Absorption of ions and water Flashcards
What is the movement in the small intestine?
Transcellular- solute crosses the two cell membranes, active transport through channels.
Paracellular - solute moves passively down concentration gradient, between cells via tight junctions
What is the volume of liquid through the GI tract?
Saliva 1.5L
2L water
2L gastric juice
2L pancreatic and bile secretions 1L small intestinal secretions
What is the volume of liquid reabsorbed in the GIT?
6.5L reabsorption in small intestine
1.9L reabsorption in large intestine
So only about 0.1L is excreted.
How does Na+ cause water to be absorbed into cells?
Water moves by osmosis towards areas of higher solute (ion)concentration.
Na+ is pumped out of the cell through the Na+/K+ pump, which decreases the concentration in the cell, so Na+ enters from the lumen down the concentration gradient.
How is water absorbed into cells?
Cl- will also enter because of Na+, causing the cell to be hypertonic, so water flows into the lumen.
Water enters via the tight junctions into the intercellular spaces.
How does Na+ absorption occur?
Na+/K+ ATPase pump, primary transport, it creates the gradient and keeps intracellular Na+ low.
Secondary transport via:
Na+/Glucose transport or Na+/amino acid transport.
Na+/H+ exchanger.
Parallel Na+/H+ and Cl-/HCO3- exchange.
What is Na+/glucose transport?
Uses the SGLT1 cotransporter to move in glucose, galactose or neutral amino acids with Na+.
High amount in the jejunum.
This is down an electrochemical gradient.
What is the Na+/H+ exchanger?
Na+ is exchanged for H+.
Down the electrochemical gradient.
In the jejunum.
Takes place when pH is neutral or alkaline (HCO3-).
What is parallel Na+/H+ and Cl-/HCO3- exchange?
Na+ is exchanged for H+, and coupled to the Cl-/HCO3- exchanger.
It is electroneutral, as the charges balance out.
Primary method of Na+ absorption in the fasted state.
Regulated by cAMP/cGMP and Ca2+ via adenylyl cyclase.
What is E coli food poisoning?
Enterotoxin binds to enterocytes
Toxin is internalised - endocytosis
Interacts with Gs G protein, which increases cAMP levels.
This leads to increased Cl- secretion, which blocks Na+ and Cl- uptake.
How does E.coli cause diarrhoea?
In the lumen, there is increased levels of Na+ and Cl-, so H2O goes into the lumen rather than being absorbed into the enterocyte - causes diarrhoea.
How is calcium absorbed?
Paracellular - passive transport, along concentration gradient.
Active transport by transcellular route.
Requires vitamin D receptor - VDR.
In duodenum
What is the process of calcium absorption?
Ca2+ enters through TRPV6 channel.
Ca2+ is toxic in the cell, so Ca2+ is bound by calbindin-D9K, which puts it in vesicles and reduces free Ca2+.
Ca2+ can then transport around enterocytes, then released into interstitial space via PMCA (plasma membrane Ca2+ ATPase.
What is the VDR?
Vitamin D binds and the VDR ensures there is transcription of the TRPV6 receptor and PMCA channel.
This promotes uptake of Ca2+ from the lumen into the body.
What is vitamin D deficiency?
Poor diet/lack of sun.
Bone softening
Demineralisation
Causes hypocalcaemia.
What is osteoporosis?
Creation of new bone tissue does not keep up with removal of old tissue.
The bone becomes brittle and weak.
Vitamin D, Ca2+ and Na+ are low.
What is Osteomalacia?
Softening of bone.
Due to vitamin D deficiency.
What is iron?
Haem iron - very bioavailable, derived from myoglobin and haemoglobin, animal sources.
Non-haem iron, inorganic, most is not absorbed.
Oxalates disrupt iron absorption, found in plants.
What is iron?
Haem iron - very bioavailable, derived from myoglobin and haemoglobin, animal sources.
Non-haem iron, inorganic, most is not absorbed.
Oxalates disrupt iron absorption, found in plants.
How is iron involved in oxygen transport?
Haemoglobin - carries oxygen.
Myoglobin
Haematopoiesis - red blood cell production.
What are the forms of iron?
Ferrous - Fe2+
Ferritin - Fe3+
In food, Fe exits as ferritin, but cannot be absorbed in this form.
How is iron absorbed?
Fe3+ is converted to Fe2+ by ferric reductase, transported into enterocyte by DMT1.
In the cells, Fe2+ is converted to Fe3+ by hephaestin, transported by ferroportin to the outside of the cell.
Fe3+ is then transported into the blood via protein carrier transferrin.
What does the Fe and transferrin do?
Erythropoiesis - synthesis of erythrocytes.
Or goes to the liver and stored as ferritin. The liver can release stored Fe3+ back to circulation through ferroportin.
What is Hepcidin?
Hepcidin is for regulation of Fe.
Inhibits ferroportin in the liver which decreases plasma Fe concentration.
Hepcidin reduces absorption of Fe from the lumen of the small intestine via inhibiting ferroportin.
