Renal control of acid and base Flashcards
Is the bicarbonate buffer system open or closed?
Open as CO2 is exhaled to balance the scales
What is the bicarbonate buffer equation?
CO2 + H2O –> H+ + HCO3-
The kidneys need to excrete acid, how do they do this as H+ cannot be excreted freely?
Na/H exchange in the proximal tubule
Active H+ ATPase pump in the collecting tubules
Why must the body reabsorb all HCO3-?
As loss of it is essentially gain of H+
What is H+ bound to in the filtrate?
bound to filtered buffers such as phosphate or ammonia
What three organs are involved in acid base regulation?
Liver, lungs and kidney
How do the lungs aid the bicarb equation?
Exhale CO2
How does the liver input into the bicarb equation?
Urea synthesis in liver (from NH4+ & CO2) produces H+ against which HCO3- is titrated
How does the kidneys input into the bicarb equation?
Kidneys excrete NH4+ and there is a net gain of HCO3- and net loss of H+
What provides the energy for PT acidification?
Energy for the process is provided by basolateral Na+K ATPase
What enzyme catalyses the bicarb equation?
carbonic anhydrase
What factors control the bicarbonate reabsorption?
Luminal HCO3- concentration
Luminal flow rate
Arterial pCO2
Angiotensin II (via decrease in cyclic AMP)
An increase in any of these four factors causes an increase in bicarbonate reabsorption
How does PTH affect bicarb absorption?
Parathyroid hormone also has an effect: an increase in hormone level increases cAMP and decreases bicarbonate reabsorption
Where is ammonium produced?
Ammonium (NH4) is produced predominantly within the proximal tubular cells.
Where does the ammonium come from?
The major source is from glutamine which enters the cell from the peritubular capillaries.
How is ammonium produced?
Ammonium is produced from glutamine by the action of the enzyme glutaminase. Further ammonium is produced when the glutamate is metabolised to produce alpha-ketoglutarate
Where is ammonium removed into the filtrate?
The thick ascending limb of the loop of Henle is the important segment for removing ammonium. Some of the interstitial ammonium returns to the late proximal tubule and enters the medulla again (ie recycling occurs).
How does a large decrease in extracellular fluid cause alkalosis?
Extracellular fluid volume depletion stimulates sodium reabsorption and increases H+ secretion (Via Na/H+ transporter) and HCO3- reabsorption.
The increased aldosterone levels also stimulate H+ secretion.
Hyperkalemia decreases ……… secretion and ………. reabsorption and tends to cause acidosis
H+
HCO3
