Renal regulation of water and acid-base balance lecture Flashcards
LO:
This session aims to discuss:
- Different renal processes regulating water balance.
- The role of Vasopressin in urine production and excretion.
- The role of kidney in maintaining body’s acid-base balance.
- Different renal-regulation associated clinical disorders.
This session relates to the following TILO:
- Urogential homeostatic mechanism: summarise the mechanisms regulating ion/water balance and acid-base homeostasis under normal and pathological conditions.
Just remember chloride always goes back into blood in this case!
Renal tubules: Transport mechanisms
Renal tubules: Transport pathways
Reabsorption in Early Proximal Convoluted Tubule
Reabsorption in Henle’s loop
Reabsorption in early Distal convoluted Tubule
Reabsorption in distal DCT & Collecting duct
Session plan
Osmosis and osmolarity
Calculate the osmolarity for 100 mmol/L glucose and 100mmol/L NaCl.
Osmosis-fluid particles move from low solute concentration to a high solute concentration until equilibrium is reached.
Osmotic pressure is what is pulling these. Not dependent on size of particles but number of particules
Osmolarity for glucose is equal to molarity, but for NaCl it dissociates into Na and Cl so osmolarity is twice the molarity.
So remember osmolarity doesn’t always equal molarity!
Body fluid distribution
2/3s ie most fluid sits inside cells
Inside extracellular, 1/4 sits in intravascular compartment (plasma)
Interstitial fluid-fluid that bathes cells
Transcellular fluid-different regions of body. =The portion of total body water contained within epithelial-lined spaces, such as the cerebrospinal fluid, and the fluid of the eyes and joints.
Water can be lost from your body in 2 ways-unregulated loss and regulated loss
Unregulated-not in our control
Regulated-renal regulation
In the body, we need to be able to control how much water we are losing, in response to our body’s needs. This is where renal regulation kicks in. Renal regulation does 2 types of water balances:
POSITIVE water balance
When water enters or leaves your body, the FIRST compartment it enters is your extracellular fluid compartment, and only overtime it equilibrates with the intracellular fluid compartment. So first compartment that is hit is your ECF compartment.
So now you’ve taken in this high water intake, this would enter your extracellular fluid compartment and so the volume of this will increase and result in a decrease in your sodium concentration and therefore a corresponding decrease in the osmolarity of the ECF compartment.
To balance this out, the kidneys will produce hyposmotic urine (the osmolarity is compared to your plasma). So the body wants to lose this extra water and so the kidney does this and then your osmolarity normalises.
NEGATIVE water balance
eg person is dehydrated or not taking in enough water. In this case your ECF volume will go down, and there will be a corresponding increase in the sodium concentration and therefore osmolarity increases. So now kidneys want to preserve water and so produce hyperosmotic urine.
In addition to this, your body also triggers thirst, so in combination of producing hyperosmotic urine and increasing water intake your body then normalises your osmolarity.
Water reabsorption
In PCT-reabsorb 67% of water, Na, Cl
In loop of Henle-ascending-can’t absorb water but salt can be, passive in thin ascending limb, active in thick ascending limb.
In thin descending limb-can’t absorb salt, but water is being passively absorbed.
- Since water is reabsorbed through the passive process of osmosis, it requires a gradient.
- The medullary interstitium needs to be hyperosmotic for water reabsorption to occur from the Loop of Henle and Collecting duct.
The reason for all of this to occur is because you want your water to be passively absorbed, and it is occuring by osmosis. Body is very smart, it doesn’t want to lose energy in absorbing this water, so what it does is, it wants the water to passively flow back into your medullary interstitium, and for this a gradient is required. You need a gradient and you need the medullary interstitium to get hyperosmotic, and then your water would flow out of the loop of Henle and into the medulla.
Next, when your filtrate moves into your DCT and then your collecting duct, variable amounts of water is reabsorbed, and that is under the control of vasopressin hormone (ADH).
Countercurrent multiplication
When filtrate arrives at loop of henle =300mOsmol/L, which is equal to the osmolarity of blood plasma.
