Renal Regulation of Acid-Base Balance -Langston 9/21

renal control of Acid-Base Balance

Rather than merely buffering, the kidneys can produce acidic urine to excrete hydrogen or they can produce alkaline urine to excrete bicarbonate. The kidneys are critical for ultimate control of acid-base balance Proteins metabolism generate acid, and thus carnivores have a net gain of acid that ultimately needs to be excreted. Herbivores may ingest more alkali and thus must excrete bicarbonate. The kidney is able to perfume either task. Bicarbonate is freely filtered at the glomerulus and typically, all of it is reabsorbed in dogs and cats. Each bicarbonate that is reabsorbed requires that a hydrogen ion is secreted. A vast amount of bicarbonate is filtered and thus bicarbonate Reabsorption is quantitatively Important H+ are secreted into the UF, above and beyond what is needed to reabsorb bicarbonate for a net acid excretion . Metabolism of proteins and phospholipids produces 40-80mEq of NONVolatile acid per day (human) that need to be secreted. Because they are nonvolatile (not H2CO3, and thus can't be excreted by lungs) it is up to the kidneys to excrete them. The volatile acid (CO2) that is produced by carb & Fat Metabolism can be excreted by lungs. With acidosis the kidney reabsorbs all the filtered bicarbonate and makes new bicarbonate to enter the blood (secreting H+ in the process) With alkalosis, the kidney do NOT resorb all the bicarbonate, allowing some to be excreted in the urine correcting alkalosis

bicarbonate Reabsorption: 85% in prox tube 10% in LOH 5% distal tube

bicarbonate Reabsorption: 85% in prox tube 10% in LOH 5% distal tube

ammonia as a buffer in prox tubule

* Ammonium ions (NH3) trap H+ in the urine *Some cell membranes are permeable to NH3, ( but not NH4+) Ammonia - NH3 Ammonium-NH4+ Urea Ammonia (NH3) is a Lipid soluble and some cell membrane are permeable to it (with out a carrier molecule). Ammonium (NH4+) is Polar, but it can bind like H+ to a carrier molecule Glutamine can diffuse into the Prox. tube cells, where it is converted to 2 Ammonium ions (NH4+) & 2 bicarbonate ions. The ammonium is secreted into the UF using the : Na+/H+ counter-transporter, and the new bicarbonate is transported into the Peritubular capillaries.

*Because Phosphate is present in the urine in high amounts and can accept H+, it makes a powerful urine buffer Reabsorbing all of the filtered bicarbonate is necessary but NOT sufficient for the net acid excretion that is needed in Dogs and Cats. Only a limited amount of Free H+ can be excreted in urine, because the urine, because the urine can only be acidified to a pH of about 4.8. Secreted H+ can be buffered by substances other than Bicarbonate. = Phosphate

*Because Phosphate is present in the urine in high amounts and can accept H+, it makes a powerful urine buffer Reabsorbing all of the filtered bicarbonate is necessary but NOT sufficient for the net acid excretion that is needed in Dogs and Cats. Only a limited amount of Free H+ can be excreted in urine, because the urine, because the urine can only be acidified to a pH of about 4.8. Secreted H+ can be buffered by substances other than Bicarbonate. One such Buffer is PHOSPHATE NaHPO4- is filtered at Glomerulus. Part is reabsorbed, but there is usually net phosphate excretion with Typically diets. Secreted H+ is buffered by filtered NaHPO4-, forming NaH2PO4. Bicarbonate that is returned to the blood is new bicarbonate (not replacement for filtered bicarbonate), allowing net acid excretion. The acid secreting mechanisms are the same as previously discussed: a Na+/H+ secondary active transporter in the Prox. segments and a H+ATPase primary active transporter in the Distal segments. Phosphate is an excellent Urinary buffer, because the Pka (the pH where buffering is Maximal ) is 6.8, the. usual pH of urine. Titratable acidity is the amount of base needed to get a 24-hour urine sample to a pH fo 7.4 . Phosphate is equated with Titratable acidity because it is the major contributor.

