Multiple choice Which of the following statements characterises the excretion of acid by the nephron? Acid can be excreted as NH4+ ions. Acid excretion mainly occurs in the early nephron. In the distal nephron, secreted hydrogen ions mainly contribute to bicarbonate reabsorption. Sodium phosphate is a urinary base. Human urine is usually alkaline in nature. Reveal answer Fill a bubble to check yourself A Correct answer Explanation When hydrogen ions are secreted by the tubules, they can combine with NH3 to form NH4+ and excreted as NH4+.