Lesson 4 of 5 / Excretion in humans
Selective reabsorption
How does the nephron change filtrate into urine?
I can explain selective reabsorption and use data to distinguish filtrate from urine.
I can explain filtration and reabsorption in urine formation.Syllabus
Syllabus K325, Topic 7(c). Explain filtration and reabsorption in urine formation.
About 5 min
Make a guess. You are not marked.
Glucose is filtered into the kidney tubule. How much glucose is in a healthy person's urine?
Show the answer
Normally none, because it is all reabsorbed.
At normal blood levels, all filtered glucose is taken back into the blood from the first part of the tubule.
The key idea
Selective reabsorption moves useful substances from the nephron fluid back into the blood; the remaining fluid becomes urine.
The proximal convoluted tubule, the first coiled section, recovers glucose and amino acids. This is a flow model, not a drawing of nephron shape. Arrow widths do not specify percentages. "Normally all" for glucose assumes usual blood concentrations and normal kidney function.
Explanation
As filtrate moves along the nephron tubule, substances cross the tubule wall into surrounding blood capillaries. This return to blood is called reabsorption. Selective means different substances and amounts are handled differently.
The first coiled part of the tubule after Bowman's capsule is the proximal convoluted tubule. This is where filtered glucose and amino acids are recovered. At usual blood concentrations in a healthy kidney, their reabsorption is normally complete. Glucose uptake by tubule cells depends on energy supplied by respiration. Needed ions are also reabsorbed.
Most filtered water returns to the blood by osmosis as water-potential differences arise. The amount finally excreted varies with the body's needs. Reabsorption of water is not simply water being actively pumped through a membrane.
Urea remains a major waste in urine, although some filtered urea can also be reabsorbed. Urine normally contains urea, water and dissolved ions, without appreciable glucose or large plasma proteins. The collecting ducts deliver the remaining fluid towards the renal pelvis.
When comparing samples, distinguish concentration from total amount. Removing much water can increase the concentration of urea in the fluid even if the amount of urea has not increased. A higher concentration does not prove that the nephron manufactured more urea.
Step by step
- 1
Identify the stage
Initial filtrate and final urine are different samples.
- 2
Follow the substance
Describe movement from tubule fluid through the wall back to blood.
- 3
Check the quantity
Use amount for a mass balance. Use concentration only with attention to fluid volume.
Worked example
A model water balance
In a simplified interval, 100 mL of water is filtered and 2 mL leaves as urine. Assuming no other addition, how much filtered water returned to blood?
One way to explain it
100 - 2 = 98 mL returned to blood by reabsorption. This is 98% of the filtered water in the example. The numbers illustrate a calculation, not a standard urine-production rate.
Why this answer works
- Start with filtered amount, not its concentration.
- Subtract the amount that leaves.
- State the model assumption and units.
Watch out for this
A student says: "More concentrated urea in urine proves the kidney produced extra urea." What is wrong with this?
Show the answer
The liver produces most urea. Reabsorbing water reduces fluid volume and can concentrate the urea that remains.
Check your understanding
Glucose is present in initial filtrate but normally absent from final urine. Which process explains this?
Show the answer
Glucose is selectively reabsorbed from the tubule fluid into blood.
It can be filtered first and then returned to blood through the tubule wall.
Exam-style questions
Check your understanding
Invented values in g per 100 cm3: plasma has glucose 0.10, urea 0.03, protein 8.0. Which values are expected in the filtrate at the start of the tubule?
Show the answer
Glucose 0.10, urea 0.03, protein 0
Small molecules are filtered at plasma levels; proteins are too large to pass through.
Quick recall
How does the nephron change filtrate into urine?