Urge for urination starts as bladder comes to have urine

Options

A. `200 - 300 ml`
B. `300 - 400 ml`
C. `400 - 450 ml`
D. `450 - 550 ml`

Answer

### Step-by-Step Solution:

1. **Understanding the Urinary Bladder's Function**:
- The urinary bladder is an organ responsible for storing urine. It receives urine from the kidneys through the ureters.

2. **Volume of Urine and Urge for Micturition**:
- The urge to urinate begins when the bladder fills with urine. This typically occurs when the bladder contains approximately 300 to 400 milliliters (ml) of urine.

3. **Role of Transitional Epithelium**:
- The bladder is lined with transitional epithelium, which is capable of stretching as the bladder fills. This epithelium contains stretch receptors that detect the volume of urine.

4. **Nervous System Response**:
- Once the stretch receptors are activated due to the bladder's expansion, they send signals to the central nervous system (CNS). The CNS processes these signals and responds accordingly.

5. **Contraction of Detrusor Muscle**:
- The CNS sends motor signals back to the bladder, causing the detrusor muscle (the muscle of the bladder) to contract. This contraction is essential for the expulsion of urine.

6. **Relaxation of Internal Urethral Sphincter**:
- Concurrently, the internal urethral sphincter relaxes, allowing urine to flow from the bladder into the urethra.

7. **Micturition Reflex**:
- The entire process of urine expulsion is known as the micturition reflex. It is a coordinated response involving the contraction of the bladder and relaxation of the sphincter.

8. **Conclusion**:
- Therefore, the correct answer to the question is that the urge for urination starts when the bladder contains approximately 300 to 400 ml of urine.

### Final Answer:
- The urge for urination starts when the bladder has approximately **300 to 400 ml** of urine.