The primary sites of gas exchange in marine teleosts, the gills, are also responsible for osmoregulation. Because gills are designed to increase surface area and minimize diffusion distance for gas exchange between the blood and water, they may contribute to the problem of water loss by osmosis and passive salt gain. This is called the osmo-respiratory compromise. To overcome this, tunas constantly drink seawater to compensate for water loss. They excrete highly concentrated urine which is approximately isosmotic to blood plasma, i.e. urine solute to plasma solute ratio is close to 1 (U/P≅1). Because of this, solely excreting urine is not sufficient to resolve the osmoregulatory problem in tunas. In turn, they excrete only the minimum volume of urine necessary to rid of solutes that are not excreted by other routes, and the salt is mostly excreted via gills. This is why the composition of solutes in urine differs significantly from that of the blood plasma. Urine has a high concentration of divalent ions, such as Mg2+ and SO42− (U/PÂ»1), as these ions are mostly excreted by the kidneys keeping their concentration in blood plasma from rising. Monovalent ions (Na+, Cl−, K+) are excreted by the gills, so their U/P ratios in the urine are below 1. The excretion of inorganic ions by structures other than kidneys is called the extrarenal salt excretion.