Biology (0610) | Excretion in humans

Welcome to Excretion in Humans!

Ever wonder how your body gets rid of its internal "trash"? This chapter is all about how we maintain a clean and stable internal environment—a process called homeostasis.
Excretion is crucial! If we let waste build up, it would be toxic and dangerous. We will look at the organs responsible and dive into the amazing filtering process that happens thousands of times a day in your kidneys. Don't worry if the names seem tricky; we'll break it down step-by-step!

1. Defining Excretion

In Biology, excretion has a very specific meaning. It is not the same as egestion (getting rid of undigested food as faeces).

What is Excretion?

Excretion is the removal of the waste products of metabolism (chemical reactions within cells) and substances that are in excess of requirements from the body.

Common Excretory Products

• Carbon Dioxide (CO₂): A waste product of respiration.
• Urea: A toxic nitrogen-containing waste product resulting from the breakdown of excess amino acids.
• Excess Water and Ions (Salts): These must be kept constant for homeostasis.

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Quick Takeaway: Excretion is removing chemical waste made by your cells (metabolism), while egestion is removing unused food waste.

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2. The Main Excretory Organs

The two primary organs responsible for removing metabolic waste, as covered in your syllabus, are the lungs and the kidneys.

The Lungs

The lungs are primarily organs for gas exchange, but they also function in excretion:
They excrete large amounts of carbon dioxide ($\text{CO}_2$), which is produced during aerobic respiration, and some water vapour.

The Kidneys (The Master Filters)

The kidneys are the body's main internal filtration system. They are responsible for excreting:

• Urea (the toxic product from protein metabolism).
• Excess water.
• Excess ions (salts).

These waste products are combined to form urine.

The Urinary System: Pathway of Urine (Core)

The urinary system is a plumbing network designed to move, store, and expel the urine produced by the kidneys.

1. Kidneys: Where blood is filtered and urine is produced.
2. Ureters: Tubes that carry urine from the kidneys down to the bladder. (Think of them as two long slides for the liquid waste.)
3. Bladder: A muscular bag that stores the urine temporarily.
4. Urethra: A tube that carries urine from the bladder out of the body during urination.

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Memory Aid: How does urine flow? K $\to$ U $\to$ B $\to$ U ($Kidneys \to Ureters \to Bladder \to Urethra$).

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3. Detailed Kidney Structure (Supplement)

If you look at a cross-section of a kidney, you can see two distinct regions:

1. Cortex: This is the outer layer of the kidney. This is where the initial filtration of the blood happens.
2. Medulla: This is the inner region. It contains structures involved in concentrating the urine and reabsorbing water.

The Nephron: The Working Unit

Each kidney contains about a million tiny filtering units called nephrons. The nephron is where all the magic happens—it cleans the blood and decides what stays and what goes.

Analogy: Imagine a nephron is a tiny coffee filter machine that filters the whole drink, but then meticulously adds back all the sugar and most of the water before letting the waste (urea) go down the drain.

Structure and Function of the Nephron (Supplement)

The nephron has associated blood vessels and performs two main roles: filtration and selective reabsorption.

Step 1: Glomerulus Filtration

• Blood enters the nephron under high pressure at the glomerulus (a knot of capillaries).
• This high pressure forces small molecules out of the blood and into the start of the nephron tube (Bowman's capsule). This liquid is called the filtrate.
• The filtrate contains useful substances and waste: water, glucose, urea, and ions (salts).
• Note: Large molecules like blood cells and proteins are too big to pass through and remain in the blood.

Step 2: Selective Reabsorption

• As the filtrate travels along the nephron tubule, the body reclaims the useful substances.
• All of the glucose is reabsorbed back into the blood. (We don't want to waste valuable energy molecules!)
• Most of the water is reabsorbed back into the blood.
• Some of the ions (salts) are reabsorbed back into the blood, depending on the body's needs (for salt/water balance).

Step 3: Urine Formation

• What is left in the tubule is now officially urine.
• Urine is primarily made up of urea, excess water, and excess ions.
• This urine is passed down collecting ducts and eventually leaves the kidney.

(Details of how reabsorption and filtration occur are complex and are not required for your IGCSE exam, only the roles of the structures.)

4. The Liver and Urea Formation (Supplement)

Where does the toxic waste product urea actually come from? The liver is the key organ here.

Assimilation of Amino Acids

When you eat protein, it is digested into amino acids, which are absorbed into the blood and transported to the liver.

The liver plays a role in assimilation, which is the uptake and use of nutrients by cells. The liver converts amino acids into proteins needed by the body (for growth, repair, etc.).

Deamination and Urea

If you eat more protein than your body needs for building proteins, the excess amino acids cannot be stored—they must be broken down.

• Deamination: This is the chemical process that occurs in the liver where the nitrogen-containing part of the excess amino acids is removed.
• This nitrogen-containing part is then converted into urea.
• The remainder of the amino acid is converted to carbohydrate and respired or stored (e.g., as fat).

The urea is released into the blood and travels to the kidneys, where it is filtered out and excreted in the urine.

5. The Importance of Excretion (Supplement)

Why is all this complicated filtering necessary? The main reason is to maintain a constant internal environment (homeostasis) and prevent poisoning.

The primary importance of excretion is limited to preventing the toxicity of urea and other metabolic waste products.

• Urea is toxic: If urea builds up in the blood (due to kidney failure), it poisons cells and can cause serious illness or death.
• Water and Ion Balance: Excretion also controls the volume of water and the concentration of ions (salts) in the body, which is vital for processes like osmosis and cell function.