Urinary concentrating ability of kidney increases if there is: September 2008
**Core Concept**
The urinary concentrating ability of the kidney is primarily regulated by the **counter-current mechanism** in the nephrons, particularly in the loops of Henle and the collecting ducts. This process involves the regulation of water and electrolyte balance, with **antidiuretic hormone (ADH)** playing a crucial role. The kidney's ability to concentrate urine is essential for maintaining proper **osmoregulation**.
**Why the Correct Answer is Right**
An increase in **antidiuretic hormone (ADH)** levels enhances the kidney's urinary concentrating ability. ADH, also known as vasopressin, acts on the **collecting ducts** to increase water permeability, allowing more water to be reabsorbed into the bloodstream. This process concentrates the urine, enabling the body to conserve water when needed.
**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because a decrease, not an increase, in ADH would lead to a reduction in urinary concentrating ability.
**Option B:** Incorrect as it is not directly related to the primary mechanism of urinary concentration.
**Option C:** Incorrect because it does not directly influence the counter-current mechanism or ADH's effect on the collecting ducts.
**Clinical Pearl / High-Yield Fact**
Remember, **ADH** plays a critical role in regulating water balance in the body, and its dysfunction can lead to conditions such as **diabetes insipidus**, characterized by an inability to concentrate urine.
**Correct Answer:** D. Increase in ADH.