12.2 Removing Waste Products

A free GCSE Biology lesson for AQA (8461) from OBA School: excretion of carbon dioxide, urea, water and ions, and the structure of the urinary system. 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

• Name the main waste products of the body and say how each one is removed
• Describe how urea is made in the liver from excess amino acids
• Explain why the water and ion content of the body must be controlled
• Label the kidneys, ureters, bladder, urethra and their blood vessels

Key words

• Excretion: Removing waste products made by the body’s own chemical reactions (metabolism).
• Deamination: Removing the nitrogen-containing (amino) part of an excess amino acid in the liver. It makes ammonia.
• Urea: A less toxic waste made in the liver from ammonia. It is carried in the blood and removed by the kidneys.
• Urine: A solution of urea, excess ions and excess water made by the kidneys.
• Ureter: The tube that carries urine from a kidney to the bladder.
• Urethra: The tube that carries urine from the bladder out of the body.
• Renal artery / vein: Renal artery: brings blood to the kidney. Renal vein: takes filtered blood away.

Waste products and how they leave the body

Your cells carry out thousands of reactions. Some products are toxic or not needed, so they must be removed. This is excretion.

• Carbon dioxide; Where it comes from Aerobic respiration in every cell; How it is removed Lungs, when you breathe out
• Urea; Where it comes from The liver, from excess amino acids; How it is removed Kidneys in urine (a little in sweat)
• Water; Where it comes from Food, drink and respiration; How it is removed Lungs (breath), skin (sweat), kidneys (urine)
• Mineral ions (e.g. sodium); Where it comes from Food and drink; How it is removed Skin (sweat), kidneys (urine)

Key fact: Water lost in breath and sweat is not controlled to balance your water level. Only the kidneys adjust how much water and how many ions are lost.

Faeces are mostly undigested food that never entered your cells, so getting rid of them is egestion, not excretion.

Making urea in the liver

Proteins in food are digested into amino acids. Your body uses them to build new proteins, but it cannot store any extra. In the liver, excess amino acids are deaminated: the part containing nitrogen is removed and becomes ammonia.

Ammonia is very toxic, so the liver converts it straight away into urea, which is much less toxic. Urea passes into the blood and is filtered out by the kidneys.

Why water and ions must be balanced

Body cells are surrounded by tissue fluid. If this fluid has a different concentration from the cytoplasm, water moves by osmosis across the partially permeable cell membrane.

• too concentrated (too little water or too many ions); Water moves… out of cells by osmosis; Effect on animal cells Cells shrink and cannot work properly.
• normal; Water moves… in and out equally; Effect on animal cells Cells keep their shape.
• too dilute (too much water); Water moves… into cells by osmosis; Effect on animal cells Cells swell and may burst (animal cells have no cell wall).

Worked example: a daily water balance

On a cool day Priya takes in 2600 cm³ of water. She loses 600 cm³ in sweat, 350 cm³ in breath and 150 cm³ in faeces. Her water content stays constant. How much does she lose in urine, and what percentage of her total loss is this?
Urine = 2600 − (600 + 350 + 150) = 2600 − 1100 = 1500 cm³
Percentage = 1500 ÷ 2600 × 100 = 57.7%

Answer: 1500 cm³, which is 57.7% (1 d.p.)

The urinary system

You have two kidneys at the back of the abdomen. Blood containing urea arrives in the renal artery (a branch of the aorta). The kidneys filter it and the cleaned blood leaves in the renal vein, which joins the vena cava.

Urine trickles down each ureter to the bladder, where it is stored. A ring of muscle (a sphincter) keeps the bladder closed until urine leaves through the urethra.

A kidney cut lengthways shows an outer cortex, an inner medulla and a hollow pelvis, where urine collects before entering the ureter.

Exam tip: Ureter comes before urethra – alphabetically and in the body: kidney → ureter → bladder → urethra.

Common mistakes

• Saying urea is made in the kidneys. It is made in the liver; the kidneys only remove it.
• Mixing up the ureter and the urethra.
• Saying urea comes from carbohydrates or fat. It comes from excess amino acids (from protein).
• Calling the removal of faeces excretion. It is egestion: faeces are mainly undigested food.
• Saying carbon dioxide is removed by the kidneys. It is breathed out by the lungs.

Revision questions

Try these, then check your answers below each one.

Question 1. Urea is made from which substance? (Choose from: Excess amino acids, Excess glucose, Excess fatty acids, Old red blood cells.)

Answer: Excess amino acids

Working: Excess amino acids (from protein) are deaminated in the liver; the ammonia formed is converted into urea.

Question 2. In the liver, excess amino acids are deaminated. Name the toxic substance this forms, which is then converted into urea.

Answer: ammonia

Working: Deamination forms ammonia, which is immediately converted into less toxic urea.

Question 3. Which tube carries urine from the bladder out of the body? (Choose from: Renal vein, Renal artery, Urethra, Ureter.)

Answer: Urethra

Working: The ureters carry urine from the kidneys to the bladder; the urethra carries it out of the body.

Related lessons

• Previous lesson: 12.1 Controlling Body Temperature
• Next lesson: 12.3 The Human Kidney
• In this chapter: 12.4 Dialysis – an Artificial Kidney
• In this chapter: 12.5 Kidney Transplants

Continue in the full course

This lesson is section 12.2 (AGCBH1202) 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.