L7 Small Intestine Flashcards
What is the structure of the small intestine?
The small intestine is comprised of three structural parts; the duodenum, jejunum and ileum. The ileum and cecum met at the ileocecal valve. This valve controls the movement of matter from the ileum into the cecum.
The duodenum and the jejunum met at the duodenojejunal flexure.
What is the general structure of the small intestine?
The small intestine has many coils. The SI have villi, finger-like projections, and the cells have microvilli also, this helps to maximise the surface area for absorption. Like the rest of the GI tract, the layers include a mucosa, muscularis mucosa, submucosa and muscularis propria.
How is peristalsis achieved?
- There are circular muscles around the circular wall of the bowel. The circular muscles contract behind the bolus while circular muscles ahead of the bolus relax
- Longitudinal muscles that runs from top to bottom. Longitudinal muscles ahead of the bolus contract, shortening adjacent segments.
What is segmentation?
Alternate contraction of neighbouring segments, churns and fragments the bolus, mixing contents with intestinal secretions.
What is a migrating motor complex?
The migrating motor complex is a distinct pattern of electromechanical activity observed in gastrointestinal smooth muscle during the periods between meals. It is thought to serve a “housekeeping” role and sweep residual undigested material through the digestive tube. Pyloric sphincter is relaxed, allowing larger things to pass. For a few minutes, a series of strong, slow, peristaltic waves will sweep down, mostly from the stomach, and along the small bowel.
What secretions occur in the small intestine?
Chemicals are secreted first in the duodenum and then into the rest of the small bowel.
Secreted from crypts of Lieberkühn (and submucosal Brunner’s glands in the duodenum).
Hormones from APUD cells (endocrine cells):
- CCK from I cells (Cholecystokinin) - Secretin from S cells in the proximal intestine - Motilin from M cells in the stomach - Gastrin from G cells in the stomach
These hormones enable motility of the GI tract
- Bicarbonate secretion from pancreatic duct cells Mucus secretion from goblet cells
- Bile serves an important role in lipid digestion, and is secreted from the gallbladder into the common bile duct, in response to CCK.
How is carbohydrate digestion achieved in the small intestine?
Uses amylase which starts in the mouth with salivary amylase and continues with secretions in the SI. Soluble amylase only breaks internal alpha 1-4 bonds. Remaining short chain carbohydrates are broken down by specific enzymes on the brush border membrane.
How are carbohydrates absorbed in the small intestine?
Sodium ions are actively pumped out of the cell and potassium into the cell. This creates a low sodium concentration inside the cell. Sodium can then transport glucose or galactose into the cell using a symport protein. Once inside the cell, the monosaccharide is transporter outside the cell by a separate receptor on the luminal membrane. This is less energy dependent and does not require sodium. Glucose and galactose are selectively absorbed by SGLT1 (sodium-glucose transporter 1). Fructose passively, via Glut5. GLUT5 is a fructose transporter expressed on the apical border of enterocytes in the small intestine. GLUT5 allows for fructose to be transported from the intestinal lumen into the enterocyte by facilitated diffusion due to fructose’s high concentration in the intestinal lumen.
How is protein digestion achieved in the SI?
Proteolysis begins in the stomach around (15%) but pepsin is inactivated in the duodenum. Enterokinase is secreted by crypt cells. Enterokinase converts trypsinogen into trypsin. Trypsin can then activate pancreatic enzymes:
- Trypsinogen into trypsin
- Chymotrypsinogen into Chymotrypsin
- Proelastase into elastase
- Procarboxypeptidase A into carboxypeptidase A
- Procarboxypeptidase B into carboxypeptidase B
Once you get to the short peptide stage, they are broken up into single amino acids and dipeptides by peptidase at the brush border.
What are endopeptidases?
Endopeptidase are proteolytic peptidases that break peptide bonds of nonterminal amino acids. This includes trypsin, chymotrypsin and elastase.
What are exopeptidase?
An enzyme which breaks the terminal peptide bond in a peptide chain such as carboxypeptidases.
What is the role of carboxypeptidase A?
Peptides with a C terminal neutral amino acids are broken down by Carboxypeptidase A.
What is the role of carboxypeptidase B?
Peptides with a basic C terminal amino acid are broken down by Carboxypeptidase B.
How is protein absorption achieved?
The process of transporting this into the cell is via the sodium gradient. Once they get into the cell, they are transported out again by specific transporters on the inner surface. Dipeptides (and sometimes tripeptides) are taken up by proton linked secondary active transporters which depends on the sodium proton exchange which depends on sodium gradient which requires energy.
What are Peyer’s patches?
M cells take in bigger chunks of proteins, sometimes whole proteins. These cells lie closely to Payer’s patches. Payer’s patches are aggregated of lymphoid cells in the bowel wall. The M cells phagocytose the big chunks of proteins and sends out the same proteins into the body undigested. This may be a way in which the body becomes immunocompetent. This is even more active in new-born babies.