1. All filtered bicarbonate is reabsorbed -this happens by secreting H+ into the UF, which binds filtered HCO3-. -A HCO3- molecule is reabsorbed into the blood (although it is NOT the same HCO3- molecule) 2 a. To excrete the ACID that is consumed/produced, H+ is secreted into UF in excess of that needed to reabsorb bicarbonate. -A small fraction of that is free H+, which can decrease the urine pH to as low as 4.8. b. Half of the net acid excretion is buffered by urinary Phosphate, the other half by Ammonium. -each H+ secreted generates a NEW HCO3-, then enters the blood.

1. All filtered bicarbonate is reabsorbed -this happens by secreting H+ into the UF, which binds filtered HCO3-. -A HCO3- molecule is reabsorbed into the blood (although it is NOT the same HCO3- molecule) 2 a. To excrete the ACID that is consumed/produced, H+ is secreted into UF in excess of that needed to reabsorb bicarbonate. -A small fraction of that is free H+, which can decrease the urine pH to as low as 4.8. b. Half of the net acid excretion is buffered by urinary Phosphate, the other half by Ammonium. -each H+ secreted generates a NEW HCO3-, then enters the blood.

1. All the filtered bicarbonate there must be reabsorbed 2. Excess H+ must be secreted. This is accomplished by two main mechanisms a. H+ is secreted into the UF and buffered by phosphate - Titratable acidity (phosphate) b.H+ is secreted into the UF and buffered by ammonium - Ammonia /ammonium Each H+ secreted generates s NEW HCO3- that enters the blood

1. All the filtered bicarbonate there must be reabsorbed 2. Excess H+ must be secreted. This is accomplished by two main mechanisms a. H+ is secreted into the UF and buffered by phosphate - Titratable acidity (phosphate) b.H+ is secreted into the UF and buffered by ammonium - Ammonia /ammonium Each H+ secreted generates s NEW HCO3- that enters the blood

defenses against changes: buffers, Lung , kidney

1. Chemical Buffers -Immediately combine with acid or base to minimize [H+] change -SECONDS -The chemical buffer systems of the body will immediately combine with acid or base to prevent changes in pH. -They do NOT remove the acid or base; they merely tie them up until balance can be restored Bicarbonate, phosphate, and Proteins are some of the main channel buffers 2. lungs -Remove CO2 (and H2CO3) -MINUTES -The lungs can act within minutes to remove CO2 (and thus H2CO3, a weak acid) to buffer the blood 3. Kidneys -Excrete (or retain H+) -DAYS -The kidneys take hours to days to buffer, but they are the most powerful system, and can correct acidosis or alkalosisi

Regulation of renal Acid base balance The most important stimuli for in creating H+ secretion by the tubules in acidosis X 3

1. an increase in PCO2 in the Extracellular fluid -PCO2 enters the tubes cells and increases formation of H+ 2. An increase in H+ concentration in the Extracellular fluid (decreased pH) 3 a possible 3rd factor is excess Aldosterone production, which stimulates Intercalated Type A cells to secrete more H+, leading to metabolic alkalosis.

1 volatiles acids 2. fixed acids 3. buffers 4. pK 5. Chemical buffers

1. breathable 2. Not breathable 3. prevent big change in pH by binding up an H+ (acid) or a base 4. ph at middle of that curve where half is weak acid and half is conjugate base. 5. superfast, lungs-fast, Kidneys-staying power

ammonia recycling .

Ammonium is generated in the proximal tubule cells and secreted into the lumen It travels thru the LOH and can be reabsorbed in the Ascending Thick limb by substituting for K+ on the Na+K+2Cl- pump. Partial dissociation of NH3 and H+ then occurs in less the less acid tubular cell. The NH3 diffuses into the medullary interstitium where it reaches relatively HIGH concentrations. It then diffuses back into those segments that have the lowest pH and therefore have the most favorable gradient : S3 segment of the Late prox. tube and , more importantly, the Medullary Collecting tube., where the secreted NH3 is trapped as NH4+, and then excreted .

Bicarbonate Reabsorption 85% prox tube 10% LOH 5 distal tubule

Bicarbonate Reabsorption 85% prox tube 10% LOH 5 distal tubule

Bicarbonate is the most important Extracellular buffer.

