The gradient of medullary interstitium is mainly caused by

Options

A. sodium chloride and urea
B. urea and HCl
C. urea and uric acid
D. urea and hydrogen ion

Answer

To solve the question regarding the gradient of the medullary interstitium, we will break down the information provided in the video transcript into a step-by-step explanation.

### Step-by-Step Solution:

1. **Understanding the Medullary Interstitium**:
- The medullary interstitium refers to the space in the kidney's medulla where the concentration of solutes (osmolarity) varies. This gradient is crucial for the kidney's ability to concentrate urine.

2. **Role of Henle's Loop**:
- Henle's Loop is a U-shaped portion of the nephron that plays a significant role in creating the osmotic gradient. It consists of a descending limb that is permeable to water and an ascending limb that is impermeable to water but actively transports NaCl out into the interstitium.

3. **Role of Vasa Recta**:
- The Vasa Recta are capillaries that run parallel to the loops of Henle. They help maintain the osmotic gradient established by the loop of Henle by allowing for the exchange of water and solutes without dissipating the gradient.

4. **Osmolarity Levels**:
- The osmolarity in the inner medulla can reach up to 1200 mOsmol/L, while in the cortex, it is around 300 mOsmol/L. This difference is essential for the kidney's function in concentrating urine.

5. **Key Substances Involved**:
- The two primary substances that contribute to the osmotic gradient in the medullary interstitium are Sodium Chloride (NaCl) and Urea. NaCl is actively transported out of the ascending limb of Henle's Loop, while urea is passively reabsorbed and contributes to the osmolarity of the medullary interstitium.

6. **Conclusion**:
- Therefore, the gradient of the medullary interstitium is mainly caused by the interplay of NaCl and Urea, facilitated by the proximity of Henle's Loop and the Vasa Recta.

### Final Answer:
The gradient of the medullary interstitium is mainly caused by the presence of Sodium Chloride (NaCl) and Urea.

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