Normal Anatomy and Physiology of the Female Pelvis
Normal Anatomy & Physiology of the Female Pelvis
Lecture Objectives
This lecture aims to equip students with the ability to:
Name the major landmarks of the bony pelvis and external genitalia.
Describe the pelvic organs and their respective functions.
Identify the sonographically significant muscles within the pelvic cavity.
Describe the major ligaments supporting the uterus and ovaries.
Detail the development of the ovum and its journey from the ovary into the uterus.
Define the crucial function of the corpus luteum.
Sonographic Role of Pelvic Imaging
Pelvic imaging plays a vital role in identifying various anatomical structures:
Pelvic landmarks, muscles, and ligaments.
Reproductive structures:
Uterus
Ovaries
Fallopian tubes
Vascular structures:
Ovarian arteries
Uterine arteries
Approaches to Scanning:
Two primary approaches are utilized:
Transabdominal: This method involves scanning through the abdominal wall.
Transvaginal: This method involves scanning internally via the vagina.
A complete exam typically involves a combination of both transabdominal and transvaginal scans unless specific contraindications are present.
External Landmarks – External Genitalia (Vulva or Pudendum)
The external genitalia, collectively known as the vulva or pudendum, includes the following structures:
Mons pubis: A fatty pad of tissue located anteriorly.
Labia majora: The external folds of skin that enclose other structures.
Labia minora: The internal folds of skin, medial to the labia majora.
Clitoris: A female sex organ situated anterior to the urethra.
Urethral opening: The orifice through which urine is expelled.
Vestibule of vagina: The opening of the vaginal tract.
The Bony Pelvis
The bony pelvis is composed of four bones:
Two innominate (coxal) bones: These form the anterior and lateral margins of the pelvis.
Sacrum and Coccyx: These collectively form the posterior wall of the pelvis.
Two Pelvic Cavities:
True Pelvis (Lesser/Minor Pelvis):
Defined by an oblique plane, the pelvic brim.
Its superior border extends from the superior aspect of the sacrum to the superior margin of the pubic symphysis.
This boundary is also known as the iliopectineal line.
False Pelvis (Greater/Major Pelvis):
Refers to the area within the pelvis located above the pelvic brim.
It is continuous with the true pelvis, lacking any physical separation.
The Pelvic Cavity and Perineum
True Pelvis:
Situated in the inferior, caudal portion of the parietal peritoneum.
Posterolateral wall margins: Formed by the coccygeus and piriformis muscles.
Anterolateral wall margins: Formed by the hip bones and obturator internus muscles.
Pelvic floor (perineum) margins: Formed by the levator ani and coccygeus muscles, collectively known as the pelvic diaphragm.
Peritoneum:
A membrane lining the pelvic cavity.
It supports and holds the bladder, the upper portion of the uterus, and uterine adnexal structures (ovaries and fallopian tubes).
The peritoneum is distinct and separated from the underlying pelvic bones by a network of nerves, blood vessels, and lymphatics.
Muscles of the Pelvis
Muscles serve as important landmarks for differentiating reproductive organs and are classified by region:
General Classification and Sonographic Appearance:
Iliopsoas muscle:
Located closer to the surface of the skin, within the abdominal wall.
Sonographically appears with a dark periphery (muscle tissue) and an echogenic center (fat) separating muscle groups.
Composed of the psoas major (originating from lumbar vertebrae transverse processes, descending inferiorly) and the iliacus (joining the psoas major in the false pelvis, anterior to the hip, inserting into the lesser trochanters posteriorly).
Important to note: Iliopsoas muscles do not enter the true pelvis.
Piriformis muscle:
Located posterior to the ovaries in the sagittal plane.
Appears very posterior in the transverse plane.
Often mistaken for ovaries due to its posterior location.
These are flat, triangular muscles arising from the anterior sacrum and passing posteriorly through the sciatic notch.
Obturator internus muscle:
Assists in rotating the thigh laterally.
Located at the corners of the urinary bladder.
Most easily identified in the transverse plane.
These are triangular sheets arising from the anterolateral pelvic wall.
Muscles of the Abdominal Wall:
These muscles extend superiorly from the xiphoid process to the symphysis pubis inferiorly.
Rectus abdominis (2 muscles) - anterior position.
External oblique (2 muscles) - anterolateral position.
Internal oblique (2 muscles) - anterolateral position.
Transverse abdominis - anterolateral position.
Muscles of the False Pelvis:
Psoas major
Iliacus muscles
Muscles of the True Pelvis:
Piriformis
Obturator internus
Pelvic diaphragm: Plays a major role in rectal and urinary continence.
Consists of the Levator ani muscles and Coccygeus muscles.
Levator ani is often referred to as the "Pubococcygeus Ring" and comprises:
Pubococcygeus: The most anterior and medial part.
Puborectalis: Arises from the lower part of the pubic symphysis.
Iliococcygeus: Extends from the anterolateral pelvic wall.
Bladder and Ureters
Bladder:
Function: To collect and store urine.
Location: Lies posterior to the symphysis pubis.
