Physio - Small Intestine And Absorption Of Nutrients, Water And Electroytes

Question 1

Describe the mucosa of the small intestine.

Answer 1

1. Contain absorptive cells- digest and absorb nutrients
2. Goblet cells - secrete mucus.

Question 2

List the cells that are found on mucosa of the small intestine that covers the villi and crypts.

Answer 2

Villi- Absorptive cells, goblet cells
Crypts- absorptive , goblet, enteroendocrine, paneth

What do the paneth and enteroendocrine cells do?

Question 3

Describe the secretion of intestinal juice in crypts.

Answer 3

1. Basolateral membrane contains a Na/K+/Cl cotransporter that diffuses these ions into the erythocyte from the interstitial fluids.
2. Cl- is diffused into the intestinal lumen of the erythrocyte via the Cl- channel in the lumen membrane.
3. Na+ and K+ is non soluble therefore, it is pushed back in the interstitial fluids.
4. The accumulated Cl- makes the intestinal lumen negative thus causing a diffusion of Na+ and K+ via pathcellular pathway.
5. HCO3- is secreted via the cl-/hco3- exchanger in the lumen membrane
6. Na+ movement is attracts water by osmosis.

Question 4

What are the three types of digestion?

Answer 4

Luminal - pancreatic enzymes and bile
Membrane - brush border enzymes
Intercellular

Question 5

Describe the digestion of carbohydrates in the small intestine.

Answer 5

1. Intestinal lumen - the action of pancreatic amylase in the IL.
2. Membrane digestion- the actions of the 4 carbohydrates at the brush border.
   — Alpha-dextrinase- acts on alpha dextrin, clipping off one glucose at a time
   — Sucrase, maltase and lactase breaks downs disaccharides into monosaccharides.

Question 6

Describe the digestion of fat in the small intestine.

Answer 6

1. Intestinal lumen
   Fats are fully digested in the lumen of the small intestine through the action of bile and pancreatic lipase.

Note: no fat- digesting enzymes in the brush border of the small intestine.

Question 7

Describe the digestion of proteins in the small intestine.

Answer 7

Step 1. Intestinal lumen - action of the pancreatic peptidases, either exopeptidases (one step only. Explain why) or endopeptidases (will move on to step 2)

Step 2. Absorbs the short bonds (from endopeptidases) into the enterocyte and split it with the use of an intercellular peptidase or
== spilt the short bonds with the use of the brush border enzymes and absorb it into the enterocyte.

Note: Aminopeptidase is the only protease at the brush border.

Question 8

Describe the digestion of nucleic acids in the small intestine.

Answer 8

Involves two steps.

1) intestinal lumen - nucleic acids can be be digested by pancreatic nucleases to split RNA and DNA into mononucleotides
2) membrane digestion - the use of brush border enzymes (nucleosidase and phophatase) to break down the nucleotides into pentose sugars, phosphates and nitrogenous bases to be absorbed into the blood.

Question 9

List the two types of cells in the smooth muscle of the GIT wall.

Answer 9

Contractile
Autorhythmic (generate slow wave potentials and basic electrical rhythm)

Question 10

Explain the depolarizing and repolarizing phase of GIT muscle motility

Answer 10

— depolarizing phase of slow wave caused by the opening of Ca+ channels
— repolarizing phase of the slow wave caused by opening of K+ channels.

Question 11

What stimulates GIT motility and increases the frequency of spike potentials of the autorhythmic cells?

Answer 11

AcH

Note: Adrenaline and nonadrenaline has opposite effects.

Question 12

State the two types of motility of the small intestine.

Answer 12

Peristalsis and segmentation

Question 13

Describe the segmentation pattern in intestinal motility.

Answer 13

= Involves the contraction of the circular muscles and relaxation of the longitudinal muscles.

= intestinal content is moved back and forth and mixed with intestinal juices.

Question 14

Describe the peristalsis pattern in intestinal motility.

Answer 14

— Pattern involves the contraction of the circular and longitudinal muscles.
— involves two segments:
A) propulsive - circular contracts, longitudinal relax
B) receiving - circular relax, longitudinal contracts

Question 15

Explain the peristaltic reflex of the small intestine.

