Mediates the transport of urea driven by a concentration gradient across the cell membranes of erythrocytes and the renal inner medullary collecting duct which is critical to the urinary concentrating mechanism (PubMed:11792714). Facilitates water transport in erythrocytes (PubMed:12133842). urea(in) = urea(out) Homotrimer; each subunit contains a pore through which urea permeates (By similarity). Identified in a complex with STOM (By similarity). Restricted to the basolateral membrane in various portions of the urothelium. Expressed in brain, kidney, heart, liver, lung, skeletal muscle, spleen, testis, ureter and urinary bladder (at protein level). Along the gastrointestinal tract, detected in colon, jejunum and stomach (at protein level). In the kidney, expressed in some microvessels of the inner and outer medulla, but not all (at protein level). Not detected in the cortex (at protein level). Detected in the urothelium all along the urinary tract, including the papilla surface, the ureter, the bladder and the urethra (at protein level). In the brain, expressed at the border of the corpus callosum and striatum in astrocytic cellular processes surrounding blood microvessels (at protein level). Detected in erythrocytes (at protein level). Down-regulated by water deprivation in urinary bladder and ureter, but not in kidney medulla, colon, testis nor brain. N-glycosylated in red blood cells, as well as in most non-erythroid tissues, except in the gastrocnemius muscle and in the gastrointestinal tract, including liver, colon and stomach. Mutant mice exhibit grossly normal appearance, activity and behavior. Plasma sodium, potassium, chloride, bicarbonate and creatinine concentrations, as well as hematocrit, are similar to wild type animals. Urea permeability in erythrocytes is 45-fold lower than that from wild-type mice. Daily urine output is 1.5-fold greater and urine osmolarity is lower than in wild-type mice. After 24 hours of water deprivation, plasma urea concentration is 30% higher and urine urea concentration 35% lower in mutant mice than in wild-type animals. Mice lacking both Aqp1 and Slc14a1 are born at the expected Mendelian ratio, but do not thrive; half of them die within ten days after birth and none are alive after two weeks. Urine osmolality is somewhat lower than that observed with mice lacking Aqp1. Besides, erythrocyte water permeability is significantly lower than in mice lacking only Aqp1. Belongs to the urea transporter family.