Renal Regulation of Acid-Base Balance Flashcards
What is the effect and cause of rhabdomyolysis?
It is due to the release of myoglobin into the bloodstream
In order to control pH what is secreted and what must be replenished?
H+ will be secreted and the kidneys must prevent the loss of HCO3- in the urine as it is used to neutralize much of the acid.
What is the pH of urine around?
It is usually low due to the secretion of H+ so therefore it is acidic
How do the kidneys secrete a large amount of H+ if they can only secrete urine with a pH of 4.0 - 4.5?
They must excrete H+ alongside buffers like Pi
What term is used to refer to the urinary buffers?
Titratable Acids
Besides the excretion of H+ as a titratable acid what other mechanisms contribute to the maintenance of the acid-base balance?
It is done through the synthesis and excretion of ammonium (NH4+)
Where does most of the HCO3- reabsorption occur?
In the proximal tubule (80%)
What is the predominant buffer in the proximal tubule?
HCO3- because the CO2 hydration reaction predominates due to presence of carbonic anhydrase in the border.
What is the level of net H+ secretion in the proximal tubule?
Net H+ secretion of H+ is low due to the neutralization of H+ with HCO3- during HCO3- ion reabsorption
What are the main buffers in the distal tubule and the collecting duct?
Phosphate and ammonium ions dominate due to the scarcity of carbonic anhydrase
What is the net secretion of H+ in the distal tubule and the collecting duct?
It is very high due to strong proton pumping and buffering with Pi and NH4+
What is Type I-Renal Tubular Acidosis?
It is distal tubule RTA and is the failure of the distal nephron to secrete H+ which leads to increased back-leaking of H+ and may be caused by H+ pump failure.
What is Type II-Renal Tubular Acidosis?
It is proximal tubule RTA and is the failure of the proximal nephron to recycle H+ due to low carbonic anhydrase and decreased HCO3- reabsorption.
What is the [HCO3-] that HCO3- is regulated at?
It is regulated at a concentration near the renal plasma threshold
What processing of HCO3- occurs in the PT?
80% reabsorption
What processing of HCO3- occurs in the DT and CD?
20% reabsorption
How does the GFR regulate HCO3- reabsorption?
Reabsorption rate of the HCO3- is matched to changes in the filtered load via GT balance
How does the systemic acid-base balance regulate HCO3- reabsorption?
Respiratory acidosis and increase in arterial PCO2 or metabolic acidosis and decreased [HCO3-] will lead to the acidification of the ICF.
This causes the insertion of the apical H+-ATPase that leads to increased H+ secretion and decreased HCO3- reabsorption.
How does aldosterone regulate HCO3- reabsorption?
Increased aldosterone will lead to increased H+ secretion in the intercalated cells of CD.
Increased aldosterone will increase Na+ reabsorption and lead to increased H+ secretion.
How does arterial [K+] regulate HCO3- reabsorption?
Increased K will increase K+-H+ exchange and lead to extracellular acidosis -> hyperkalemic metabolic acidosis
Decreased K will lead to hypokalemic metabolic alkalosis.
How does the arterial [Cl-] regulate HCO3- reabsorption?
Increased [Cl-] will decrease HCO3- reabsorption and decreased will cause the opposite
How does the extracellular fluid volume regulate HCO3- reabsorption?
During expansion of ECF, HCO3- reabsorption will be inhibited to to dilution of plasma [HCO3-]
How does the Na+ balance regulate the HCO3- reabsorption?
Retaining Na+ will decrease HCO3- reabsorption and losing Na+ will increase HCO3- reabsorption
What is the minimal urinary pH around and how does it reach such impressively low levels?
It is around a pH of 4.4 and it demonstrates the ability of the distal nephron to secrete H+ against a strong acid gradient.
What are sources of free H+ in the urine?
Fixed acids - H2SO4
Titratable acids - H3PO4
What acid is not a source of free H+ in the urine?
Carbonic acid
What is the role of HPO4(2-) as a titratable acid?
It can combine with secreted H+ ions and make H2PO4- and the negatively charged anion is lipid insoluble so there is no back diffusion.
What is the role of diffusion trapped ions and how do they “trap?”
NH3 reacts with secreted H+ to form NH4+ and NH4+ is charged and lipid impermeable, which traps the H+. NH3 is made from deamination reactions and it is freely permeable across tubular cells.
What happens to NH4+ in chronic acidosis?
NH3 production increased in acidosis which allows for increased H+ secretion as an adaptation to the acidosis
How is HCO3- and H+ balanced through NH4+?
For each secreted molecule of NH4+ a new molecule of HCO3- is added to the ECF
