The kidney: structure, ultrafiltration and reabsorption

Structure and urea

Gross structure: blood enters in the renal artery and leaves in the renal vein. The outer cortex contains the Bowman's capsules and convoluted tubules; the inner medulla contains the loops of Henle and collecting ducts; urine drains into the pelvis and then the ureter to the bladder.

A nephron: Bowman's capsule (around the glomerulus), proximal convoluted tubule, loop of Henle (descending and ascending limbs), distal convoluted tubule, collecting duct.

Urea is made in the liver. Excess amino acids cannot be stored, so they are deaminated: the amino group is removed as ammonia, which is very toxic and is converted to less toxic urea (in the ornithine cycle). The rest of the molecule is respired or stored as glycogen or fat.

Ultrafiltration

The afferent arteriole entering the glomerulus is wider than the efferent arteriole leaving it, so blood in the glomerulus is under high hydrostatic pressure. Water and small molecules (glucose, amino acids, urea, ions) are forced out into the Bowman's capsule through:

1. pores (fenestrations) in the capillary endothelium

2. the basement membrane, which acts as the main filter

3. gaps between the podocytes lining the capsule

Blood cells and large plasma proteins stay in the blood. About 180 dm³ of filtrate is formed each day, but only about 1.5 dm³ of urine leaves the body.

Selective reabsorption and the loop of Henle

• Proximal convoluted tubule: all glucose and amino acids, most ions and much water are reabsorbed. Cells have microvilli (large surface area) and many mitochondria. Na+ is actively pumped out of the cells into the blood, so Na+ diffuses in from the filtrate, bringing glucose and amino acids with it by co-transport. Water follows by osmosis.
• Loop of Henle (countercurrent multiplier): the ascending limb actively transports Na+ and Cl- out into the medulla but is impermeable to water. This makes the tissue fluid of the medulla very concentrated (low water potential). The descending limb is permeable to water, so water leaves it by osmosis. Because the two limbs carry fluid in opposite directions, the concentration gradient is built up along the whole loop.
• Collecting duct: passes through the concentrated medulla, so water can leave by osmosis, producing concentrated urine (controlled by ADH).

Animals in dry habitats, such as desert rodents, have long loops of Henle and produce very concentrated urine.

Worked example

About 180 dm³ of filtrate are formed per day and 1.5 dm³ of urine is produced. What percentage of the filtrate is reabsorbed?

1. Reabsorbed = 180 - 1.5 = 178.5 dm³
2. Percentage = 178.5 / 180 x 100 = 99.2%

Answer: About 99.2%.

Check yourself

1. Filtrate forms at 125 cm³ per minute and urine at 1.0 cm³ per minute. What percentage of the filtrate is reabsorbed? Give your answer to 1 decimal place.
   Show answer

   99.2 %. 124 / 125 x 100 = 99.2%.

2. Filtrate forms at 125 cm³ per minute. What volume forms in 24 hours, in dm³?
   Show answer

   180 dm³. 125 x 1440 = 180 000 cm³ = 180 dm³.

3. Why is blood in the glomerulus under high pressure?
   • The afferent arteriole is wider than the efferent arteriole
   • The glomerulus has thick muscular walls
   • The renal vein is blocked
   • The heart pumps directly into it
   Show answer

   The afferent arteriole is wider than the efferent arteriole. A wide inlet and narrow outlet raise pressure.

4. Which substance is normally not present in the glomerular filtrate?
   • Glucose
   • Urea
   • Sodium ions
   • Plasma proteins
   Show answer

   Plasma proteins. Large proteins cannot pass the basement membrane.

5. How are cells of the proximal convoluted tubule adapted for reabsorption?
   • Thick walls with no membranes
   • Microvilli and many mitochondria
   • Few mitochondria and no carrier proteins
   • Impermeable to glucose
   Show answer

   Microvilli and many mitochondria. Large surface area and ATP for active transport.

6. What happens in the ascending limb of the loop of Henle?
   • Water leaves by osmosis
   • Glucose is reabsorbed
   • Urea is made
   • Na+ and Cl- are actively pumped out, but water cannot follow
   Show answer

   Na+ and Cl- are actively pumped out, but water cannot follow. It is impermeable to water, so the medulla becomes concentrated.

7. How is urea formed?
   • Glucose is broken down in the kidney
   • Excess amino acids are deaminated in the liver and the ammonia is converted to urea
   • Fatty acids are oxidised in muscles
   • Proteins are filtered unchanged
   Show answer

   Excess amino acids are deaminated in the liver and the ammonia is converted to urea. Deamination then the ornithine cycle.

8. Why do desert mammals often have long loops of Henle?
   • A longer loop builds a steeper concentration gradient, so more water is reabsorbed
   • They need to excrete more water
   • Long loops filter more blood
   • They do not produce urea
   Show answer

   A longer loop builds a steeper concentration gradient, so more water is reabsorbed. More concentrated medulla gives more concentrated urine.

Practise with instant feedback Print this lesson

Sources

• Pearson Edexcel International Advanced Level in Biology specification (XBI11/YBI11, first teaching 2018)
• ABPI Schools: Kidney structure and function

Last updated 7 November 2026. Spotted a mistake? Tell us.