Excretion - The Renal System
Section: Biology | Syllabus: Cambridge Lower Secondary Checkpoint Science (0893)
What is Excretion?
- All living organisms produce waste products during metabolism
- Excretion is the removal of these toxic waste products from the body
Excretion vs. Egestion
- Excretion and egestion are different processes
| Feature | Excretion | Egestion |
|---|---|---|
| What? | Removal of metabolic waste | Removal of undigested food (faeces) |
| Made where? | Inside body cells | Never entered body cells (stayed in the gut) |
| Examples | CO₂, urea, water | Fibre, undigested matter |
Main Excretory Products in Humans
- Humans excrete several different waste products through different organs
| Waste Product | Produced From | Excreted By |
|---|---|---|
| Carbon dioxide (CO₂) | Respiration | Lungs (in exhaled air) |
| Urea | Breakdown of excess amino acids (proteins) | Kidneys (in urine) |
| Water | Respiration, metabolism | Kidneys (urine), lungs, skin (sweat) |
| Mineral salts | Excess from diet | Kidneys (urine), skin (sweat) |
The Human Excretory System
- The excretory system, also called the renal system, is responsible for removing urea and excess water and salts from the blood
- the kidneys are the main organs of this system
Main Organs and Structures
The main organs of the human excretory (renal) system
| Structure | Function |
|---|---|
| Kidneys (2) | Filter blood to remove urea and excess water/salts; make urine |
| Renal artery | Brings oxygenated blood containing urea TO the kidney |
| Renal vein | Takes filtered blood (low urea) AWAY from the kidney |
| Ureter (2) | Carries urine from kidney to bladder |
| Bladder | Stores urine temporarily |
| Urethra | Carries urine from bladder out of the body |
- There is an anatomical difference between the male and female excretory system: males have a longer urethra than females
- this is a key difference examiners expect you to be able to identify when comparing diagrams of male and female excretory systems
How Kidneys Work
- The kidneys filter blood to remove urea and control water and salt levels
- this process involves filtration, reabsorption, and excretion
The Kidney Filtration Process
- Step 1: blood enters the kidney through the renal artery, at high pressure
- Step 2: filtration occurs in millions of tiny nephrons, the filtering units
- Step 3: useful substances — glucose, most water, minerals — are reabsorbed back into the blood
- Step 4: the remaining waste — urea, excess water, excess salts — becomes urine
- Step 5: urine flows through the ureter to the bladder
- Step 6: cleaned blood leaves the kidney through the renal vein
- Kidneys filter blood to remove urea, which is excreted in urine
How blood is filtered as it passes through the kidney
Urea Formation
- Urea is a toxic waste product that must be removed from the body
- it is produced when the liver breaks down excess amino acids
How Urea is Formed
- Step 1: we eat proteins, which are digested into amino acids
- Step 2: amino acids are absorbed into the blood
- Step 3: the body uses some amino acids for growth and repair
- excess amino acids cannot be stored
- Step 4: the liver breaks down excess amino acids, producing toxic urea
- Step 5: urea dissolves in blood plasma
- Step 6: the kidneys filter out urea and excrete it in urine
- Urea is toxic — if it builds up in the blood, it causes serious harm
- this is why the kidneys must remove it continuously
Composition of Blood, Filtrate, and Urine
- Different substances are either filtered, reabsorbed, or excreted during kidney function
| Substance | In Blood (entering kidney) | In Filtrate (after filtering) | In Urine (leaving kidney) |
|---|---|---|---|
| Water | ✓ High | ✓ High | ✓ Medium (excess removed) |
| Urea | ✓ Present | ✓ Present | ✓ HIGH (concentrated) |
| Glucose | ✓ Present | ✓ Present | ✗ NONE (all reabsorbed) |
| Proteins | ✓ Present | ✗ NONE (too large to filter) | ✗ NONE |
| Red blood cells | ✓ Present | ✗ NONE (too large to filter) | ✗ NONE |
| Mineral salts | ✓ Present | ✓ Present | ✓ Present (excess excreted) |
- Glucose: present in blood → present in filtrate → not in urine, since it's completely reabsorbed
- Urea: present in blood → present in filtrate → high in urine, since it's concentrated and excreted
Functions of the Kidney
- The kidneys perform three main functions essential for maintaining homeostasis in the body
Excretion
- Removes urea from the blood
- Excretes it in urine
Osmoregulation (Water Balance)
- Controls the amount of water in the blood
- Drink lots: kidneys make dilute urine, to remove the excess water
- Drink little, or a hot day: kidneys make concentrated urine, to conserve water
Salt Balance
- Controls mineral salt levels in the blood
- excess salts are excreted in urine
How the kidneys adjust urine concentration depending on water intake
Kidney Failure and Treatment
- When kidneys fail, toxic wastes build up in the blood
- without treatment, kidney failure can be fatal
What Happens if Kidneys Fail?
- Urea builds up in the blood — this is poisonous
- Water and salt balance is lost
- Can be fatal without treatment
Treatment Options
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