Answer 15

Sensory neurons detect chemical and mechanical stimulation resulting in:
1) electrical signal sent to the interneurons on the oral and anal side.
2) the oral side sends signal to the excitatory neurons to release AcH and Sp to cause muscle contractions.
3) the anal side send signal to inhibitory neurons to release VIP and NO to cause muscle relaxation.

Question 16

What are the structural features that contribute to the large absorptive surface of the small intestine?

Answer 16

Microvilli (increase absorptive surface x 20)
Villi and crypts (increase absorptive surface x 10)
Circular folds (increase absorptive surface x3)

Question 17

State the molecules that are diffused via the trans cellular and paracellular pathway.

Answer 17

Transcellular- fatty acids, glycerols, fat soluble vitamins and B6

Paracellular - water and ions.

Question 18

Describe the absorption of carbohydrates (monosaccharides) in the small intestine.

Answer 18

1. Fructose moves through the luminal membrane via facilitated diffusion via the GLUT 5 transporter
2. Glucose and galactose moved through the luminal membrane via simple active transport via the SGLT.
3. Monosaccharides move through the basolateral membrane via facilitated diffusion via the GLUT 2 transporter.

Question 19

Describe the absorption of proteins in the small intestine.

Answer 19

1. Amino acids and short peptides are absorbed in the luminal membrane via secondary active transport.
2. Absorption of amino acids is coupled with Na+
3. Absorption of short peptides are coupled with H+
4. Amino acids cross the basolateral membranes via facilitated diffusion.

Question 20

Describe the absorption of fats in the small intestine.

Answer 20

1. Fatty acids and glycerol cross the luminal membrane of enterocyte via simple diffusion.
2. In the cytosol, fatty acids and glycerols resynthesize into chylomicrons.
3. Chylomicrons enter the basolateral membrane by exocytosis and enter lymphatic capillary.

Note: fatty acids and glycerols enter the absorptive intestinal surface via the micelle (acts like a shuttle)

Question 21

Describe the absorption of fat- soluble vitamins.

Answer 21

1. Fat soluble vitamins (FSV) enter the luminal membrane via the micelle (acts like a shuttle).
2. FSV are absorbed in the luminal membrane via simple diffusion
3. The FSV are resynthesized into chylomicrons.
4. The chylomicrons are absorbed in the basolateral membrane via exocytosis and into the lymph capillaries.

FSC= vitamin D, K, E, A

Question 22

All vitamins are most absorbed where?
That includes fat soluble and water soluble vitamins

Answer 22

Jejunum and ileum.

Question 23

Describe the absorption of water soluble vitamins in the small intestine.

Answer 23

1. B6 is absorbed via simple diffusion
2. B12 is absorbed via facilitated diffusion
3. Other water soluble vitamins are absorbed via secondary active transport via Na+ transporters.

Question 24

Discuss the absorption of calcium in the small intestine.

Answer 24

Calcium is absorbed in the duodenum, ileum, jejunum.
1. Ca2+ is absorbed in the luminal membrane via facilitated diffusion.
2. Ca2+ binds to calbindin to be transported into the basolateral membrane.
3. Ca+ is absorbed into the blood via secondary active transport with a Na+ and H+ coupler.

Question 25

Describe the absorption of iron in the small intestine. (REVIEW)

Answer 25

Iron is only absorbed in the duodenum. Only transcellular pathway is involved.
1. Fe2+ is absorbed in the aptical membrane via facilitated diffusion with heme.
2. In the cell, heme is removed via heme oxygenase.
3. During the 1st pool, fe is absorbed into blood via facilitated diffusion and ferroportin.
4. In the 2nd pool, fe2+ is bounded with ferritin and stored within a cell.

Question 26

Describe the aborption of Na+ in the small intestine.

Answer 26

Na+ is absorbed in the luminal membrane via the Na+-K+ATPase pump. Absorbed mainly in jejunum.

Question 27

Discuss the absorption of HCO3-

Answer 27

HCO3- is absorbed into basolateral membrane via facilitated diffusion then into the blood.

Note: absorbed in the jejunum.

Question 28

Discuss the absorption of Cl- in the small intestine

Answer 28

Cl- is absorbed via the luminal membrane via the Cl-HCO3- transporter
2. Cl- is absorbed w/ a Cl- transporter into the basolateral membrane via facilitated diffusion then into the blood.