Small Intestine

Small Intestine

It is divided into duodenum, jejunum and ileum. Mucosa: characteristic features- Plicae circularis (valves of Kerkring) Villi & Microvilli Goblet cells (few) Crypts of Lieberkuhn (intestinal glands) Glands are lined by columnar cells, goblet cells, Paneth cells & enteroendocrine cells

The Small Intestine

Plays key role in digestion and absorption of nutrients 90% of nutrient absorption occurs in the small intestine

Segments of the Intestine

Figure 24–16

Segments of the S.I.

S.I. Runs from pyloric sphincter to the ileocecal valve; 3 segments:The Duodenum is the 25 cm (10 in.) long segment of small intestine closest to stomach“Mixing bowl” that receives chyme from stomach, digestive secretions from pancreas and liverThe Jejunum is the 2.5 meter (8.2 ft) long middle segment and is the location of most chemical digestion and nutrient absorptionThe Ileum is he final 3.5 meter (11.48 ft) long segment, joins large intestine at ileocecal valve

Intestinal Folds and Projections

Structural modifications of the small intestine wall increase surface area Plicae = Largest; deep transverse (circular) folds in intestinal lining; permanent features (they do not disappear when small intestine fills) Intestinal Villi: a series of fingerlike projections of mucosa Villi are covered with simple columnar epithelium which themselves are have many plasma membrane projections called microvilli All serve to increase surface area for absorption (altogether by 600x)

Intestinal Histology

Absorptive columnar cellsGoblet cells between columnar epithelial cells eject mucins onto intestinal surfacesEnteroendocrine cells in intestinal glands produce intestinal hormones:gastrincholecystokininSecretinPeyer’s patches are found in the submucosa


Lacteals
Each villus lamina propria has ample capillary supply (to absorb nutrients) and nerves In addition, each villus has a central lymph capillary called a lacteal. These are larger than the blood capillaries and thus can absorb larger particles into the body, such as lipid droplets. Muscle contractions move villi back and forth to facilitate absorption and to squeeze the lacteals to assist lymph movement

Small Intestine

Small Intestine
Submucosa: contains blood vessels, lymphatics and Meissner’s plexus. Muscularis externa: Outer longitudinal and inner circular layers of smooth muscle.Serosa/Adventitia

Duodenum

Presence of Brunner’s glands in submucosa

High power view of the Duodenal Mucosa

Jejunum
Villi are tongue shaped.Absence of Brunner’s glands.

Ileum

Presence of lymphoid aggregations in lamina propria known as Peyer’s patches.Villi are short & finger like.


The Large Intestine
Also called large bowel Horseshoe-shaped, about 1.5 meters long and 7.5 cm wide Extends from end of ileum to anus Lies inferior to stomach and liver Frames the small intestine Functions Reabsorption of water [the last 15-20%] Compaction of intestinal contents into feces Absorption of important vitamins produced by bacteria Storage of fecal material prior to defecation

Parts of the Large Intestine

Cecum: the pouchlike first portion Has wormlike appendix projecting from it Colon: the largest portion Rectum: the last 15 cm of digestive tract Anal canal

The Colon

Has a larger diameter (this is why it is called large) and thinner wall than small intestine The wall of the colon forms a series of pocketlike pouches (haustra) giving it a sgmented appearance Haustra permit expansion and elongation of colon

Colon Muscles

3 longitudinal bands of smooth muscle (taeniae coli) run along outer surfaces of colon deep to the serosa (similar to outer layer of muscularis externa) Muscle tone in taeniae coli creates the haustra

Large Intestine

It consists of: appendix, colon, rectum and anal canal. Mucosa: Absence of Plicae circulares and villi Presence of Microvilli Presence of Crypts of Lieberkuhn Presence of Goblet cells in large number Submucosa Muscularis externa: Inner circular layer - thin compared to small intestine. Outer longitudinal layer- forms Taenia coli. Adventitia: Appendices epiploicae (peritoneum forms pouch like processes filled with fat)

Large Intestine

Magnified view of a villus

Large Intestine

Mucosa and Glands of the Colon
Figure 24–24

Vermiform Appendix

A small blind-ending diverticulum. Large accumulations of lymphoid tissue in lamina propria which may extend into submucosa. Intestinal villi are usually absent. Crypts are poorly formed. Muscularis externa is thin. Absence of taenia coli.

Characteristics of the Colon

Lack villi Abundance of goblet cells Presence of distinctive intestinal glands in crypts deeper than glands of small intestine dominated by goblet cells Mucosa of the large intestine does not produce enzymes Provides lubrication for fecal material Large lymphoid nodules are scattered throughout the lamina propria and submucosa The longitudinal layer of the muscularis externa is reduced to the muscular bands of taeniae coli

Vermiform Appendix

Rectum
Intestinal glands are straight, like test tubes. A continuous coat of longitudinal muscle is present. Absence of taenia. Absence of appendices epiploicae.

Anal Canal

Epithelium: upper part-simple columnar, middle part-stratified squamous non-keratinized, lower part-covered by true skin. Mucosa has characteristic longitudinal folds-Anal columns. Small mucosal folds between the anal columns -Pectinate line. Crypts disappear below this line. Muscularis externa-circular muscle forms involuntary internal anal sphincter.

Ano-rectal Junction

MCQ
Q3. Plica circularis is a feature of: Oesophagus Stomach Small intestine Large intestine

MCQ
Q4. Taenia coli is present in: Oesophagus Stomach Small intestine Large intestine

MCQ
Q5. Abundant lymphoid tissue in lamina propria is a feature of: Oesophagus Stomach Duodenum Appendix