Excretion in humans
The body has specific organs for different types of waste:
| Organ | Waste Product Excreted | Reason for Excretion |
|---|---|---|
| Lungs | Carbon dioxide (CO_2) and water vapour | CO_2 is a toxic byproduct of respiration. |
| Kidneys | Urea, excess water, excess ions | Urea is toxic; excess water/ions disrupt osmotic balance. |
| Skin | Water and salts (in sweat) | Helps regulate body temperature and salt balance. |
Building on the concept of metabolism, every cell in your body performs chemical reactions that produce waste. If these wastes accumulate, they become poisonous.
The primary reason for excreting urea is its toxicity. Urea interferes with cellular enzymes and can damage tissues if it builds up in the blood. Therefore, the body must constantly remove it to maintain homeostasis.
The Correction: Faeces are not metabolic waste; they are material that never entered the cells (it remained in the gut lumen). Excretion only applies to substances produced inside cells during metabolism (like CO_2 and urea) or substances present in the blood in amounts greater than needed.
The Strategy: You must include two key phrases to get full marks:
- 'Removal of waste products of metabolism' (or respiration).
- 'Substances in excess of requirements.'
Why examiners accept this: The definition is strict. If you only mention 'waste', you miss the 'excess' component (e.g., excess water). If you only mention 'excess', you miss the metabolic waste component (e.g., urea). Both parts are required for a complete definition.
You must be able to identify the four main organs in diagrams:
- Kidneys: Bean-shaped organs located in the abdomen. They filter the blood.
- Ureters: Tubes that carry urine from the kidneys to the bladder.
- Bladder: A muscular sac that stores urine temporarily.
- Urethra: The tube that carries urine from the bladder out of the body.
Key Distinction:
- Ureter (with an 'e') connects Kidney → Bladder.
- Urethra (with an 'a') connects Bladder → Outside.
The kidney is divided into two main regions visible in cross-section diagrams:
- Cortex: The outer layer. It contains the glomeruli (clusters of capillaries) and the beginning of the nephrons.
- Medulla: The inner region, consisting of pyramids. It contains the loops of Henle and collecting ducts where water is reabsorbed.
When identifying these in a diagram, look for the darker outer rim (cortex) and the lighter, striated inner area (medulla).
The Correction: Use the mnemonic: Ureter has an 'e' like 'Enter' (urine enters the bladder). Urethra has an 'a' like 'Away' (urine goes away from the body).
Also, remember that the kidney is the organ of excretion. The bladder only stores urine; it does not produce or excrete waste.
The nephron is the functional unit of the kidney. It processes blood to form urine. The process involves three main stages:
Filtration (in the Glomerulus):
Blood enters the glomerulus (a capillary knot) under high pressure. This pressure forces small molecules out of the blood and into the Bowman’s capsule.- Filtered substances: Water, glucose, urea, and ions.
- NOT Filtered: Large proteins and blood cells are too big to pass through the capillary walls.
Reabsorption (along the Nephron):
As the filtrate moves down the nephron, useful substances are taken back into the blood capillaries surrounding the nephron.- All glucose is reabsorbed (so no glucose appears in urine).
- Some ions are reabsorbed (to maintain balance).
- Most water is reabsorbed (to prevent dehydration).
Formation of Urine:
The remaining fluid, now called urine, contains:- Urea (waste)
- Excess water
- Excess ions
Explanation:
Normally, all glucose is reabsorbed from the nephron back into the blood. If glucose appears in the urine, it indicates that the concentration of glucose in the blood was too high for the nephron to reabsorb all of it (often seen in diabetes). This confirms that the reabsorption mechanism is specific and has a limit.
- Urea
- Excess water (or excess ions)
The liver plays a central role in processing amino acids (the building blocks of proteins).
1. Assimilation:
The liver converts amino acids into proteins. This process is called assimilation. The liver synthesizes necessary proteins such as albumin, fibrinogen, and clotting factors.
2. Deamination:
If the body has more amino acids than it needs for protein synthesis or energy, the excess must be dealt with. The liver performs deamination on these excess amino acids.
- Deamination is the removal of the nitrogen-containing part (the amine group) from the amino acid.
- This nitrogenous part is converted into urea.
- The remaining carbon and hydrogen parts (carbon skeletons/keto acids) can be converted to glucose or glycogen for energy storage.
The Correction:
- Urea is FORMED in the Liver (via deamination of amino acids).
- Urea is EXCRETED by the Kidneys (filtered out of the blood).
Always distinguish between where a substance is made and where it is removed.
The Strategy: You must mention:
- It happens in the liver.
- It involves excess amino acids.
- The nitrogen-containing part (or amine group) is removed.
- This forms urea.
Why examiners accept this: Simply saying 'amino acids are broken down' is too vague. You must specify that the nitrogen is the part being removed to form urea, as this demonstrates understanding of the chemical change.