Bicarbonate is the most important Extracellular buffer. Proteins are important intracellular buffers, especially hemoglobin in RBCs, which accounts for 80% of the non-bicarbonate buffering of whole blood. Phosphate is an important intracellular and urinary buffer. Bone carbonate has a vast amount of buffering capacity for chronic acidosis but does not respond quickly . No matter how many buffers are present, there is only one [H+] and one pH. This is the isohydric Principle. The ratio of any buffer can be predicted with Henderson -Hasselbalch equation and law of mass actions

Bicarbonate system

Bicarbonate pK far from body pH Effective buffer despite this Concentration in blood Open system

Buffers are made of : *

Buffers are made of : *Weak Acid & Conjugate base *Weak Base & conjugate acid A buffer binds the H+ provided in the Strong acid or OH- from the strong base. Instead of the strong acid causing a big pH change, there is a much smaller change in pH. Buffers work best within 1 pH unit of their pK. Buffers accept or donate protons to minimize changes in pH.

Carnivores produce acid from metabolism of protein Herbivores may eat more alkali than acid.

Carnivores produce acid from metabolism of protein Herbivores may eat more alkali than acid.

Cells in the Last Distal tubule 1. Principal cells -Reabsorb Na+ -Secrete K+ 2. Intercalated Type A Cells -Secrete H+ -Reabsorb K+ 3. Intercalated Type B cells -Secrete HCO3- **Primarily Active transport to Secrete H+ is necessary for finely tuned acid-Base balance . This occurs in the Distal and Collecting Tuves.

Cells in the Last Distal tubule 1. Principal cells -Reabsorb Na+ -Secrete K+ 2. Intercalated Type A Cells -Secrete H+ -Reabsorb K+ 3. Intercalated Type B cells -Secrete HCO3- **Primarily Active transport to Secrete H+ is necessary for finely tuned acid-Base balance . This occurs in the Distal and Collecting Tuves.

Chemical Buffers bind H+ or Release H+ Bicarbonate -is one of the most important chemical buffers. -it is present in high quantities in the blood Protein -are important intracelluar buffers. For example, Hemoglobin is a protein found in high concentrations in RBCs and provides a lot of the buffering capacity of blood. -In fact, Hemoglobin provides 80% of the non-Bicarbonate buffering of blood -the pK of most proteins is around 7.4 which is a normal ph, which means they are functioning in their most effective range. Phosphate is an important buffer in both the intracelluar compartment and the urine. Bone is a major buffer with chronic acidosis

Chemical Buffers bind H+ or Release H+ Bicarbonate -is one of the most important chemical buffers. -it is present in high quantities in the blood Protein -are important intracelluar buffers. For example, Hemoglobin is a protein found in high concentrations in RBCs and provides a lot of the buffering capacity of blood. -In fact, Hemoglobin provides 80% of the non-Bicarbonate buffering of blood -the pK of most proteins is around 7.4 which is a normal ph, which means they are functioning in their most effective range. Phosphate is an important buffer in both the intracelluar compartment and the urine. Bone is a major buffer with chronic acidosis

Excreting Bicarbonate to Correct Alkalosis

Cow/Herbivores The kidney, being quite versatile, can excrete Bicarbonate when necessary. This might include when Metabolic Alkalosis develops or in Ruminant consuming a diet high in Alkali/low in protein. Not all of the filtered bicarbonate is reabsorbed in that setting. If more bicarbonate secretion is needed, Intercalated TYPE B cells can accomplish this. They have membrane TRANSPORTERS similar to Type A cells, but the transporter's Orientation and location are SWITCHED. The H+ATPASE pump is located on the basolateral membrane and pumps H+ back into the blood, where as the HCO3-/Cl- Counter-transporter shuttles HCO3- into the UF in lumen-see pic

Each acid can be described by how much it dissociates (the ratio of acid relative to its dissociated ions) and this dissociation constant is called K pK is the value at which a buffer pair has equal amounts of weak acid and conjugate base. Buffers are most effective within +/- 1 pH unit of the pK The pK of phosphate is the same as typical urine pH, making it a highly effective urinary buffer. CO2 works well as a buffer despite having a pK far from blood pH because it is present in large amounts & because CO2 can be eliminated by the lungs (open system) keeping the equation from reaching equilibrium

