkuhn Mucous from goblet cells Shedded epithelial cells

SUCCUS ENTERICUS Daily secretions 3 L Colourless fluid Sp. Gravity – 1.010 pH 7.6 Water 98.5% Solids 1.5%

SUCCUS ENTERICUS Inorganic – 0.7% Cations: Na+, K+, Ca2+, Mg2+ Anions: Cl-, HCO3-, PO43- Organic – 0.8% Enzymes

Intestinal Enzymes Enterokinase (Converts Trypsinogen to Trypsin) Disaccharide splitting enzymes: Maltase, invertase, lactase &  limiting dextrinase Proteolytic enzymes: Erepsin, Nuclease Intestinal Lipase Cholesterol esterase Lecithinase Alkaline Phosphatase

Disaccharide splitting enzymes Sucrose Invertase Glucose + Fructose Maltose Maltase Glucose + Glucose Lactose Lactase Glucose + Galactose  limit dextrins  limiting dextrase Glucose

Protein splitting enzymes Peptones & polypeptides Erepsin Amino acids Nucleic acids Nuclease Nucleotides & nucleosides Nucleotidase & Nucleosidase Purine & pyrimidine

Fat splitting enzymes Cholesterol esters Cholesterol esterase Free cholesterol Lecithin & Isolecithin Phospholipase simple phospholipids Organic phosphate Alk. Phosphatase Free phosphate

REGULATION Neural: Parasympathetic Sympathetic: Role not clear Vagus – Ach - Stimulates Sympathetic: Role not clear Local Plexus: ↑ secretion Mechanical – Irritation - ↑ secretion

Humoral regulation All GI hormones stimulates Gastrin, CCK – PZ, Secretin, VIP, GIP, Glucagon Somatostatin – inhibit glucagon stimulated secretion Prostaglandin – Stimulate secretion

FUNCTIONS OF SUCCUS ENTERICUS DIGESTIVE FUNCTION PROTECTIVE FUNCTION ACTIVATOR FUNCTION

L.I. as seen in Barium enema

MOTILITY Types of movements 1. Mixing movements Segmentation (Haustral contractions) 2. Propulsive Mass Peristalsis (mass movement)

Mixing movements Large circular constrictions at regular intervals due to contractions of circular muscle Longitudinal muscle (aggregated to form teniae coli) also contract Bag-like ‘haustrations’ are formed After few minutes, new haustral contractions occur in other nearby areas Fecal matter is slowly dug into & rolled over Also propel the contents slowly towards anus

Propulsive movements Haustral contractions slowly move contents Mass movements rapidly move contents en masse towards rectum Similar to peristalsis, but large areas show contraction Last for 10-30 minutes, come 1-3 times a day; usually after eating breakfast Causes desire to ‘defecate’ if rectum is filled

Gastroileal reflex gastrin ↑ motility of ileum Food In Stomach Vagovagal ↓ tone of reflexes ileocaecal sphincter

Mass movements in colon Gastrocolic reflex Gastric distension Neural (ENS,ANS) Hormonal (gastrin) Mass movements in colon Distension of rectum Desire to defecate

GASTROCOLIC REFLEX FOOD IN STOMACH MASS PERISTALSIS DISTENSION OF RECTUM

Absorption & Secretion in colon Minimal absorption occurs in colon Unabsorbed carbohydrates short chain fatty acids (by colonic bacteria) absorbed Absorbs sodium by an active mechanism (influenced by Aldosterone) Chloride is absorbed in exchange to bicarbonate, which is secreted Absorption of water (90٪ of water delivered from SI) Only Mucus is secreted by colon

Other functions Distal colon functions principally as storage organ Synthesis of Vitamin K, folic acid & B-complex vitamins

Colonic bacteria Colon contains 500 different species of bacteria Mostly harmless & beneficial; some pathogenic Synthesize Vitamin K, B group vitamins Compete with pathogenic microbes for nutrition Promote colonic motility Maintain mucosal integrity Produce gases by fermentation of unabsorbed carbohydrate

Defecation Process of emptying of rectum when distended Is under voluntary control Most of the time, rectum is empty of feces Desire to defecate occurs when a mass peristalsis forces the feces into the rectum & defecation reflex begins The act can be postponed voluntarily

Internal anal sphincter Anal sphincters Internal anal sphincter (smooth muscle) Rectum Pelvic diaphragm External anal Sphincter (skeletal Muscle)

Anal sphincters Continuous dribble of fecal matter through the anus is prevented by tonic contraction of 2 sphincters: Internal anal sphincter- thickening of the circular muscle; lies immediately inside anus; supplied by pelvic parasympathetic nerves External anal sphincter- Striated voluntary muscle; surrounds the internal sphincter & extends distal to it; supplied by somatic pudendal nerve (under voluntary control)

Higher centers Sacral Spinal cord Pudendal nerve Afferents Sacral Spinal cord Parasympathetic efferents Pudendal nerve Distension receptors (SOMATIC) Both sphincters inhibited

Defecation reflex Receptors: distension receptors of rectum Afferent pathway: parasympathetic nerves Center: sacral spinal cord (S2, S3, S4) Efferent pathway: pelvic parasympathetic, somatic pudendal Effector structures: Descending colon, sigmoid colon, rectum (all contracted), anal sphincters (both relaxed)

Other factors involved in voluntary defecation Taking deep breath to move diaphragm down Closure of glottis Contraction of abdominal muscles Relaxation of pelvic floor

Constipation Is a pathological decrease in colonic motility resulting in prolonged evacuation of colonic contents & delayed defecation Frequency of defecation varies from person to person; In constipation, there is a decrease in that frequency Results in formation of dry, hard fecal matter Causes abdominal discomfort, dull headache, anorexia due to rectal & colonic distension Causes: Emotions, low-bulk diet, colonic obstruction, neurological, ignoring urge to defecate

OUT COME Describe composition and function of small/large intestinal secretion Regulation of small intestinal secretion Defecation, Constipation.