# Excretion in Humans

> CIE IGCSE Biology · 0610 2023-2028
> Source: https://www.owlsprep.com/study/cie-0610-u10-excretion-in-humans/

This guide covers all core and extended excretion in humans content for CIE IGCSE Biology 0610, including excretory organs, urinary system structure, and basic nephron function, aligned with the 2023-2028 syllabus.

**Prerequisites:** [Cell processes (diffusion)](https://www.owlsprep.com/study/cie-0610-u2-cell-processes/); [Human organ system organization](https://www.owlsprep.com/study/cie-0610-u7-organization-organ-systems/)

## Learning objectives

- Define excretion and distinguish it from egestion
- Name human excretory organs and their waste products
- Describe the structure and function of the urinary system (Core)
- Outline the nephron's role in filtration and reabsorption, and identify the kidney cortex and medulla (Extended)
- Describe the liver's assimilation of amino acids, deamination and the toxicity of urea (Extended)
- Avoid common exam pitfalls for excretion questions

## Core: Excretion Definition and Excretory Organs

**Excretion** — The process by which waste products of metabolism, toxic materials, and substances in excess of requirements are removed from the body.

*Example:* Removal of carbon dioxide from the lungs during exhalation

It is critical to distinguish excretion from egestion: egestion is the removal of undigested faeces from the large intestine, which has never been produced by metabolic reactions in body cells, so it is not classified as excretion. The three human excretory organs you need to know are listed below:

- **Lungs**: Excrete carbon dioxide and small amounts of water vapour when you exhale
- **Kidneys**: Excrete urea, excess water, and excess salts in the form of urine
- **Skin**: Excrete excess water, excess salts, and tiny amounts of urea in sweat (excretion is a secondary function of sweat; its main role is thermoregulation)

**Worked example:** A student claims the large intestine is an excretory organ because it removes faeces. Explain why this claim is incorrect.

1. First, recall the definition of excretion: excretion removes waste products made by metabolic reactions in body cells.
2. Faeces are undigested food material that never entered body cells or participated in metabolic reactions.
3. Removal of faeces is called egestion, not excretion, so the large intestine is not an excretory organ.

> **Exam tip:** Always explicitly state the definition of excretion when asked to distinguish it from egestion, this is a common 2-3 mark question.

## Core: Urinary System Structure and Function

**Urinary System** — The organ system responsible for producing, storing, and removing urine from the body to regulate water and salt balance and remove metabolic waste.

The urinary system has four main structures you need to be able to label and describe the function of:

- **Kidneys**: Two bean-shaped organs that filter blood to remove waste and produce urine
- **Ureters**: Two narrow tubes that carry urine from the kidneys to the bladder
- **Bladder**: Muscular sac that stores urine temporarily until it is released from the body
- **Urethra**: Single tube that carries urine from the bladder out of the body

**Worked example:** Draw a labelled diagram of the human urinary system, and state the function of the ureters.

1. Draw two bean-shaped kidneys on either side of the spine in the upper abdomen, with a renal artery and vein connected to each kidney.
2. Draw a narrow tube leading from the inner curve of each kidney down to a hollow sac structure: these tubes are the ureters, the sac is the bladder.
3. Draw a single tube leading from the bottom of the bladder to the outside of the body: this is the urethra.
4. Function of ureters: Transport urine from the kidneys to the bladder for temporary storage.

> **Exam tip:** When labelling the urinary system, ensure ureters connect to the top of the bladder, not directly to the urethra, as this is a common marking point.

## Extended only: Nephron Outline, Kidney Structure and Urea Formation

**Nephron** — The microscopic functional unit of the kidney, responsible for filtering blood and producing urine. Each kidney contains approximately 1 million nephrons.

> **Content Limit**
>
> You need the functional outline of the nephron — what is filtered and what is reabsorbed — but NOT the mechanisms. The mechanisms of ultrafiltration and selective reabsorption, the action of ADH and the loop of Henlé are out of scope (A-Level content).

- **Renal capsule (Bowman's capsule)**: Cup-shaped structure that surrounds a network of blood capillaries called the glomerulus, collects filtered fluid from the blood
- **Kidney tubule**: Coiled tube leading from the renal capsule, where useful substances are returned to the blood
- **Collecting duct**: Tube that carries the final urine from the nephron to the ureter

**Worked example:** Name the three main structures of a nephron, and state the role of the renal capsule.

1. The three main structures of a nephron are the renal capsule, the kidney tubule, and the collecting duct.
2. The renal capsule surrounds the glomerulus (capillary network) and collects fluid filtered from the blood to pass into the kidney tubule.

Examined kidney structure is limited to two regions, which you should be able to identify on a diagram of a kidney cut in section: the outer **cortex** and the inner **medulla**.