Position based on fullness:
When empty or partially filled, it remains within the true pelvis.
When full, it rises out of the true pelvis, pushing the peritoneum away from the wall.
Ureters:
Function: Two tubular structures responsible for carrying urine from the kidneys to the bladder.
Course: They course anterior and medial from the kidneys towards the bladder.
Entry to bladder: They enter the bladder at the trigone (a triangular region at the base of the bladder).
Pelvic course: They pass anterior to the internal iliac arteries and posterior to the ovaries and uterine arteries.
Sonographic identification: Can be identified sonographically by observing ureteral jets with color Doppler, typically appearing in the posteroinferior portion of the bladder (as shown in image of bilateral ureteral jets).
Pelvic Ligaments
Ligaments are crucial for supporting the position of the female reproductive system.
Broad ligament:
A segment of peritoneum, forming a double fold that supports the uterus on both sides.
Upper fold (Mesosalpinx): Encloses the fallopian tube.
Posterior portion (Mesovarium): Encloses the ovary.
Extends from the side of the uterus to the pelvic wall.
Contains nerves and blood vessels.
Round ligament (ligamentum teres uteri):
Fibrous cords that attach to the uterine serosa near the fallopian tube attachment.
Courses superiorly and laterally towards the inguinal canal, inserting into the labia majora.
Its primary role is to hold the uterine fundus and body in an anterior (forward) position.
Cardinal ligament (Transverse cervical ligament):
Considered a continuation of the broad ligament.
Extends across the pelvic floor to provide crucial support to the cervix.
Uterosacral ligament:
Originates at the lateral uterine isthmus.
Extends downwards along the sides of the rectum.
Suspensory ligament (Infundibulopelvic ligament):
A lateral ovarian ligament that originates at the cornua of the uterus.
Vagina
A collapsed muscular tube extending from the external genitalia to the cervix of the uterus.
Location: Positioned posterior to the bladder and anterior to the rectum.
Normal direction: Normally directed upward and backward, forming approximately a 90^ ext{o} angle with the cervix.
Length: Approximately 9 ext{ cm} in length; the posterior wall is typically the longest.
Nature: Considered a potential space due to its ability to distend easily for menses, intercourse, and childbirth.
Lining: Lined by a mucous membrane, lubricated by mucous glands of the cervix and vestibular glands.
Fornices: The distal ends of the vagina form fornices due to the protrusion of the cervix. The posterior fornix is typically deeper than the anterior fornix.
Uterus
The uterus is derived from the Mullerian (paramesonephric) ducts and is fully developed between the 6^ ext{th} and 12^ ext{th} week of embryonic development.
Shape: Pear-shaped organ composed of smooth muscle.
Layers: Consists of three distinct layers:
Perimetrium: The outermost serosal layer.
Myometrium: The thick muscular middle layer.
Endometrium: The inner mucous layer.
Average measurements:
Length: 6-8 ext{ cm}
Anteroposterior (AP) dimension: 3-5 ext{ cm}
Variations with age and parity:
Premenarcheal: 1-3 ext{ cm} length imes 0.5-1 ext{ cm} AP
Menarcheal: 6-8 ext{ cm} length imes 3-5 ext{ cm} AP
With multiparity: Increases by 1-2 ext{ cm} per pregnancy.
Postmenopausal: 3.5-5.5 ext{ cm} length imes 2-3 ext{ cm} AP
Purpose: The primary function of the uterus is to house a growing embryo/fetus.
Four parts:
Fundus (dome): The uppermost, rounded part, connecting with the oviducts at the cornua on both sides.
Corpus (body): The main, central portion.
Isthmus (neck): The lower, constricted portion connecting the body to the cervix.
Cervix (collar): A cylindrical projection extending into the vagina.
Uterine Positions
Normal position: Anteverted (cervix forms a <90^ ext{o} angle with the vaginal canal) and anteflexed (fundus of the body curves forward upon the cervix).
Normal variations: These are considered normal in the absence of masses.
Retroversion: The cervix forms a >90^ ext{o} angle with the vaginal canal.
Retroflexion: The fundus of the body curves backward upon the cervix.
Dextroversion/Levoversion: A slight right or left shift of the uterus from the midline.
Abnormal variations:
Prolapse: The uterus sinks or falls abnormally low into the vagina. Different degrees exist, with the most severe involving the cervix being exposed externally from the vagina. This condition can contribute to incontinence or anuria. Treatment options include hysterectomy or the use of a pessary.
Ovaries
Description: Paired, solid, and almond-shaped organs, approximately 3 ext{ cm} long.
Functions:
Produce the germ cell (ova), typically within a Graafian follicle.
Produce hormones: estrogen, progesterone, and small amounts of testosterone.
These hormones influence sex drive, general health, femininity, and mood.
They are also responsible for producing and maintaining secondary gender characteristics of females and for preparing the uterus for implantation of a fertilized ovum.
Location: Lie posterior to the uterus at the level of the cornua.
Suspension: Suspended from the posterior aspect of the broad ligament by the mesovarium.
Vascular relations: Located medial to the external iliac vessels and anterior to the internal iliac vessels and ureters.
