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◄ Respiration and kidney physiology
The nephron, the kidney’s basic structural
and functional unit, is responsible for the purification and filtration of the
blood. During filtration, bicarbonates
leave the blood and become part of nephron filtrate,
including water, electrolytes, glucose, amino acids, vitamins, small proteins,
creatinine, and urea. As these
substances pass through the nephron, many of them are selectively reabsorbed and returned to the blood,
including sodium and bicarbonate ions.
Other substances are secreted into
the filtrate from the surrounding cells and capillaries, such as hydrogen and
ammonium ions.
Carbon dioxide plays the key role both in
the return of bicarbonates from the filtrate back into the blood, and the
synthesis of new bicarbonates lost through the buffering of unutilised hydrogen
ions, generated during protein metabolism.
Carbon dioxide and H2O, in the filtrate, diffuse into the tubular cells that surround the filtrate, to form carbonic acid: CO2 + H2O ↔ H2CO3. Just as in the case of
red blood
cells, carbonic acid dissociates into hydrogen and bicarbonate ions:
H2CO3 ↔ H+ + HCO3‾. The
bicarbonates in tubular cells are transported into the surrounding capillaries,
and are thus fully reclaimed for general circulation (bicarbonate reclamation). The hydrogen ions in these cells are
transported into the filtrate in exchange for sodium ions. Sodium ions in the tubular cells, together
with the bicarbonate ions, are co-transported
to the capillaries, and thus returned to general circulation. And, the hydrogen ions, now in the filtrate,
combine with more bicarbonate ions in the filtrate to form carbonic acid:
H+ + HCO3‾ ↔ H2CO3.
The carbonic acid in the filtrate
dehydrates into CO2 and H2O, which then diffuse into the
same tubular cells, where once again they form carbonic acid in the tubular
cells, and the cycle begins anew. A
nearly identical process, also requiring CO2, provides for the
synthesis of new bicarbonates that replace the ones lost in the buffering of
acids generated during protein metabolism (bicarbonate
regeneration). In this case,
however, H+
in the filtrate is
combined with sodium phosphate and excreted, rather than being utilised in the
formation of H2O reabsorbed by tubular cells.
Overbreathing
results in CO2 deficit in the kidneys, which means that less
bicarbonate is recovered from the filtrate, and new bicarbonate is no longer
formed. This may mean that bicarbonate
ions, crucial to the buffering of metabolic acids, such as lactic acid produced
during exercise, are depleted. The
consequences may include (1) compromised physical endurance in sports and fitness
enthusiasts, and (2) the appearance of fatigue symptoms associated with chronic
stress, where adequate buffering of even small amounts of lactic acid is
compromised. The exchange of hydrogen
ions for sodium ions is also reduced, and may contribute to development of
sodium deficiency and its associated symptoms.
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Behavioral Physiology Institute,
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