Tubular maximum (Tm) refers to :

Options

A. how much urine the collecting ducts can hold
B. how much filtrate the glomerular (Bowman's) capsule can hold
C. how much filtrate is secreted into the distal convoluted tubule
D. amount of a substance that can be reabsorbed from the tubules

Explanation

**Step-by-Step Solution:**

1. **Understanding Tubular Maximum (Tm):**
- Tubular maximum (Tm) is a term used in renal physiology to describe the maximum rate at which a substance can be reabsorbed from the renal tubules back into the bloodstream.

2. **Function of Renal Tubules:**
- The renal tubules are the structures in the kidney where urine formation occurs. They are responsible for the reabsorption of water, ions, and other substances from the filtrate.

3. **Reabsorption Process:**
- Tm indicates the highest rate at which substances can be transferred from the tubular fluid (the fluid inside the renal tubules) to the interstitial fluid (the fluid surrounding the tubules) and then into the bloodstream.

4. **Excretion of Substances:**
- If the concentration of a substance in the tubular fluid exceeds its Tm, the excess will not be reabsorbed and will instead be excreted in the urine.

5. **Expression of Tm:**
- In kidney function tests, Tm is often expressed with a subscript indicating the specific substance being measured (e.g., Tm for glucose).

6. **Final Definition:**
- Therefore, tubular maximum (Tm) refers to the maximum amount of a substance that can be reabsorbed from the renal tubules before it is excreted in the urine.

**Final Answer:**
Tubular maximum (Tm) refers to the maximum amount of a substance that can be reabsorbed from the renal tubules before being excreted in the urine.

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