GI 6: Small + Large Intestine Motility and Secretion

Physiological anatomy of small intestine

− mucosa of the intestine is organized into finger-like villi with the associated villus crypts (between adjacent villi)

Villi

projections into the lumen

- covered predominantly with mature, absorptive enterocytes

o Function: absorption

Crypts (of Lieberkuhn)

tubular invaginations in the epithelium around the villi

- lined largely with younger epithelial cells, which are involved primarily in secretion

- at the base are stem cells, which continually divide and provide the source of all the epithelial cells

SI Secretion

Secretion of fluid and electrolytes into the intestinal Lumen

- carried out mainly by cells located within the Crypts of the mucosa

SI Absorption

Absorption from the Lumen into the Plasma

- performed primarily by enterocytes located within the upper 2/3 of the Villus

Secretions of small intestine

• 1-2 L/day of intestinal juice

• Secretions are typically Isotonic and slightly Alkaline due to a relatively high HCO3- component

Composed mainly of:

• H2O

• Electrolytes (Na+, K+, Cl-, HCO3-)

• Mucus

Secretions of Colon

secretes a similar, but lower volume (0.2 L/day) fluid as compared to the SI

• generally richer in Mucus

• possess higher concentration of K+ & HCO3-

Functions of Intestinal secretions

1. Maintain Chyme Fluidity

- facilitates digestion, absorption & motility

2. Play a role in diluting ingested noxious agents & microorganisms

3. High HCO3- aids in Buffering

- Acid Chyme (sm. intestine)

- bacterially produced acids (colon).

4. Mucus component protects the intestinal lining from mechanical abrasions

Regulation of secretions of small intestine

major physiological stimuli of intestinal secretions:

- tactile stimulation of mucosal cells

- distension of the intestinal wall

• most secretions are normally reabsorbed by the intestines themselves

Toxins effect on secretion

Clinically, a few toxins (e.g. Cholera toxin) may greatly stimulate intestinal secretion to levels that exceed the reabsorptive capacity of the intestines

- causes production of a watery diarrhea that can lead to dehydration, electrolyte imbalance and perhaps death

Components of Intestinal Secretion: SI

• 1-2 L/day

• H2O

• Na+, K+, Cl-, HCO3-

• Mucus

• Alkaline

• Isotonic

Components of Intestinal Secretion: Colon

• Similar Sm. Int.

• 0.2 L/day

• ↑ Mucus

• ↑ K+; HCO3-

Overview of Secretion Function and Regulation

Function:

• Chyme Fluidity:

- Dig./Absorp.

- Motility

- Dilute Toxins

• Abrasion

• Acid Buffer

Regulation:

• Tactile (Mucosa)

• Distention (Wall)

Chloride secretion

- Cl– normally enters enterocytes from the interstitial fluid via Na+–K+–2 Cl– cotransporters in their basolateral membranes

- secreted into the intestinal lumen via channels that are regulated by various protein kinases

Chloride Apical Fluid Secretion

Cl- transport across the Apical membrane occurs through two types of Cl-Channels:

1. Cl- channel activated via a cAMP-dependent phosphorylation pathway

2. Cl- channel activated by increased intracellular Ca++ levels (Ca++)

CTFR

Cystic Fibrosis Transmembrane Conductance Rectifier

- Cl- channel activated via a cAMP-dependent phosphorylation pathway

- stimulated by a variety of secretagogues, including Cholera toxin and a number of Hormones

found:

- salivary glands

- pancreatic ducts

- pulmonary airways

- implicated in Cystic Fibrosis

Cl- channel activated by increased intracellular Ca++ levels (Ca++)

- Parasympathetic Neural stimulation of intestinal fluid secretion acts through activation of this type of Cl- channel

- Stimulation is mediated by acetylcholine (ACh) through the enteric nervous system

Sodium secretion

- when stimulated, one or both channels secrete Cl- into the Lumen

- negative electrical potential (-) created by the luminal movement of Cl- acts as a driving force for the paracellular movement of Na+ into the Lumen

- result is the secretion of NaCl into the Lumen

Water secretion

- secretion of NaCl into the Lumen creates an Osmotic Gradient that attracts the passive movement of H2O into the Lumen

- net result is the secretion of an Isotonic Fluid (NaCl & H2O) into the intestinal crypts and Lumen

Physiological anatomy of the Large Intestine (Colon)

- marked proximally by the Ileocecal Sphincter & Valve

- marked distally by the External Anal Sphincter

anatomically distinguished (orad → caudad) into the:

