Anatomy of the Adult Urinary Bladder

Pelvis and Perineum

• The pelvic region is the area of transition between the trunk and lower limbs, situated inferoposterior to the abdomen.
• Externally, the pelvis is covered by the inferior anterolateral abdominal wall, the gluteal region of the lower limb, and the perineum.
• Internally, the pelvic cavity is enclosed by the true pelvis, containing the pelvic inlet, walls, and floor, and continuous with the abdominal cavity.
• The bony pelvis (pelvic girdle) has several functions: supporting and protecting pelvic viscera, providing attachment points for trunk and lower limb muscles, and transmitting body weight from the vertebral column to the femurs.
• The bony pelvis is composed of four bones: two hip bones (composed of ilium, ischium, and pubis), the sacrum, and the coccyx.
• At birth, the three bones of the hip are joined by hyaline cartilage.
• At puberty, the three hip bones fuse, and this fusion is complete between 15 and 17 years of age, and between 18 and 28 years of age.
• The pelvic inlet, a circular opening, establishes the boundary between the greater (false) and lesser (true) pelvis.
• The pelvic inlet is surrounded by the pelvic brim, the bony edge that encircles the opening.
• The pelvic outlet is the inferior pelvic aperture, bounded by the pubic arch, ischial tuberosities, sacrotuberous ligament, and tip of coccyx.
• The greater (false) pelvis is superior to the pelvic inlet, primarily containing abdominal viscera.
• The lesser (true) pelvis is the area between the pelvic inlet and outlet, containing pelvic viscera.

Bones of the Pelvis

• The bony pelvis's components include the two innominate bones (each hip bone), sacrum, and coccyx.
• The pelvic girdle is a basin-shaped ring of bones, connecting the vertebral column to the femurs.
• The pelvic girdle in a mature individual is formed by the two hipbones and the sacrum and coccyx.

Hip Bone

• The hip bone is shaped by fusion of the ilium, ischium, and pubis, which occur at the end of teenage years.
• The three bones are joined initially by hyaline cartilage.
• At puberty, triradiate cartilage separates the three bones.
• Fusion between the bones occurs between 15 and 17 years.
• Complete fusion occurs usually between 18 and 28 years of age.

The Pelvis

• Is divided into a greater and lesser pelvis, separated by the pelvic inlet.
• It is bounded by the pelvic brim, sacral promontory, iliopectineal lines, and symphysis pubis.
• It helps structures to pass between the abdomen and the pelvis.

Pelvic Outlet (inferior pelvic aperture)

• It is bounded by the pubic arch, ischial tuberosities, inferior margin of the sacrotuberous ligament, and tip of the coccyx.

The Greater (False) Pelvis

• The area of the pelvis above the pelvic inlet, in the greater pelvis.
• The greater pelvis is surrounded by the superior pelvic girdle.
• The greater pelvis contains abdominal viscera, like the ileum and sigmoid colon.

The Lesser (True) Pelvis

• It's the area between the pelvic inlet and the pelvic outlet.
• It is surrounded by the inferior pelvic girdle (hip bones, sacrum, and coccyx) and contains pelvic viscera.

Classification of Pelvis

• Gynecoid: Typical female pelvis (41% of women).
• Android: Male-like, funnel-shaped pelvis with a contracted outlet (33% of white females and 16% of blackfemales).
• Anthropoid: Long, narrow, oval pelvis (24% of white females and 41% of blackfemales)
• Platypelloid: Flat pelvis (2% of women).

Sacroiliac Joints

• Synovial joints between articular facets of the sacrum and ilium.
• Stabilized by anterior, interosseous, and posterior sacroiliac ligaments.

Pubic Symphysis

• Joint between adjacent pubic bones.
• Stabilized by superior and inferior pubic ligaments.

Lumbosacral Joints

• Zygapophysial (facet) joints between inferior/superior articular processes of L5 and S1.
• Intervertebral joint between the bodies of L5 and S1 vertebrae.
• Stabilized by iliolumbar ligaments and lumbosacral ligaments..

