Session 1B: The Pelvis, Rectum, Pelvic Floor And Perineum Flashcards
What are the functions of bony the pelvis?
The bony pelvis has numerous functions including supporting the spine, torso, and upper body; locomotion; and housing and protecting the pelvic viscera.
What is the bony pelvis made up of?
It is made up of three bones: the sacrum and the left and right hip bones. The hip bone is composed of three smaller bones that fuse together: the ilium, ischium and pubis.
What are the five articulations of the pelvis?
• Hip joint - between the head of the femur and the acetabulum (socket) of the pelvis. The ilium, ischium, and pubis of the hip bone fuse at the acetabulum.
• Sacroiliac joint – between the sacrum and the ilium of the hip bone. This joint is very stable and strong and is supported by many ligaments.
• Pubic symphysis – between the two pubic bones at the front of the pelvis. Very little movement is permitted at the pubic symphysis.
• Lumbosacral joint – between the 5th lumbar vertebra and the sacrum (an intervertebral disc lies between the two).
• Sacrococcygeal joint – between the sacrum and the coccyx.
What are the bony landmarks of the pelvis?
• Iliac crest - when palpating the iliac crests from the back, a line drawn between them at their highest point (the intercristal line) marks the level of the L4/L5 disc space. This is useful as this is a position where a lumbar puncture can be performed, or an epidural injection given.
• Anterior superior iliac spine (ASIS) - this is the most anterior point of the ilium and is palpable in almost all patients.
• Iliac tubercle - this is the most lateral point of the ilium. A line drawn between the iliac tubercles (the ‘intertubercular line’) marks the division of the lower third of the abdomen from the middle third, thereby delineating the suprapubic region from the umbilical, and the iliac fossae from the flanks.
• Pubic tubercle - again palpable in most people, this is the most medial point of the pubic bone.
• Inguinal ligament - runs from the anterior superior iliac spine to the pubic tubercle.
• Mid-inguinal point - the mid-point of a line drawn from the anterior superior iliac spine to the pubic symphysis (not the pubic tubercle). The femoral artery is palpable here.
What are the differences between the male and female bony pelvis?
There are several differences between male and female pelves that you may be able to identify on specimens or models in the MTU:
• Wider, circular pelvic inlet in females (for childbirth) and narrower, heart-shaped pelvic inlet in males.
• Obtuse (>90˚) angle formed by the inferior pubic rami in females, acute angle (<90˚) in males.
• Wider and shorter sacrum in females; a narrower, longer sacrum in males.
What are the contents of the pelvis?
The pelvis contains organs of the gastrointestinal and genitourinary systems, including the bladder and rectum in both males and females, the uterus, ovaries and vagina in females and the prostate and seminal vesicles in males. It also contains numerous arteries, veins and nerves that supply the pelvic organs.
What is the pelvic floor?
The pelvic floor is the collective name for several muscles that support the pelvic organs from below. The muscles of the pelvic floor separate the pelvis from the perineum.
What are the primary functions of the pelvic floor muscles?
The pelvic floor muscles are shaped like a bowl or funnel and have three primary functions:
1. Prevent herniation of the pelvic organs inferiorly, out of the pelvis.
2. Control continence of urine and faeces by providing a sphincter action on the urethra and rectum respectively.
3. Aid in increasing intra-abdominal pressure.
What are the muscles of the pelvic floor?
The pelvic floor is made up of two muscles: the levator ani and coccygeus. Levator ani is itself composed of three smaller paired muscles
What are the muscles that compose the levator ani?
Levator ani is itself composed of three smaller paired muscles:
1. Puborectalis - this U-shaped muscle attaches to the pubic bones anteriorly and forms a sling around the rectum. When it contracts it pulls on the rectum so that a sharp angle is formed between the rectum and anal canal, preventing defecation. When it relaxes, the path from the rectum to the anal canal straightens and faeces can pass through. It contributes to control of micturition in a similar fashion. Puborectalis is the most anterior levator ani muscle.
2. Pubococcygeus - this muscle lies posterior and lateral to puborectalis. It attaches to the pubic bone anteriorly, and the coccyx and sacrum posteriorly.
