Anatomy and Physiology of the Uterus and Fallopian Tubes

ℹ️ About

• The uterus is a hollow, thick-walled muscular organ situated in the true pelvis between the bladder anteriorly and rectum posteriorly.
• It communicates superiorly with the fallopian (uterine) tubes and inferiorly with the vagina via the cervix.
• Its primary roles are menstruation, implantation, fetal development, and parturition.
• It is composed of three distinct layers: perimetrium (outer), myometrium (middle), and endometrium (inner).

🔬 Uterus – Gross Anatomy

• Parts: Fundus (above tubal openings), body, isthmus, and cervix.
• Position: Normally anteverted and anteflexed (tilted forward over the bladder).
• Size (non-pregnant): ~7–8 cm long, 5 cm wide, ~40 g in weight.

🧱 Layers of the Uterine Wall

• Perimetrium: Outer serosal layer (visceral peritoneum).
• Myometrium: Thick smooth muscle layer arranged in:
  • Outer longitudinal fibres (merge with supporting ligaments)
  • Middle circular/spiral fibres (rich vascular supply; key for haemostasis postpartum)
  • Inner longitudinal/oblique fibres
• Endometrium: Inner mucosal layer with:
  • Functional layer (shed during menstruation)
  • Basal layer (regenerates functional layer)

🔄 Physiology of the Uterus

• Menstrual cycle: Oestrogen stimulates proliferative growth; progesterone induces secretory transformation.
• Implantation: Occurs in the secretory phase when endometrium is thick, vascular, and glandular.
• Labour: Coordinated myometrial contractions mediated by oxytocin and prostaglandins.
• Postpartum: Spiral muscle fibres constrict uterine vessels, reducing haemorrhage.

🤰 Changes in Pregnancy

• Weight increases from ~40 g to ~1 kg at term.
• Growth occurs primarily via hypertrophy of existing smooth muscle cells (rather than hyperplasia).
• Oestrogen promotes myometrial growth; progesterone maintains uterine quiescence.
• Stretching and hormonal effects increase uterine blood flow dramatically.

🩸 Blood Supply

• Primarily from the uterine arteries (branches of the internal iliac arteries).
• Additional supply from ovarian arteries (branches of the abdominal aorta).
• Uterine artery runs in the base of the broad ligament and crosses over the ureter (“water under the bridge”).
• Extensive anastomosis supports pregnancy and protects against ischaemia.

🧵 Cervix

• Cylindrical lower portion of the uterus (~2–3 cm long).
• Internal os (to uterine cavity) and external os (to vagina).
• Rich in collagen and connective tissue with limited smooth muscle.
• Softens in late pregnancy due to prostaglandins and collagen remodelling.

🌸 Fallopian (Uterine) Tubes – Anatomy

• Paired muscular tubes ~10–12 cm long.
• Extend from uterine cornua to the ovaries.
• Four parts:
  • Infundibulum: Funnel-shaped end with fimbriae.
  • Ampulla: Widest part; usual site of fertilisation.
  • Isthmus: Narrow medial segment.
  • Intramural (interstitial): Passes through uterine wall.

⚙️ Fallopian Tube Physiology

• Oocyte capture: Fimbriae sweep ovulated oocyte into tube.
• Fertilisation: Usually occurs in the ampulla.
• Transport: Coordinated ciliary beating and smooth muscle peristalsis move embryo toward uterus.
• Hormonal regulation: Oestrogen enhances ciliary activity; progesterone slows transport.

🩺 Clinical Correlations

• Ectopic pregnancy most commonly occurs in the ampulla.
• Pelvic inflammatory disease can damage cilia → infertility.
• Uterine fibroids arise from the myometrium.
• Cervical incompetence may cause mid-trimester loss.