017 Physiology of the GI tract: absorption 1

Question 1

Where does most of our nutrient absorption occur in small intestine?

Answer 1

Jejunum

Question 2

What absorption takes place in the ileum?

Answer 2

Bile salts and vitamins

Question 3

What are changes in villi structure along the SI?

Answer 3

In the duodenum, villi are much shorter and wider.
In the jejunum, villi are long fingerlike projections that increase SA by 10 fold.
In the ileum, villi are shortened again.

Question 4

What is the microvilli?

Answer 4

Cell that form the brush border membrane increasing surface area by 20 fold.

Question 5

What is the estimated surface area for absorption?

Answer 5

Approximately 200m2

Question 6

What are the 3 pathological mechanism for malabsorption syndrome?

Answer 6

Intraluminal mechanisms
Intestinal mechanisms
Post epithelial mechanisms

Question 7

What is coeliac disease?

Answer 7

Intolerance to gluten. Gluten causes complexes that bind to epithelium and activate T lymphocytes, which produces toxins that cause lysis in the enterocytes. As you move up the villi, enterocytes become damaged. Hence the decrease in villi length, normal absorption cannot occur.

Question 8

What does the duodenum absorb?

Answer 8

Iron -10%

Calcium -30%

Question 9

Where is phosphate absorbed?

Answer 9

Jejunum

Question 10

In which regions is water absorbed?

Answer 10

All regions of the intestine.

Question 11

Describe the differences in Na+ concentration from lumen to blood plasma

Answer 11

Na+ concentration is both high in the lumen (-140mM) and blood, whilst low in the enterocyte (-15mM).

Question 12

What is the significance of SGLT1?

Answer 12

Mutations in this gene causes glucose galactose malapsorption. This is also responsible for absorbing 260 molecules of H2O per glucose (5L per day).

Question 13

How much fluid is normally absorbed in the small intestine?

Answer 13

8.5L

Question 14

How much fluid is secreted in the intestines per day?

Answer 14

1.5L

Question 15

Describe intestinal fluid secretion

Answer 15

Chloride secretion occurs from the cells lining the crypts of lieberkuhn in all intestinal segments. In the basal lateral membrane, the NKCC symporter transports Cl into the enterocyte. This is moved out of the cell into the lumen by CFTR carrier protein, pulling NA+ and H2O along with it due to osmotic gradient.

Question 16

What stimulates the intestinal fluid secretion?

Answer 16

Secretagoges e.g. VIP and parasympathetic nervous system

Question 17

How does Cholera toxin cause secretory diarrhoea?

Answer 17

Cholera bacteria secretes a toxin. There is a receptor on the apical part of the enterocyte in the small intestine. This stimulates the enzyme adenylyl cyclase, which catalyzes ATP to form cAMP. The increase in cAMP will enhance expression of CFTR protein. This drives Cl-, hence Na+ and H2O secretion. Patients with this disease may secrete 12L per day.

Question 18

What is the difference between secretory diarrhea and osmotic diarrhea?

Answer 18

Osmotic diarrhea - inability to absorb carbohydrate, resulting in delivery of carbohydrates to the colon and causing water to be drawn into the colon due to osmotic gradient. e.g. SGLT1 malfunction.
Secretory diarrhea - the excessive stimulation of the CFTR protein causing Cl- transportation out into the intestinal lumen, causing water to be drawn into the intestine due to osmotic gradient.

Question 19

How can we treat patients with secretory diarrhoea?

Answer 19

Give patients fluid containing sources of glucose and NaCl. Secretory diarrhoea affects secretion from the crypts. SGLT1 is not affected, so by giving fluid, we are essentially replenishing lost fluids and electrolytes.

Question 20

Where does fluid absorption take place in relation to fluid secretion?

Answer 20

Fluid absorption takes place on the villi. Fluid secretion takes place in the crypts of lieberkuhn.

Question 21

Which protein digestion enzymes are found in the villi enterocytes?

Answer 21

Endopeptidase

Aminopeptidase

Question 22

Which enzymes are used for protein digestion in the stomach?

Answer 22

Pepsin

Question 23

Which protein digestion enzymes are found in pancreas secretions?

Answer 23

Trypsin, Chymotrypsin, Carboxypeptidase

Question 24

Which enzymes are used digest peptides into single amino acids?

Answer 24

Dipeptidase

Tripeptidase

Question 25

Explain protein absorption from gut lumen into the blood.

Answer 25

Na+/H+ antiporters transport Na+ down the concentration gradient into the enterocyte, whilst transporting H+ into the lumen. The PepT1 symporter then uses energy of H+ concentration gradient to drive small peptides into the cell, where they are hydrolized and enter through basal lateral membrane through amino acid specific channels.

Question 26

What does Amylase break down?

Answer 26

Starch/glycogen into maltose

Question 27

What does Maltase break down?

Answer 27

Maltose into glucose

Question 28

What does sucrase break down?

Answer 28

Sucrose into glucose and fructose

Question 29

What does lactase break down?

Answer 29

Lactose into glucose and galactose

Question 30

Discuss the presence of intestinal GLUT2 during digestive phase and its significance in diabetes and obesity.

Answer 30

Between meals, SGLT1 is the dominant transporter of glucose at the brush border membrane. Its high affinity allows it to absorb low levels of glucose in the lumen. During the digestive phase, GLUT2 is the dominant transporter at the brush border membrane. This provides a high capacity route for glucose absorption. It seems that GLUT2 is up-regulated in recent models of obese and diabetic human patients.

Question 31

Who qualifies for bariatric surgery?

Answer 31

Those with BMI >40 or BMI >35

Question 32

How many bariatric surgery procedures are performed annually in England?

Answer 32

8000

Question 33

Where does protein absorption mainly occur

Answer 33

Jejunem and upper ileum

Question 34

Name some luminal digestive enzymes

Answer 34

Trypsin, chemo trypsin, aminopeptidase, carbixylopeptidase

Question 35

Name some brush border protein digestion enzymes

Answer 35

Endopeptidase, aminopeptidase

Question 36

Where is Maltese located

Answer 36

Brush border digestion

Question 37

What are the 3 common types of bariatric surgery

Answer 37

Adjustable gastric band: a band that can adjust the cardiac sphincter of the stomach
Route en Y gastric bypass: cutting out a portion of the stomach and the duodenum, so that a small pouch of the stomach is just directly connected to the jejunem
Vertical sleeve gastrectomy: just cutting out a huge chunk of the stomach