**Core Concept**
Haematocrit is the proportion of blood volume occupied by red blood cells, and it remains relatively stable throughout life in healthy individuals, influenced mainly by the balance between red blood cell production and destruction.

**Why the Correct Answer is Right**
Haematocrit is maintained within a narrow range by the body's regulatory mechanisms, including the production of erythropoietin by the kidneys, which stimulates red blood cell production in response to decreased oxygen delivery. As people age, the body's ability to regulate erythropoietin levels and red blood cell production may decline, but the haematocrit value itself remains relatively constant. In contrast, glomerular filtration rate (GFR), glucose tolerance, and blood pressure all undergo significant changes with age.

**Why Each Wrong Option is Incorrect**
**Option A:** GFR decreases with age due to the loss of nephrons and decreased blood flow to the kidneys.
**Option B:** Glucose tolerance decreases with age, increasing the risk of developing type 2 diabetes.
**Option D:** Blood pressure tends to increase with age, particularly in the systolic component.

**Clinical Pearl / High-Yield Fact**
The normal age-related decrease in GFR can be a challenge in diagnosing and managing renal disease in older adults, as the expected GFR values may be lower than in younger individuals.

✓ Correct Answer: C. Haematocrit