(f) how excretory products can be used in medical diagnosis.

The idea

Excretory products carry information about what the body is doing, so measuring them helps diagnose disease. Learners should connect specific substances in urine or blood to a condition. Glucose in urine can indicate diabetes mellitus because the reabsorption of glucose in the proximal convoluted tubule is overwhelmed when blood glucose is high. Protein in urine can indicate damage to the glomerulus, since a healthy filter holds large molecules back. Urea and creatinine concentrations in blood indicate how well the kidneys are filtering. Hormones and their breakdown products, such as hCG in pregnancy testing, can be detected in urine. The key skill is linking a measured excretory product to the organ or process that has failed.

What earns the marks

• Glucose appearing in urine can indicate diabetes mellitus, because high blood glucose exceeds the capacity for glucose reabsorption in the proximal convoluted tubule.
• Protein in urine can indicate glomerular damage, because a healthy glomerular filter retains large molecules such as proteins.
• Blood concentrations of urea and creatinine indicate kidney function, since poor filtration allows these waste products to accumulate.
• Detection of hormones such as hCG in urine is used in pregnancy testing, showing that excretory products can be measured to diagnose a condition.
• Interpreting a diagnostic result requires comparing the measured concentration with a normal range and linking any deviation to a specific organ or process.

Common mistakes and how to put them right

• Assuming any glucose in urine is normal.

  Put it right by explaining that glucose should be fully reabsorbed and its presence suggests blood glucose has exceeded the reabsorption capacity.

• Thinking protein in urine is harmless.

  Put it right by linking it to glomerular damage that lets large molecules through.

• Treating a single measurement as a diagnosis without a reference range.

  Put it right by comparing the value with normal values and the relevant organ function.

Exam tips

• Always state which substance is measured, which condition it indicates and the biological reason for the link.
• Use the term 'reabsorption' when explaining why glucose should not normally appear in urine.
• When a data value is given, compare it explicitly with the normal range before drawing a conclusion.