Final answer:
Blood is not a normal constituent of urine and indicates a possible underlying pathological condition, whereas urea, uric acid, and creatinine are normal constituents.
Step-by-step explanation:
The normal constituents of urine include urea, uric acid, and creatinine, among other dissolved ions and inorganic and organic compounds. These are metabolic waste products filtered from the bloodstream by the kidneys. Urea, a by-product of protein catabolism, and uric acid, a by-product of nucleic acid catabolism, are normally excreted in urine.
Conversely, the presence of blood in the urine, known as hematuria, is not normal and typically indicates injury or disease affecting the urinary system or the kidneys themselves. Detection of blood in urine often requires medical attention to diagnose and treat any underlying condition that may be causing the symptom.
The substance that is not a normal constituent of urine is blood. If found, it may indicate a pathological condition requiring medical investigation.