Osmoregulation in the Kidneys (BTEC Unit 5 Biology Revision)

Maintaining water levels in the body is important

Ever wondered how your body stops you from becoming too dehydrated or too diluted?

That’s the job of your kidneys — specifically a process called osmoregulation.

In exams, you need to understand:

• How the kidney controls water balance

• The role of the nephron

• How hormones like ADH regulate this process

What is Osmoregulation?

Osmoregulation is the control of water and solute concentration in the body.

Why is it important?

• Keeps blood water potential stable

• Prevents cells from:

  • Shrinking (too little water)

  • Bursting (too much water)

Your body must maintain a balanced internal environment (homeostasis). Each kidney contains millions of tiny filtering units called nephrons.

Key parts of the nephron:

• Glomerulus → filters blood

• Bowman’s capsule → collects filtrate

• Loop of Henle → creates concentration gradient

• Collecting duct → adjusts water reabsorption

Structure of a nephron

Step 1: Ultrafiltration

• Blood enters the glomerulus

• High pressure forces:

  • Water

  • Glucose

  • Ions

  • Urea

into the Bowman’s capsule. Large molecules (proteins, cells) stay in the blood

Ultrafiltration removes substances at pressure

Step 2: Selective Reabsorption

Useful substances are reabsorbed:

• Glucose → fully reabsorbed

• Ions → selectively reabsorbed

• Water → variable (this is key for osmoregulation)

Step 3: Counter-current Multiplier

Descending limb:

• Permeable to water

• Water leaves by osmosis

• Filtrate becomes more concentrated

Ascending limb:

• Impermeable to water

• Actively pumps out Na⁺ and Cl⁻ ions

• Surrounding tissue becomes very salty

Water leaves the descending loop and NaCl leaves the ascending loop

Step 4: The Role of ADH (Antidiuretic Hormone)

This is released by the pituitary gland depending on blood water levels.

When you are dehydrated:

• Blood is more concentrated

• More ADH released

• Collecting duct becomes more permeable to water

• More water reabsorbed

• Small volume of concentrated urine

When you are overhydrated:

• Blood is diluted

• Less ADH released

• Collecting duct is less permeable

• Less water reabsorbed

• Large volume of dilute urine

Summary table

Condition	ADH Level	Water Reabsorbed	Urine Produced
Dehydrated	High	High	Small, concentrated
Overhydrated	Low	Low	Large, dilute

Example Question:

Explain how the kidney regulates water balance (4 marks)

Model Answer:

• The Loop of Henle creates a concentration gradient in the medulla

• ADH controls the permeability of the collecting duct

• More ADH increases water reabsorption by osmosis

• This produces concentrated urine when dehydrated