- Cecum

- Ascending

- Transverse

- Descending

- Sigmoid Colon

- Rectum

- Anal Canal

- colorectal Sphincter separates the rectum and anal canal

Colon motility

relies on the actions of its thick smooth muscle layers

1. outer longitudinal layer is organized into three distinct flat bands called Teniae Coli

2. inner circular layer is continuous through the anal canal, where it increases in thickness to form the internal anal sphincter

3. smooth-striated muscle transition region leads to purely Striated Muscle bundles that comprise the external anal sphincter

Entry of food from SI to LI: Ileocecal Valve

mainly functions to Prevent Fecal Reflux from the colon into the ileum

- its lips protrude into the lumen, acting as a one-way valve

- when colon pressure pushes fecal contents backward, the valve closes, preventing reflux into the ileum

- physiologically, this is important in order to the maintain Sequestration of Most Bacteria within the Colon

Entry of food from SI to LI: Ileocecal Sphincter

primarily functions to Control Ileal Emptying Rate into the colon

- normally remains mildly constricted, causing a general slowing of ileal emptying into the cecum

- physiologically, this is important because the ileum is the “Point of No Return” in terms of Digestion & Absorption of Major Nutrients since the colon does not largely participate in nutrient digestion or absorption.

- basal constrictive tone of the ileocecal sphincter is maintained by intrinsic myogenic control

Entry of food from SI to LI: Distension

distension of adjacent proximal (Ileum) or distal (Colon) intestinal regions modulate sphincter tone and ileal emptying rate:

Ileal Distension:

- decreases Sphincter Tone

- SPEEDS Ileal Emptying Rate into the colon

- prevents build-up of material in the ileum

Colon Distension:

- increases Sphincter Tone

- SLOWS Ileal Emptying Rate into the colon

- prevents excess filling of the colon

Absorption in Large Intestine

digestion and absorption of most nutrients is almost complete by the time intestinal material enters the colon

- however, significant amounts of H2O & Electrolytes, remain to be absorbed

- in addition, Waste material that is indigestible (Fiber) or produced within the tract (bacteria; mucus) must be transported, stored, and then eliminated from the colon

H2O Absorption

While the large bowel is important for optimal H2O absorption, MOST H2O entering the tract is actually absorbed by the SMALL Intestine

- 7-10 L of ingested or secreted H2O enter the GI tract/day, but only 600 ml is reabsorbed the colon

- however, colonic material still remains highly Fluid

Total Absorption in Large Intestine

reabsorbs all but 100 ml

- causes a relatively Hard Solid material (Feces) to normally reach the rectum for evacuation

- in spite of the relatively small amount of fluid normally reabsorbed, the colon has a large absorptive capacity (5-7 L/day)

- acting as a “reserve capacity” when small intestinal absorption and/or secretion become impaired

Motility Functions of Large Intestine

Colonic motility actions function in the:

• Mixing of material for Mucosal Absorption (H2O & electrolytes)

• Propulsion of contents from the Ileum → Rectum → Anal Canal

• Storage (Feces)

• Evacuation (Defecation) of waste material

Motility Functions of Large Intestine: Haustrations

Segmenting Contractions

• the most common colonic motility action

• local constricting contractions

• act to Mix intestinal contents with little net propulsion of material

• Colonic segmenting contractions exert Greater Force

• 2-4x Longer in Duration (12-60 sec.) than those of the small intestine.

• Contractions deeply constrict the lumen, causing the characteristic Haustrations

Motility Functions of Large Intestine: Mass Movements

Peristaltic Contractions

- serve in the Propulsion of intestinal contents toward the rectum

compared to small intestine peristalsis (8-12x/min; 4-5 sec., 1-4 cm):

- Less Frequent (1-3x/day)

- Longer in Duration (10-30 min.)

- propel material a Greater Distance (20 cm)

Large Intestine: Neural Reflexes

GASTROCOLIC REFLEX:

• Stomach Distension (e.g. After a Meal)

• Colon Mass Movement

• Coordinated w/ Gastroileal Reflex

DUODENALCOLIC REFLEX:

• Duodenal Distension (e.g. After Meal)

• Colon Mass Movement

• Augments Gastrocolic & Gastroileal

• Defecation Sensation After Eating

Large Intestine: Inter-regional Neural Reflexes

All reflexes are mainly regulated Extrinsically and are largely responsible for the Defecation Sensation that commonly develops shortly after eating a meal

Composition of Feces

Normal Human Feces is about 75% Water and 25% Solids

Note: more than half of the components of feces is of Non-Dietary Origin

- most dietary carbohydrates, fats & protein are digested & absorbed

- there is a relatively Low Composition Variability of feces despite significant variability in diet composition