Next salt is actively reabsorbed from your ascending limb into medullary interstitium, and as you can see the osmolarity here goes down and the osmolarity of the interstitium increases.
Following this, water passively flow out of your thin decending limb to equilibrate with your interstitium, so the osmolarity in the thin descending limb increases.
Next fresh filtrate arrives at loop of henle (remeber this is isoosmotic to plasma, so again it would be 300). Then you’ve got a shift down, so these 400s go down and the fresh 300s move in.
So next what your nephron does is active salt reabsorption, so salt is actively pumped into the interstitium. So the osmolarity decreases in the thick ascending limb and the osmolarity of the interstitium increases.
Next water passively flows out of the thin descending limb to equilibriate with the interstitium.
So that is what keeps happening, as new filtrate keeps arrives at your nephron, this process keeps happening, so salt is actively reabsorbed and water keeps passively flowing out. And after going through this process several times, you can see a gradient has already started developing. You had something like 300 here and now we can see there is a gradient in the interstitium in the last pic, and when this process happens a few more times, we see that a gradient of 300 to 1200 can be produced, and this gradient helps water to passively flow out of your loop of henle and also from your collecting duct, and then the water is reabsorbed by your body.
Salt isn’t the only thing responsible for this gradient, urea is also responsible.
Urea recycling
Urea-plays role in developing medullary interstitial gradient
Urea is also reabsorbed into the medullary interstitium and that also contributes to increasing the osmolarity of your medullary interstitium, and that also helps in water reabsorption, which occurs both from your thin descending limb and your collecting duct.
vasa recta-series of blood capillaries that supply blood to your nephrons.
So what happens is your filtrate arrives at the collecting duct and there are 2 urea transporters, so you have UT-A1, which is present on the apical cell membrane, and you have UT-A3, which is present on the basolateral cell membrane.
So you have urea, which has arrives here and through the function of these 2 urea transporters, it is pumped out into your interstitium, and as the urea is pumped out, the osmolarity can go as high as 600 mmol/L, which is a really high concentration. Now this urea which has reached your medullary interstitium, it can be absorbed by both your vasa recta and your loop of Henle. It enters you loop of Henle from the thin descending side.
Urea that have reached medullary interstitia can enter both vasa recta via UT-B1 (UT-B1 is present on both your apical and basolateral sides. Think Basa recta, v sounds like b so transporter is UT-B1) and thin descending limb via UT-A2.
So as you can see, your urea, when it reaches the collecting duct, it enters your interstitium, increasing osmolarity. Then it is reabsorbed by your loop of henle, so it keeps getting recycled in your rephron. And what that helps to achieve is, first of all it helps in the water reabsorption, as you can see the osmolarity in the interstitium has risen, which will help in water reabsorption. So the first thing that happens is urine concentration increases, because your water is being reabsorbed.
And secondly, your urea excretion will require less water. So all of this urea which is getting concetrated in the nephron, when it reaches the collecting duct, because it is concentrated in cells of the nephron as well, it will require less water to be excreted from the body. And so we don’t have to spend a lot of fluid in getting urea excreted out of our body. So urea recycling acheives both of these purposes. So because urea is absorbed back into the thin descending limb, it is being recycled, so by the time it reaches the collecting duct its concentration has started developing here. And since water has been reabsorbed there is less water in the collecting duct, and if you remember as we go deeper into the medulla, the gradient is increasing (300 to 1200). So further down collecting duct you have less water and more urea so therefore this urea is then excreted from your body in less water.
AVP-is able to increase abundance of UT-A1 and UT-A3, so it can increase permeability of collecting ducts to urea, and that could increase the amount of urea that is in the medullary interstitium and that would obviously help water reabsorption.
Quiz
=False
Osmolarity of 100 NaCl will be 200 as it will disociate into Na and Cl. So it would be the same as 200 Na
False
First enters ECF, after time it equibrilates with ICF
True
Vasopressin/Anti-Diuretic Hormone (ADH)
If plasma osmolarity rises, detected by osmoreceptors in hypothalamus which are very sensitive so they stimulate avp release so that will increase the number of aquaporin channels and this increases water reabsorption.
