Formation of Urine:

• Urea, excess water, and ions not needed by the body pass down the renal tubule into the kidney’s pelvis.

• The fluid, now termed urine, flows through the ureter to the bladder.

Storage and Excretion:

• The bladder expands to hold approximately 400 cm3 of urine.

• A sphincter muscle contracts to prevent urine from escaping the bladder.

• When the sphincter relaxes voluntarily (controlled by adults) or reflexively (in babies), the bladder’s muscular walls expel urine through the urethra.

• Most children can voluntarily control the sphincter muscle by the age of 3.

Role of ADH (Anti-Diuretic Hormone)

What is ADH?

Hormone released by the pituitary gland to regulate water balance.

How ADH Works:

Dehydration:
Hypothalamus detects low blood water → signals pituitary to release ADH.
ADH makes collecting duct walls more permeable → more water reabsorbed into blood.
Result: Small volume of concentrated urine , , .
Overhydration:
Less/no ADH released → collecting ducts less permeable.
Result: Large volume of dilute urine , .
Exam Tip: Link ADH to negative feedback (maintaining stable internal conditions) .

Factors Affecting Urine Composition

Hydration Levels:

Dehydrated → concentrated urine (more urea, ions; less water).
Hydrated → dilute urine (less urea; more water) , .
Diet: High protein → more urea; high salt → more ions .
Health: Diabetes (high glucose in urine) or kidney failure (abnormal filtrate)

Question
Explain how the body responds to dehydration using ADH. Use the terms hypothalamus, pituitary gland, and negative feedback.

Answer:

Dehydration → hypothalamus detects high blood solute concentration → pituitary releases ADH → collecting duct becomes permeable → water reabsorbed → urine concentrated.
Negative feedback loop maintains water balance.

Question
A person drinks 2 liters of water in 30 minutes. Describe how their kidneys produce urine and the role of ADH in this process.

Answer:

Overhydration → low ADH → less water reabsorption → large volume of dilute urine.

Question

Explain why a person with diabetes mellitus produces glucose-rich urine. How does this affect urine volume?

Answer:

High blood glucose → exceeds nephron’s reabsorption capacity → glucose in urine (osmotic effect) → more urine produced.