(3.6) More on controlling ion concentrations and removing waste products

and the problems resulting from malfunctioning kidneys and kidney failure (plus mention of associated liver function)

The ions essential for life are absorbed into the blood stream in the gut from digested food.

If the water or ion content of the body is wrong, too much water may move into or out of the cells and damage them.

The ion content of the body – ions are lost via the skin when we sweat (you can taste it on your skin) and excess ions are lost via the kidneys in the urine.

For example, the kidney controls the sodium ion (Na⁺) concentration from digested food containing salt.

Any excess of any ion needs to be removed e.g. by the kidneys and subsequent excretion of urine.

Water and ions, such as sodium (Na⁺) enter the body when we eat and drink and are then absorbed into the bloodstream.

Any imbalance in the ion concentrations of the cells will result in the wrong quantity of water being absorbed by osmosis and this reduces the correct functioning of the cell.

 

It is the kidney maintains the ion concentration balance in your blood.

If the ion or water content of the body is not right it can upset the balance between ions and water. It can result in too little or too much water being absorbed into the cells by osmosis. The wrong amount of water in cells damages them inhibiting their normal function.

Ions are lost in sweat, but this is not a regulated system, which is why the correct balance of ions is maintained by the kidneys.

The appropriate amount of ions is reabsorbed into the blood after filtration in the kidney and any excess ions removed from the body in the urine.

As already described, humans need to remove waste products from their bodies to keep their internal environment relatively constant.

 

Know and understand that waste products that have to be removed from the body include urea, produced in the liver by the breakdown of amino acids and removed by the kidneys in the urine, which is temporarily stored in the bladder.

We take in protein in our food e.g. meat and cheese, which is broken down into amino acids (H₂NCHRCOOH, R is variable). The amino acids are then converted (via DNA/RNA coding) into the proteins we actually need.

Unfortunately, we cannot store amino acids in our body, so the excess must be dealt with in some way. The amino acids can be converted into fats and carbohydrates (C, H and O compounds) which can be stored in the body, but neither of these groups of molecules contain nitrogen - whose excess must be removed in some way by the liver and kidneys.

This conversion occurs in the liver (can also be done by the kidney) and involves a process called deamination. Toxic ammonia (NH₃) is produced as a waste product of the process and this must be converted by the liver into non-toxic urea, formula O=C(NH₂)₂. The urea is soluble and transported to the kidneys, where it can be filtered out and excreted from your body in urine.

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Summary of the three primary kidney functions

in terms of cleaning the blood of waste products and maintaining the correct concentrations of various substances.

(i) The kidney removes urea, formed from the waste of excess amino acids.

The body can't store proteins, so excess amino acids are converted into storable fats and carbohydrates and the waste product of the process is urea.

Urea is a poisonous substance so on its release from the liver into the bloodstream it is filtered out by the kidneys before storage in the bladder prior to excretion in urine.

 

(ii) The kidney adjusts the water content of the blood

e.g. to avoid you becoming dehydrated.

(iii) The kidney also adjusts the concentration of ions like sodium (Na⁺), potassium (K⁺) and chloride (Cl^–) in the blood.

A healthy kidney produces urine by:

first filtering the blood, excess ions such as sodium are removed, though some ions are lost in sweat,

reabsorbing all the sugar,

reabsorbing the dissolved ions needed by the body,

reabsorbing as much water as the body needs,

releasing urea, excess ions and water as urine.

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Malfunction of kidney - problems

Obviously, if any of the above functions of the kidney are not working correctly there will be problems! So, some notes on malfunctioning kidneys and kidney failure

Urea, the waste product from excess amino acids, is released by the liver and filtered out by the kidneys and excreted via the bladder in urine.

People whose kidneys do not function properly may die because toxic substances accumulate in their blood.

If the kidneys are not working properly-efficiently then potentially toxic substances including ammonia can build up in your blood stream and your body is unable to deal with the problem.

If you lose your ability to control the levels of ions and water in your body it can have fatal consequences.

If kidney function deteriorates to the point of complete kidney failure your life is in danger.

Their lives can be saved by using dialysis machines or having a healthy kidney transplanted.

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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 in controlling ion concentrations and removing the waste product urea

Homeostasis: The Big Picture

Homeostasis is the maintenance of a stable internal environment in the body.

It ensures that conditions such as temperature, water levels, blood glucose, and ion concentrations remain within narrow limits for cells and enzymes to function properly.

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Control of Ion Concentrations

Why It Matters:

• Ions like sodium (Na⁺), potassium (K⁺), and calcium (Ca²⁺) are essential for:

  • Nerve impulses

  • Muscle contraction

  • Enzyme activity

• Too many or too few ions can disrupt cell function and osmotic balance.

How the Body Controls Ion Levels:

• Ions are absorbed from food and lost through sweat and urine.

• The kidneys regulate ion levels by:

  • Filtering excess ions from the blood.

  • Reabsorbing the right amount of ions back into the blood.

  • Excreting the rest in urine.

This selective reabsorption ensures the correct ion balance in the blood and tissues.

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Removal of Urea: Excretion

What Is Urea?

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

• It is transported in the blood to the kidneys for removal.

How the Kidneys Remove Urea:

1. Ultrafiltration:

   • Blood enters the glomerulus under high pressure.

   • Small molecules like urea, water, ions, and glucose are filtered into the Bowman’s capsule.

2. Selective Reabsorption:

   • Useful substances (e.g. glucose, some water and ions) are reabsorbed in the tubules.

   • Urea is not reabsorbed.

3. Excretion:

   • Urea, excess water, and ions form urine.

   • Urine travels through the ureters to the bladder, then out via the urethra.

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Why This Is Important in Human Biology

• Demonstrates how the excretory system maintains chemical balance.

• Shows how the kidneys, liver, and circulatory system work together.

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

• Reinforces the concept of homeostasis and negative feedback.

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Exam Tips

• Use key terms: nephron, ultrafiltration, selective reabsorption, urea, ions, homeostasis, deamination.

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

• Understand how the kidneys respond to different hydration and ion levels.

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Keywords, phrases and learning objectives for this part on kidney function

In the context of homeostasis, be able to give detailed descriptions of how the kidney controls ion concentrations in the blood and kidney's function to remove of the waste products e.g. urea from deamination in liver to remove toxic ammonia.

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