HRR: absorption of water, sodium, vitamins, calcium, iron Flashcards
Where is most water reabsorbed?
The small intestine
What are villi involved in?
Absorption
What are crypts involved in?
Secretion
Describe the paracellular pathway of water reabsorption.
Water squeezes between tight junctions and into the intercellular space.
Describe the transcellular pathway of water reabsorption.
Water is absorbed through the cells, often depending on the absorption of sodium and possibly involving aquaporins.
What molecules are often involved in paracellular transport of water in the small intestine?
Glucose or amino acids coupled with sodium.
Describe how glucose can aid in paracellular water reabsorption in the small intestine.
- SGLT1 brings sodium and glucose into the cell. 2. The sodium potassium pump removes sodium in exchange for potassium from the intercellular space. 3. The movement of sodium results in chloride moving into the intercellular space as well, which drags water through the tight junctions and into the intercellular space, where it will then move toward interstitium and capillaries.
Describe water absorption in the colon.
- ENaC brings sodium into the colonocytes. 2. Sodium potassium pump exchanges sodium for potassium from the intercellular space. 3. The osmotic change in the intercellular space draws water in through tight junctions.
What are the main methods of absorption of sodium?
Na-H exchangers, sodium channels (ENaC), sodium coupled nutrient transporters.
What serves as a driving force for the apical entry of sodium?
Na-K pump.
Describe the Na-H exchanger.
NHE3 brings sodium in from the lumen and pushes a proton out in return. The Na-K pump will then exchange sodium for potassium in order to keep the sodium in the cell balanced.
Where do we see highest Na absorption from Na-H channels?
The jejunum, followed by duodenum.
Where do we see the highest Na absorption from ENaC?
The distal colon.
ENaC activity is enhanced by?
Mineralocorticoids.
Where do we see the highest sodium absorption via sodium-coupled nutrient transporters?
The jejunum, followed by the ileum.
A patient comes in super dehydrated. What solutes would we give?
Sodium, glucose, amino acids. All of these will help us reabsorb more water!
Do bacterial infections impact nutrient-coupled Na absorption?
No! Giving solutes can help as a result.
What are the fat-soluble vitamins?
A, D, E, and K.
How are fat-soluble vitamins absorbed?
They’re incorporated into chylomicrons for export into lymphatic capillaries.
What is the transporter for cobalamin?
Cubam.
Describe the absorption method of cobalamin into the enterocyte.
- Cobalamin is bound to proteins in food. It gets released via acidic stomach pH and pepsin, forming free cobalamin. 2. Haptocorrin is secreted by gastric glands and binds to cobalamin. 3. Gastric parietal cells secrete IF. 4. Proteases and HCO3 secreted by the pancreas into the duodenum release cobalamin via degradation of haptocorrin. 5. IF and cobalamin form a complex, which is then absorbed by enterocytes in the ileum via binding to cubam.
Describe what happens to cobalamin once it is in the enterocyte.
- The endocytosed vessel containing IF-CBL complex fuses with a lysosome and CBL is released. 2. Cobalamin binds transcobalamin II in a secretory vessel. 3. TCII and cobalamin are secreted and enter portal circulation.
How is calcium reabsorbed?
Transcellularly or paracellularly.
Describe active transcellular absorption of calcium.
- Calcium enters duodenal enterocyte via a channel. 2. Calcium binds to calbindin within the enterocyte. 3. NCX1, a sodium-calcium exchanging pump, or a Ca-H pump puts calcium in the interstitial space.
What is the role of vitamin D in calcium absorption?
It is necessary for synthesis of calbindin, NCX1, and the Ca-H pump.
In what form can we absorb iron?
Fe2+.
Is heme or nonheme iron easier to absorb?
Heme iron.
Describe the absorption of nonheme iron.
- Ferric iron is reduced to Fe2+. 2. Fe2+ is transported with a proton into the duodenal enterocyte. 3. Nonheme iron binds mobilferrin. 4. Fe2+ is released from mobilferrin and exits the cell via ferroprotin. 5. Fe2+ binds to a plasma transferrin to travel in the blood.
Describe the absorption of heme iron.
- Ferric iron is reduced to Fe2+. 2. Heme iron absorbs into the duodenal enterocyte via a heme transporter. 3. Iron is released from heme. 4. Nonheme iron binds mobilferrin. 5. Fe2+ is released from mobilferrin and exits the cell via ferroprotin. 6. Fe2+ binds to a plasma transferrin to travel in the blood.
