019 Physiology of the GI tract: absoption 2

Question 1

How much iron is circulating in our blood per day?

Answer 1

Approximately 3mg

Question 2

What are our main iron storage systems in our body?

Answer 2

Red blood cells, Liver

Question 3

How much iron is absorbed and egested per day?

Answer 3

1-2mg/day

Question 4

What is transferrin?

Answer 4

Iron is toxic in excess. Hence, iron is bound to transferrin when transported around our body.

Question 5

How is the bioavailability of non-haem iron promoted by?

Answer 5

Acidic pH environment e.g. ascorbic acid (vitamin C) and gastric acid.

Question 6

Where does iron absorption take place?

Answer 6

Duodenum

Question 7

Explain duodenal iron uptake of heme iron.

Answer 7

Heme iron is taken into the enterocyte by specific heme facilitated transporter.

Question 8

What is ferric reductase?

Answer 8

This reduces non-heme iron from its ferric (Fe3+) to its ferris (Fe2+) form.

Question 9

Explain duodenal iron uptake of non-heme iron.

Answer 9

Non-heme iron must be first reduced from Fe3+ (ferric) to Fe2+ (ferris) by ferric reductase. It is transported into the enterocyte by DMT1.

Question 10

Explain the uptake of both haem and non-haem iron into the blood.

Answer 10

Within the cell, both types of iron are either (1) transported across by FPN ferroportin facilitated transporter or (2) bound to ferritin and stored within the cell.

Question 11

What is ferriportin?

Answer 11

This is the facilitated transporter that moves Fe2+ from within the enterocyte into the blood stream at the basal lateral membrane.

Question 12

What occurs after Fe2+ is transported into the blood?

Answer 12

The ferroxidase hephaestin oxidizes iron from its ferris form back into the ferric form, of which it binds to transferrin to be transferred around the blood.

Question 13

What two protein is the regulator of iron homeostasis?

Answer 13

Ferroportin found on the basal lateral membrane on the enterocytes, and hepcidin produced in the liver.

Question 14

During inflammation or high iron stores, what happens to iron absorption?

Answer 14

Iron absorption decreases because activation of HFE stimulates the enzyme hepcidin in the liver that inhibits duodenal iron absorption and lowers levels of ferroportin.

Question 15

During hypoxia, what happens to iron absorption?

Answer 15

Low oxygen levels cause inhibition of HFE protein. This lowers the production of hepcidin, hence allows more absorption of iron absorption in duodenum and production of ferroportin.

Question 16

What is HFE (Human hemochromatosis protein)?

Answer 16

This is stimulated by high iron stores or chronic inflammation and activates the iron regulator protein hepcidin.

Question 17

What is ferritin?

Answer 17

This is a universal intracellular protein that stores iron.

Question 18

What can we use to determine how much iron is stored?

Answer 18

Ferritin levels

Question 19

How do we calculate the % transferrin saturation?

Answer 19

Serum iron/ total iron binding capacity (TIBC)

Question 20

In iron deficiency anaemia, describe the levels of iron, TIBC, % transferrin saturation, and ferritin.

Answer 20

Iron - low
TIBC - high
% Tranferrin saturation - low
Ferritin - low

Question 21

In haemochromatosis,

describe the levels of iron, TIBC, % Transferrin saturation, and Ferritin

Answer 21

Iron - high
TIBC - low
% Tranferrin saturation - high
Ferritin - high

Question 22

In anaemia due to inflammation

describe the levels of iron, TIBC, % Transferrin saturation, and Ferritin

Answer 22

Iron - low
TIBC - low
% Tranferrin saturation - low
Ferritin - normal/high

Question 23

Why do we have low transferrin levels during haemochromatosis?

Answer 23

As there are high iron stores, the body does not need to signal for stronger tranferrin production.

Question 24

Why does anaemia of chronic disease have high levels of ferritin?

Answer 24

In chronic illness, ferroportin is affected. Hence, all the iron is stored within the cells and not transported into the blood.

Question 25

What % of ingested iron is absorbed per day?

Answer 25

10% (1-2mg)

Question 26

What % of ingested calcium is absorbed per day?

Answer 26

30% (0.3g)

Question 27

Where does calcium absorption take place?

Answer 27

Duodenum

Question 28

Where are iron levels regulated?

Answer 28

Small intestines

Question 29

Where are calcium levels regulated?

Answer 29

Kidneys and skeleton

Question 30

How is the bioavailability of calcium promoted by?

Answer 30

Hormone calcitriol, which is a vitamin D metabolite.

Question 31

Describe the process of intestinal Ca2+ uptake during low Ca2+ diet.

Answer 31

Ca2+ is absorbed via TRPV6 transcellularly. The Ca2+ binding protein Calbindin-D moves Ca2+ to the PMCA transporter to be released into the blood stream.

Question 32

During a high Ca2+ diet, how else is Ca2+ absorbed along with TRPV6 protein?

Answer 32

Passive paracellular Ca2+ transport also occurs in a high Ca2+ diet

Question 33

How does calcitriol stimulate Ca2+ absorption?

Answer 33

Calcitriol binds to vitamin D receptor within the enterocyte and stimulates activity of both TRPV6 and PMCA.

Question 34

When intestinal calcium absorption alone does not suffice, how does the body respond to regulate Ca2+ levels?

Answer 34

Parathyroid cells have a Ca2+ level detector. When Ca2+ levels are low, it will release a parathyroid hormone (PTH). This hormone stimulates skeletal bone to release Ca2+, Kidneys to reabsorb Ca2+, and further production of calcitriol.

Question 35

What is the parathyroid hormone?

Answer 35

This is released by the parathyroid gland in response to low Ca2+ levels. It stimulates release of Ca2+ by bones, reabsorption of Ca2+ in the kidney, and more production of calcitriol.

Question 36

What is the differences between small intestine and colonic villi?

Answer 36

Longer villi occurs in the small intestine; colon contains single layer of colonial epithelial cells.

Question 37

How much fluid is reabsorbed in the colon per day?

Answer 37

0.4-1L

Question 38

What electrolytes are absorbed and secreted into the colon lumen?

Answer 38

Na+ uptake coupled with K+ secretion. Cl- uptake coupled with HCO3- secretion. There is an overall effect of NaCl uptake, which also draws in water.

Question 39

What are the two methods of Na+ uptake in the colon?

Answer 39

Stimulation by aldosterone, and by short chain fatty acids that are produced by colonic microflora.

Question 40

How does the colon maintain K+ homeostasis especially during kidney damage?

Answer 40

Normally, K+ is reabsorbed into the blood via K+ facilitated channels in the basal lateral membrane. However during high K+ levels, K+ may be excreted out of BK channels found on apical sites of colonic enterocytes.

Question 41

How does Na+ absorption take place in the colon?

Answer 41

The primary active transport ATPase generates the Na+ gradient at the basal lateral membrane. This allows the symporter SCFA/Na+ to absorb Na+ along with SCFA into the enterocyte. Under control of aldosterone, ENaC is also stimulated to absorb aldosterone.

Question 42

What is ENac protein?

Answer 42

This is the Na+ facilitated channel that is controlled by aldosterone found on the apical part of the colonic enterocyte.

Question 43

What is the difference between colonic Cl- absorption and small intestine Cl- absorption?

Answer 43

In the colon, Cl- is absorbed transcellularly, whilst in the small intestine, Cl- is absorbed paracellularly.

Question 44

What is a prebiotic?

Answer 44

An ingested digestion - resistant oligosaccharide that maintains healthy gut bacterial population.

Question 45

What is a probiotic?

Answer 45

A live microbial food supplement that improves gut microbial balance.