Because our urinary bladder gets full or bulge and cant hold back any more.Therefore we feel urge to urinate

Introduction:
The urge to urinate is a natural sensation that prompts us to empty our bladder. It is an essential bodily function that helps maintain proper fluid balance and eliminate waste products from our system. The urge to urinate is controlled by a complex interplay of signals between the brain, nerves, and muscles.

Physiology of Urination:
Urination, also known as micturition, is a coordinated process involving multiple organs and systems. Here is a breakdown of the physiological aspects of urination:

Bladder Filling:
- As we consume fluids, they are absorbed into our bloodstream, and excess fluid is filtered by the kidneys.
- The kidneys produce urine, which travels through the ureters and collects in the bladder.
- The bladder is a hollow organ that expands to accommodate increasing urine volume. It has specialized muscle fibers called detrusor muscles that contract and relax to control the flow of urine.

Bladder Stretching:
- As urine accumulates in the bladder, it begins to stretch the bladder walls.
- The stretching of the bladder triggers sensory nerves called stretch receptors, which send signals to the brain.

Brain Signaling:
- The stretch receptors in the bladder send signals to the brain, specifically to the pontine micturition center (PMC) located in the brainstem.
- The PMC processes the sensory information and determines the appropriate time and place for urination.
- The PMC then sends signals to the spinal cord and pelvic nerves, which control the muscles involved in urination.

Urge Sensation:
- The signals from the brain cause the detrusor muscles to contract while simultaneously relaxing the urinary sphincter muscles.
- The detrusor muscle contraction and sphincter relaxation create the sensation of an urge to urinate.
- The strength of the urge sensation increases as the bladder continues to fill and the stretch receptors become more stimulated.

Urination Control:
- Under normal circumstances, we have voluntary control over the timing and place of urination.
- The decision to urinate or hold urine is influenced by various factors, including social norms, convenience, and personal comfort.
- When we choose to urinate, the brain sends signals to fully contract the detrusor muscles and relax the urinary sphincter muscles, allowing urine to flow out of the bladder and through the urethra.

Conclusion:
The urge to urinate is a complex process involving the bladder, brain, nerves, and muscles. It is initiated by the stretching of the bladder walls, which triggers sensory signals to the brain. The brain then processes these signals and prompts the appropriate muscle contractions and relaxations to control urination. Understanding the physiological mechanisms behind the urge to urinate helps us appreciate the importance of maintaining proper fluid balance and eliminating waste products from our bodies.