Cambridge O Level Biology · Syllabus 5090 · Excretion
Selective Reabsorption
What is Selective Reabsorption?
The return of useful substances from the nephron tubule to the blood in the surrounding capillaries, after filtration has taken them out of the blood. All of the glucose is reabsorbed, some of the ions are reabsorbed and most of the water is reabsorbed, while urea is not reabsorbed and stays in the tubule. The process is described as selective because only certain substances, and only certain quantities of them, are returned to the blood; what remains in the tubule after reabsorption becomes urine.
This definition is part of the Excretion chapter in Cambridge O Level Biology.
Selective Reabsorption in context
Excretion is the removal from organisms of toxic materials, the waste products of metabolism, and substances in excess of requirements. The two metabolic wastes you must know are carbon dioxide, made by respiration in every cell and excreted by the lungs in expired air, and urea, made in the liver from excess amino acids and removed from the blood by the kidneys in urine. The kidney does this in three steps: filtration at the glomerulus forces small molecules out of the blood, selective reabsorption returns all the glucose, some ions and most of the water to the blood, and what is left — urea, excess water and excess ions — is urine.
Common mistakes with Selective Reabsorption
- “Filtrate and urine are the same thing.” Why wrong Filtrate contains all the glucose that was filtered, plus far more water and ions. Urine contains none of that glucose. Say instead Filtrate becomes urine only after selective reabsorption has returned the useful substances to the blood.
- “Selective reabsorption removes useful substances from the blood.” Why wrong The direction is backwards. Reabsorption puts substances back into the blood, having previously lost them at filtration. Say instead Selective reabsorption returns useful substances from the tubule to the blood in the surrounding capillaries.
Questions students ask about Selective Reabsorption
What is selective reabsorption?
Selective reabsorption is the return of useful substances from the nephron tubule to the blood in the surrounding capillaries. All of the glucose, some of the ions and most of the water are reabsorbed. Urea is not reabsorbed and stays in the tubule.
What is the difference between filtrate and urine?
Filtrate is the fluid in the Bowman's capsule immediately after filtration; it contains water, glucose, urea and ions. Urine is what remains after selective reabsorption has returned all the glucose, some ions and most water to the blood, so urine contains urea, excess water and excess ions but no glucose.