Which of the following statements is true regarding the urinary tubule in the excretory system?

Analyze the question: We need the true statement about the urinary tubule (nephron tubule) in the kidney.

Concept: The nephron has two main parts:

• Renal (Malpighian) corpuscle: the glomerulus and Bowman's capsule. This is where ultrafiltration of blood occurs.
• Renal tubule (urinary tubule): the proximal convoluted tubule, loop of Henle, distal convoluted tubule and collecting duct. Here selective reabsorption and tubular secretion take place.

Through reabsorption and secretion, the tubule adjusts how much water, salts (Na⁺, K⁺, Cl⁻), glucose and H⁺ stay in the body or leave it. This is osmoregulation and electrolyte balance, helped by hormones such as ADH and aldosterone.

Evaluate each option:

• A. Production of urine: Partly true in a loose sense, since urine is the final product after the filtrate is modified in the tubule. But the whole nephron produces urine, starting with filtration, so this is not the best answer.
• B. Regulates water and electrolyte balance: True. This is the main function of the tubule: reabsorbing water and useful salts and secreting excess ions.
• C. Connects kidneys to bladder: False. That is the ureter.
• D. Site of filtration of blood: False. Filtration occurs in the glomerulus/Bowman's capsule, not in the tubule.

Answer: B.

Tip: Remember the sequence: Filtration is in the corpuscle, Reabsorption and secretion are in the tubule, and Transport to the bladder is by the ureter. If an option mentions filtration, think glomerulus. If it mentions water, salt or pH balance, think tubule.