Which part of the nephron is associated with ultra-filtration?

Step 1: Analyze the question The question asks which part of the nephron is responsible for ultra-filtration, the first stage of urine formation.

Step 2: Explain the concept Ultra-filtration happens in the renal corpuscle, which is made up of:

• the glomerulus, a knot of capillaries, and
• the Bowman's capsule, a cup-shaped structure that surrounds the glomerulus.

Blood enters the glomerulus at high pressure. This is because the afferent arteriole (entering) is wider than the efferent arteriole (leaving). The pressure forces water, glucose, amino acids, salts and urea through the capillary walls into the Bowman's capsule. Large molecules such as blood cells and plasma proteins are too big to pass through, so they stay in the blood. The fluid that collects is called the glomerular filtrate.

Step 3: Check each option

• A. Distal convoluted tubule: adjusts salts and water (selective reabsorption and secretion). It does not filter.
• B. Proximal convoluted tubule: reabsorbs glucose, amino acids and most of the water and salts. It does not filter.
• C. Collecting duct: reabsorbs water, controlled by ADH, and carries urine to the pelvis. It does not filter.
• D. Bowman's capsule: the site where the filtrate is collected after ultra-filtration. Correct.

Answer: D. Bowman's capsule

Exam tip: Match each part of the nephron to its job:

• Bowman's capsule = filtration
• Proximal convoluted tubule = reabsorption of glucose
• Distal convoluted tubule = fine-tuning of salts and water
• Collecting duct = water balance and urine transport

Think "Bowman's = Blood is filtered."