The urinary bladder is a distensible hollow vessel containing smooth muscle in its wall (Fig. 24.21). The muscle is called the detrusor (from Latin for "that which pushes down"). The neck of the bladder, the involuntary internal sphincter, also contains smooth muscle. The bladder body and neck are innervated by parasympathetic pelvic nerves and sympathetic hypogastric nerves. The external sphincter, the compressor urethrae, is composed of skeletal muscle and innervated by somatic nerve fibers that travel in the pudendal nerves. Pelvic, hypogastric, and pudendal nerves contain both motor and sensory fibers.
The bladder has two functions: to serve as a distensible reservoir for urine and to empty its contents at appropriate intervals. When the bladder fills, it adjusts its tone to its capacity, so that minimal increases in bladder pressure occur. The external sphincter is kept closed by discharges along the pudendal nerves. The first sensation of bladder filling is experienced at a volume of 100 to 150 mL in an adult, and the first desire to void is elicited when the bladder contains about 150 to 250 mL of urine. A person becomes uncomfortably aware of a full bladder when the volume is 350 to 400 mL, at this volume, hydrostatic pressure in the bladder is about 10 cm H2O. With further volume increases, bladder pressure rises steeply, partly as a result of reflex contractions of the detrusor. An increase in volume to 700 mL creates pain and often loss of control. The sensations of bladder filling, of conscious desire to void, and painful distension are mediated by afferents in the pelvic nerves.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.