A level Biology exam revision resources written by A level Examiners


The kidney is supplied with blood by the renal artery. As the blood passes through the kidney it is filtered, waste is removed and useful substances are reabsorbed. The filtered blood returns to the heart through the renal vein. The waste substances removed by the kidney pass to the bladder through tubes called ureters.

The part of the kidney which is responsible for filtering the blood and reabsorbing the useful substance is called a nephron. There are a million nephrons present in each kidney. The figure 3, below, shows the location of a nephron within a kidney.

Figure 3 : The kidney
The Kidney

The sequence of filtration and selective reabsorption that occurs in the nephron is explained in this section. Use figure 4 and the number labels to follow the explanations, provide below.

Figure 4
The Kidney
  1. The Bowman's capsule
    • Blood enters a knot of capillaries called the glomerulus. The blood is under pressure as it has been pumped by the left ventricle (of the heart).
    • Ultrafiltration occurs. As the blood passes through the Bowman's capsule, small molecules are forced through into the kidney tubule. The solution entering the kidney tubule is called the filtrate

      Table 1 : Components of blood
      The kidney

  2. The proximal convoluted tubule
    • The cells lining the proximal tubule have two key visible adaptations, which are shown in figure 5, below. They also have many carrier proteins present in their membranes.

      Figure 5
      The kidney
    • The microvilli increases the surface area for absorption, and the mitochondria provide ATP for active transport.
    • As the filtrate passes through the proximal tubule, 100% of the glucose is reabsorbed, 100% of the amino acids are reabsorbed and 80% of the mineral ions are reabsorbed.
    • This reabsorption makes the water potential of the proximal tubule cells more negative that the filtrate. Water therefore moves by osmosis out of the filtrate into the proximal tubule cells.

  3. The loop of Henle

    Figure 6
    The loop of Henle
    • The ascending limb of the loop of Henle is impermeable to water so only ions leave from this limb. Na+ and Cl- are both actively transported out of the ascending limb.
    • Some Na+ and Cl- diffuse back into the descending limb.
    • This ions movement creates an ion gradient in the medulla.
    • Some water leaves the descending limb by osmosis.

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  4. The distal convoluted tubule
    • The final reabsorption of Na+ and Cl- occurs by active transport here.
    • The hormone aldosterone controls this process.

  5. The collecting duct
    • The concentration of Na+ and Cl- in the medulla surrounding the collecting duct is high due to the action of the loop of Henle.
    • Therefore the water potential of the medulla is more negative than the water potential inside the collecting duct.
    • Water can therefore always leave the collecting duct by osmosis.

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