= Renal papilla
A <renal papilla> is the tip of a <renal pyramid>, projecting into the receiving space of a <renal calyx>. The converging <collecting duct system> opens at this surface, allowing <urine> to leave the medullary <tissue> and enter the collecting passages. A papilla is a region of the <kidney> itself; the surrounding calyx is part of its drainage route.<Longitudinal section through Kidney by the Manipal Museum of Anatomy and Pathology>[voiceover and labeled specimen: pyramid apex opening into minor calyx]{ref}
Trace a drop of tubular fluid toward this boundary. Before it exits the <collecting duct>, exchange across the lining <epithelium> can still modify its composition. After it enters the <renal calyx>, it has left that microscopic tubular pathway and moves into a larger collecting passage. This spatial distinction prevents confusion between making <urine> and routing the already collected output.
The <renal papilla> also marks the deep end of the <renal medulla>'s concentrating environment. A section through the <kidney> connects the broad base of a <renal pyramid>, facing the <renal cortex>, to its narrower papillary outlet. The shape helps organize many converging passages, but shape alone does not explain the <membrane transport> that concentrates their contents.<Anatomy and Physiology 2e by J. Gordon Betts and others>[Section25.3: internal anatomy and renal pyramids]{ref}
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