Adam suffers from severe emphysema. Gas exchange in his lungs is not adequate, and he requires supplemental oxygen. But even though his blood carbon dioxide levels are elevated, his blood pH is close to normal. Considering the urinary system, explain how this can occur.
Dr Sarah can you respond to this?
This is a very complex process. Here is a good review for you.
"4.5.3 Chronic Respiratory Acidosis: Renal Bicarbonate Retention
With continuation of the acidosis, the kidneys respond by retaining bicarbonate.
If the respiratory acidosis persists then the plasma bicarbonate rises to an even higher level because of renal retention of bicarbonate.
Thus in a chronic respiratory acidosis there are TWO factors present which elevate the plasma bicarbonate:-
Firstly: The acute physicochemical change and consequent buffering esp by intracellular protein. (Immediate onset - as occurs with an acute respiratory acidosis.)
Secondly: The renal retention of bicarbonate as renal function is altered by the elevated arterial pCO2 and additional bicarbonate is added to the blood passing through the kidney. (Slow onset)
Studies have shown that an average 4 mmol/l increase in [HCO3-] occurs for every 10mmHg increase in pCO2 from the reference value of 40mmHg. For example, if arterial pCO2 has risen from 40mmHg to 60mmHg (due to decreased alveolar ventilaton) and remained elevated for several days, then this chronic rise of "2 tens" (i.e. 60-40=20mmHg rise = 2 rises of 10mmHg) results in a rise of plasma bicarbonate by 8 from its reference value of 24mmol/l up to 32 mmol/l. Consequently, we would predict that if this chronic respiratory acidosis was the only base disorder present, then plasma bicarbonate would be 32mmol/l.
The renal response in underway by 6 to 12 hours with a maximal effect reached by 3 to 4 days. This maximal effect is not sufficient to return plasma pH to normal, but because of the additional renal contribution, the pH is returned towards normal much more than occurs in an acute respiratory acidosis.
The response occurs because increased arterial pCO2 increases intracellular pCO2 in proximal tubular cells and this causes increased H+ secretion from the PCT cells into the tubular lumen. This results in:
1)increased HCO3 production which crosses the basolateral membrane and enters the circulation (so plasma [HCO3] increases.)
2)increased Na+ reabsorption in exchange for H+ and less in exchange for Cl- (so plasma [Cl-] falls)
3)increased 'NH3' production to 'buffer' the H+ in the tubular lumen (so urinary excretion of NH4Cl increases)"
if you need more, here is the source Dr Sarah, MD gave you : http://www.anaesthesiamcq.com/AcidBaseBook/ab4_5.php
In order to understand how Adam's blood pH remains close to normal despite elevated carbon dioxide levels, we need to consider the urinary system's role in maintaining acid-base balance.
The urinary system plays a crucial role in regulating the body's pH balance through the process of urine formation and excretion. One of its main functions is to remove excess acids or bases from the bloodstream, thus helping to maintain the blood's pH within a narrow range.
In Adam's case, his emphysema causes impaired gas exchange in his lungs. As a result, the removal of carbon dioxide is compromised, leading to elevated levels of this waste gas in his blood. Carbon dioxide can combine with water to form carbonic acid (H2CO3), which can lower blood pH and make it acidic.
To compensate for this acidosis, the kidneys respond by increasing the excretion of hydrogen ions (H+) in the urine. Hydrogen ions are acidic, so by eliminating them, the kidneys help restore the blood's pH balance towards normal. The kidney achieves this primarily through the process of tubular secretion.
Additionally, the kidneys can also regulate the reabsorption of bicarbonate ions (HCO3-) in the renal tubules. Bicarbonate acts as a buffer in the blood, helping to neutralize excess acids. By adjusting the reabsorption of bicarbonate, the kidneys can assist in maintaining the blood's pH within the appropriate range.
In summary, in Adam's case, the urinary system compensates for the impaired gas exchange in his lungs by increasing the excretion of hydrogen ions and adjusting the reabsorption of bicarbonate ions. This process helps maintain his blood pH close to normal despite elevated carbon dioxide levels.