IGCSE Biology glossary
Selective reabsorption
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Definition
The process in the kidney tubules where useful substances from the filtrate (all glucose, most water, some ions and amino acids) are reabsorbed back into the blood. Glucose and amino acids are reabsorbed by active transport in the proximal convoluted tubule, while water is reabsorbed by osmosis.
What to include in an exam answer
ALL glucose is reabsorbed (so normally no glucose in urine). Most water is reabsorbed. Urea is NOT reabsorbed (it is the waste product that needs to be excreted). If glucose appears in urine, it may indicate diabetes.
Example in context
Explain why glucose is normally absent from urine (2 marks): All glucose that passes into the filtrate by ultrafiltration is reabsorbed back into the blood by active transport in the proximal convoluted tubule. Therefore no glucose remains in the fluid that becomes urine.
Related syllabus topic
Excretion in humans