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A renal calyx is a cup-like collecting passage within the kidney that receives urine leaving a renal papilla. A minor calyx lies around a papillary outlet; these passages join into a major calyx, which drains toward the renal pelvis and then the ureter. The calyx is therefore part of the outflow route, downstream of the collecting duct system, rather than a site where the glomerulus first filters blood.^[ref]

The branching arrangement supplies a simple mass balance. Suppose three upstream minor calyx passages deliver steady volumetric flow rates of $0.2$, $0.3$ and $0.1mL/min$ to one outlet. With no storage or side flow, that outlet carries $0.6mL/min$. These illustrative values describe how branches combine; they are not normal human flow measurements.

A longitudinal anatomical section makes the distinction visible: locate the pointed renal papilla first, then the hollow receiving space around it. The projecting tissue and its receiving passage are adjacent structures with different roles.^[ref]

The renal pelvis is the expanded collecting region of the kidney that receives urine from the major calyx passages and continues into the ureter. It lies downstream of the collecting duct system and the renal calyx network. Its function is to collect and route their combined output, rather than to carry out glomerular filtration.^[ref]

Trace the drainage sequence from a single renal pyramid: its collecting ducts open at a renal papilla, the receiving minor calyx joins a larger major calyx, and the latter drains toward the renal pelvis. A longitudinal anatomical specimen exposes these nested connections directly. The surrounding kidney tissue and the space receiving its output are different structures.^[ref]

For a steady illustrative mass balance, suppose two incoming passages supply $0.4$ and $0.6mL/min$. If pelvic storage is unchanged and there is no other inflow or loss, the ureter must carry $1.0mL/min$. Sampling there measures pooled output: it cannot, by itself, reveal which upstream nephron produced a particular change in urine composition.

A ureter is a muscular tube carrying urine from a kidney's collecting region to the urinary bladder. Coordinated contractions of its smooth muscle propel urine along the tube, so transport is not simply a consequence of gravity.^[ref]

It should be distinguished from the urethra, which carries urine outward from the bladder. The distinction separates two different transport segments and explains why a description of blockage or flow must specify its location. Urine production in the kidney, passage through the ureter and storage in the bladder are different stages of the urinary system.

The urethra carries urine from the urinary bladder to the exterior. Its length, neighboring structures and reproductive relations differ in male and female anatomy. It should be distinguished from the ureters, which connect kidneys to bladder.

The urinary bladder is a distensible muscular organ that stores urine arriving through the ureters. Emptying requires coordinated activity of the detrusor muscle and outlet structures. Storage and production are different functions: most urine formation occurs in the kidneys.

The urinary system forms, transports, stores and eliminates urine. The kidneys perform filtration and regulated transport, the ureters convey urine to the urinary bladder, and the urethra carries it outward. Regulation of body fluids and movement of the resulting urine are different functions.