Each acid can be described by how much it dissociates (the ratio of acid relative to its dissociated ions) and this dissociation constant is called K pK is the value at which a buffer pair has equal amounts of weak acid and conjugate base. Buffers are most effective within +/- 1 pH unit of the pK The pK of phosphate is the same as typical urine pH, making it a highly effective urinary buffer. CO2 works well as a buffer despite having a pK far from blood pH because it is present in large amounts & because CO2 can be eliminated by the lungs (open system) keeping the equation from reaching equilibrium

Extra H+ needs to be excreted in the urine. Part of the H+ will be buffered by filleted HPO4-, but that will not be enough. The prox. tube can increase ammonia generation from glutamine by 10 fold, thus providing a mechanism to secrete more acid. Urine pH will be ACIDIC

Extra H+ needs to be excreted in the urine. Part of the H+ will be buffered by filleted HPO4-, but that will not be enough. The prox. tube can increase ammonia generation from glutamine by 10 fold, thus providing a mechanism to secrete more acid. Urine pH will be ACIDIC

Extracellular Buffers

Extracellular Buffers HCO3/CO2 Inorganic Phosphates Plasma Proteins Intracellular Buffers Proteins Organic phosphates

fixed acids

Fixed Acids -are initially Buffered, then excreted by Kidneys Normal catabolism of proteins (creating sulfuric acid) and phopholipids (creating phosphoric acid) adds acid to the body. These acids must be excreted to maintain balance. Dz states may cause production of additional acids, such as lactic acid (Anaerobic Metabolism) or Ketoacids (Dibetes mellitus). Fixed acids are initially buffered, then excreted by the kidneys *Proteins—->sulfuric acid *Phospholipids—> phosphoric acid Abnormal (in large quantities) Anaerobic Metabolism —->lactic acid Diabetes mellitus —->ketoacids

Prox. tube cells make Ammonium-high capacity Ammonia buffers H+ in lumen

Prox. tube cells make Ammonium-high capacity Ammonia buffers H+ in lumen

Henderson-Hasselbalch equation

Isohydric Principle -No matter how many buffers are present, there is only 1 [H+] and One pH -This is the isohydric Principle. -the ratio of any buffer can be predicted with HHE and law of Mass action. -We use the Bicarbonate-carbonic acid paint to predict pH in body fluids Law of Mass Action -The velocity of a reaction is proportional to the product of the concentration of the reactants -Weak acids (and bases) are partially dissociated in solution and the amount of dissociate is described by a constant K and K2. HH Equation

Secondary Active Transport can secrete hydrogen ions against a modest gradient. 85% of Bicarbonate is reabsorbed in the proximal tubule, 10% in the LOH, 5% in the Distal segments. In the proximal tube, Ascending thick LOH, and early distal tubule H+ is secreted into the UF by secondary Active transport with a Na+ counter-transporter. this pump is fueled by the Na+K+ATPASE pump on the basolateral side, which creates a gradient favorable for Na t enter the cell. The H+ combines with HCO3- which was filtered at the Glomerulus, forming H2CO3. The H2CO3 then dissociates int CO2 and H2O. This rxn occurs spontaneously, but occurs much faster with carbonic Anhydrase,which is present in the brush border of the Prox. tube. The Co2 diffuses into the tube cell and recombines with H2O under the influence of Carbonic Anhydrase into H2CO3. This dissociates into H+ and HCO3-. The HCO3- is reabsorbed across the basolateral membrane into the blood stream . The net result is that filtered bicarbonate is reabsorbed , although the module is not the exact on filtered.

