explain the mechanism of excretion by the lungs, kidney, and skin
The lungs are important for excretion of carbon dioxide (the respiratory acid) and there is a huge amount of this to be excreted: at least 12,000 to 13,000 mmols/day.
In contrast the kidneys are responsible for excretion of the fixed acids and this [ is also a critical role even though the amounts involved (70-100 mmols/day) are much smaller. The main reason for this renal importance is because
there is no other way to excrete these acids and it should be appreciated that the amounts involved are still very large when compared to the plasma [H+] of only 40 nanomoles/litre.
There is a second extremely important role that the kidneys play in acid-base balance, namely the reabsorption of the filtered bicarbonate. Bicarbonate is the predominant extracellular buffer against the fixed acids and it important that its plasma concentration should be defended against renal loss.
In acid-base balance, the kidney is responsible for 2 major activities:
* Reabsorption of filtered bicarbonate: 4,000 to 5,000 mmol/day
* Excretion of the fixed acids (acid anion and associated H+): about 1 mmol/kg/day.
Both these processes involve secretion of H+ into the lumen by the renal tubule cells but only the second leads to excretion of H+ from the body.
The renal mechanisms involved in acid-base balance can be difficult to understand so as a simplification we will consider the processes occurring in the kidney as involving 2 aspects:
* Proximal tubular mechanism
* Distal tubular mechanism
There are no new answers.