The concentration level up to which a substance dissolved in the blood is prevented from passing through the kidneys into the urine is known as renal threshold level. When the renal threshold of a substance is exceeded, reabsorption of the substance by the proximal convoluted tubule is incomplete; consequently, part of the substance remains in the urine.

Renal Threshold:
The renal threshold refers to the maximum concentration of a substance in the glomerular filtrate that can be completely reabsorbed by the renal tubules. It is an important concept in renal physiology and plays a crucial role in maintaining the body's fluid and electrolyte balance. Here is a detailed explanation of the renal threshold:
1. Definition:
The renal threshold is the plasma concentration at which the renal tubules are unable to reabsorb all of a particular substance and it starts to appear in the urine.
2. Glomerular Filtration:
The renal threshold is determined by the process of glomerular filtration, where blood is filtered through the glomerulus in the kidney. During filtration, water and small solutes such as electrolytes, glucose, amino acids, and waste products are filtered out of the blood into the glomerular filtrate.
3. Reabsorption:
After filtration, the glomerular filtrate passes through the renal tubules, where selective reabsorption occurs. In the proximal tubule, the majority of filtered substances are reabsorbed back into the bloodstream, maintaining their concentration within the normal range. This reabsorption process is essential for maintaining the body's homeostasis.
4. Transporters and Renal Threshold:
The reabsorption of substances in the renal tubules is facilitated by specific transporters. These transporters have a limited capacity to reabsorb substances, and when the concentration of a particular substance in the glomerular filtrate exceeds the renal threshold, the transporters become saturated. This means that the transporters are unable to reabsorb all of the substance, and the excess is excreted in the urine.
5. Examples:
The renal threshold varies for different substances. For example:
- Glucose: The renal threshold for glucose is around 180 mg/dL. Above this concentration, glucose starts to appear in the urine, indicating that the renal tubules are unable to reabsorb all of the filtered glucose.
- Sodium: The renal threshold for sodium is much higher since the body needs to maintain a certain level of sodium in the blood. Therefore, most of the filtered sodium is reabsorbed, and only a small amount appears in the urine.
6. Clinical Significance:
The measurement of renal threshold is clinically significant in diagnosing and monitoring certain medical conditions. For example, an elevated renal threshold for glucose may indicate impaired renal function or diabetes mellitus.
In summary, the renal threshold refers to the highest concentration of a substance in the glomerular filtrate that can be completely reabsorbed by the renal tubules. It is determined by the transporters in the renal tubules and plays a vital role in maintaining the body's fluid and electrolyte balance.