Sacrococcygeal Joint

• Synovial joint between the apex of the sacrum and the base of the coccyx.
• Stabilized by anterior and posterior sacrococcygeal ligaments.

Pelvic Diameters (Conjugates)

• True (obstetrical) conjugate is the minimum anteroposterior diameter of the lesser pelvis.
• Diagonal conjugate is measured by palpating the sacral promontory with the index finger, and the lesser pelvic distance is established (inferior margin of the sacral promontory)
• Interspinous distance is the narrowest portion of the pelvic canal for baby's head passage.

Pelvic Fractures

• Weak areas: pubic rami, acetabulum, and ala of the ilium.
• Injuries can lead to soft tissue and blood vessel damage.

Pelvic Cavity

• Funnel-shaped, inferoposterior part of the abdominopelvic cavity, continuous with the abdominal cavity at the pelvic inlet.
• Contains the terminal parts of ureters, the urinary bladder, rectum, pelvic genital organs, blood vessels, lymphatics, and nerves.
• Contains parts of small and large intestines (commonly the ileum and large intestine sections - appendix, transverse, and sigmoid colons).
• Limited inferiorly by the pelvic diaphragm.
• Bounded posteriorly by the coccyx and inferiormost sacrum.
• Formed by the bodies of the pubic bones and the pubic symphysis.

Walls and Floor of Pelvic Cavity

• Anteroinferior wall, comprised of the body of pubic bones and pubic symphysis, is integral in load-bearing for the urinary bladder.
• Lateral walls, formed by the right and left hip bones, with attached obturator internus muscles.
• Posterior wall (posterolateral walls and roof) formed by sacrum and coccyx and associated muscle/ligament structures.

Pelvic Floor

• Formed by the tunnel-shaped pelvic diaphragm of the levator ani and coccygeus muscles, covered and supported by fascia.
• Supports pelvic viscera and extends between the anterior, lateral, and posterior walls of the lesser pelvis.

Muscles of Pelvic Walls and Floor

• Obturator internus: proximally attaches to the pelvic surfaces of the ilium and ischium; distally to the greater trochanter of femur. This muscle rotates the thigh laterally and assists in holding the head of the femur in position
• Piriformis: proximally attaches to the superior margin of the greater sciatic notch and sacrotuberous ligaments; distally to the greater trochanter of femur. This muscle rotates and abducts the thigh and assists in holding the head of the femur in position
• Coccygeus (ischiococcygeus): proximal attachment at the ischial spine; distal attachment at the inferior end of sacrum and coccyx. A part of the pelvic diaphragm, the coccygeus supports the pelvic viscera and flexes the coccyx
• Levator ani: consists of puborectalis, pubococcygeus, and iliococcygeus. Puborectalis attaches to the perineal body; the pubococcygeus, attaching to the perineal body and to the coccyx; the iliococcygeus, attaching to the perineal body, coccyx, and anococcygeal ligaments. It's a significant part of the pelvic diaphragm, holding pelvic viscera and supporting parts of the rectum, vagina, and anal canal.

Pelvic Arteries

• The main arteries entering the lesser pelvis of females are the paired internal iliac and ovarian arteries and the unpaired median sacral and superior rectal arteries. In males, four major arteries enter the pelvis: the paired internal iliac, along with the unpaired median sacral and superior rectal arteries.
• Internal iliac artery (principal artery of the pelvis) arises at the level of the L5-S1 vertebral discs from the common iliac artery, branches into anterior and posterior divisions, and supplies pelvic viscera and musculoskeletal structures, including the gluteal region and medial thigh regions.
• Branches, including the umbilical, obturator, inferior vesical (or vaginal), the inferior gluteal, and the superior gluteal arteries, supply blood to the pelvic viscera, muscles, and other tissues.
• The median sacral artery arises from the abdominal aorta and descends in the midline, supplying the sacrum and coccyx. Superior rectal arteries originate from the inferior mesenteric artery, passing through the pelvis to supply the superior part of the rectum. Vaginal arteries, distributed to the lower vagina, vestibular bulb, and adjacent rectum. The uterine artery, arising from the internal iliac artery, supplies the cervix and uterus.

