Filtration of waste products in humans happens in the kidneys. There is a large number of filtration units present inside the kidneys to help them in doing this job. These filtration units are called:

Correct — C. The filtration unit of the kidney is the nephron, and each human kidney holds roughly a million of them packed close together.

One nephron is a whole apparatus: a knot of capillaries called the glomerulus, the cup that receives its filtrate, and a long tubule that takes back water, glucose and salts before what remains leaves as urine.

The stem asks for the unit that does the filtering, not for a part of it, so the answer is option (c).

Why the others are wrong

• (a)
  Bowman’s capsules — Bowman's capsule is only the cup at the head of a nephron, catching filtrate pushed out of the glomerulus. It is a component of the filtration unit, not the unit itself.
• (b)
  alveoli — Alveoli are the air sacs of the lungs, where oxygen and carbon dioxide cross into and out of the blood. They are respiratory structures with no role in the kidney.
• (d)
  capillaries — Capillaries are the body's smallest blood vessels and run everywhere. The glomerulus is a capillary tuft inside a nephron, but capillaries in general are not a filtration unit.

Filtration in the kidney happens in two stages inside one unit. Blood under pressure in the glomerulus is forced across a membrane into Bowman's capsule, carrying water, salts, glucose, amino acids and urea with it.

The tubule then takes back what the body still needs — most of the water, all the usable glucose, much of the salt — by selective reabsorption.

What is left is urine, which drains through collecting ducts to the ureter and on to the urinary bladder. Because the filtering and the recovery happen along one structure, the nephron is the level at which excretion is taught.

Key facts

• Each human kidney contains roughly a million nephrons.
• A nephron is made up of a glomerulus, a Bowman's capsule and a tubule.
• Bowman's capsule surrounds the glomerulus and receives the filtrate from it.
• Selective reabsorption in the tubule recovers glucose, most of the water and much of the salt.

Common traps

• Answering Bowman's capsule, which is a part of the nephron rather than the whole unit.
• Confusing alveoli, the exchange units of the lung, with the filtering units of the kidney.

No directly related past PYQ was found.