Secondary Active Transport can secrete hydrogen ions against a modest gradient. 85% of Bicarbonate is reabsorbed in the proximal tubule, 10% in the LOH, 5% in the Distal segments. In the proximal tube, Ascending thick LOH, and early distal tubule H+ is secreted into the UF by secondary Active transport with a Na+ counter-transporter. this pump is fueled by the Na+K+ATPASE pump on the basolateral side, which creates a gradient favorable for Na t enter the cell. The H+ combines with HCO3- which was filtered at the Glomerulus, forming H2CO3. The H2CO3 then dissociates int CO2 and H2O. This rxn occurs spontaneously, but occurs much faster with carbonic Anhydrase,which is present in the brush border of the Prox. tube. The Co2 diffuses into the tube cell and recombines with H2O under the influence of Carbonic Anhydrase into H2CO3. This dissociates into H+ and HCO3-. The HCO3- is reabsorbed across the basolateral membrane into the blood stream . The net result is that filtered bicarbonate is reabsorbed , although the module is not the exact on filtered.

Secretion of H+ and Reabsorption of HCO3- in distal segments

Secondary active transport can NOT Secrete sufficient H+ to account for the necessary net NON-Volatile acid excretion to achieve balance. Primary active transport using a H+ ATPase pump occurs in the Intercalated (Type A) cells. This mechanism can Secrete H+ against a large gradient. H+ is secreted into the lumen by H+ATPase and combines with filtered Bicarbonate. Intracellular bicarbonate that is created is reabsorbed with a Cl- Exchanger. As in the earlier segments, each filtered HCO3- buffers a secreted H+ and a different HCO3- is reabsorbed.

Stored blood does not have the respiratory system to blow off CO2 and thus provide buffering. As a result, banked blood can be profoundly Acidemic . Luckily a transfusion is a small fraction of the total blood volume (usually) and once in the body, all the body systems jump into buffer it and make it safe. The tech picked up the wrong sample.

Stored blood does not have the respiratory system to blow off CO2 and thus provide buffering. As a result, banked blood can be profoundly Acidemic . Luckily a transfusion is a small fraction of the total blood volume (usually) and once in the body, all the body systems jump into buffer it and make it safe. The tech picked up the wrong sample.

The extensive muscle activity is like Strenuous exercise, and the muscles convert to Anaerobic metabolism. lactic acid builds up, CO2 is inadequately cleared and acidosis develops. Incidentally, K+ can be released from the damaged cells, causing HyperKalemia. These Abnormalities will resolve shortly after the end of the seizure, if the lungs and kidneys are normal .

The extensive muscle activity is like Strenuous exercise, and the muscles convert to Anaerobic metabolism. lactic acid builds up, CO2 is inadequately cleared and acidosis develops. Incidentally, K+ can be released from the damaged cells, causing HyperKalemia. These Abnormalities will resolve shortly after the end of the seizure, if the lungs and kidneys are normal .

H+ concentration is tightly regulated

The hydrogen ion concentration impacts almost all enzymes systems in the body, and thus hydrogen concentration must be tightly regulated. Hydrogen is present in the blood in Extremely small quantities. Ex: Na+ is present at 145MEq/L, whereas Hydrogen is present at 0.00004mEq/L, which is 3.5 million times lower. = Hydrogen must be precisely controlled.!

The lungs can blow off CO2, a Volatile Acid. This is considered an open system, as there is not a fixed limit to the buffering capacity . increase H+ + HCO3- <—> H2CO3. <——> H2O + CO2—->> -an increase in H+ (acidosis) will shift this equation to the right, and the lungs will respond by increasing ventilation to remove CO2 (PANTING) Conversely, increasing the CO2 (poor ventilation) will drive the equation to the LEFT , causing acidosis (hold breath) increase H+ + HCO3- <——> H2CO3. <—-> H2O + CO2

The lungs can blow off CO2, a Volatile Acid. This is considered an open system, as there is not a fixed limit to the buffering capacity . increase H+ + HCO3- <—> H2CO3. <——> H2O + CO2—->> -an increase in H+ (acidosis) will shift this equation to the right, and the lungs will respond by increasing ventilation to remove CO2 (PANTING) Conversely, increasing the CO2 (poor ventilation) will drive the equation to the LEFT , causing acidosis (hold breath) increase H+ + HCO3- <——> H2CO3. <—-> H2O + CO2

Impact of pK on Hydrogen Excretion: Phosphate & Ammonium pK

The phosphate system has a PKA of 6.8 which is close to the typical pH of urine, making Phosphate a highly effective buffer in urine. The Ammonia/Ammonium system has a PKA of 9.2, which is far from an achievable urine pH. However, since NH3 diffuses into the UF and becomes trapped as NH4+, more NH3 can keep diffusion in, making this a powerful buffering mechanism despite the Pka

There is a limit to how low the urine pH can get, because he Tubules can not keep secreting H+ into the lumen if it is not buffered., because the concentration gradient will prevent it. However because of the buffering ability of phosphate ammonia, a lot of acid can be excreted with Minimal Chang in urine pH.