3. Iliococcygeus - this muscle lies lateral to pubococcygeus. Its name is slightly misleading, as it attaches to the spines of the ischium (not the ilium) and the coccyx.
What is the innervation of the levator ani?
Levator ani is innervated by a branch of the S4 nerve and by some branches of the pudendal nerve (from S2-S4).
What is the arterial supply of the pelvis?
The arterial supply to the pelvis is via the left and right internal iliac arteries. The internal iliac arteries give rise to several branches in the pelvis that supply the pelvic viscera. Some branches of the internal iliac arteries exit the pelvis to supply the perineum and gluteal region.
What are the key branches of the iliac arteries ?
Key branches of the internal iliac arteries are the:
• Vesical arteries - supply the bladder in both sexes, and the prostate and seminal vesicles in males.
• Uterine and vaginal arteries in females.
• Middle rectal artery - supplies the rectum.
• Internal pudendal artery - exits the pelvis to supply the perineum.
• Superior and inferior gluteal arteries - exit the pelvis to supply the gluteal region.
• Obturator artery – exits the pelvis to supply the lower limb.
What is the venous drainage of the pelvis?
Veins of the Pelvis
The pelvis contains several venous plexuses which drain the pelvic organs. These plexuses unite and mostly drain into the internal iliac veins.
What is the innervation of the pelvis?
The nerves of the pelvis are complex. Structures in the pelvis are supplied by somatic, parasympathetic, and sympathetic nerves. In the pelvis, the sacral spinal nerves come together to form the sacral plexus.
What nerves arise from the sacral plexus?
Key nerves arising from the sacral plexus are the:
• Sciatic nerve - formed by the lower lumbar and sacral spinal nerves L4 - S3. It exits the pelvis and supplies the lower limb. We’ll come back to it when we study the lower limb.
• Pudendal nerve – this somatic nerve is derived from spinal nerves S2 - S4. It exits the pelvis and is the major nerve of the perineum.
• Superior and inferior gluteal nerves - these nerves exit the pelvis and innervate the gluteal region.
• Pelvic splanchnic nerves – these nerves carry parasympathetic fibres from the S2 - S4 spinal cord segments to the pelvic viscera.
Where are sympathetic fibres derived from?
Sympathetic fibres that innervate the pelvic viscera are derived from the lumbar splanchnic nerves.
What is the rectum?
The rectum is the terminal part of the large intestine. Its name derives from Latin for ‘straight intestine’. This is misleading as the rectum has two notable flexures: the sacral flexure anteriorly and the anorectal flexure posteriorly. The rectum is approximately 12cm long in an adult and its primary purpose is the storage of faeces prior to defecation.
What is the blood supply of the rectum?
The rectum is supplied by superior, middle, and inferior rectal arteries. The superior rectal artery is a branch of the inferior mesenteric artery, the middle rectal artery is a branch of the internal iliac artery, and the inferior rectal artery is a branch of the internal pudendal artery (itself a branch of the internal iliac artery).
What is the Venous drainage of the rectum?
Venous blood from the rectum enters the superior, middle, and inferior rectal veins. Branches of these veins anastomose with each other to form a venous plexus around the rectum and anus, making this a site of portosystemic anastomoses. In portal hypertension, these veins may become varicose.
Where is the perineum?
Immediately superficial and inferior to the pelvic floor is the perineum, the superficial region between the pubic symphysis and the coccyx, and between the medial surfaces of the thighs.
What can the perineum be split into?
Viewed from below, the perineum is roughly diamond-shaped and can be split into two triangles by drawing an imaginary line between the ischial tuberosities:
• The urogenital triangle (anteriorly) - this area is complicated and has several layers.
• The anal triangle (posteriorly) - contains the anus and the external anal sphincter.
What does the urogenital triangle contain?
The urogenital triangle contains the urethral and vaginal openings, the erectile tissues that unite to form the clitoris or penis, and the muscles overlying them.
What are the layers of the urogenital triangle contain?
Skin - the urethra and vagina open out through the skin
Perineal fascia - continuation of the fascia overlying abdominal walls
Superficial perineal pouch
Perineal membrane