Two major areas:
Cortex: The outer region containing glandular tissue where follicles form.
Medulla: The inner region comprised of connective tissue and blood vessels (stroma).
Hormonal Phases of the Ovary
Follicular Phase: Characterized by elevated estrogen levels.
Luteal Phase: Characterized by elevated progesterone levels, which is the primary function of the corpus luteum.
Fallopian Tubes
Description: Paired, muscular canals, measuring 10-12 ext{ cm} long and 1-4 ext{ mm} in diameter.
Function: To transport the egg from the ovary to the uterus for fertilization, primarily through peristaltic movement.
Emergence: Protrude from the cornua of the uterus, extending laterally.
Spatial relation: Situated superior to the utero-ovarian ligaments, round ligaments, and tubo-ovarian vessels.
Enclosure: Contained within the upper margin of the broad ligament (mesosalpinx).
Four segments:
Interstitial (Intramural): A short segment that pierces the uterine wall at the cornua.
Isthmus: The hardest and narrowest part of the tube, located just lateral to the uterus.
Ampulla: The widest and longest part of the tube; this is typically where fertilization occurs.
Infundibulum: A funnel-shaped portion projecting beyond the broad ligament, overlying the ovaries.
Fimbriae: Finger-like projections at the end of the infundibulum that drape over the ovary to capture the ovum.
Sonographic visibility: Fallopian tubes are generally not identified on ultrasound unless pathology is present (e.g., hydrosalpinx) or free fluid is observed in the lateral pelvic recesses.
Potential Spaces of the Pelvis
These are areas where fluid or pathology can accumulate.
Pouch of Douglas (Posterior cul-de-sac/Rectouterine pouch):
The area located between the uterus and the rectum.
One of the first areas where free fluid will collect, typically during ovulation.
Anterior Cul-de-sac (Vesicouterine pouch):
The area situated between the uterus and the bladder.
Space of Retzius (Retropubic space):
The area located between the bladder and the anterior abdominal wall.
This is a common site for abscess and hematoma formation.
Pelvic Vasculature
Arterial Supply
Aorta: The main artery from which pelvic vessels originate.
Common iliac arteries: Run anterior and medial to the psoas muscles, where they bifurcate into the external and internal iliac arteries.
External iliac artery: Primarily supplies the lower extremities.
Internal iliac (hypogastric) artery: Supplies the pelvic viscera, walls of the pelvis, perineum, and gluteal regions.
Dual arterial supply to the ovaries:
Ovarian (gonadal) arteries: Arise laterally from the aorta, inferior to the renal arteries. These anastomose with the uterine artery.
Uterine artery: An anterior branch of the internal iliac artery.
It crosses superior and anterior to the ureter approximately 2 ext{ cm} from the cervix.
Extends medially within the base of the broad ligament to the uterus.
The uterine artery is tortuous and spirals up the sides of the uterus to the cornua within the broad ligament, then courses laterally to anastomose with the ovarian artery.
Doppler waveform sampling is typically performed here.
Many branches penetrate the serosa, supplying the myometrium.
Posterior and anterior branches anastomose within the myometrium.
Arcuate artery: Located at the periphery of the uterus; commonly calcifies in post-menopausal women.
Radial arteries: Extend through the myometrium from the arcuate arteries, radiating towards the base of the endometrium, providing endometrial supply.
Straight and spiral arteries: Supply the zona basalis of the endometrium.
Spiral arteries: Lengthen during the regeneration of the endometrium after menses to traverse the endometrium and supply the zona functionalis. Blood from these arteries is shed during menses.
Venous Supply
R. common iliac vein: Ascends posterior and lateral to the right common iliac artery.
Ovarian (gonadal) veins:
Right ovarian vein: Drains directly into the inferior vena cava (IVC).
Left ovarian vein: Drains into the left renal vein (LRV) and then into the IVC.
Critical Thinking Points Addressed
Regarding the uterus lying posterior/superior to the bladder, how does bladder filling change the uterine fundal position?
As the bladder fills, it expands, pushing the uterus superiorly and posteriorly. In an anteverted/anteflexed uterus, this might cause the fundus to appear more vertical or even slightly flatten its anteflexion. A full bladder also serves as an acoustic window, improving visualization of pelvic structures.
What is the relationship of the uterus when compared to the bladder?
The uterus is typically located posterior and superior to the bladder. The anterior cul-de-sac (vesicouterine pouch) is the potential space between these two organs.
Which vessel branches from the uterine artery to course around the periphery of the uterus?
The arcuate artery branches from the uterine artery and courses around the periphery of the uterus.
Summary of Key Points
This lecture covered the essential normal anatomy and physiology of the female pelvis, including:
Identification of major landmarks of the bony pelvis and external genitalia.
Sonographically significant muscles of the pelvic cavity.
Detailed descriptions of pelvic organs (uterus, ovaries, fallopian tubes) and their functions.
Major ligaments that support the uterus and ovaries.
The developmental process and passage of the ovum from the ovary into the uterus.
The definition and function of the corpus luteum in hormone production.