1097

Humans are ureotelic, but we also excrete uric acid. The uric acid in human urine comes largely from the metabolism of nucleic acids. If uric acid levels in the extracellular fluid rise too high, uric acid can precipitate in joints, causing the age-old malady gout. Because solubility goes down with temperature, uric acid usually precipitates first in the extremities, especially the big toe. Pain in the big toe is a telltale symptom of gout. The risk for developing gout goes up with excessive consumption of protein, beer, and spirits (less so with wine) and in general with obesity and insulin resistance. Therefore in the past it was called “the disease of rich men.” Gout has been around for a very long time; the first documented case was in Egypt around 2500 B.C.E.

Humans can also excrete ammonia, which is an important mechanism for regulating the pH of the extracellular fluids. As you will see later in this chapter, excreted ammonia buffers the urine and enables the excretion of excess hydrogen ions.

Species that live in different habitats at different developmental stages may use more than one mechanism of nitrogen excretion. The tadpoles of frogs and toads, for example, excrete ammonia across their gill membranes, but adult frogs and toads generally excrete urea. Some adult amphibians that live in arid habitats excrete uric acid.