GI 11: Large Intestine

the large Intestine is marked proximally by the _____ and distally by the _____

proximally: Ileocecal Sphincter & Valve

distally: External Anal Sphincter

the large Intestine is anatomically distinguished how?

from orad → caudad:

- Cecum

- Ascending

- Transverse

- Descending

- Sigmoid Colon

- Rectum

- Anal Canal

Colorectal Sphincter

separates the rectum and anal canal

Large Intestine Smooth Muscle

1. outer longitudinal layer

- organized into three distinct flat bands called Teniae Coli

2. inner circular layer

- 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

Haustra

result from properties of the colon:

1. structural (smooth muscle thickening)

2. functional (segmenting colonic contractions)

give the colon a characteristic segmented appearance

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 importance of the Ileocecal Valve

important in order to maintain Sequestration of Most Bacteria within the Colon

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 importance of the Ileocecal Sphincter

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

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

Haustrations

Segmenting Contractions

• the most common colonic motility action

• local constricting contractions

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

Force of Haustrations

• 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

Mass Movements

Peristaltic Contractions

- serve in the Propulsion of intestinal contents toward the rectum

Mass Movement Frequency

compared to small intestine peristalsis

- Less Frequent (1-3x/day)

- Longer in Duration (10-30 min.)

- propel material a Greater Distance (20 cm)

SI Peristalsis: 8-12x/min; 4-5 sec., 1-4 cm

Inter-regional Reflex

coordinated with

a. Gastroileal Reflex and

b. Duodenalcolic Reflex

serve to coordinate movement of contents along tract

Duodenalcolic Reflex

Coordinated with the actions of the gastrocolic & gastroileal reflexes

• Duodenal Distension (e.g. After Meal)

• Colon Mass Movement

• Augments Gastrocolic & Gastroileal

• Defecation Sensation After Eating

Gastrocolic Reflex

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

• Colon Mass Movement

• Coordinated w/ Gastroileal Reflex

Duodenal Distension

augments stimulation of Colon Mass Movements

Composition of Feces

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

- More than half of the components of feces is of Non-Dietary Origin