Controlling water balance (AQA GCSE Biology): Revision Notes
Controlling water balance
What is water balance?
The urinary system keeps the right amount of water in your body. It also removes waste products that could harm you.
Your body cells need the correct water balance to work properly. If cells lose or gain too much water through osmosis, they stop working efficiently.
Water balance is crucial for cellular function - even small changes in water content can significantly affect how well your cells operate.
How does your body lose water?
Your body loses water in several ways:
- Through your lungs - when you breathe out (exhale)
- Through your skin - when you sweat
- Through urine - water, ions and urea leave your body
The body cannot control water loss from the lungs or skin. But it can control how much water leaves in urine. This is why the kidneys are so important for maintaining water balance!
How do kidneys control water balance?
The kidneys are the main organs that control water balance. They do this by making urine in two steps:
How Kidneys Control Water: A Two-Step Process
Step 1: Filtration
- Blood gets filtered to remove waste and excess water
- This happens in tiny units inside each kidney
Step 2: Selective reabsorption
- Useful substances like glucose get taken back into the blood
- The right amount of water gets reabsorbed
- Waste products like urea stay in the urine
Structure of the urinary system
The urinary system has several important parts that work together to philtre blood and eliminate waste:
- Kidneys - philtre blood and make urine
- Renal arteries - carry blood from the body to the kidneys
- Renal veins - carry cleaned blood back to the body
- Ureters - tubes that carry urine from kidneys to bladder
- Bladder - stores urine until you're ready to go to the toilet
- Urethra - tube that carries urine outside the body
- Muscle - keeps the exit from the bladder closed until you decide to urinate
Each part of the urinary system has a specific role - from filtering and cleaning the blood to storing and eliminating waste products.
Osmotic changes and cells
When the concentration of body fluids changes, it affects your cells - especially red blood cells:
- High concentration - red blood cells lose water and become crinkled
- Low concentration - red blood cells gain water and eventually burst
This is why maintaining water balance is so important! When cells gain or lose too much water due to osmotic changes, they can be damaged or destroyed, which can seriously affect your health.
What's in plasma compared to urine?
Your kidneys change the composition of your blood by selective reabsorption:
- Urea concentration - 60 times higher in urine than in plasma
- Salt concentration - about 2 times higher in urine
- Glucose - found in plasma but none in healthy urine (kidneys reabsorb all glucose)
The concentrations in urine vary depending on how much water your body needs to keep or get rid of. When you're dehydrated, urine becomes more concentrated to conserve water.
Key Points to Remember:
- The urinary system maintains water balance and removes waste products
- Water is lost through lungs, skin and urine - but only urine loss can be controlled
- Kidneys philtre blood and selectively reabsorb useful substances like glucose and the right amount of water
- Urea is concentrated in urine (60 times more than in plasma) while glucose is completely reabsorbed
- Osmotic changes can damage cells, so water balance is essential for proper cell function