There is a limit to how low the urine pH can get, because he Tubules can not keep secreting H+ into the lumen if it is not buffered., because the concentration gradient will prevent it. However because of the buffering ability of phosphate ammonia, a lot of acid can be excreted with Minimal Chang in urine pH.

Titratable acidity is the amount of base needed to get a 24-hour urine sample to a pH fo 7.4 . Phosphate is equated with Titratable acidity because it is the major contributor.

Titratable acidity is the amount of base needed to get a 24-hour urine sample to a pH fo 7.4 . Phosphate is equated with Titratable acidity because it is the major contributor.

Types of Acid x 1 of 2 : Volatile Acid

Volatile Acid = CO2 -Facilitated by Carbonic Anhydrase (enzyme catalyze rxn. ) Cells make CO2 from aerobic metabolism and add it to the blood. Inside RBCs, Carbonic Anhydrase combines CO2 with H2O —>H2CO3, H2CO3 dissociates into H+ and HCO3-. In the lungs, CA reverses the reaction, the CO2 is expired (its Volatile) and Balance is restored.

Weak vs Strong

Weak vs Strong Strong acids are ones that rapidly dissociate & dissociate extensively -This rapidly releases a large amount of H+ Weak acids do NOT dissociate as fully & thus releases less H+ Strong bases react rapidly & strongly with H+, where as weak bases bind H+ more weakly .

When acid-producing diet: - Prox tube reabsorbs most filtered bicarbonate; -Distal tube reabsorbs any that get past prox. tube Hydrogen is secreted. Buffers in UF include : Phosphate (limited amt) and secreted/recycled Ammonia (adjustable amt) Hydrogen secretion in distal tube is ACTIVE (ATP) to allow secretion of large amounts With Alkaline diet : -Bicarbonate can be secreted

When acid-producing diet: - Prox tube reabsorbs most filtered bicarbonate; -Distal tube reabsorbs any that get past prox. tube Hydrogen is secreted. Buffers in UF include : Phosphate (limited amt) and secreted/recycled Ammonia (adjustable amt) Hydrogen secretion in distal tube is ACTIVE (ATP) to allow secretion of large amounts With Alkaline diet : -Bicarbonate can be secreted

While phosphate is an excellent buffer, it is not being filtered as much with CKD (decreased GFR= Decreased filtration of everything.) Additionally once all the filtered phosphate has to titrated Secreted H+, it has no more acid secreting power.

While phosphate is an excellent buffer, it is not being filtered as much with CKD (decreased GFR= Decreased filtration of everything.) Additionally once all the filtered phosphate has to titrated Secreted H+, it has no more acid secreting power.

definitions of Acids and bases

pH is based on the ratio of HCO3-, to PCO2, not the absolute amount of either. A Hydrogen ion (H+) is a single proton released from a hydrogen atom. An acid is a molecule that can RELEASE a Hydrogen (H+). ex: HCL dissociates into H+ & Cl- and thus is an ACID A base is a molecule that can accept a hydrogen ion. ex: HCO3- is a base, as it can COMBINE with H+ and form H2CO3. A base is sometimes referred to as ALKALI, which are Alkaline metals combined with a Hydroxl ion (OH-) A STRONG ACID -is one that rapidly dissociates completely & releases LARGE amounts of H+ A WEAK ACID -is one that dissociated LESS completely and release LESS H+. A STRONG BASE =rapidly and strongly reacts with H+ Because normal [H+] is such a small number, it is measured with a logarithm scale using pH units. pH is the: -LOG [H+], which is the same as saying the log of the inverse of the Hydrogen ion Concentration. Because it is an inverse, a lower pH means a higher [H+] and vice versa

see pic

see pic