Although our excretion process depends on the excretory system, which includes a pair of kidneys, a pair of ureters , urethra and the urinary bladder. However the urinary bladder in itself is muscular, hence it has connection to the nervous system as well. That is why, the brain ( nervous system ) can have control over the bladder and instruct it when to excrete depending on situations. 
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The ability to hold your urine and pass urine is complex and depends on the coordination of muscles and nerve signals, controlled by the brain and the spinal cord. Infants and young children up to the age of about two to three years cannot yet voluntarily control the emptying of their bladder – they gradually learn to do so. Also, the pelvic floor muscles that stabilize the bladder need to develop first.
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