12.3 The Human Kidney

A free GCSE Biology lesson for AQA (8461) from OBA School: filtration, selective reabsorption, and how ADH controls the water content of the blood. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Describe how the kidneys make urine by filtration and selective reabsorption
• Interpret data on the composition of blood plasma, filtrate and urine
• Explain how the volume and concentration of urine change with water intake and loss
• Explain how ADH controls the water level of the blood by negative feedback (Higher tier)

Key words

• Nephron: One of about a million tiny tubules in each kidney. Each one filters blood and makes urine.
• Filtration: Small molecules are forced out of the blood under high pressure into the kidney tubule.
• Filtrate: The liquid filtered out of the blood: water, glucose, ions and urea, but no protein or cells.
• Selective reabsorption: Useful substances (all the glucose, some ions, water as needed) are taken back into the blood from the tubule.
• ADH: Anti-diuretic hormone. Released by the pituitary gland; makes the kidney tubules reabsorb more water (Higher tier).
• Negative feedback: A change from normal causes a response that reverses the change.

Inside the kidney: nephrons

Each kidney contains about a million nephrons. Blood arrives at a nephron through a knot of capillaries called the glomerulus, which sits inside a cup-shaped capsule. The capsule leads into a long tubule and then a collecting duct.

Step 1: filtration

Blood in the glomerulus is under high pressure. Small molecules – water, glucose, mineral ions and urea – are squeezed out into the capsule. This liquid is the filtrate.

Proteins and blood cells are too large to pass through, so they stay in the blood. Filtration is not selective: it depends only on size.

Key fact: The kidneys filter about 180 dm³ of liquid from the blood every day, but you make only about 1.5 dm³ of urine. Almost all of the filtrate goes back into the blood.

Step 2: selective reabsorption

As the filtrate flows along the tubule, useful substances are taken back into the blood. This is selective – only what the body needs is reabsorbed.

• All the glucose is reabsorbed (by active transport).
• Some mineral ions – as many as the body needs.
• Water – as much as the body needs (by osmosis).
• Most urea stays in the tubule. Urea, excess ions and excess water leave as urine.

• Protein; Blood plasma (g/dm³) 72; Filtrate (g/dm³) 0; Urine (g/dm³) 0
• Glucose; Blood plasma (g/dm³) 0.9; Filtrate (g/dm³) 0.9; Urine (g/dm³) 0
• Urea; Blood plasma (g/dm³) 0.3; Filtrate (g/dm³) 0.3; Urine (g/dm³) 18
• Sodium ions; Blood plasma (g/dm³) 3.3; Filtrate (g/dm³) 3.3; Urine (g/dm³) 4.6

Worked example: reading the table

How many times more concentrated is urea in urine than in the filtrate? Explain why.
18 ÷ 0.3 = 60 times.
Urea is not reabsorbed, but most of the water in the filtrate is. The same urea ends up in a much smaller volume of water.

Answer: 60 times more concentrated, because water is reabsorbed but urea is not

Worked example: percentage reabsorbed

A person filters 180 dm³ a day and makes 1.5 dm³ of urine. What percentage of the filtrate is reabsorbed?
180 − 1.5 = 178.5 dm³ reabsorbed. 178.5 ÷ 180 × 100 = 99.2%

Answer: 99.2% (1 d.p.)

Controlling water with ADH (Higher tier)

The water level of the blood is controlled by the hormone ADH. Receptors in the brain detect the concentration of the blood. The pituitary gland releases ADH into the blood, and it travels to the kidneys.

ADH acts on the kidney tubules (collecting ducts), making them more permeable to water, so more water is reabsorbed back into the blood. This is negative feedback: the response reverses the change.

Exam tip: Anti-diuretic means "against making lots of urine". More ADH → less urine; less ADH → more urine.

Common mistakes

• Saying the kidney filters out only the waste. Filtration removes all small molecules, then useful ones are reabsorbed.
• Saying protein is in the filtrate. Protein molecules (and blood cells) are too big to be filtered.
• Saying glucose is normally in urine. All the glucose is reabsorbed in a healthy person.
• Getting ADH backwards: more ADH means more water reabsorbed and a smaller volume of urine.
• Saying ADH is made by the kidneys. It is released by the pituitary gland and acts on the kidney tubules.

Revision questions

Try these, then check your answers below each one.

Question 1. Which substance is filtered out of the blood in the kidney but is ALL reabsorbed in a healthy person? (Choose from: Protein, Sodium ions, Glucose, Urea.)

Answer: Glucose

Working: All the glucose is reabsorbed. Urea mostly stays in the tubule, protein is not filtered, and only as many sodium ions as needed are reabsorbed.

Question 2. True or false? Proteins are found in the filtrate made by a healthy kidney.

Answer: False

Working: False. Protein molecules (and blood cells) are too large to be filtered, so they stay in the blood.

Question 3. Blood plasma contains 0.30 g/dm³ of urea. The urine contains 21 g/dm³. How many times more concentrated is urea in the urine?

Answer: 70

Working: 21 ÷ 0.30 = 70 times. Water is reabsorbed but urea is not, so urea becomes concentrated.

Related lessons

• Previous lesson: 12.2 Removing Waste Products
• Next lesson: 12.4 Dialysis – an Artificial Kidney
• In this chapter: 12.1 Controlling Body Temperature
• In this chapter: 12.5 Kidney Transplants

Continue in the full course

This lesson is section 12.3 (AGCBH1203) of our free GCSE Biology course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Biology course.