Acid-Base: L41 - Respiratory and Renal pH Control Mechanisms Flashcards
Discuss replacement of HCO3- when HCO3- is being filtered by the glomerulus.
Because bicarbonate is being filtered in the kidneys, it must be replaced to retain pH balance. The cells in the proximal tubule are responsible for 80% of the replacement of bicarbonate.
Carbon dioxide is a by-product of metabolism in the tubular epithelial cell. It combines with water to form carbonic acid. Carbonic acid then dissociates into H+ and bicarbonate. H+ is secreted via Na+-H+ exchanger into the lumen to combine with the bicarbonate that is being filtered, to form carbonic acid which then goes to CO2 and H20 that is excreted. In the membrane bicarbonate is transported via Na+-HCO3- cotransporter into the interstitial fluid, ‘replacing’ the HCO3- lost in the urine.
Discuss the formation of HCO3- due to non-volatile acid and excretion of titratable acid.
The kidney must also produce bicarbonate to replace that lost in the neutralisation of the non-volatile acids made by metabolism. The kidney does this in two ways: excretion of titratable acid and synthesis and excretion of NH4. Fluid that reaches the distal tubule is low on bicarbonate (most of it was replaced in the PT), so the H+ that is secreted by the tubular epithelial cells is more likely to attach to buffers like HPO4- to make a titratable acid that is then secreted in the urine. The HCO3 from the dissociation is moved into the blood, thus a new molecule of bicarbonate is produced.
Discuss the synthesis and excretion of NH4 used to maintain pH balance.
Glutamine is synthesised by the liver under low HCO3- conditions (acidosis). Filtered glutamine then transports across to the epithelial cells along with Na+. Glutamine also moves into the epithelial cells from interstitial fluid without sodium. Glutamine dissociates into NH4+ and HCO3-. NH4+ is secreted and excreted using Na+ co-transport, whilst HCO3- moves into the interstitial fluid.
What happens once the synthesised HCO3- moves into interstitial fluid?
Combines with H+ ions to form CO2 + H20, increasing the pH.
What is the Henderson-Hasselbalch equation and how does this reflect respiratory and renal control of acid-base?
pH=6.1+ log HCO3-/ (0.03xpCO2). Usual values are 24mmol/L of bicarbonate and 40mmHg for carbondioxide, giving pH=7.4. Respiratory control is shown in pCO2 and renal control is shown in bicarbonate concentration, this equation shows that if either respiratory or renal function is disrupted, it can be compensated by the other function so that pH remains constant.
Describe metabolic acidosis.
Metabolic acidosis is bicarbonate deficit, and is due to either too much H+ or a lack of bicarbonate. It can occur by diseases such as uncontrolled diabetes or starvation, and results in abnormally large amounts of acid entering blood.
Describe metabolic alkalosis.
Metabolic alkalosis is when there is bicarbonate excess, and is due to either a lack of H+ or too much bicarbonate. It can occur due to ingestion of large amounts of alkali or excess vomiting (loss of stomach acid H+).
Describe respiratory acidosis.
Too much H+ in blood, means too much CO2, which occurs when you hypoventilate as you are not breathing out the H+. This can be due to respiratory conditions such as pneumonia or emphysema that causes retention in CO2.
Describe respiratory alkalosis.
Means there is a carbonic acid deficit, due to hyperventilation (fever or anxiety) which can result in excess loss of carbonic acid.
How does the body compensate for metabolic and respiratory acidosis?
For metabolic acidosis:
Respiratory system can -
Breathing rate can increase which results in decreased PCO2 in plasma.
Renal system can -
You have too much H+ so you want to secrete more of it into tubules, as well as decrease GFR so you retain more HCO3-, less of it is filtered.
If you have respiratory acidosis:
Can perform the renal actions above.
How does the body compensate for metabolic and respiratory alkalosis?
Metabolic alkalosis:
Renal system:
Increased GFR to filter more bicarbonate and offload more of it into urine.
Decrease secretion of H+.
Respiratory system:
Need more H+ so you retain more CO2 by less breathing.
If you have respiratory alkalosis you can perform the renal system actions above.
