Mechanisms of Acid-Base Balance Flashcards
What is the normal extracellular concentration of H+?
40 nmol/L
What is the normal plasma pH range?
7.35-7.45
Outside what range of plasma pH is considered incompatible with life?
<6.8
>7.8
What is the urine pH range?
4 - 8.5
What controls the PCO2?
Alveolar ventilation
What controls plasma HCO3- concentration?
Renal excretion of H+ and reabsorption of HCO3-
What are the two main buffers: intracellular and extracellular?
Intracellular: H2PO4- —–> H+ + HPO42-
Extracellular: H2CO3 —–> H+ + HCO3-
Describe where most of the buffering takes place in the four kinds of acid-base imbalance.
Metabolic Acidosis - 80-85% intracellular
Metabolic Alkalosis - 30-35% intracellular
Respiratory Acidosis + Alkalosis - almost ALL intracellular
What are the four sources of H+ ions in the body?
Physiological - carbohydrate and fat metabolism, sulfur containing amino acids
Pathological - hypoxia —-> anaerobic respiration —-> lactic acid
Or diabetes —–> ketoacids
Volatile acids - from carbohydrate and fat metabolism (results in CO2 production - CO2 is lost through respiration)
Non-Volatile acids - from protein metabolism (these H+ are excreted by the kidneys)
In which three regions of the nephron does renal H+ excretion take place?
Proximal convoluted tubule
Thick ascending limb of the loop of Henle
Outer medullary collecting duct
What transporters are involved in the renal excretion of H+ in the proximal convoluted tubule and in the collecting duct?
PCT
Na+/H+ exchanger
HCO3-/Na+ cotransporters (basolateral membrane)
Collecting Duct
H+ ATPase
Cl-/HCO3- exchanger (basolateral membrane)
Describe the process of excretion of H+ and retention of HCO3-.
H+ goes out of the cell via a Na+/H+ exchanger
H+ reacts with HCO3- in the filtrate to form H2CO3
Carbonic anhydrase converts H2CO3 to H2O + CO2
H2O + CO2 are absorbed into the tubular cell and react to form H2CO3
Carbonic anhydrase in the cell converts H2CO3 —-> H+ + HCO3-
H+ moves out via the Na+/H+ exchanger
HCO3- is reabsorbed into the blood
Where does bicarbonate reabsorption take place and which areas reabsorb more bicarbonate?
80% takes place in the proximal convoluted tubule
20% takes place in the thick ascending limb of the loop of Henle and in the outer medullary collecting duct
State two primary stimuli for increase in H+ secretion.
Decrease in plasma bicarbonate concentration
Increase in PaCO2
State two secondary stimuli for an increase in H+ secretion.
NOTE: secondary means that it is not directed at maintaining acid-base balance
Angiotensin II secretion Aldosterone secretion Decrease ECF volume Hypokalaemia Increase in filtered load of bicarbonate
How are the stimuli for a decrease in H+ secretion different?
They are the opposite
Describe bicarbonate production.
Amino acids are broken down in the liver to produce glutamine and urea
Glutamine is taken to the kidneys and broken down to produce ammonium and alpha ketoglutarate.
Alpha-ketoglutarate is then converted to HCO3-.
What happens to the ammonia produced in the tubular cells from the breakdown of glutamine?
It moves into the tubular fluid either as ammonium or as NH3 + H+
Further down the nephron, H+ will enter the tubular fluid and react with the NH3 to form NH4+
What could cause metabolic acidosis?
Addition of non-volatile acids
Loss of non-volatile alkalis
Failure to reabsorb sufficient HCO3-
What is the most common cause of metabolic alkalosis?
Loss of non-volatile acids (e.g. vomiting)
What two things can the kidney change to compensate for respiratory acidosis or alkalosis?
NH4+ excretion and HCO3- reabsorption
