Intestines Flashcards
Where does most of the absorption occur in the GI tract?
Small intestines
What are the two routes of absorption?
- Directly into the bloodstream (to the liver)
2. Entering the lymphatic vessels (usually for large molecules)
Divisions of the small intestines
- Duodenum
- Jejunum
- Ileum
Blood supply to the intestines
*Supplied by the inferior + superior mesenteric arteries
- Superior mesenteric: supplies most of S.I. and some of L.I.
- Inferior mesenteric: supplies rest of L.I. (2/3)
What is the purpose of the anastomoses in the intestines?
- Ensures there is always blood –> GI tract
- Prevents bacteria in peritoneum (infection)
Venous drainage of intestines
- Inferior mesenteric v. dumps into splenic v.
- Pancreaticoduodenal v. drains pancreas + parts of duodenum
- Splenic v. + superior mesenteric v. + pancreaticoduodenal v. dump into hepatic portal v.
Layers of the S.I.
- Muscularis
- Submucosa
- Mucosa
- Plicae circularis: foldings of mucosa, allow for increased SA for absorption
- Villi: increases SA
- Microvilli: found on epithelial cells to increase SA, there are also brush border enzymes to break down food
What are villi?
- Projections from the mucosal surface
- There are BVs in the villi (allows nutrients to be transported away)
- Lacteals are also found in villi (modified lymphatic vessels that allow for nutrients to come in)
- GI tract then dumps into left subclavian v.
Features of the duodenum
- Short!
- Has the most dense plicae circularis; has a lot of enzymes so the plicae circulars acts like agitators to get food to mix with enzymes
- Major site for calcium + iron absorption
- In the first portion of duodenum, there are Brunner’s glands
- Secretes bicarb-rich protein to protect from acidic chyme that comes from the stomach
Why don’t you need Brunner’s glands after the first 10 cm of duodenum?
Because of the hepatopancreatic ampulla (receives secretions for the liver + pancreas to make bicarb rich secretion)
Features of the ileum
- S.I. gets smaller and smaller since it is just residual matter now
- No more plicae circularis
- Contains lymphoid nodules (MALT, Peyer’s patches)
- Found under the mucosal layer (has B + T cells)
- This is where Vit B12 is absorbed (DNA synthesis)
- Ends at ileocaecal value
What is the gastroileal reflex?
When peristalsis occurs at the ileum to make way for stuff that is coming from stomach –> S.I.
How is digestion regulated in the intestine?
- Nerve impulses that cause release of pancreatic juices (bicarb containing mucus + enzymes)
- Hormone release from enteroendocrine cells that line the S.I.
Secretin
Induces secretion of bicarb rich juice
CCK
Induces secretion of enzyme rich juice
Patterns of intestinal contractions
- Fed pattern
- During feeding; irregular contractions to mix food with digestive juices (mechanical + chemical, segmentation - circular muscles)
- Fasting pattern
- Peristalsis to move food from duodenum –> colon
- Prevents bacterial overgrowth
What breaks down carbohydrates?
Amylase!
Found in mouth + pancreas + absorptive cells lining S.I.
What breaks down proteins?
Pepsin from the stomach
Trypsin, chymotrypsin + carboxypeptidase from pancreas (released in inactive form, activated in S.I.)
Peptidases found in brush border of epithelial cells
What breaks down fats?
Bile salts from the liver - emulsifies the fat
What breaks down nucleic acids?
Enzymes released by the pancreas + found at the brush border
Pancreatic proteases
- Secreted as inactive enzymes
- Trypsin is one of the proteases & it activates the other ones
- Trypsinogen is activated by membrane-bound enteropeptidase (brush border enzyme)
- Trypsin breaks down chymotrypsinogen & procarboxypeptidase to protect the pancreas from auto digestion
Ways that absorption occur in the S.I.
- Active transport (using energy)
- Secondary active transport (chemical gradient)
- Facilitated diffusion (transport)
- Diffusion
Intestines and reabsorption
- The intestines are responsible for fluid balance
- Electrolytes absorbed in small (mostly) & large intestine
- Calcium + iron absorbed in duodenum
- Lots of bicarb ions absorbed in large intestine
Bicarb reabsorption & poop
- Diarrhea = loss of bicarbs = acidosis
- Constipation = more bicarb absorption = alkalotic
Alterations in the large intestines
- Smoother mucosa (no folds)
- Teniae coli: gives lumen a triangle shape
- Thinner circular muscle (less pinching in L.I.)
- Haustra: longitudinal action (moves feces with wall contact for absorption)
Regions of the L.I. (x4)
- Cecum
- Colon
- Rectum
- Anal canal
Which parts of the L.I. are retroperitoneal?
Ascending + descending colons
Transverse colon is suspended by the mesocolon
Intestinal glands
Intestinal glands are covered by mucus cells that secrete to lubricate + ease passage of feces
Parts of the anal canal
- Anal columns: longitudinal folds (highly vascular)
- Dilation of vascular sinusoids = hemorrhoids
- Internal anal sphincter: smooth muscle
- External anal sphincter: skeletal muscle
The defecation reflex
- Gastrocolic reflex stretches rectal wall
- Parasympathetic reflex is initiated (contracts rectal wall & relaxes internal anal sphincter)
- Defecation can occur (but you can stop it, cortex –> U.M.N –> L.M.N)
- Stretch info of sigmoid colon is shared to the cortex
