Systems

Excretion in humans

Excretion is the removal of metabolic waste. In humans the kidney is the organ that clears urea and balances water and salts.

CAPS

Grade 11 · Life Sciences · Systems

Life Sciences is two papers. Use the strand to know which one.

  • Where it is examined

    Grade 11 works through energy transformations, gas exchange, excretion, population ecology and human impact. The same two-paper pattern is used.

  • How to write it

    Name the structure, then say what it does. A labelled diagram only scores if the labels are the biological names. “It helps the body” is not a function.

  • How a marker reads it

    A common mix-up is to say the glomerulus chooses useful molecules, or that the kidney produces urea.

A 2-mark “explain” wants two facts, or one fact and its reason. Extra sentences that contradict the first one can cancel a mark.

nephron

Diagram

Worked theory

Worked theory. A learner who has played in the sun and not drunk. Blood is too concentrated. ADH rises. The collecting ducts reabsorb more water. Urine comes out small in volume and dark. After a litre of water, ADH falls, less water is reabsorbed, and urine is pale and plentiful. Glucose in the urine is not normal. It means the tubule could not reabsorb all of it, which is a sign to test for, not a usual step of the nephron. The kidney did not “make” urea. The liver made urea from excess amino acids. The kidney only removed it.

More worked examples

  • Two litres of water, then a match

    After two litres of water, ADH falls. The collecting ducts let water go. In the next hour the urine is pale and there is a lot of it. The same person, after a football match with no drink, has high ADH. The collecting ducts keep water. Urine is a small dark volume. Glucose in that urine would be a third case: the filter let it through, as it always does, and the tubule failed to take it all back. Healthy nephrons return all the glucose.

  • A dialysis morning

    When the nephrons have failed, a machine three times a week does the job of filtration: wastes cross into a fluid the body does not keep. The person’s own kidneys do not regrow between sessions. Skip a week and urea rises. Dialysis is a replacement, not a cure, and it does not put glucose back the way a healthy tubule does. A learner who says the machine is a new kidney has skipped what it cannot do.

In ordinary words

Blood reaches the nephron under pressure. At the glomerulus, water, urea, glucose and salts are filtered. Large proteins and blood cells stay in the blood. Along the tubule, all the glucose and the salts and water the body still needs are reabsorbed. Urea stays in the fluid and becomes urine. ADH, from the pituitary, makes the collecting duct keep more water when the blood is too concentrated. Sweat and exhaled carbon dioxide are also excretion. Faeces are not. Faeces were never inside the body’s cells as metabolic waste.

In South Africa

Dark urine after a hot taxi-rank afternoon, pale urine after drinking, is ADH in ordinary life.

Part 1 / 4

What it is

Excretion is the removal of metabolic waste. In humans the kidney is the organ that clears urea and balances water and salts.

Diagram
Diagram

More worked examples

Board walkthroughs on this topic. Open one, copy the working, then try the next.

Example 1 / 20

Plasma contains 0,1% glucose; normal urine contains 0%. Explain where glucose is removed.

How we got there

Read this like a teacher at the board. We go slowly, name every number, and say why we take each step.

Explain in another language · 11

1. What is the question asking?

Read the question twice. Circle the unknown. Underline every number and unit. The paper is asking: Plasma contains 0,1% glucose; normal urine contains 0%. Explain where glucose is removed.

Before any working — hunt first

Given

Numbers and units already in the sentence: 0,1% · 0%. Copy each one into a column. A name without a number is not given.

Missing

After you copy the given, the letter that still has no number is missing. The paper is pointing at: Explain where glucose is removed. Circle that word — that is the missing variable.

Find

We are required to find: Explain where glucose is removed. Write that as a symbol on the left, with a question mark, before you touch a formula.

Calculate

What we need to calculate: Filtration then reabsorption. Then substitute the given, one line at a time, to get Explain where glucose is removed.

In other words — as if you have never seen this before

If you have never seen this before: Filtration then reabsorption. We will name a method in words, write the formula with no numbers, then substitute one line at a time.

2. What do we already know?

Glucose in plasma vs urine.

3. Which method — and why this one?

Filtration then reabsorption.

4. Step-by-step working

  1. 01 / 08

    Step A — Read the question twice

    Slow down. Circle the word that names the unknown. Underline every number and its unit. Do not grab a formula yet — first decide what the paper is actually asking.

    Why this step: If you can say the method in a sentence, you will pick the right formula. First-timers skip this and grab the first equation they remember.

  2. 02 / 08

    Step A2 — Hunt: Given, Missing, Find, Calculate

    Given: Numbers and units already in the sentence: 0,1% · 0%. Copy each one into a column. A name without a number is not given. Missing: After you copy the given, the letter that still has no number is missing. The paper is pointing at: Explain where glucose is removed. Circle that word — that is the missing variable. Find: We are required to find: Explain where glucose is removed. Write that as a symbol on the left, with a question mark, before you touch a formula. Calculate: What we need to calculate: Filtration then reabsorption. Then substitute the given, one line at a time, to get Explain where glucose is removed.

    Why this step: Do not name the letters yet. First hunt: what already has a number (Given), what has no number (Missing), the word the paper wants (Find), and which formula still contains the missing letter (Calculate).

  3. 03 / 08

    Step B — Write the given in a column

    Glucose in plasma vs urine.

    Why this step: A column with units stops you mixing cm with m, or minutes with seconds. Markers can also see you started correctly.

  4. 04 / 08

    Step C — Name the method in words

    Filtration then reabsorption.

    Why this step: If you can say the method in a sentence, you will pick the right formula. First-timers skip this and grab the first equation they remember.

  5. 05 / 08

    Explain

    Glucose is filtered in the Bowman's capsule, then all of it is actively reabsorbed in the proximal convoluted tubule.

    Why this step: We only do one job per line so a marker can follow the marks — and so you can find a slip without starting over.

  6. 06 / 08

    Step G — Sense-check the size

    Ask: is this the kind of number a Grade 10–12 paper would want? A speed of 3 000 ms for a learner walking, or a probability bigger than 1, means a unit or algebra slip. Estimate first: round the numbers and do the sum in your head.

    Why this step: Markers write ‘unrealistic’ in the margin when the method is right but the size is wild. A 10-second head check often finds a missing 10² or a unit that never changed.

  7. 07 / 08

    Step F — Box the answer with its unit

    It is filtered then fully reabsorbed in the proximal tubule.

    Why this step: A number without a unit is half a mark on a Physics paper, and a marker can miss a Maths answer that is buried in working.

  8. 08 / 08

    Step H — What the marker is looking for

    On an NSC memo you usually earn: (1) a method mark for naming the right formula or idea in words, (2) an accuracy mark for substituting the given numbers with units, (3) an accuracy mark for the boxed answer with its unit. Write those three lines even if you are unsure of the arithmetic — blank pages score zero.

    Why this step: First-timers chase the final number and leave the method blank. The memo still pays out for the idea and the substitution even when the last line is wrong.

5. The answer

It is filtered then fully reabsorbed in the proximal tubule.

Copy onto your answer sheet — in this order

Answer: It is filtered then fully reabsorbed in the proximal tubule.

6. Check it like a marker

Glucose in urine suggests diabetes.

7. A trap first-timers fall into

First-timers often grab the first formula they remember, mix units (cm with m, minutes with seconds), or drop a minus sign. Write the formula in words first: “this equals that, because…”

Ntsako · fx-991ZA PLUS. Tap to calculate.