CDS Biology · Human Physiology
Excretion and Reproductive Organs
Where urea comes from, how the kidneys, ureters, bladder and urethra get rid of it, and what each organ of the male reproductive system does.
Why this matters
Six CDS questions from 2017 (II) to 2025 (II). Four are about urea and urine: where urea is made, from which food, why spilled urine smells, and the path urine takes. Two are about the male reproductive organs.
Concept 1 of 2: Urea, the kidneys and the path of urine
When the body breaks down extra protein, the nitrogen part comes off as ammonia, which is poisonous. The liver quickly turns it into urea, which is much safer. The blood carries the urea to the kidneys, which filter it out with water to make urine. Urine then runs down to the bladder, is stored, and leaves the body through the urethra.
Definition
Excretion of urea:
- Urea is made in the liver, from the ammonia released when amino acids (from proteins) are broken down. Carbohydrates and fats have no nitrogen, so they make no urea.
- The kidneys filter urea, salts and extra water out of the blood. Each kidney has about a million filtering units called nephrons.
- Path of urine: kidney → ureter → urinary bladder → urethra.
- Fresh urine hardly smells. Spilled urine smells sharp because bacteria break urea down into ammonia.
- When both kidneys fail, the blood is cleaned by a machine: haemodialysis.
| Organ | Its part in removing urea |
|---|---|
| Liver | Makes urea from the ammonia of broken-down proteinsQ Asked twice: CDS 2019 (I) (urea is produced in the liver) and 2022 (II) (from proteins). |
| Kidney | Filters urea and water from the blood to make urine |
| Ureter | Carries urine from the kidney to the bladder |
| Urinary bladder | Stores urine |
| Urethra | Carries urine out of the body |
| Bacteria on the floor | Break urea into ammonia, the smell of stale urineQ CDS 2017 (II): the smell is from bacteria turning urea into ammonia. |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2020 · CDS (I) 2020 — General Knowledge · Q27Easy
Example 1 · Human Physiology · Excretion and Reproductive Organs
Which one of the following is the correct sequence of organs that occur in the path of urine flow in human body?
Urea is made in the liver, not the kidney
The kidneys remove urea; the liver makes it. Blood, lungs and the digestion of starch have nothing to do with making urea.
Ureter before bladder, urethra after
The ureters lead from the kidneys to the bladder; the urethra leads from the bladder out. The two names are close, so check which tube is on which side of the bladder.
Concept 2 of 2: The male reproductive organs
Sperm are made in the testes, which hang outside the body in the scrotum because sperm form best a little below body temperature. From there sperm move to the epididymis to mature, then along the vas deferens. On the way, the seminal vesicles and the prostate add a fluid that feeds the sperm and lets them swim. The cervix, by contrast, is the neck of the womb in a woman.
Definition
The male system:
- Testes (in the scrotum): make sperm (in the seminiferous tubules) and the hormone testosterone.
- Epididymis: stores the sperm while they mature.
- Vas deferens: carries sperm towards the urethra.
- Seminal vesicles and prostate gland: add fluid that nourishes the sperm and makes them motile (able to swim). Sperm plus this fluid is semen.
- Urethra: common passage for urine and semen.
The female system: ovaries, oviducts (fallopian tubes), uterus, cervix (the neck of the uterus) and vagina.
| Male organ | What it does |
|---|---|
| Testis | Makes sperm and testosterone |
| Scrotum | Keeps the testes a little cooler than the body |
| Epididymis | Stores sperm while they matureQ CDS 2018 (II): epididymis, vas deferens, seminal vesicles and prostate together serve maturation and motility, not spermatogenesis. |
| Vas deferens | Carries sperm towards the urethra |
| Seminal vesicles and prostate | Add fluid that feeds the sperm and lets them swim |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2025 · CDS (II) 2025 — General Knowledge · Q44Easy
Example 2 · Human Physiology · Excretion and Reproductive Organs
Which one of the following is not a part of the male reproductive system in human beings ?
Sperm are made only in the testis
The epididymis, vas deferens, seminal vesicles and prostate mature, carry and activate sperm. Spermatogenesis itself happens in the testis, so any option that gives it to the other organs is wrong.
The cervix is female
The cervix is the narrow neck of the uterus. The testis, scrotum and prostate are male.
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Reference tables (2)
Urea, the kidneys and the path of urine6 rows
| Organ | Its part in removing urea |
|---|---|
| Liver | Makes urea from the ammonia of broken-down proteinsQ Asked twice: CDS 2019 (I) (urea is produced in the liver) and 2022 (II) (from proteins). |
| Kidney | Filters urea and water from the blood to make urine |
| Ureter | Carries urine from the kidney to the bladder |
| Urinary bladder | Stores urine |
| Urethra | Carries urine out of the body |
| Bacteria on the floor | Break urea into ammonia, the smell of stale urineQ CDS 2017 (II): the smell is from bacteria turning urea into ammonia. |
The male reproductive organs5 rows
| Male organ | What it does |
|---|---|
| Testis | Makes sperm and testosterone |
| Scrotum | Keeps the testes a little cooler than the body |
| Epididymis | Stores sperm while they matureQ CDS 2018 (II): epididymis, vas deferens, seminal vesicles and prostate together serve maturation and motility, not spermatogenesis. |
| Vas deferens | Carries sperm towards the urethra |
| Seminal vesicles and prostate | Add fluid that feeds the sperm and lets them swim |
Watch out for (4)
- Urea is made in the liver, not the kidney→ Urea, the kidneys and the path of urine
- Ureter before bladder, urethra after→ Urea, the kidneys and the path of urine
- Sperm are made only in the testis→ The male reproductive organs
- The cervix is female→ The male reproductive organs
Test yourself on Human Physiology
15 past CDS questions from this chapter, timed at 15 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.