CDS (I) 2020 General Knowledge · Question 27 of 120

Correct — A, Kidney, ureter, urinary bladder, urethra. The human excretory system is a pair of kidneys, a pair of ureters, a urinary bladder and a urethra, and urine moves through them in that order. NCERT puts it in one sentence: urine produced in the kidneys passes through the ureters into the urinary bladder, where it is stored until it is released through the urethra. Filtration happens in the nephrons of the kidney, so the kidney has to come first; the bladder is the store, so it must sit between the tubes that fill it and the tube that empties it; and the urethra is the single exit, so it must come last.

• (b)Kidney, urinary bladder, ureter, urethra — Puts the bladder before the ureter. The ureter is the tube that carries urine down to the bladder, so it cannot come after it.
• (c)Kidney, ureter, urethra, urinary bladder — Swaps the last two. The urethra is the exit passage, so nothing lies beyond it — urine cannot flow from the urethra back into the bladder.
• (d)Urinary bladder, kidney, urethra, ureter — Starts at the bladder, which stores urine rather than making it, and scrambles the rest. Urine originates in the kidney.

Kidneys filter nitrogenous waste — chiefly urea and uric acid — out of the blood. The working unit is the nephron, in which a cluster of thin-walled capillaries sits in the cup-shaped end of a coiled tube; useful substances such as glucose, amino acids, salts and much of the water are reabsorbed as the filtrate runs along that tube. The urine formed in each kidney passes down a ureter to the urinary bladder, which is muscular and under nervous control, and leaves through the urethra.

Two anchors decide the whole order. The kidney is where urine is made, so it opens the sequence; the urethra is the single outlet, so it closes it. That alone rules out options (c) and (d). Between the remaining two, the question is whether the tube from the kidney or the bladder comes next, and the names help — ureter and urethra are easy to mix up, but the ureter is the upper tube from kidney to bladder and there are two of them, while the urethra is the single lower tube out of the body. Fixing that pair is the whole item.

• The human excretory system consists of a pair of kidneys, a pair of ureters, a urinary bladder and a urethra.
• Urine formed in the kidneys passes through the ureters into the bladder, where it is stored until release through the urethra.
• The nephron is the basic filtration unit of the kidney.
• Glucose, amino acids, salts and much of the water are selectively reabsorbed as the filtrate flows along the nephron tubule.
• The bladder is muscular and under nervous control, which is why the urge to urinate can usually be held.

• Interchanging ureter and urethra, which is the single commonest slip on this topic.
• Placing the bladder before the ureters because it is the organ students picture first.
• Forgetting that ureters come in a pair while the urethra is single.

As a correct-sequence item on the urinary tract, or as a label-the-structure question on the excretory system.

• practice — not a real PYQ

  The basic filtration unit of the human kidney is called the

  • (a)neuron
  • (b)nephron
  • (c)alveolus
  • (d)villus

  Answer(b) nephron — a capillary cluster set in the cup-shaped end of a coiled tubule.

• practice — not a real PYQ

  Which one of the following is reabsorbed almost completely from the filtrate as it passes along the nephron tubule of a healthy person?

  • (a)Urea
  • (b)Uric acid
  • (c)Glucose
  • (d)Creatinine

  Answer(c) Glucose — it is a useful substance and is selectively reabsorbed, which is why it does not normally appear in urine.