Lesson 2 of 5 / Excretion in humans
The urinary system
Which tubes carry blood, and which carry urine?
I can identify the kidneys, ureters, bladder and urethra and explain their functions.
I can identify urinary organs and state their functions.Syllabus
Syllabus K325, Topic 7(b). Identify urinary organs and state their functions.
About 4 min
Make a guess. You are not marked.
Which organ actually removes urea from your blood?
Show the answer
The kidneys.
The kidneys filter blood and form urine. The bladder only stores urine until you urinate.
The key idea
Kidneys form urine; ureters deliver it to the bladder; the urethra carries it out of the body.
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
Identify what is flowing
A blood vessel contains blood; a ureter contains urine.
- 2
Trace the urinary route
Start at a kidney, then pass through its ureter to the bladder and urethra.
- 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.
Watch out for this
A student says: "The bladder removes urea from the blood." What is wrong with this?
Show the answer
The kidneys process blood and form urine. The bladder mainly stores the urine that arrives through the ureters.
Check your understanding
Which structure carries urine from the bladder to the outside?
Show the answer
Urethra
The urethra is the exit route from the bladder.
Quick recall
Which tubes carry blood, and which carry urine?