Decrease in blood vol/pressure, want to be able to conserve these, so this is detected by baroreceptors, but these are less sensitive than osmoreceptors, AVP production is stimulated.
Factors such as nausea, angiotensin 2 and nicotine also stimulate ADH (NAN stimulates adh)
When have a lot of alcohol, feel dehydrated as ADH is inhibited so have dilute urine. Think A and E decrease AVP seceretion.
Mechanism of action of ADH
V2 receptor of basolateral membrane for AVP. Aquaporin 3 and 4 channels are always present on your basolateral cell membrane.
AVP comes via blood vessel and binds to V2 receptor, so G protein signalling cascade is activated which results in the activation of protein kinase A. Protein kinase A increases secretion of aquaporin 2 channels in this vesicle form which is then transported to your apical cell membrane. So water is reabsorbed through AQP2 and then into blood via AQP3 or 4 channels.
It is able to up or down regulate number of aquaporin 2 and 3 channels on both apical and basolateral membranes
Diuresis (Increased dilute urine excretion)
Low ADH:
So isosmotic fluid (to blood ie around 300) enters loop of Henle.
Thick ascending limb: Nacl is actively reabsorbed in thick ascending limb. So Na/K pump actively pumps out 3 na ions and pumps 2 k ions in, so inside the cell there is low sodium, and using that gradient your sodium from your tubular fluid, enters the cell in downward way because there is low concentration in cell and high concentration in tubular fluid. And along with that potassium and chloride also transported into the cell.
Potassium and chloride then leave the cell by K-Cl symporter, and that again is reabsorbed by the blood.
So by the use of these transporters, salt is reaborbed by the blood. so by the time the fluid enters the DCT, it is now hypoosmotic.
DCT:
Inside DCT, sodium and chloride are again being actively reabsorbed, but since we don’t have ADH or have a small amount of ADH, the Aquaporin 2 channel is absent so water is not reabsorbed.
In DCT, again have downward gradient for Na-cl symporter. The energy that is released from the downhill movement of Na is used by chlorine, and it leaves the cell through the K-Cl symporter. So that is again how your NaCl is reabsorbed by the body.
Collecting Duct:
So fluid is hypoosmotic. Inside collecting duct NaCl is actively reabsorbed, and even though there is very little/zero ADH, water still manages to be reabsorbed, because it can go through paracellular pathways and there is still a small amount of aquaporin 2, 3 and 4 present.
Inside the collecting duct, you’ve got your principal cells and they reabsorb sodium again through the use of the sodium-potassium ATPase pump and channels.
So by the time your urine is excreted from your body, because it has gone through a lot of salt reabsorption and very little water reabsorption, the osmolarity of this is around 50mOsm/L
So osmolarity of urine is very low, could be due to diseased state or just as you had had lots of water. The volume of urine excreted could be as high as 18L.
Antidiuresis (Concentrated urine in low volume excretion)
eg due to dehydration or disease
When salt is being actively reabsorbed by your ascending limb, your DCT and your collecting duct, you ADH boosts the number of these different symporters and channels and therefore supports sodium reabsorption. (Note-it seems to just boost transporters of on apical side ie triple transporter, NaCl symporter and sodium channels.)
So when your hypoosmotic fluid, which is in your DCT, because ADH levels are present in high numbers, water is being reabsorbed both in DCT and in your cortical collecting duct and your medullary collecting duct.
And when it goes through your medullary collecting duct, (as gradient established) water is reabsorbed in huge amounts, so by time urine leaves your body, the osmolarity of the urine could be as high as 1200mOsm/L and the volume could be as low as 0.5L per day, so would lose a very small amount of water.
