How does the body remove waste, and how does the kidney clean the blood?
Explain excretion and describe how the kidney filters the blood and reabsorbs useful substances
A focused answer to the O-Level Biology outcome on excretion. The meaning of excretion, the waste products removed, and how the kidney filters the blood and reabsorbs useful substances to make urine.
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What this dot point is asking
SEAB wants you to define excretion, name the main waste products the body removes and where they come from, and describe how the kidney cleans the blood by filtration and selective reabsorption to make urine. You should be able to explain why useful substances such as glucose do not appear in healthy urine.
The answer
What excretion is
Excretion is the removal of the waste products of metabolism (the chemical reactions in the body) from the body. It is not the same as removing undigested food (faeces), which was never absorbed and is called egestion.
Main excretory products and their organs:
- Carbon dioxide (from respiration) is excreted by the lungs.
- Urea (from the breakdown of excess amino acids) is excreted by the kidneys in urine.
- Excess water and salts are removed by the kidneys (and some in sweat).
Where urea comes from
Excess amino acids cannot be stored. They are broken down in the liver, a process called deamination, which produces urea. The urea is carried in the blood to the kidneys to be removed.
How the kidney makes urine
Each kidney cleans the blood in two main stages:
1. Filtration. Blood enters the kidney at high pressure. Small molecules are forced out of the blood into the kidney tubule: water, glucose, salts and urea. Large molecules such as proteins and the blood cells are too big and stay in the blood. The liquid filtered out is called the filtrate.
2. Selective reabsorption. As the filtrate passes along the tubule, useful substances are reabsorbed back into the blood: all the glucose (by active transport), most of the water (the amount controlled to balance the body), and some salts. What is left, mainly water, urea and excess salts, continues on as urine.
The urine passes to the bladder and is removed from the body.
Why glucose is not in healthy urine
Glucose is filtered out of the blood during filtration, but because it is useful it is all reabsorbed back into the blood. So in a healthy person, none is left to appear in the urine. (In untreated diabetes, blood glucose is so high that not all of it is reabsorbed, so glucose appears in the urine.)
Examples in context
Example 1. Glucose in the urine and diabetes. In untreated diabetes, blood glucose is so high that the kidney cannot reabsorb all of it, so glucose appears in the urine. A urine glucose test is therefore one way to detect diabetes.
Example 2. Drinking lots of water. When you drink a lot, the kidney reabsorbs less water, so you make more dilute urine. When you are dehydrated, it reabsorbs more water, making less, more concentrated urine. This helps keep the body's water content steady.
Try this
Q1. Define excretion. [2 marks]
- Cue. The removal from the body of the waste products of metabolism, such as carbon dioxide and urea.
Q2. Name the waste product removed by the kidney and state where it is made. [2 marks]
- Cue. Urea, made in the liver from the breakdown of excess amino acids.
Q3. Explain why a healthy person has no glucose in their urine. [2 marks]
- Cue. Glucose is filtered out of the blood but is all reabsorbed back into the blood by active transport, so none is left to appear in the urine.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original5 marks(a) Define excretion. (b) Name the waste product removed by the kidneys and where it is made. (c) Describe the two main stages by which the kidney makes urine.Show worked answer →
(a) Excretion is the removal of the waste products of metabolism (chemical reactions) from the body.
(b) The main waste product removed by the kidneys is urea; it is made in the liver from the breakdown of excess amino acids (deamination).
(c) Two stages: first, filtration, in which blood at high pressure is filtered in the kidney, so that water, glucose, salts and urea pass out of the blood into the tubule (large molecules such as proteins and blood cells stay in the blood). Second, reabsorption, in which useful substances such as all the glucose, most of the water and some salts are reabsorbed back into the blood; the remaining liquid (water, urea and excess salts) becomes urine.
Markers reward a correct definition of excretion, urea made in the liver from excess amino acids, and the two stages of filtration and selective reabsorption.
Original4 marksGlucose is found in the liquid filtered in the kidney but is not normally present in the urine of a healthy person. Explain why.Show worked answer →
During filtration, small molecules including glucose pass out of the blood into the kidney tubule, so glucose is present in the filtered liquid.
However, glucose is a useful substance the body needs, so it is all reabsorbed back into the blood from the tubule, by active transport. Because all the glucose is reabsorbed, none is left to pass out in the urine of a healthy person.
Markers reward glucose being filtered out of the blood, then all of it being reabsorbed (by active transport) back into the blood, so none appears in the urine.
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