K325 / 2027

Lesson 2 of 5 / Excretion in humans

The urinary system

Which tubes carry blood, and which carry urine?

In this lesson: Identify the kidneys, ureters, bladder and urethra and explain their functions.

About 4 min

The key ideaKidneys form urine; ureters deliver it to the bladder; the urethra carries it out of the body.

Urinary system front view with kidneys highlighted. Two ureters connect the kidneys to one bladder; a urethra leaves the bladder.KidneyKidneyUreterUreterBladderUrethra
The kidneys process blood through many nephrons and form urine. Blood enters through renal arteries and leaves through renal veins.

Green arrows show urine flow. Red and blue vessels indicate the separate blood supply schematically; the ureters are the two descending tan tubes. Anatomical right is on the viewer's left.

Explanation

The kidneys are a pair of organs near the back of the abdomen. Each receives blood through a renal artery and returns blood through a renal vein. These vessels are part of the circulatory system.

Inside each kidney, many microscopic nephrons process fluid from the blood. Filtration and reabsorption help remove urea and adjust the amounts of water and ions leaving as urine. The remaining blood stays within blood vessels.

Urine collects towards the central renal pelvis and enters a ureter. There is one ureter from each kidney. These tubes carry urine to the bladder, which stores it until urination.

The urethra carries urine from the bladder to the outside. Ureter and urethra are different names for different structures, so trace the route before labelling a diagram.

Blood leaving a healthy kidney still contains water, useful solutes and blood cells. A kidney does not convert all incoming blood into urine. Nor does urine travel back to the heart through the renal vein.

Step by step
  1. 1

    Identify what is flowing

    A blood vessel contains blood; a ureter contains urine.

  2. 2

    Trace the urinary route

    Start at a kidney, then pass through its ureter to the bladder and urethra.

  3. 3

    Give a function

    Form, carry, store and remove urine are four different roles.

Worked example

Label a tube by its connections

A diagram shows a tube connecting the left kidney to the bladder. Name it and state its function.

One way to explain it

It is the left ureter. It carries urine from the left kidney to the bladder for storage.

Why this answer works
  • The connections identify the tube.
  • The urethra begins at the bladder, not at the kidney.
  • The tube carries urine rather than blood.
Is this true? "The bladder removes urea from the blood."

The kidneys process blood and form urine. The bladder mainly stores the urine that arrives through the ureters.

Try a question

Which structure carries urine from the bladder to the outside?
You can return to this lesson any time.