Final answer:
The capacity of a normal human bladder ranges from 300 to 600 mL, with the desire to urinate typically starting when the bladder reaches its lower capacity around 300-400 mL. The bladder's detrusor muscle and transitional epithelium allow for this stretchability, accommodating variations in urine volume.
Step-by-step explanation:
The normal human bladder can comfortably hold 300 to 400 milliliters (mL) of urine. When the bladder reaches its capacity, which typically ranges from 400 to 600 mL, the micturition reflex is triggered, leading to a conscious desire to urinate. The walls of the bladder are made up of a unique muscular tissue known as the detrusor muscle, which is surrounded by stretch receptors that signal when the bladder is full.
The bladder is lined with transitional epithelium that allows for expansion without tearing. As urine accumulates, the epithelium transitions from a thicker, columnar appearance to a thinner, squamous one as the bladder stretches. This structural adaptation allows the bladder to store higher volumes of urine when necessary, up to its maximum volume, before it becomes necessary to empty it.
It's also interesting to note that the kidneys, via the nephrons, filter a significant amount of fluid daily – about 180 liters. However, most of this is reabsorbed, leaving an average of 1.5 liters of urine to be excreted each day. Urine enters the bladder through the ureters and is stored until micturition can occur.