(3.2) The three functions of the kidneys (and mention of associated liver function)

The kidneys are bean-shaped organs found in the abdomen, one on either side of the aorta.

The kidneys are really important in control of ...

(a) Water control

The kidneys help to regulate the water content of the body by controlling how much water is reabsorbed and how much is lost in urine.

Water is lost through the skin in sweating and the lungs when breathing out, but these losses cannot be controlled by any organ in the body, including the kidneys.

We also lose water when the kidneys remove urea in urine.

So, this water loss must be balanced by water we consume in food and drinks.

But, the kidney will excrete excess water too.

Water balance is strongly related to ion concentrations in the body fluids.

 

(b) Ions and small molecules concentration control

Ions such as sodium, iron and calcium are taken into our body in food and drinks and then reabsorbed into the bloodstream.

If the ion content of the body is wrong. it can upset the balance between ion concentration and water and too much water can be absorbed into cells by osmosis - so the wrong balance can lead to cell damage e.g. extreme dehydration from lack of water OR excess hydration (caused by some illegal drugs) and both situations can be fatal.

Important ions like sodium and chloride, are lost in sweat, which tastes salty as a consequence, but this situation is not regulated by the body.

Hence the importance of the kidneys in maintaining water AND ion balance in the body.

The correct amount of ions are reabsorbed and excess ions are removed in urine.

So, the kidneys are also an important high pressure 'filtration' system for removing unwanted materials and waste products out of the blood passing through the kidneys, which would otherwise cause harmful effects on the body if not removed.

The kidneys filter out these substances as the blood through - hence the process name of filtration.

Glucose, ions and other useful substances and appropriate amount of water are re-absorbed back into the blood stream - this process is called selective reabsorption.

This process is essential to adjust the levels of ions to their appropriate concentration.

 

(c) Urea removal

What else does the kidney remove to maintain the right balance of various substances?

After the filtration, the principal waste substance removed by the body in the urine is urea from the blood.

Urea is made from excess nitrogen, often from amino acids from metabolised food and urea is less toxic than ammonia.

Neither proteins or amino acids can be stored in the body, so excess amino acids are converted into fats and carbohydrates, but these molecules do not contain nitrogen, so the nitrogen part of amino acids must removed in some way.

This process, called deamination, happens in the liver and produces the waste product ammonia.

However, ammonia is toxic and is converted to urea and transported to the kidneys where it is filtered out and excreted in urine.

See (3) Details on how the kidney works in conjunction with the urinary and blood systems

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Key points Source of information is based on textbooks & syllabus-specifications for students taking the AQA GCSE, Edexcel GCSE and OCR GCSE level biology examinations (~US grades 9-10).

Key points about kidney function

Exam-focused set of revision notes for biology students on the three key functions of the kidney—including water control, ion and small molecule concentration, and removal of urea—and why they matter in human biology.

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Overview: What Do the Kidneys Do?

The kidneys are vital organs in the urinary (excretory) system.

Each kidney contains around one million nephrons, which are microscopic filtering units.

Their main job is to filter the blood, removing waste and maintaining the correct balance of water and chemicals.

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1. Water Control (Osmoregulation)

Why It Matters:

• Cells need a stable water balance to function properly.

• Too much water → cells swell and burst.

• Too little water → cells shrink and become dehydrated.

How the Kidneys Help:

• The kidneys adjust how much water is reabsorbed into the blood.

• Controlled by the hormone ADH (Anti-Diuretic Hormone):

  • More ADH → more water reabsorbed → concentrated urine.

  • Less ADH → less water reabsorbed → dilute urine.

This process is part of homeostasis, keeping internal conditions stable.

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2. Ion and Small Molecule Concentration Control

Why It Matters:

• Ions like sodium, potassium, and calcium must be kept in balance.

• Imbalances can affect nerve impulses, muscle function, and enzyme activity.

How the Kidneys Help:

• Filter out excess ions from the blood.

• Selectively reabsorb the right amount of ions and small molecules (like glucose).

• Prevents harmful changes in blood plasma concentration.

Glucose is always reabsorbed in a healthy kidney - its presence in urine may indicate diabetes.

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3. Removal of Urea (Excretion)

What Is Urea?

• A toxic waste product formed in the liver during deamination (breakdown of excess amino acids).

• Transported in the blood to the kidneys.

How the Kidneys Help:

• Urea is filtered out of the blood in the nephrons.

• It is not reabsorbed and is excreted in urine.

Urea must be removed to prevent toxicity and damage to cells.

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Why This Is Important in Human Biology

• Shows how the body maintains internal balance (homeostasis).

• Demonstrates the interaction between the excretory, endocrine, and circulatory systems.

• Helps explain the effects of kidney failure, dehydration, and electrolyte imbalance.

• Links to real-world health issues like diabetes, high blood pressure, and uremia.

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Exam Tips

• Use key terms: nephron, filtration, selective reabsorption, ADH, urea, homeostasis.

• Be able to label diagrams of the nephron and urinary system.

• Understand how the kidneys respond to different hydration levels.

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Keywords, phrases and learning objectives for this part on varied functions of the kidney

Be able to describe three functions of the kidneys including water control, ion and small molecule concentrations and the removal of urea.

Be able to explain why ammonia must be converted to urea (deamination) for excretion.

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