Question

1. Explain why the respiratory system and the renal system have such different time frames in terms of their physiology for PH response.

2. Regarding the change in the kidneys that characterizes the renal response to an alkalosis, describe:

            a. tubular transport of hydrogen ions:

            b. tubular transport of bicarbonate ions:

Answer 1

1) * The respiratory tract can adjust the blood upward in minutes by exhaling CO2 from the body.

* The renal system can also adjust blood PH through the excretion of Hydrogen( H+) ions and the conservation of bicarbonate, but this process takes hours to days to have an effect.

* Postthypercapnia- Hypoventilation ( decreased respiratory rate) causes hypercapnia ( increased levels of CO2) which results in respiratory acidosis.

* Once carbondioxide levels return to baseline, the higher bicarbonate levels reveal themselves putting the patient into metabolic alkaloids.

* The kidneys can regulate reabsorption of carbonic acid in the tubule, increasing or reducing acid secretion.

* So, urine that is more acidic than normal may mean the body is ridding itself of excess dietary acid and thus making blood PH more alkaline.

* Ammonia is another way the kidney can regulate PH balance.

2.(a) Tubular transport of Hydrogen ions:

* Tubular secretion occurs at proximal convoluted tubule and distal convoluted tubule, hydrogen ion is secreted by means of active transport and co- transport I.e antiporter and ammonia diffuses into renal.

* In PCT approximately 80% of H- ion secretion is mediated by Na+/H+ exchange and 20% by H(+)- ATPase.

* Tubular transport of secretion can be either active or passive or co-transport.

* Substances mainly secreted into renal tubules are:- H+, K+, NH3, urea, creatinine.

b) Tubular transport of bicarbonate ions:

* Bicarbonate is partly reabsorbed passively following sodium reabsorption.

* It is also reabsorbed by buffering Hydrogen ions.Within the renal tubule hydrogen ions react with bicarbonate to form carbonic acid.

* About 85- 90% of the filtered bicarbonate is reabsorbed into the proximal tubule and the rest is  reabsorbed by the interrelated cells of the distal tubule and collecting ducts.

*The H+ leaves the proximal tubule cell and enters the PCT lumen by two mechanisms: via a Na+ - H+ antiporter ( major route).

* The kidneys maintain homeostasis through the excretion of waste products. Acidosis causes more bicarbonate to be reabsorbed from the tubular fluid, while the collecting ducts secrete more hydrogen to generate more bicarbonate, and more NH3 buffer is formed.