Intestines

Question 1

Where does most of the absorption occur in the GI tract?

Answer 1

Small intestines

Question 2

What are the two routes of absorption?

Answer 2

1. Directly into the bloodstream (to the liver)

2. Entering the lymphatic vessels (usually for large molecules)

Question 3

Divisions of the small intestines

Answer 3

• Duodenum
• Jejunum
• Ileum

Question 4

Blood supply to the intestines

Answer 4

*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)

Question 5

What is the purpose of the anastomoses in the intestines?

Answer 5

• Ensures there is always blood –> GI tract

- Prevents bacteria in peritoneum (infection)

Question 6

Venous drainage of intestines

Answer 6

• 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.

Question 7

Layers of the S.I.

Answer 7

• 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

Question 8

What are villi?

Answer 8

• 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.

Question 9

Features of the duodenum

Answer 9

• 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

Question 10

Why don’t you need Brunner’s glands after the first 10 cm of duodenum?

Answer 10

Because of the hepatopancreatic ampulla (receives secretions for the liver + pancreas to make bicarb rich secretion)

Question 11

Features of the ileum

Answer 11

• 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

Question 12

What is the gastroileal reflex?

Answer 12

When peristalsis occurs at the ileum to make way for stuff that is coming from stomach –> S.I.

Question 13

How is digestion regulated in the intestine?

Answer 13

• Nerve impulses that cause release of pancreatic juices (bicarb containing mucus + enzymes)
• Hormone release from enteroendocrine cells that line the S.I.

Question 14

Secretin

Answer 14

Induces secretion of bicarb rich juice

Question 15

CCK

Answer 15

Induces secretion of enzyme rich juice

Question 16

Patterns of intestinal contractions

Answer 16

1. Fed pattern
   
   • During feeding; irregular contractions to mix food with digestive juices (mechanical + chemical, segmentation - circular muscles)
2. Fasting pattern
   
   • Peristalsis to move food from duodenum –> colon
   • Prevents bacterial overgrowth

Question 17

What breaks down carbohydrates?

Answer 17

Amylase!

Found in mouth + pancreas + absorptive cells lining S.I.

Question 18

What breaks down proteins?

Answer 18

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

Question 19

What breaks down fats?

Answer 19

Bile salts from the liver - emulsifies the fat

Question 20

What breaks down nucleic acids?

Answer 20

Enzymes released by the pancreas + found at the brush border

Question 21

Pancreatic proteases

Answer 21

• 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

Question 22

Ways that absorption occur in the S.I.

Answer 22

1. Active transport (using energy)
2. Secondary active transport (chemical gradient)
3. Facilitated diffusion (transport)
4. Diffusion

Question 23

Intestines and reabsorption

Answer 23

• 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

Question 24

Bicarb reabsorption & poop

Answer 24

• Diarrhea = loss of bicarbs = acidosis

- Constipation = more bicarb absorption = alkalotic

Question 25

Alterations in the large intestines

Answer 25

- 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)

Question 26

Regions of the L.I. (x4)

Answer 26

1. Cecum 2. Colon 3. Rectum 4. Anal canal

Question 27

Which parts of the L.I. are retroperitoneal?

Answer 27

Ascending + descending colons Transverse colon is suspended by the mesocolon

Question 28

Intestinal glands

Answer 28

Intestinal glands are covered by mucus cells that secrete to lubricate + ease passage of feces

Question 29

Parts of the anal canal

Answer 29

- Anal columns: longitudinal folds (highly vascular) - Dilation of vascular sinusoids = hemorrhoids - Internal anal sphincter: smooth muscle - External anal sphincter: skeletal muscle

Question 30

The defecation reflex

Answer 30

- 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