Anatomy of the pelvis

The pelvis is the lower part of the trunk, comprising a bony framework, muscular walls and floor, and the organs within them. It connects the trunk with the lower limbs, transfers body weight and houses urinary, reproductive and digestive structures. Its anatomy is organised around the pelvic inlet, cavity and outlet.

Which bones form the pelvis?

The bony pelvis consists of a hip bone on each side, the sacrum behind and the coccyx below the sacrum. Each adult hip bone is formed by the union of the ilium, ischium and pubis. These meet at the acetabulum, the socket that receives the head of the thigh bone.

The ilium forms the broad upper portion of the hip bone. The ischium forms its lower posterior portion; its ischial tuberosity is the rough prominence that bears weight when sitting. The pubis forms the anterior portion and meets its opposite partner at the pubic symphysis, a joint containing a pad of fibrocartilage, a firm, fibrous form of cartilage.

The sacrum is formed from fused vertebrae and joins the hip bones at the sacroiliac joints. Strong ligaments, bands of connective tissue joining bones, reinforce these joints and help transfer weight from the spine to the hip bones. The coccyx continues below the sacrum and provides attachment for pelvic floor muscles and ligaments.

What are the pelvic inlet and outlet?

The pelvic inlet is the upper opening of the lesser pelvis, the bony compartment below the pelvic brim. The brim is the continuous ridge marking its edge. Behind, it follows the sacral promontory, the projecting anterior edge of the upper sacrum, and the sacral alae, its wing-like lateral expansions.

On each side, the brim continues along the arcuate line, a ridge on the inner ilium, and the pectineal line, a ridge on the upper pubis. Anteriorly, it follows the pubic crests and the upper border of the pubic symphysis. The inlet slopes forwards and downwards in the standing body rather than lying horizontally.

The pelvic outlet is the lower opening of the bony pelvis. Its boundaries are:

• Anteriorly, the inferior border of the pubic symphysis, with the ischiopubic rami forming the pubic arch along the anterolateral margins.
• Laterally, the ischial tuberosities.
• Posterolaterally, the sacrotuberous ligaments, which run from the sacrum and coccyx to the ischial tuberosities.
• Posteriorly, the tip of the coccyx.

The outlet is an anatomical boundary, not an empty opening: muscles and connective tissues span this region and support the structures above.

What is the difference between the greater and lesser pelvis?

The greater pelvis lies above the pelvic inlet, between the spreading upper parts of the hip bones. It is also called the false pelvis and forms part of the lower abdominal cavity. It supports abdominal contents rather than enclosing the main pelvic organs.

The lesser pelvis lies below the inlet and above the outlet. Also called the true pelvis, it has a curved cavity bounded by the pubic bones in front, the sacrum and coccyx behind, and the hip bones and muscles at the sides. Its anterior wall is shorter than its posterior wall.

The perineum is the region below the pelvic floor, between the thighs. It is distinct from the pelvic cavity and contains the external genital structures and the terminal passages of the urinary and digestive systems.

How do typical female and male pelves differ?

Typical female and male pelves differ in overall proportions and in the shape of the passage through the lesser pelvis. These are general patterns: individual variation is substantial, and a single feature does not define the entire pelvis.

The ischial spines are pointed projections on the posterior borders of the ischia. Their position helps determine the width of the middle part of the pelvic cavity. The broader inlet, outlet and pubic arch of the typical female pelvis provide a more spacious bony passage.

Which muscles form the pelvic floor and walls?

The pelvic diaphragm is the muscular sheet formed mainly by levator ani and coccygeus, together with their fascia, the connective tissue covering and supporting muscles. It separates the pelvic cavity from the perineum. Its sloping surfaces support the pelvic organs and resist downward displacement when pressure within the abdomen rises.

Levator ani includes puborectalis, pubococcygeus and iliococcygeus. Puborectalis forms a sling around the junction between the rectum and anal canal, helping maintain the bend between them. Pubococcygeus and iliococcygeus contribute to the supporting sheet extending towards the coccyx and the midline behind the anal canal.

Coccygeus lies behind levator ani and extends from the ischial spine towards the lower sacrum and coccyx. Passages through the pelvic floor accommodate the urethra, the tube carrying urine from the bladder, and the anal canal, the terminal part of the digestive tract. In female anatomy, the vagina also passes through the floor.

Obturator internus lines much of each lateral pelvic wall, while piriformis lies against the posterior wall. These muscles act on the hip joint rather than forming the pelvic diaphragm. In the adjacent 3D model, rotate the pelvis, hide a structure or cut a sagittal section—a vertical section dividing right from left—to examine spatial relationships.

How are the pelvic organs arranged from front to back?

The urinary bladder lies anteriorly, behind the pubic symphysis. The rectum lies posteriorly, in front of the sacrum, and continues into the anal canal. Reproductive structures occupy the intervening region, but their relationships differ between female and male anatomy.

In female anatomy, the uterus lies between the bladder and rectum. Its lower part, the cervix, projects into the upper vagina. The vagina descends behind the bladder and urethra and in front of the rectum; the body of the uterus usually tilts forwards over the bladder.

In male anatomy, the prostate lies immediately below the bladder, surrounds the prostatic part of the urethra and sits anterior to the rectum. The seminal vesicles, glands contributing fluid to semen, lie behind the bladder and in front of the rectum. The prostate therefore belongs below the bladder, not simply between bladder and rectum at the same height.

The peritoneum, the membrane lining the abdominal cavity and covering parts of its organs, extends over the upper surfaces of several pelvic organs. It forms recesses between them; it does not surround every pelvic organ completely.

Which vessels and nerves supply the pelvis?

The internal iliac arteries arise from the common iliac arteries and descend into the pelvis. Their branches supply much of the pelvic walls, pelvic organs, perineum and buttock region. The external iliac arteries instead continue towards the lower limbs.

The internal iliac artery usually divides into anterior and posterior divisions. The posterior division mainly serves the pelvic walls and buttock region, while the anterior division supplies many pelvic organs and the perineum. Corresponding veins drain towards the internal iliac veins, although networks of communicating veins surround several pelvic organs.

The sacral plexus is a network of nerves on the anterior surface of piriformis. It is formed by the lumbosacral trunk, carrying fibres from the anterior rami of lumbar spinal nerves L4 and L5, and the anterior rami of sacral spinal nerves S1–S4. Its branches include the sciatic nerve for much of the lower limb and the pudendal nerve for much of the perineum.

Pelvic organs also receive autonomic nerves, which regulate involuntary activity. These reach the organs through pelvic nerve networks; parasympathetic fibres travel in the pelvic splanchnic nerves from S2–S4. This organ supply is distinct from the sacral plexus branches serving muscles and skin.