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NDA Biology · Human Physiology

Excretion and Reproduction

The nephron filters blood at Bowman's capsule to make urine; the kidney also releases renin to control blood pressure; sperm form through meiosis from the primary spermatocyte.

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Why this matters

3 PYQs — small but reliable. Two anchors: Bowman's capsule is the filtration site of the nephron, and the kidney's enzyme renin starts the blood-pressure cascade. One reproduction fact: the first meiotic division of spermatogenesis occurs in the primary spermatocyte.

Concept 1 of 2

The nephron — filtration and renin

Intuition

The kidney's working unit is the nephron. Blood is filtered at a cup-shaped structure — Bowman's capsule — surrounding a knot of capillaries (the glomerulus). The filtrate is then refined into urine as it travels down the tubule. The kidney also has a hormone job: it releases the enzyme renin to raise blood pressure when needed.

Definition

The nephron's key parts and the renin role:

  • Bowman's capsule (+ glomerulus) — the site of ultrafiltration; blood is filtered here.
  • Tubule (PCT, loop of Henle, DCT) + collecting duct — reabsorb water, salts and useful molecules; the rest becomes urine.
  • Renin — an enzyme secreted by the kidney that converts plasma angiotensinogen → angiotensin, raising blood pressure (the RAAS cascade).
Bowman’s capsule(filtration)Tubuleloop of Henle(reabsorption)Urine

Worked example

In which part of the nephron is blood first filtered, and what is the rest of the tubule mainly for?
  1. Blood arrives at the glomerulus, a tuft of capillaries.
  2. It is filtered into the surrounding Bowman's capsule — this is ultrafiltration.
  3. The filtrate then flows through the tubule, where useful water and solutes are reabsorbed.
  4. What remains is urine, collected by the collecting duct.
Answer:Filtration happens at Bowman's capsule; the tubule reabsorbs useful substances.
Practice this conceptself-check · 3 quick reps

Try it yourself

The kidney secretes an enzyme that converts angiotensinogen into angiotensin. Name the enzyme and its overall effect.

Practice — Level 1 (3 reps)

Quick reps to lock in the method. Try each, then check.

  1. 1.
    Which part of the nephron filters blood?
  2. 2.
    Which kidney enzyme acts on angiotensinogen?
  3. 3.
    What does renin ultimately raise?

From the bank · past-year question

Example 1Human PhysiologyEASY
Which part of nephron of the human kidney is responsible for filtration of blood for excretion ?

[Q87 · Sep · 2024]

Bowman's capsule filters; it doesn't carry urine away

Distractors offer the collecting duct, ureter or renal vein. Filtration specifically happens at Bowman's capsule; the ureter merely carries finished urine out of the kidney.
Drill 1 more on the nephron — filtration and renin

Concept 2 of 2

Spermatogenesis — where meiosis happens

Intuition

Sperm are made in stages. Diploid stem cells (spermatogonia) divide by mitosis, then one of them grows into a primary spermatocyte — and it is THIS cell that undergoes the first meiotic division, halving the chromosome number.

Definition

The sequence and the meiosis checkpoint:

  • Spermatogonium (diploid) — divides by mitosis.
  • Primary spermatocyte — undergoes meiosis I (the FIRST meiotic division) → two secondary spermatocytes.
  • Secondary spermatocyte — undergoes meiosis II → spermatids → mature sperm.
  • Sertoli cells — nurse cells that nourish developing sperm (they do not divide into sperm).

Worked example

During spermatogenesis, in which cell does the first meiotic division take place?
  1. Spermatogonia divide by mitosis, not meiosis.
  2. One enlarges into a primary spermatocyte.
  3. The primary spermatocyte undergoes meiosis I — the first meiotic division.
  4. Its products (secondary spermatocytes) then undergo meiosis II.
Answer:The primary spermatocyte (it undergoes meiosis I).
Practice this concept3 quick reps

Practice — Level 1 (3 reps)

Quick reps to lock in the method. Try each, then check.

  1. 1.
    Which cell undergoes the first meiotic division in sperm formation?
  2. 2.
    How do spermatogonia divide?
  3. 3.
    What is the role of Sertoli cells?

From the bank · past-year question

Example 2Human PhysiologyEASY
In which cell does the first meiotic cell division take place during spermatogenesis?

[Q94 · Apr · 2026]

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Mastery check — 1 interleaved questions

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Example 1Human PhysiologyMODERATE
Kidney secretes an enzyme, which changes plasma protein angiotensinogen into angiotensin. The enzyme is

[Q66 · Apr · 2017]

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