ADH-related clinical disorders
Central/pituitary diabetes insipidus-could be genetic or acquired due to trauma or infection
Low ADH so produce lots of urine so thirst would be triggered and would drink lots of water. This could be treated by giving external ADH eg desmopressin (ddAVP)
SIADH-high ADH, due to trauma, infection or even cancer, in this case body will conserve lots of water so blood volume will increase and sodium conc in blood will go down. Urine would be hyperosmolar.
Can be corrected by inhibitor of ADH receptor, as if ADH receptor was inhibited, then the aquaporin channels would not be inserted so water would not be absorbed by body and it could release excess water. Treatment=Fluid restriction and Vaptans, which are nonpeptide vasopressin receptor antagonists (VRA)
Nephrogenic diabetes insipidus-again could be genetic or acquired. Correct amount of ADH being produced but issue is in collecting duct. Could be ADH can’t bind to V2 receptor, so it does not lead to proper signalling pathway, or your aquaporin 2 gene is mutated, so it is not producing enough or correct aquaporin 2 channels.
Corrected using NSAIDs or Thiazide Diuretics-diuretics reduce rate of filtration so less blood being filtered so amount of urine produced is less
True
True-can regulate both 2, 3 and 4 aquaporin channels
False as have a lot more ADH being secreted so water is being conserved by body so going to be hypoosmotic
Section 2: Renal regulation of acid-base balance
Acid-base balance
Through food we eat and how it is metabolised in body, both acid and base is continuously added to our fluid compartments.
Lots of base or bicarbonates are actually lost in faeces, so that results in the net addition of metabolic acid to your body-could be as much as 50-100 mEq/day. So this metabolic acid needs to be neutralised otherwise it would have big impact on pH of blood. This is done by your bicarbonates, so your bicarbonates neutralise this acid, resulting in the production of the sodium salts of these strong acids. And these strong acids are then excreted in your kidneys and the carbon dioxide which is produced along with it, is then released by the lungs.
Now your bicarbonate in the ECF compartment is around 350 mEq, as you can see 50-100 mEq is added to our body per day, so this volume (350) would pretty much run out in 4-7 days, therefore our body needs to add more bicarbonate ions to our body, and that is again done by our kidneys.
See role of kidneys: almost 100% of bicarbonates are reabsorbed by body
Role of Bicarbonate ion as a buffer
Carbonic acid is produced as a result of the hydration of CO2 and this results in 1 bicarbonate ion and 1 proton.
Bicarbonate buffer system is being managed by both kidneys and lungs, so system is regulated by BOTH!
Henderson-Hasselbach equation gives the effect on pH, because of bicarbonate and carbon dioxide.
When partial pressure of CO2 rises in our body, it leads to more protons, so that would lead to acidosis. And if PCO2 goes down it will cause protons to go down so could also lead to alkalosis. This proton concentration is inversely proportional to bicarbonate ie if bicarbonate ion conc increases it could cause alkalosis (proton conc decreases).
If acid base disorder is caused by CO2 then it is respiratory acid-base disorder, but if caused by changes in bicarbonate ions, then we call it metabolic acid-base disorders
80% of bicarbonate reaborbed in PCT, 10% in thick ascending limb and remaining 10% combination of DCT and collecting duct.
Reabsorption of bicarbonate ion
In the tubular fluid you have presence of both proton and bicarbonate ions, through the action of CA it produces water and CO2. In PCT, CO2 is entering cell by diffusion, then combines with H20 by action of enzyme carbonic anhydrase, produces a proton and a bicarbonate ion. This proton then exits cell via NHE3 antiporter which uses downhill energy gradient released by sodium to transport the proton into the tubular fluid. Second mechansim is your pump so proton is being pumped into tubular fluid by H+ ATPase. Bicarbonate leaves the cell through the use of sodium bicarbonate symporter, and this then enters your blood.
Similar process occurs in thick ascending limb
There are 2 types of intercalated cells- alpha secretes proton, beta secrete bicarbonates
Majority of the time, the role of alpha intercalated cell is more important as we want to lose all this extra acid. But role of beta intercalated cell becomes really importent in the case of alkalosis, where the body wants to lose the extra bicarbonates.
note the hydrogen potassium exchanger on the alpha intercalated cell-as hydrogen ions are secreted potassium is being reabsorbed.
