IGCSE Biology / Homeostasis and excretion
Ultrafiltration and selective reabsorption
Edexcel International GCSE Biology (4BI1), chapter 8: Homeostasis and excretion, specification point 2.72B, 2.74B, 2.75B, 2.77B. Free lesson.
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The nephron filters small molecules from the blood under pressure, then selectively returns useful substances to the blood. This leaves urea and excess substances in urine.
How to label a nephron
A nephron is one of the kidney’s filtering units. Blood enters through the afferent arteriole and leaves through the narrower efferent arteriole. The glomerulus is a knot of capillaries inside Bowman's capsule, which catches the filtrate. The tubules then carry it through the kidney. The proximal convoluted tubule is where most reabsorption happens. The loop of Henle dips from the outer cortex into the inner medulla and back. It helps set up the conditions for water reabsorption. The distal convoluted tubule fine-tunes salt levels. The collecting duct carries urine through the…
How ultrafiltration works
Ultrafiltration is filtering under pressure. The afferent arteriole is wider than the efferent arteriole, so blood builds up at high pressure in the glomerulus. This pressure forces small molecules through the capillary wall and into Bowman's capsule. Water, glucose, amino acids, salts and urea pass through because their molecules are small enough. Blood cells and large plasma proteins stay in the blood because they are too large to pass through. The liquid collected in Bowman's capsule is called the filtrate. It contains useful substances as well as waste, because filtration is by…
What selective reabsorption returns to the blood
Selective reabsorption puts useful substances from the filtrate back into the blood. Most of this happens in the proximal convoluted tubule. It is selective because different substances are reabsorbed in different amounts. All glucose is reabsorbed by active transport, which uses energy to move it against its concentration gradient. Tubule cells have many mitochondria to supply this energy. All amino acids are reabsorbed in the proximal convoluted tubule. Some salts are reabsorbed, according to what the body needs. Most water is reabsorbed by osmosis, following salts and glucose…
How urine is formed
Further along the nephron, the collecting duct reabsorbs more water. ADH affects how much water is taken back. What remains is urine: water containing urea and excess salts dissolved in it. A large volume of fluid is filtered each day, but most of it is reabsorbed. About 180 litres are filtered daily, while about 1.5 litres of urine are produced. The kidney does not simply filter waste out of the blood. It filters small molecules, including useful ones, then returns useful substances by selective reabsorption.