IP Biology Notes: Excretion in Humans (Upper Sec 07)

Study guide26 Nov 2025, 00:00 ZUpdated 17 Jul 2026

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Ezekiel Tan·Academic Advisor (Biology)

Free IP Biology notes on kidney structure, urine formation, and dialysis basics for Sec 3 to Sec 4.

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For Integrated Programme students: Your current school materials, teacher instructions, and assessment scope take precedence because IP topic sequence and depth vary by school. This is an Eclat IP guide, not the O-Level / SEC G3 exam-track guide.

How this chapter applies

Eclat core: excretion, the urinary system, nephron function, ultrafiltration, selective reabsorption, urine formation, kidney failure, and dialysis form the main route.
School-sensitive extension: detailed Loop of Henle mechanisms, counter-current language, hormonal water-balance detail, and calculations should be used only where the current school teaches them.
2027 national comparison: K325 Topic B7 requires the kidneys, ureters, bladder, and urethra, plus nephron function and dialysis at outline level.
Check your school: nephron labels, the depth of pressure explanation, and the treatment of ADH or water balance can differ. Keep the urinary-system route distinct from nephron detail.
Exam-track route: use the separate O-Level and SEC G3 Biology notes for K325 topic ownership.

The core idea is simple: Excretion removes metabolic waste from the body.

Use it as a working check: Do not confuse excretion with egestion. Kidneys remove urea, excess salts, and excess water by ultrafiltration and selective reabsorption.

Then go one layer deeper: Example: glucose is small enough to enter the filtrate, but a healthy nephron reabsorbs it in the proximal convoluted tubule so it does not appear in urine.

What you must know

Urinary route: kidneys form urine, ureters carry it to the bladder, the bladder stores it, and the urethra carries it out of the body.
Excretion = removal of metabolic waste ( $\ce{CO2}$

Sources

Add Eclat Institute as a Preferred Source on Google.

Substance	Enters filtrate at Bowman's capsule?	Main later fate	Exam trap
Blood cells and plasma proteins	No	Stay in blood because they are too large to pass the filtration barrier.	Saying protein is normally found in filtrate or urine.
Glucose and amino acids	Yes	Fully reabsorbed in the proximal convoluted tubule in a healthy person.	Saying glucose is "filtered out" and then wasted.
Urea	Yes	Mostly remains in filtrate and is excreted in urine.	Treating urea like a useful nutrient to reabsorb.
Water and mineral ions	Yes	Reabsorbed in controlled amounts according to body needs.	Forgetting that water balance changes with ADH.

Stage	What happens	Key mechanism	Common trap
Glomerulus to Bowman's capsule	Small molecules enter the filtrate; blood cells and plasma proteins remain in blood.	High blood pressure and filtration barrier size.	Saying useful substances are "chosen" here. This stage mainly separates by size.
Proximal convoluted tubule	Glucose and amino acids are fully reabsorbed; much water and some ions return to blood.	Active transport, diffusion, and osmosis.	Putting complete glucose reabsorption in the collecting duct.
Loop of Henle and distal tubule	Water and ion balance is adjusted as filtrate moves onward.	Osmosis and ion movement across tubule walls.	Treating every part of the tubule as doing the same job.
Collecting duct	Water reabsorption changes with ADH, so urine becomes more concentrated or more dilute.	ADH changes collecting duct permeability to water.	Saying ADH removes urea directly.
Ureter onward	Final urine leaves the kidney for storage and later removal.	Flow from ureter to bladder to urethra.	Calling urine formation egestion instead of excretion.

Substance	Dialysis fluid setup	Result during dialysis	Common trap
Urea	No urea in dialysis fluid	Urea diffuses from blood into dialysis fluid.	Saying urea is actively pumped out.
Glucose	Same glucose concentration as normal blood	No net glucose loss from the blood.	Using pure water, which would remove useful glucose.
Mineral ions	Same useful ion concentration as normal blood	Excess ions can diffuse out, but needed ions are retained.	Saying all salts must be removed.
Proteins and blood cells	Membrane pores are too small for them	They remain in the blood.	Saying proteins diffuse back from dialysis fluid.

IP Biology Notes: Excretion in Humans (Upper Sec 07)

How this chapter applies

What you must know

Sources

Detailed notes

Nephron process-order checkpoint

Worked walkthroughs

Dialysis gradient checkpoint

Pitfalls and fixes

Practice drills

Quick applications

Exam cues

Next steps