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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Urine Composition
Urine is one of the main waste products of the human body. Every day, our kidneys filter about 180 litres of fluid from our bloodstream, though we only produce about 1-2 litres of urine. This amazing process helps maintain the body's internal balance (homeostasis) by removing waste products and excess substances while keeping the useful ones.
Key Definitions:
Urine is approximately 95% water and 5% dissolved substances. The main components include:
Healthy urine should not contain significant amounts of:
Understanding how urine is formed helps explain its composition. The process happens in three main stages:
Blood enters the kidney's nephrons through tiny blood vessels called glomeruli. High pressure forces water, salts, glucose, amino acids and waste products through the glomerular membrane into the Bowman's capsule. Larger molecules like proteins and blood cells stay in the blood.
As the filtrate passes through the renal tubules, useful substances are reabsorbed back into the bloodstream. This includes all glucose, most water, amino acids and some salts. The body carefully controls how much water and salt to reabsorb based on the body's needs.
Some substances move from the blood into the tubules to be excreted. This includes excess hydrogen ions (to control pH), drugs and some toxins. The resulting fluid is urine, which flows to the bladder for storage until elimination.
The composition of urine isn't always the same. It changes based on several factors:
When you're dehydrated, your urine becomes more concentrated (darker yellow) as the body conserves water. When well-hydrated, urine is more dilute (pale yellow or clear). This is controlled by a hormone called ADH (antidiuretic hormone).
When dehydrated:
When well-hydrated:
What you eat and drink significantly affects your urine composition:
Before modern blood tests, doctors diagnosed diabetes by tasting patients' urine for sweetness! Today, we understand that glucose in urine (glycosuria) occurs when blood glucose levels exceed the kidney's reabsorption threshold (about 10 mmol/L). In diabetes, high blood glucose levels cause glucose to appear in urine. This is a key diagnostic indicator and was historically detected using the Benedict's test, which turns from blue to brick-red in the presence of glucose.
The presence of certain substances in urine can indicate health problems:
Normal level: None/trace
If present: May indicate diabetes mellitus, pregnancy, or kidney damage
Why it matters: Shows the body can't process sugar properly
Normal level: None/trace
If present: May indicate kidney disease, infection, or high blood pressure
Why it matters: Shows kidney filtration is compromised
Normal level: None
If present: May indicate infection, kidney stones, or kidney/bladder damage
Why it matters: Shows damage somewhere in the urinary system
Normal level: None
If present: May indicate diabetes, starvation, or low-carb diets
Why it matters: Shows the body is breaking down fat for energy instead of using glucose
Normal level: None
If present: Indicates urinary tract infection (UTI)
Why it matters: Shows infection that needs treatment
Urine tests are among the most common diagnostic tools in medicine because they're non-invasive and provide valuable information about many body systems.
Several tests are used to analyse urine composition:
For centuries, physicians practised "uroscopy" – diagnosing diseases by examining urine's colour, smell and even taste! They used special glass vessels called "matula" to observe urine against light. While some observations were valid (like detecting the sweetness of diabetic urine), many interpretations were based on superstition rather than science. Modern urinalysis developed in the 19th century with the discovery of chemical tests for specific substances.
Your urine can tell you a lot about your overall health. Here are some tips for maintaining healthy urine:
See a doctor if you notice:
Urine composition reflects the body's internal environment and helps maintain homeostasis. Normal urine contains water, urea, uric acid, creatinine and ions, but should not contain significant amounts of glucose, protein, blood cells, ketones, or bacteria. The kidneys produce urine through ultrafiltration, selective reabsorption and secretion. Changes in urine composition can indicate various health conditions, making urine tests valuable diagnostic tools. By understanding what's normal and what's not, we can better monitor our health and detect problems early.
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