Taking back what is needed
Selective re-absorption.
Almost all comes back
A healthy adult's kidneys filter a very large amount of fluid every day, but only a little leaves the body as urine.
What do you think happens to the rest?
What this lesson covers
The idea
As urine flows along the tube, glucose, amino acids, salts and a major amount of water are selectively re-absorbed; the water re-absorbed depends on the body's excess water and the dissolved waste to be excreted.
Almost all comes back
A healthy adult's kidneys filter a very large amount of fluid every day, but only a little leaves the body as urine.
What do you think happens to the rest?
- Most of it is taken back into the blood
- It is stored in the kidney
- It is made into bile
How much is taken back?
Slide the amount of urine passed in a day. The book says it is a litre or two.
Taking back
As the urine flows along the coiled tube, useful things such as glucose, amino acids and salts are taken back into the blood, together with a major amount of water. This is selective re-absorption. How much water is taken back depends on how much excess water there is in the body and on how much dissolved waste has to be excreted.
As urine flows along the tube, glucose, amino acids, salts and a major amount of water are selectively re-absorbed; the water re-absorbed depends on the body's excess water and the dissolved waste to be excreted.
Notes
As urine flows, glucose, amino acids, salts and a major amount of water are selectively re-absorbed; the water taken back depends on excess water in the body and the dissolved waste to be excreted.
Check yourself
Which of these are selectively re-absorbed as urine flows along the tube?
On what does the amount of water re-absorbed depend?
In a healthy adult the initial filtrate is about 180 L a day. If 2 L is excreted as urine, how many litres are re-absorbed?
Answer: 178 L
180 L − 2 L = 178 L is re-absorbed.
Where does the re-absorption take place?
- Glucose, amino acids, salts and a major amount of water — correct. Yes!
- Only urea and uric acid. Urea and uric acid are nitrogenous wastes that the kidneys remove from the blood.
- Only the Bowman's capsule. The Bowman's capsule is part of the nephron; it collects the filtrate.
- On the length of the ureter. The book says the amount depends on the excess water in the body and the dissolved waste to be excreted.
- On how much excess water is in the body and how much dissolved waste is to be excreted — correct. Yes!
- On the colour of the urine. The book says the amount depends on the excess water in the body and the dissolved waste to be excreted.
- In the urinary bladder. The urinary bladder stores the urine until it is passed out.
- In the Bowman's capsule, where the filtrate is first collected. The substances are re-absorbed as the urine flows along the tube, after the capsule has collected the filtrate.
- As the urine flows along the tube of the nephron — correct. Yes!