The three main regions of the small intestine are the duodenum, the jejunum, and the ileum. The small intestine is where digestion is completed and virtually all absorption occurs. These two activities are facilitated by structural adaptations that increase the mucosal surface area by 600-fold, including circular folds, villi, and microvilli. There are around 200 million microvilli per square millimeter of small intestine, which contain brush border enzymes that complete the digestion of carbohydrates and proteins. Combined with pancreatic juice, intestinal juice provides the liquid medium needed to further digest and absorb substances from chyme. The small intestine is also the site of unique mechanical digestive movements. Segmentation moves the chyme back and forth, increasing mixing and opportunities for absorption. Migrating motility complexes propel the residual chyme toward the large intestine.
The main regions of the large intestine are the cecum, the colon, and the rectum. The large intestine absorbs water and forms feces, and is responsible for defecation. Bacterial flora break down additional carbohydrate residue, and synthesize certain vitamins. The mucosa of the large intestinal wall is generously endowed with goblet cells, which secrete mucus that eases the passage of feces. The entry of feces into the rectum activates the defecation reflex.
anal canal
final segment of the large intestine
anal column
long fold of mucosa in the anal canal
anal sinus
recess between anal columns
appendix
(vermiform appendix) coiled tube attached to the cecum
ascending colon
first region of the colon
bacterial flora
bacteria in the large intestine
brush border
fuzzy appearance of the small intestinal mucosa created by microvilli
cecum
pouch forming the beginning of the large intestine
circular fold
(also, plica circulare) deep fold in the mucosa and submucosa of the small intestine
colon
part of the large intestine between the cecum and the rectum
descending colon
part of the colon between the transverse colon and the sigmoid colon
duodenal gland
(also, Brunner’s gland) mucous-secreting gland in the duodenal submucosa
duodenum
first part of the small intestine, which starts at the pyloric sphincter and ends at the jejunum
epiploic appendage
small sac of fat-filled visceral peritoneum attached to teniae coli
external anal sphincter
voluntary skeletal muscle sphincter in the anal canal
feces
semisolid waste product of digestion
flatus
gas in the intestine
gastrocolic reflex
propulsive movement in the colon activated by the presence of food in the stomach
gastroileal reflex
long reflex that increases the strength of segmentation in the ileum
haustrum
small pouch in the colon created by tonic contractions of teniae coli
haustral contraction
slow segmentation in the large intestine
hepatopancreatic ampulla
(also, ampulla of Vater) bulb-like point in the wall of the duodenum where the bile duct and main pancreatic duct unite
hepatopancreatic sphincter
(also, sphincter of Oddi) sphincter regulating the flow of bile and pancreatic juice into the duodenum
ileocecal sphincter
sphincter located where the small intestine joins with the large intestine
ileum
end of the small intestine between the jejunum and the large intestine
internal anal sphincter
involuntary smooth muscle sphincter in the anal canal
intestinal gland
(also, crypt of Lieberkühn) gland in the small intestinal mucosa that secretes intestinal juice
intestinal juice
mixture of water and mucus that helps absorb nutrients from chyme
jejunum
middle part of the small intestine between the duodenum and the ileum
lacteal
lymphatic capillary in the villi
large intestine
terminal portion of the alimentary canal
left colic flexure
(also, splenic flexure) point where the transverse colon curves below the inferior end of the spleen
main pancreatic duct
(also, duct of Wirsung) duct through which pancreatic juice drains from the pancreas
major duodenal papilla
point at which the hepatopancreatic ampulla opens into the duodenum
mass movement
long, slow, peristaltic wave in the large intestine
mesoappendix
mesentery of the appendix
microvillus
small projection of the plasma membrane of the absorptive cells of the small intestinal mucosa
migrating motility complex
form of peristalsis in the small intestine
motilin
hormone that initiates migrating motility complexes
pectinate line
horizontal line that runs like a ring, perpendicular to the inferior margins of the anal sinuses
rectal valve
one of three transverse folds in the rectum where feces is separated from flatus
rectum
part of the large intestine between the sigmoid colon and anal canal
right colic flexure
(also, hepatic flexure) point, at the inferior surface of the liver, where the ascending colon turns abruptly to the left
saccharolytic fermentation
anaerobic decomposition of carbohydrates
sigmoid colon
end portion of the colon, which terminates at the rectum
small intestine
section of the alimentary canal where most digestion and absorption occurs
tenia coli
one of three smooth muscle bands that make up the longitudinal muscle layer of the muscularis in all of the large intestine except the terminal end
transverse colon
part of the colon between the ascending colon and the descending colon
Valsalva’s maneuver
voluntary contraction of the diaphragm and abdominal wall muscles and closing of the glottis, which increases intra-abdominal pressure and facilitates defecation
villus
projection of the mucosa of the small intestine
Watch this animation that depicts the structure of the small intestine, and, in particular, the villi. Epithelial cells continue the digestion and absorption of nutrients and transport these nutrients to the lymphatic and circulatory systems. In the small intestine, the products of food digestion are absorbed by different structures in the villi. Which structure absorbs and transports fats?
Answers may vary.
By watching this animation, you will see that for the various food groups—proteins, fats, and carbohydrates—digestion begins in different parts of the digestion system, though all end in the same place. Of the three major food classes (carbohydrates, fats, and proteins), which is digested in the mouth, the stomach, and the small intestine?
Answers may vary.
1. In which part of the alimentary canal does most digestion occur?
A) stomach
B) proximal small intestine
C) distal small intestine
D) ascending colon
B
2. Which of these is most associated with villi?
A) haustra
B) lacteals
C) bacterial flora
D) intestinal glands
B
3. What is the role of the small intestine’s MALT?
A) secreting mucus
B) buffering acidic chyme
C) activating pepsin
D) preventing bacteria from entering the bloodstream
D
4. Which part of the large intestine attaches to the appendix?
A) cecum
B) ascending colon
C) transverse colon
D) descending colon
A
1. Explain how nutrients absorbed in the small intestine pass into the general circulation.
Nutrients from the breakdown of carbohydrates and proteins are absorbed through a capillary bed in the villi of the small intestine. Lipid breakdown products are absorbed into a lacteal in the villi, and transported via the lymphatic system to the bloodstream.
2. Why is it important that chyme from the stomach is delivered to the small intestine slowly and in small amounts?
If large quantities of chyme were forced into the small intestine, it would result in osmotic water loss from the blood into the intestinal lumen that could cause potentially life-threatening low blood volume and erosion of the duodenum.
3. Describe three of the differences between the walls of the large and small intestines.
The mucosa of the small intestine includes circular folds, villi, and microvilli. The wall of the large intestine has a thick mucosal layer, and deeper and more abundant mucus-secreting glands that facilitate the smooth passage of feces. There are three features that are unique to the large intestine: teniae coli, haustra, and epiploic appendages.