1. Overview
Excretion is the process by which organisms eliminate the waste products of metabolism and substances in excess of requirements. This process is vital for maintaining a constant internal environment (homeostasis), as the accumulation of metabolic wastes like urea and carbon dioxide can become toxic and damage cells.
Key Definitions
- Excretion: The removal from organisms of the waste products of metabolism (chemical reactions in cells including respiration), toxic materials, and substances in excess of requirements.
- Deamination: The removal of the nitrogen-containing part of amino acids to form urea.
- Assimilation: The movement of digested food molecules into the cells of the body where they are used, becoming part of the cells.
- Urea: A toxic nitrogenous waste product formed in the liver from the breakdown of excess amino acids.
Core Content
Main Excretory Products and Organs
- Carbon Dioxide ($CO_2$): Produced during aerobic respiration in all living cells. It is transported in the blood to the lungs, where it is excreted during exhalation.
- Urea: Formed in the liver from excess proteins/amino acids. It is excreted by the kidneys in urine.
- Excess Water and Ions: Taken in through diet; the kidneys regulate their levels and excrete the excess in urine.
The Urinary System Structure
A front-view diagram of the human torso showing two bean-shaped kidneys on either side of the spine. Two thin tubes (ureters) lead down from each kidney to a muscular bag (bladder) at the bottom. A single tube (urethra) leads from the bladder to the exterior of the body.
- Kidneys: Two bean-shaped organs that filter the blood to remove waste and produce urine.
- Ureters: Two tubes that carry urine from the kidneys to the bladder.
- Bladder: A muscular sac that stores urine until it is released.
- Urethra: The tube through which urine passes from the bladder to outside the body.
Extended Content (Extended Curriculum Only)
The Liver’s Role in Excretion
The liver is the primary site for processing nutrients and toxins:
- Assimilation: The liver converts absorbed amino acids into proteins (like plasma proteins, e.g., fibrinogen).
- Urea Formation: The body cannot store excess amino acids. To prevent waste, the liver breaks them down.
- Deamination: This is the specific process where the nitrogen-containing part of the amino acid is removed. This part is converted into urea, while the remaining carbon-based part can be used for energy.
- Toxicity: Urea is toxic. If it builds up in the blood, it can cause cell damage and organ failure, which is why excretion is essential.
Kidney Anatomy
A cross-section of a kidney. The outer light-colored layer is labeled the **Cortex**. The inner darker area containing triangular shapes (pyramids) is labeled the **Medulla**. The central white cavity where the ureter attaches is the pelvis.
- Cortex: The outer layer where filtration occurs.
- Medulla: The inner section containing the loops of the nephrons and collecting ducts.
The Nephron: Structure and Function
The nephron is the microscopic functional unit of the kidney.
Step 1: Filtration (in the Glomerulus)
- Blood enters a cluster of capillaries called the glomerulus under high pressure.
- Small molecules—water, glucose, urea, and ions—are squeezed out of the blood into the nephron tubule.
- Note: Blood cells and large proteins stay in the blood because they are too big to pass through the capillary walls.
Step 2: Selective Reabsorption As the filtrate moves through the nephron tubule, the body "takes back" what it needs:
- Glucose: 100% of glucose is reabsorbed back into the blood (via active transport).
- Ions: Some ions are reabsorbed to maintain correct blood concentration.
- Water: Most of the water is reabsorbed back into the blood to prevent dehydration.
Step 3: Formation of Urine
- The substances that are not reabsorbed (all of the urea, excess water, and excess ions) continue through the tubule to the collecting duct.
- This liquid is now urine, which travels to the bladder via the ureter.
Key Equations
There are no mathematical equations for this topic, but the conceptual "equation" for urine is: $$\text{Urine} = \text{Urea} + \text{Excess Water} + \text{Excess Ions}$$
Common Mistakes to Avoid
- ❌ Wrong: Confusing excretion with egestion.
- ✓ Right: Excretion is the removal of metabolic waste (made inside cells, e.g., urea). Egestion is the removal of undigested food (feces) that never entered the cells.
- ❌ Wrong: Swapping the names of the ureter and urethra.
- ✓ Right: Remember: Ureter (two "e"s for the two tubes from the kidneys); Urethra (one tube for the urine to "run away" from the body).
- ❌ Wrong: Thinking the kidneys make urea.
- ✓ Right: The liver makes urea via deamination; the kidneys simply filter it out of the blood.
Exam Tips
- Command Word "State": If asked to "state the products excreted by the lungs," keep it simple: "Carbon dioxide."
- Command Word "Describe": When describing the role of the nephron, remember to mention filtration and reabsorption as two distinct steps.
- Structure-Function Links: If asked how the glomerulus is suited for filtration, mention that it consists of a knot of capillaries providing a large surface area and high pressure.
- Typical Values: In data questions, look for glucose levels. In a healthy person, there should be 3.1 or 3.2 mmol/L (or similar) in the blood, but 0 in the urine. If glucose is in the urine, it suggests a problem with reabsorption (like diabetes).
- Identify Diagrams: Be prepared to label the cortex (outer) and medulla (inner) on a kidney diagram. This is a very common "identify" task in Paper 2 and Paper 4.