into tubular fluid & excrete Most H + binds to bicarbonate, ammonia, and phosphate buffers Bound and free H + excreted in urine, i.e., H + expelled from body Other buffer systems H + concentration by binding H +
Secretion and Neutralization of H + by the Kidney 24-32 K + Na + NaHCO 3 HCO 3 – Na + Urine Carbonic Anhydras e CO 2 Aerobic Respiration Na + HCO 3 – + CO 2 – H 2 CO 3 H 2 CO 3 H 2 O Antiport Diffusion thru channel Diffusion thru lipid membrane Key + CO 2 2 1 8 5 10 11 6 7 9 4 3 1 In blood: H + + HCO 3 – H 2 CO 3 2 H 2 CO 3 H 2 O + CO 2 , which enter the tubule cell 3 4 Carbonic Anhydrase combines H 2 O + CO 2 to re-form H 2 CO 3 5 H 2 CO 3 HCO 3 – + H + ; HCO 3 – returns to blood 6 Na + -H + antiport exchanges H + for Na + 7 from glomerular filtrate NaHCO 3 Na + + HCO 3 – ; Na + transported into tubule cell 8 Na + leaves cells by Na + -pump at basolateral side into blood 9 HCO 3 – + H + from tubule H 2 CO 3 10 At brushborder, H 2 CO 3 H 2 O + CO 2 again; Carbonic Anhydrase catalyzes CO 2 enters t. cell and H 2 O passes out in urine - thereby, excreting the H + that was originally in blood! CO 2 diffuses from blood into t. cell PLUS CO 2 from Aerobic Respiration 11 Glomerular filtrate H + H CO 3 H + H 2 O H + H 2 O peritubular capillary Renal tubule cells. e.g., PCT , DCT Tubular fluid Carbonic Anhydrase Caveat - Step #6 If tubular fluid H + conc. increases to pH of 4.5, H + secretion inhibits/even stops Na + /H+ exchange. This then would limit kidney's ability to secrete and excrete excess H+ How is this prevented? 3 buffer systems: Bicarbonate, Phosphate & Ammonia
24-33 Diffuses thru channel Antiport Key Tubular fluid K + Na + +Na H 2 PO 4 Na + Na + Urine HCO 3 – + HCO 3 – HCO 3 – N H 4 Cl + Glomerular filtrate NH 3 H 2 CO 3 H + Amino acid catabolism H + H 2 CO 3 Diffusion thru membrane H + + NH 3 + Cl – H + + Na 2 HPO 4 peritubular capillary Renal tubule cells e.g., PCT, DCT Diffusion thru channel Dibasic Sodium Phosphate Ammonia Regulation of Urine H+ Concentration If tubular fluid H + conc. increases to pH of 4.5, excess H + inhibits Na + /H+ exchange and limits kidney's ability to secretes an excrete excess H+ How is this prevented? 3 buffer systems: Bicarbonate, Phosphate & Ammonia
24-34 Renal Control of pH Narrative Phosphate system dibasic sodium phosphate , Na 2 HPO 4 in glomerular filtrate • Reacts with some of H + replacing a Na + in buffer to form Na H 2 PO 4 which passes into urine - excreted! Na 2 H 1 PO 4 + H 1 + Na 1 H 2 PO 4 + Na 1 + Ammonia ( NH 3 ) from amino acid catabolism acts as a base to neutralize acid NH 3 + H 1 + and Cl − N H 4 Cl NH 4 Cl, ammonium chloride is weak acid
24-35 Acidosis, ECF pH below 7.35 H + diffuses into cell; drives K + out K + concentration in ECF H + buffered by protein in ICF loss positive charges in ICF RMP hyperpolarized RMP hyperpolarized Cells less excitable Acidosis depresses the CNS Nerve/muscle cells difficult to stimulate, CNS depression (confusion, disorientation, coma, death) Alkalosis, ECF pH above 7.45 H + diffuses out of cell; K + diffuses in Net gain of cations in ICF RMP Depolarized RMP depolarized Cells more excitable Alkalosis overstimulates excitatory cells Nerves overstimulated, muscles; spasms, tetany, convulsions, respiratory paralysis
pH 7.45 7.35 Alkalosis Acidosis 8.0 6.8 Normal Death Death H 2 CO 3 HCO 3 – 24-36 Can live only a few hours, if blood pH < 7.0 or > 7.7
24-37 Compensation for Acid–Base Imbalances
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- pH, ICF, Bicarbonate, buffer solution