019 Physiology of the GI tract: absoption 2 Flashcards
How much iron is circulating in our blood per day?
Approximately 3mg
What are our main iron storage systems in our body?
Red blood cells, Liver
How much iron is absorbed and egested per day?
1-2mg/day
What is transferrin?
Iron is toxic in excess. Hence, iron is bound to transferrin when transported around our body.
How is the bioavailability of non-haem iron promoted by?
Acidic pH environment e.g. ascorbic acid (vitamin C) and gastric acid.
Where does iron absorption take place?
Duodenum
Explain duodenal iron uptake of heme iron.
Heme iron is taken into the enterocyte by specific heme facilitated transporter.
What is ferric reductase?
This reduces non-heme iron from its ferric (Fe3+) to its ferris (Fe2+) form.
Explain duodenal iron uptake of non-heme iron.
Non-heme iron must be first reduced from Fe3+ (ferric) to Fe2+ (ferris) by ferric reductase. It is transported into the enterocyte by DMT1.
Explain the uptake of both haem and non-haem iron into the blood.
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.
What is ferriportin?
This is the facilitated transporter that moves Fe2+ from within the enterocyte into the blood stream at the basal lateral membrane.
What occurs after Fe2+ is transported into the blood?
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.
What two protein is the regulator of iron homeostasis?
Ferroportin found on the basal lateral membrane on the enterocytes, and hepcidin produced in the liver.
During inflammation or high iron stores, what happens to iron absorption?
Iron absorption decreases because activation of HFE stimulates the enzyme hepcidin in the liver that inhibits duodenal iron absorption and lowers levels of ferroportin.
During hypoxia, what happens to iron absorption?
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.
What is HFE (Human hemochromatosis protein)?
This is stimulated by high iron stores or chronic inflammation and activates the iron regulator protein hepcidin.
What is ferritin?
This is a universal intracellular protein that stores iron.
What can we use to determine how much iron is stored?
Ferritin levels
How do we calculate the % transferrin saturation?
Serum iron/ total iron binding capacity (TIBC)
In iron deficiency anaemia, describe the levels of iron, TIBC, % transferrin saturation, and ferritin.
Iron - low
TIBC - high
% Tranferrin saturation - low
Ferritin - low
In haemochromatosis,
describe the levels of iron, TIBC, % Transferrin saturation, and Ferritin
Iron - high
TIBC - low
% Tranferrin saturation - high
Ferritin - high
In anaemia due to inflammation
describe the levels of iron, TIBC, % Transferrin saturation, and Ferritin
Iron - low
TIBC - low
% Tranferrin saturation - low
Ferritin - normal/high
Why do we have low transferrin levels during haemochromatosis?
As there are high iron stores, the body does not need to signal for stronger tranferrin production.
Why does anaemia of chronic disease have high levels of ferritin?
In chronic illness, ferroportin is affected. Hence, all the iron is stored within the cells and not transported into the blood.
What % of ingested iron is absorbed per day?
10% (1-2mg)
What % of ingested calcium is absorbed per day?
30% (0.3g)
Where does calcium absorption take place?
Duodenum
Where are iron levels regulated?
Small intestines
Where are calcium levels regulated?
Kidneys and skeleton
How is the bioavailability of calcium promoted by?
Hormone calcitriol, which is a vitamin D metabolite.
Describe the process of intestinal Ca2+ uptake during low Ca2+ diet.
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.
During a high Ca2+ diet, how else is Ca2+ absorbed along with TRPV6 protein?
Passive paracellular Ca2+ transport also occurs in a high Ca2+ diet
How does calcitriol stimulate Ca2+ absorption?
Calcitriol binds to vitamin D receptor within the enterocyte and stimulates activity of both TRPV6 and PMCA.
When intestinal calcium absorption alone does not suffice, how does the body respond to regulate Ca2+ levels?
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.
What is the parathyroid hormone?
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.
What is the differences between small intestine and colonic villi?
Longer villi occurs in the small intestine; colon contains single layer of colonial epithelial cells.
How much fluid is reabsorbed in the colon per day?
0.4-1L
What electrolytes are absorbed and secreted into the colon lumen?
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.
What are the two methods of Na+ uptake in the colon?
Stimulation by aldosterone, and by short chain fatty acids that are produced by colonic microflora.
How does the colon maintain K+ homeostasis especially during kidney damage?
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.
How does Na+ absorption take place in the colon?
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.
What is ENac protein?
This is the Na+ facilitated channel that is controlled by aldosterone found on the apical part of the colonic enterocyte.
What is the difference between colonic Cl- absorption and small intestine Cl- absorption?
In the colon, Cl- is absorbed transcellularly, whilst in the small intestine, Cl- is absorbed paracellularly.
What is a prebiotic?
An ingested digestion - resistant oligosaccharide that maintains healthy gut bacterial population.
What is a probiotic?
A live microbial food supplement that improves gut microbial balance.
