The nephron and urea
Introduced| English | Meaning |
|---|---|
| nephrons/ˈnefrɒnz/ | A microscopic kidney unit that filters blood plasma and modifies the resulting fluid through reabsorption and secretion. |
| ultrafiltration/ˌʊltrəfɪlˈtreɪʃn/ | The pressure-driven separation of water and small solutes from larger molecules across a filtration barrier. |
| selective reabsorption/sɪˈlektɪv riːbˈsɔːpʃn/ | The return of useful substances from kidney filtrate to the blood according to the body's needs. |
| osmoregulation/ˌɒzmɔːɡjʊˈleɪʃn/ | The control of water balance and dissolved solute concentrations in an organism. |
Inside the kidney's filter
- Each kidney contains about a million tiny filtering units called nephrons 肾单位.
- They clean the blood, keep what's useful, and remove what's not.
During ultrafiltration in the nephron, which stays in the blood?
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Large things (cells, proteins) are not filtered out.
The tiny filtering unit of the kidney is the ______.
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Each kidney has about a million nephrons.
Filtration
- High blood pressure forces small molecules (water, glucose, salts, urea) out of the blood — ultrafiltration 超滤.
- Big things (blood cells, proteins) stay in the blood.
A nephron filters the blood, then reabsorbs the useful substances; what is left is urine
The nephron at work
Each nephron filters the blood, then takes back what the body needs — what is left becomes urine.
How much glucose is normally reabsorbed back into the blood?
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Glucose is useful, so all of it is reabsorbed.
Selective reabsorption returns urea from the nephron to the blood.
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False. Selective reabsorption returns useful substances such as glucose and needed water to the blood. Urea stays in the nephron and leaves in urine.
Reabsorption
- Useful substances are taken back into the blood — selective reabsorption 选择性重吸收.
- All the glucose, and as much water as the body needs, are reabsorbed.
The liver makes urea from excess amino acids; the kidneys excrete it
Urine is mainly made of:
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Useful substances are reabsorbed; waste forms urine.
What's left is urine
- What remains — urea, excess water and salts — becomes urine.
- The amount of water reabsorbed adjusts to keep the body's water balance right (osmoregulation 渗透调节).
Worked example. Glucose is filtered out of the blood at the nephron, but it's useful — so all of it is reabsorbed. Finding glucose in someone's urine can be a sign of diabetes.
You've got it
- the nephron filters blood: ultrafiltration pushes out small molecules, keeps cells/proteins
- selective reabsorption returns useful substances (all glucose + needed water)
- what's left (urea, excess water/salts) is urine
Write from glucose that returns
- Small molecules are forced out of the blood in the nephron.
- Protein molecules and blood cells stay in the blood.
What happens to glucose, and what is left in urine?
Filtration takes small molecules, and reabsorption keeps the useful ones.
What happens to glucose in a healthy nephron?
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Selective reabsorption returns all the glucose.