The Human Urinary System

The four structures you need to identify

Cambridge requires you to identify four structures in diagrams and images of the urinary system: the kidneys, the ureters, the bladder and the urethra. Rather than memorising them as a disconnected list, it helps to learn them as a single route that urine travels along, from the moment it’s formed to the moment it leaves the body.

Kidneys

There are two kidneys. At this level, their role is to remove urea, excess water and excess ions from the blood and form urine — the internal detail of how they actually do that, cortex, medulla and the nephron, is Supplement content covered on the kidney and nephron page. Here, the kidneys are simply where the route begins.

Ureters

There are two ureters, one leading from each kidney. Each ureter carries urine from its kidney to the bladder — that’s the whole of its job, a transport tube and nothing more.

Bladder

The bladder stores urine before it’s released from the body. It’s the one structure in this route whose job is holding rather than moving.

Urethra

The urethra carries urine from the bladder to the outside of the body — the final stage of the route, and the only structure through which urine actually leaves.

The complete route

Putting all four together, the direction of urine flow is:

kidneys → ureters → bladder → urethra → outside the body

On an unfamiliar diagram, this order is more useful than trying to recall the shape of each structure from memory. Find the two structures that look symmetrical and paired — those are the kidneys — then follow the tubes leading away from them; the ones running to a single central sac are the ureters, the sac itself is the bladder, and the single tube leading away from that sac is the urethra.

Ureter versus urethra — the mix-up examiners expect

These two names differ by one letter and describe two completely different jobs, which is exactly why Cambridge tests the distinction directly. A ureter connects a kidney to the bladder, and there are two of them. The urethra connects the bladder to the outside of the body, and there’s only one. If a question asks you to label or describe either structure, check which end of the bladder you’re talking about before you commit to a name.

Supporting structures that aren’t the required labels

The renal artery and renal vein are useful for orientation, since the kidney’s whole job depends on processing blood: the renal artery carries blood to the kidney, and the renal vein carries blood away from it. But blood vessels are not part of the urinary tract, and they aren’t substitutes for any of the four required labels above — keep urine-carrying tubes and blood-carrying vessels as two separate systems, even though they sit right next to each other inside the kidney.

For what the kidneys are actually removing from that blood, and why, see what is excretion?; for how the kidney does it internally, see kidney and nephron.