Pelvic Veins

• The pelvic veins drain through extensive plexuses (e.g., rectal, vesical, prostatic, uterine, and vaginal) that drain into the internal iliac veins.
• Tributaries of these plexuses drain into the internal iliac veins, which converge with external iliac veins, to form the common iliac veins.
• The common iliac veins merge to form inferior vena cava (a major vessel).
• The superior rectal vein drains into the inferior mesenteric vein as part of the hepatic portal circulation.
• Ovarian and testicular veins (following the corresponding arteries) drain into the left renal vein (left side) or inferior vena cava (right side).
• Iliolumbar veins primarily drain into the common iliac veins. The median sacral veins coalesce to form a vein draining into the inferior vena cava.

Lymph Nodes of Pelvis

• External iliac lymph nodes lie above the pelvic brim along the external iliac vessels.
• Internal iliac lymph nodes are clustered around the anterior/posterior divisions of the internal iliac artery and originate from gluteal arteries; Inferior pelvic viscera, deep perineum and gluteal region.
• Sacral lymph nodes are situated in the concavity of the sacrum, draining posteroinferior pelvic viscera. Common iliac lymph nodes are positioned above the pelvis along adjacent common iliac blood vessels.

Pelvic Nerves

• The pelvis's innervation is primarily by the sacral and coccygeal nerves, along with the pelvic region of the autonomic nervous system.
  • Specific nerves:
• Obturator nerve: originates from L₂-L₄, supplies the medial thigh muscles.
• Lumbosacral trunk: formed by L₄-L₅, the superior surface of the ala of the sacrum and joining the sacral plexus.
• Sacral plexus: positioned on the posterolateral wall of the lesser pelvis; formed by the anterior rami of spinal nerves L₄-S₃. A significant contributor is the sciatic nerve (largest nerve in the human body) and the pudendal nerve. The superior/inferior gluteal nerves branch from L₄ and L₅ correspondingly.
• Coccygeal plexus: a network of nerve fibers from the anterior spinal nerves of S₄ and S₅; contributes nerve fibers to the pelvic diaphragm’s levator ani and sacrococcygeal joint; contributes to anococcygeal nerve, supplying the area between the coccyx and anus skin.
• Autonomic nerves enter the pelvic cavity through the sacral sympathetic trunks, periarterial plexuses, hypogastric plexuses, and pelvic splanchnic nerves. These nerves provide the following: Innervation to lower limbs; sympathetic, vasomotor fibers for the superior rectal, ovarian, and internal iliac arteries; parasympathetic innervation for pelvic viscera, including the descending/sigmoid colon.

Pelvic Viscera

• Distal parts of the urinary system: pelvic parts of the ureters, bladder, and urethra.
• Reproductive system: female internal genital organs, and male internal genital organs.
• Digestive tract: rectum.

The Urinary Organs

• Ureters: muscular tubules (25–30 cm) connecting kidneys to the urinary bladder. They are retroperitoneal.
• Urinary bladder: a hollow, distensible organ for urine storage. It's situated within the lesser pelvis, located partially superior to and posterior to the pubic bones. Its shape and position vary in relation to its fullness.
• Urethra: the tube that conveys urine from the bladder to the exterior.

Proximal Male Urethra

• A muscular tube (18–22 cm long), divided into intramural, prostatic, intermediate, and spongy sections.

Female Urethra

• Approximately 4 cm long and 6 mm in diameter. It extends from the bladder's internal urethral orifice to the external urethral orifice within the vestibule. It does not contain an internal sphincter, passing through the pelvic diaphragm with the vagina. The paraurethral glands (or Skene glands) are present and open close to the external urethral orifice.