any of the triangular sections of tissue that constitute the medulla, or inner substance, of the kidney. The pyramids consist mainly of tubules that transport urine from the cortical, or outer, part of the kidney, where urine is produced, to the calyces, or cup-shaped cavities in which urine collects before it passes through the ureter to the bladder. The point of each pyramid, called the papilla, projects into a calyx. The surface of the papilla has a sievelike appearance because of the many small openings from which urine droplets pass. Each opening represents a tubule called the duct of Bellini, into which collecting tubules within the pyramid converge. Muscle fibres lead from the calyx to the papilla. As the muscle fibres of the calyx contract, urine flows through the ducts of Bellini into the calyx. The urine then flows to the bladder by way of the renal pelvis and a duct known as the ureter.
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