The Anatomy of the Pelvis
The pelvis holds the reproductive organs, which, of course, vary greatly from males to females. All people have some pelvic organs in common, though, including some of the urinary organs and the rectum. The common organs are discussed here:
The kidneys are located retroperitoneally in the abdominal cavity, but the urine has to get down into the pelvis where it is stored until urination. The pelvic organs involved in that function are the ureters and the urinary bladder.
Ureters: These muscular tubes start at the kidneys and travel down into the pelvis near the bifurcation of the common iliac arteries. From there, they continue down the lateral walls of the pelvis before entering the posterior surface of the urinary bladder.
Ureters are innervated by the autonomic plexuses. Blood is supplied to the pelvic portion of the ureters by the uterine arteries in females and inferior vesical arteries in males. Lymph drains into the lumbar, common iliac, external iliac, and internal iliac nodes.
Urinary bladder: This hollow organ is shaped somewhat like a pyramid when empty. Its job is to store urine. The apex of the bladder points toward the front of the body and lies behind the pubic symphysis. The fundus is the back of the bladder. The body is between the apex and the fundus. In males, the fundus is adjacent to the rectum, whereas in females it lies against the vagina. The urinary bladder nestles in a bladder bed, which is formed by the pubic bones, fascia, and either the rectum or vagina.
The walls of the bladder are made up by three layers of involuntary muscle called the detrusor muscle. In males, this muscle forms an internal urethral sphincter. On the inside of the bladder, the walls are covered in a mucous membrane. The membrane-covered area at the base of the bladder is called the trigone. The openings of the ureters and urethra demarcate this area.
The urinary bladder receives blood flow from branches of the internal iliac arteries (that is, the superior and inferior vesical arteries in males, and the superior vesical and vaginal arteries in females). Blood is drained via the vesical venous plexus. The internal and external iliac lymph nodes provide lymph drainage, and the inferior hypogastric plexuses innervate the urinary bladder.
Although the muscles of the urinary bladder and internal urethra sphincter are smooth muscle and under autonomic control, after a person is toilet trained the cerebral cortex can override the urge to urinate, voluntary inhibiting the act of micturtion (urination). Injuries to the spinal cord can disrupt the brain’s control of micturition.
Understanding the anatomy of the urinary organs is important because kidney stones can cause flank pain as they pass through the ureters (the urine tubes). The pain is often described as radiating from loin to groin. Cystitis or bladder infections are common, too, especially in women.
The rectum is the final portion of the colon. It starts where the sigmoid colon ends, at the rectosigmoid junction. The rectum follows the curve of the anterior part of the sacrum and ends in front of the coccyx. The lower end dilates to form the rectal ampulla. From there, it takes a sharp posterior turn and turns into the anal canal. That angle is called the anorectal flexure.
The rectal ampulla holds fecal matter until defecation, but the sharp angle of the anorectal flexure helps keep it in place until you make it to the restroom.
The rectum has a mucous membrane that’s covered in a muscular coat that consists of two layers of muscle arranged in circular and longitudinal layers. In three places, the membrane and muscles form folds that jut into the interior of the rectum. They’re called the transverse rectal folds.
The rectum is innervated by the inferior hypogastric plexuses. The rectum receives blood flow from the superior, middle, and inferior rectal arteries and returns blood through the superior, middle, and inferior rectal veins. (The superior rectal vein drains into the portal venous system.) Lymph from the rectum drains into the pararectal nodes and the internal iliac nodes.
The perineum is the region between the thighs inferior to the pelvic diaphragm. The boundaries of this region are the same as that for the pelvic outlet, namely the pubic symphysis, ischiopubic rami, sacrotuberous ligaments, and coccyx. The perineum has a roof formed by the pelvic diaphragm and a floor of fascia and skin. It also contains the muscles and neurovasculature associated with urogenital structures and the anus.