Gastrointestinal tract Physiology Dr Suaad M Ghazi MBCh

Gastrointestinal tract Physiology Dr. Suaad M. Ghazi MBCh. B, MSc, Ph. D

Objectives of Lecture 9 1. Explain secretions. the intestinal 2. Explain the functions of the intestine. major large

Intestinal secretions Fluid secretion Mucus

[A] Fluid secretion: ♦ Both the small intestine and the large intestine secrete fluid from the crypt cells. ♦ Secretion is necessary for lubrication. ♦ Intestinal fluid secretion is activated by a number of bacterial enterotoxins that cause secretory diarrhea. ♦ The key step in the mechanism of fluid secretion from the crypt enterocytes is opening of the Cl− channels in the luminal cell membrane.

Intestinal Na. Cl and water secretion.

Intestinal secretion of water and electrolytes

There are two types of Cl− channels 1. c. AMP-activated Cl− channels Ø Enteric nervous system (ENS) neurotransmitter VIP is an important stimulator. Ø Na+ secretion follows passively via the paracellular pathway. Ø Water secretion occurs by osmosis. 2. Ca 2+-activated Cl- channels Ø Acetylcholine from ENS neurons (stimulator). Ø Serotonin from the enterochromaffin (stimulator).

Summary of intestinal absorption and secretion

♦ Enterotoxic Escherichia coli (traveler’s diarrhea) and Vibrio cholera are produced enterotoxins. ♦ Cholera toxin causes an irreversible in c. AMP levels in the enterocytes located in the crypts of Lieberkühn of the small intestine. ♦ in c. AMP an irreversible opening of chloride channels on the luminal membrane.

♦ Movement of chloride ions into the gut lumen a secondary movement of sodium ions to maintain electrical neutrality. ♦ Water follows the osmotic gradient created by sodium and chloride in fluid loss into the gut lumen. Severe diarrhea follows.

♦ Loss of gastric juice hypokalemic, metabolic alkalosis. ♦ Excessive loss of fluid from the GIT dehydration and electrolyte and acid-base disturbances. ♦ H+ ion and K+ ion concentration of gastric juice exceeds that of the plasma.

♦ Excess fluid loss metabolic alkalosis + hypokalemia. ♦ Because the pancreas, liver, ileum, and colon secrete bicarbonate, excessive loss metabolic acidosis. ♦ The colon secretes K+ the acidosis is accompanied by hypokalemia.

♦ The small intestinal glands normally secrete 1 to 2 L of intestinal juice daily. ♦ Intestinal juice is slightly alkaline (7. 4– 7. 8), and isotonic with blood plasma. ♦ It is largely water and some mucus, which is secreted both by the duodenal glands and by goblet cells of the mucosa. ♦ The major stimulus for intestinal juice production comes from hypertonic or acidic chyme, distension or irritation of the intestinal mucosa.

♦ Whenever a segment of large intestine becomes intensely irritated or infected, the mucosa then secretes large amounts of water and electrolytes. ♦ This acts to dilute the irritating factors and to cause rapid movement of the feces toward the anus. ♦ The usual result is diarrhea with loss of large quantities of water and electrolytes.

[B] Mucus: ♦ Mucus in small and large intestine contains bicarbonate ions to protect the mucosa of small intestine from acidic chyme and the acidic end products of bacterial action in the colon. ♦ Small intestinal mucus is secreted by Brunner’s glands, (within the duodenum) and by Goblet cells (intestinal epithelium).

The mucus is secreted also in response to: v Parasympathetic vagal stimulation. v Decrease p. H of the duodenum to 4 or less. v GIT hormones especially secretin. ♦ Brunner’s glands are inhibited by sympathetic stimulation which may lead to peptic ulceration.

In the large intestine the rate of secretion of mucus is regulated by: v Direct tactile stimulation. v Local nervous reflexes to the mucous cells in the crypts of Lieberkuhn. v Parasympathetic innervation to the distal two thirds of large intestine marked in secretion of mucus with an in motility.

♦ Extreme parasympathetic stimulation by emotional disturbances much mucus secreted into the large intestine bowel movement of ropy mucus every 30 min; the mucus contains little or no fecal material.

♦ Mucus in the large intestine protects the wall against excoriation and it provides the adherent medium for holding fecal matter together. ♦ It protects the intestinal wall from the great amount of bacterial activity inside the feces, the alkalinity of the secretion and provides a barrier from fecal acids.

THE LARG INTESTINE

THE LARGE INTESTINE

The contractile activity of the large intestine serves two functions: [1] It enhances the efficiency of water and electrolyte absorption. [2] It promotes the excretion of fecal material remaining in the colon.

Movement of the colon [1] Mixing movements (segmentation, and haustration). [2] Peristalsis. [3] Mass movements.

Large Intestine Motility ♦ Large intestine muscle inactive most of the time & muscle contractions sluggish & short-lived. Haustral contractions Combined contraction (circular & long) similar to segmentation occur every 30 min (approx) individual haustra contract & food propelled into next haustra aids water absorption

HAUSTRAL CONTRACTIONS Food residue Haustra

Mass movement 1. A constrictive ring occurs at a distended or irritated point in the colon, usually in the transverse colon. 2. Then rapidly thereafter, the 20 or more centimeters of colon distal to the constriction contract almost as a unit, forcing the fecal material in this segment en masse down the colon. During this process, the haustration disappear completely.

3. After the contraction is completed, then relaxation occurs before another mass movement takes place. ♦ The mass movements have forced the feces into the rectum the desire for defecation is felt.

Mass movements Ø long, slow powerful contractile waves. Ø occur over large areas of the colon 3 - 4 times daily. Ø forces food residue towards rectum. Ø typically occur just after eating gastrocolic reflex.

MASS MOVEMENTS Food residue Rectum

Mass movements is facilitated by: v Gastrocolic and duodenocolic reflexes ♦ As a result of distension of stomach and duodenum respectively. ♦ These reflexes are transmitted mainly through extrinsic nerves and only weakly through myenteric plexus. ♦ This explains the appearance of mass movement and urge to defecate after meals. ♦ They are greatly suppressed when the extrinsic autonomic nerves to the colon have been removed.

v Irritation of the colon can also initiate intense mass movement. v Intense stimulation of the parasympathetic nervous system. v Overdistension of a segment of the colon.