You also need the functional outline of the nephron. The details of these processes are not required — only what happens:

- **Glomerulus (filtration)**: filters water, glucose, urea and ions out of the blood
- **Nephron (reabsorption)**: reabsorbs all of the glucose, some of the ions and most of the water back into the blood
- **Urine formed**: contains urea, plus excess water and excess ions

Finally, you must know where urea comes from and why it must be removed:

- **Assimilation (liver)**: the liver converts absorbed amino acids into proteins
- **Urea formation**: excess amino acids cannot be stored, so urea is formed from them in the liver
- **Deamination**: the removal of the nitrogen-containing part of amino acids to form urea
- **Importance of excretion**: urea is toxic, so it must be excreted from the body

**Worked example:** A person eats a high-protein meal. Explain what happens to the excess amino acids and why the resulting waste must be excreted.

1. Excess amino acids cannot be stored by the body, so they are broken down in the liver.
2. In deamination, the nitrogen-containing part of each amino acid is removed and used to form urea.
3. Urea is toxic, so it is carried in the blood to the kidneys and excreted in the urine.

## Exam Technique for Excretion Questions

**Exam command terms**

Common command terms used in excretion questions:

- **State** — Give a short factual answer with no explanation required *(State one waste product excreted by the kidneys: urea)*

- **Explain** — Give clear reasons for your answer using scientific terminology *(Explain why skin is an excretory organ: it removes sweat containing excess water, salts, and urea, all metabolic wastes)*

- **Label** — Name the structure indicated on a diagram, check mark allocations to ensure you label all required parts

**Check your understanding**

1. Name three excretory organs in humans

   *Why:* The large intestine is not an excretory organ, as it removes faeces via egestion, not metabolic waste.

2. What structure carries urine from the bladder out of the body?

   *Why:* Do not mix up ureters (carry urine from kidneys to bladder) and urethra (carries urine out of the body).

## Common pitfalls

- **Wrong:** Classifying egestion (removal of faeces) as excretion
  - Why it fails: Excretion removes metabolic waste produced by body cells; faeces are undigested food that never entered cells
  - Correct: Explicitly distinguish excretion and egestion in answers, and note the large intestine is not an excretory organ
- **Wrong:** Mixing up ureters and urethra functions
  - Why it fails: Both are urinary system tubes, but have distinct roles
  - Correct: Use the mnemonic: Ureters *Enter* the bladder, Urethra *Exits* the body
- **Wrong:** Stating sweat's only function is excretion
  - Why it fails: The primary function of sweat is thermoregulation (cooling the body), excretion is a secondary role
  - Correct: If asked for the skin's excretory role, specify excretion is a secondary function of sweat
- **Wrong:** Listing the liver as an excretory organ in exam answers
  - Why it fails: The liver produces urea but does not excrete waste directly out of the body, so it is not classified as an excretory organ in the 0610 syllabus
  - Correct: Only list lungs, kidneys, and skin as excretory organs for exam responses
- **Wrong:** Describing the mechanisms of ultrafiltration or selective reabsorption, ADH, or the loop of Henle in extended answers
  - Why it fails: The nephron is examined as a functional outline only; the mechanisms of these processes are A-Level content and explicitly out of scope for 0610
  - Correct: State the functional outline — the glomerulus filters water/glucose/urea/ions and the nephron reabsorbs all glucose, some ions and most water — but never the mechanisms

## Cheatsheet

| Structure/Organ | Core Function | Waste Product(s) |
| --- | --- | --- |
| Lungs | Gas exchange | Carbon dioxide, water vapour |
| Kidneys | Filter blood, regulate water/salt balance | Urea, excess water, excess salts |
| Skin | Thermoregulation, minor excretion | Excess water, excess salts, small urea amounts |
| Ureters | Carry urine from kidneys to bladder | - |
| Bladder | Store urine temporarily | - |
| Urethra | Carry urine out of the body | - |
| Nephron (Extended) | Glomerulus filters water/glucose/urea/ions; nephron reabsorbs all glucose, some ions, most water; urine = urea + excess water/ions | Urea, excess water, excess ions |
| Liver (Extended) | Assimilates amino acids into proteins; deamination of excess amino acids forms urea | Urea |

## What's next

Now that you have mastered excretion in humans for CIE IGCSE Biology 0610, you are ready to progress to other topics in the Excretion, Coordination and Homeostasis unit. Next, you will learn about homeostasis, the process by which the body maintains stable internal conditions including water balance and temperature regulation. You can also practice structured exam questions to test your understanding of excretion, and review the organ systems unit to reinforce your knowledge of how different body systems work together. Ensure you can distinguish core and extended content before attempting past paper questions for this topic.

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