Cambridge O Level Biology · Syllabus 5090 · Excretion
Nephron
What is Nephron?
The microscopic functional unit of the kidney, of which each human kidney contains about a million. A nephron consists of a Bowman's capsule containing a glomerulus, a nephron tubule that includes the loop of Henle, and a collecting duct, with blood vessels running alongside it throughout. Blood is filtered under pressure from the glomerulus into the Bowman's capsule, useful substances are then selectively reabsorbed from the tubule back into the blood in the surrounding capillaries, and the fluid remaining passes into the collecting duct as urine. A nephron is made of many cells; it is not a single cell.
This definition is part of the Excretion chapter in Cambridge O Level Biology.
Common mistakes with Nephron
- “Whole blood enters the nephron.” Why wrong Blood is cells plus plasma. The cells stay behind, and so do the large plasma proteins, so what enters the nephron is neither blood nor complete plasma. Say instead Only small molecules dissolved in the plasma pass into the capsule; the blood itself remains in the capillary.
Examiner tips on Nephron
- Four words that lose marks here. “The blood is filtered into the capsule” — whole blood does not enter the nephron, only part of the plasma does. “The waste is filtered out” — glucose is filtered too, and glucose is not waste. “The proteins are filtered and then reabsorbed” — large plasma proteins never leave the blood in the first place. “Urine collects in the capsule” — it is filtrate until reabsorption has happened.
Questions students ask about Nephron
What are the parts of a nephron?
The Bowman's capsule, the glomerulus inside it, the nephron tubule, the loop of Henle, the collecting duct, and the blood vessels associated with them. Each kidney contains about a million nephrons, and each nephron is made of many cells rather than being a single cell.
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.
Why does normal urine contain no glucose?
Glucose is small enough to be filtered, so it does enter the nephron and is present in the filtrate. All of it is then reabsorbed into the blood by active transport as the filtrate passes along the tubule, so none of it reaches the collecting duct.