New bicarbonate ion production
Talked about the bicarbonate ions being reabsorbed by your nephron, but that is not enough to be able to take care of all the needs or the neutralisation of all the acid that is being produced in our body.
Kidney needs to go a step further and produce NEW bicarbonate ions as well and this is done via 2 different methods:
The kidneys can generate (produce) new HC03- through (a) urinary excretion of ammonium (NH4+) salts and (b) urinary excretion of titratable acids.
First is done in PCT= ammoniagenesis
What happens here is your glutamine produces 2 molecules of ammonia, 1 divalent anion which gives 2 molecules of bicarbonate that are reabsorbed by your blood.
Now these ammonia ions need to be excreted by body as if these ammonia ions enter into blood circulation they would be converted into molecule of urea and one molecule of proton molecule, by our liver. Then that proton molecule would need to be neutralised by a bicarbonate ion, and therefore the net addition of bicarbonate here would be nullified, so there would be no new gain of bicarbonate ions. So it is really important that kidneys excrete out these ammonia ions.
These ammonia ions are transported into the tubular fluid by 2 different methods. First one is by substituting the proton into the Na-H+ antiporter and just transporting it into the tubular fluid.
A second way is diffusion in the form of NH3 gas. Once this gas reaches the tubular fluid, it is protonated and again you have the ammonia ion, which is then excreted by your kidney.
The second method is in collecting duct:
Remember in this alpa intercalated cell, we had pumped all of these protons out, so these protons are then neutralised by a buffer in the urinary system, which is not bicarbonate. So if you’ve got a phosphate buffer here which another more common urinary buffer, and then this ion produced is then excreted out. So how are we gaining a new bicarbonate ion? Here with action of carbonic anhydrase, we produce one molecule of proton, and one molecule of bicarbonate. This proton, because it is being neutralised by a urinary buffer which is other than a bicarbonate. This bicarbonate ion which is being produced and transported to the blood is in effect a new bicarbonate.
Through the use of these 2 different methods our kidneys are able to support the acid base balance and provide extra bicarbonate ions.
Acid-base imbalance
Metabolic acidosis-due to change in bicarbonate ions, ie when bicarbonate ions go down, due to HH equation, pH will go down.
Lungs increase ventilation by hyperventilation, so there is decreased CO2 so there is decreased H+ ion and so pH will go up and it will be balanced out.
Long term, kidneys also play a role, they increase the bicrabonate reabsorption and production to help with acidosis.
Metabolic alkalosis-bicarbonate goes up so pH goes up, so go for hypoventilation, so CO2 goes up, so H+ goes up so this will decrease pH and balance out metabolic alkalosis. Kidneys respond to this by increasing bicarbonate ion secretion.
Respiratory (ie due to effect of CO2) acidosis-acute process-increased CO2 enters into cell and by action of CA it is hydrated, and it produces 1 molecule of proton and 1 molecule of bicarbonate. This proton molecule is taken care of by the cellular proteins, so in effect you get a new bicarbonate ion, which is then transported into blood to balance out decreased pH. (Ie acute process=cellular buffering)
The chronic process-kidneys take care of this long term increasing bicarbonate reabsorption and production (by increasing excretion of the acid and ammonia ions so you get more bicarbonate ions available for body)
Respiratory alkalosis-Taken care by intracellular buffering, it shift equation to the left by producing more CO2 and helping in decraesing pH.
Long term-body decreases bicarbonate production and reabsorption, as need less.
1) pH is low so acidosis
2) Analyse PCO2 and HCO3-
When there is reduced bicarbonate ion, this is metabolic acidosis as otherwise pCO2 would be high. Why is pCO2 less? Because lungs have kicked in and started to increase ventilation to decrease CO2 and decrease hydrogen ions.
=Respiratory alkalosis
As pCO2 is low and bicarbonate is also down as renal compensation has kicked in.
