Reproductive Physiology & Fetal Development

Fetal Circulation

• Fetal circulation is similar to adult circulation but modified due to non-functional lungs.
• The pulmonary circuit is bypassed to reach the systemic circuit sooner.
• Reddish/pinkish colors indicate oxygenated blood, blue indicates carbon dioxide-rich blood; purple indicates mixed blood.
• Arteries carry blood away from the heart, veins carry it to the heart.
• One umbilical vein carries oxygenated blood from the placenta to the fetus; two umbilical arteries carry deoxygenated blood from the fetus to the placenta.
• The umbilical vein goes under the liver and connects to the inferior vena cava.
• The inferior vena cava carries deoxygenated blood, so mixing occurs when the umbilical vein joins it.
• Blood is shunted from the right to the left side of the heart to bypass the pulmonary circuit.
• Umbilical arteries originate from the internal iliac arteries.
• After birth, umbilical vessels disintegrate and become adult structures.

Placental Exchange

• Maternal blood high in oxygen and nutrients comes from uterine arteries.
• Oxygen and nutrients diffuse through the placental membrane into the umbilical vein due to concentration gradients.
• The umbilical vein has the highest oxygen concentration (~80% saturation).
• The umbilical vein passes under the liver and connects to the inferior vena cava.
• The ductus venosus is a shunt that also connects the umbilical vein to the inferior vena cava, becoming the ligamentum teres in adults (attaches liver to umbilicus).
• The inferior vena cava is positioned to direct oxygen-rich blood towards the left atrium.

Bypassing the Pulmonary Circuit

• Blood from the right side of the heart is shunted to the systemic circuit via two routes.
• Foramen ovale: A hole between the right and left atria facilitates blood flow from the inferior vena cava directly to the left atrium.
• Ductus arteriosus: Connects the pulmonary trunk to the aorta.

Blood Flow and Mixing

• Blood from the superior vena cava (deoxygenated) mixes with oxygenated blood from the inferior vena cava in the right atrium.
• This mixed blood enters the right ventricle and pulmonary trunk.
• Some blood flows to the lungs for development.
• Pulmonary veins return blood (now with more carbon dioxide) to the left atrium, mixing with oxygenated blood from the foramen ovale.
• The ductus arteriosus shunts blood from the pulmonary trunk to the aorta after the brachiocephalic, left common carotid, and left subclavian arteries to ensure oxygenated blood reaches the head.
• Mixing occurs in the inferior vena cava (umbilical vein + deoxygenated blood), right atrium (superior vena cava + inferior vena cava), right ventricle, and after the left subclavian artery (ductus arteriosus).
• The highest oxygen concentration is in the umbilical vein; the highest carbon dioxide concentration is in the vena cavae.

Umbilical Arteries

• Blood travels through the dorsal aorta to the internal iliac arteries, where the umbilical arteries originate.
• Umbilical arteries carry carbon dioxide and waste to the placenta for removal.
• The mother's system removes carbon dioxide and waste.
• Oxygen saturation decreases from the umbilical vein to the umbilical arteries due to mixing.

Fetal Development (Second and Third Trimesters)

• Primarily involves growth; the foundation is laid in the first trimester.
• The mother's body adapts to accommodate the growing fetus, with organs being displaced.
• The uterus can reach the xiphoid process, making breathing difficult.

Maternal Physiological Changes

• Increased pressure in the abdominal-pelvic cavity.
• Swelling in feet and ankles due to pressure on pelvic blood vessels (especially veins).
• Hormone relaxin loosens ligaments, including the round ligament and uterosacral ligament, and the pubic symphysis.
• Lordosis: Accentuated lumbar curve due to a shift in the center of gravity.

Labor and Delivery

• Initiation by the fetus: Adrenal glands release cortisol, stimulating the placenta to increase oxytocin receptors in the myometrium.
• Increased estrogen levels from the placenta.
• Oxytocin from the mother and fetus stimulates uterine contractions.
• Prostaglandins are released from the placenta, causing further uterine contractions and cervical dilation.
• Positive feedback loop: Uterine contractions $\rightarrow$ baby's head pushes on cervix $\rightarrow$ more oxytocin release $\rightarrow$ stronger contractions.

Stages of Labor

• Dilation phase: Cervix dilates (up to 10 cm), amniotic sac ruptures.
• Expulsion phase: Baby is pushed out of the uterus and vagina (crowning).
• Placental stage: Placenta is expelled from the uterus; maternal and fetal blood may mix.
• The placenta must be fully removed to prevent infection and hemorrhage.

Transition to Adult Circulation

• Making the baby cry helps to make sure that the breathing occurs.
• Cutting the umbilical cord and initiating breathing changes fetal circulation to adult circulation.
• Foramen ovale closes and becomes the fossa ovalis.
• Ductus arteriosus constricts.
• Ductus venosus degrades over time.

Fetal Structures and Their Adult Derivatives

• Ductus arteriosus $\rightarrow$ ligamentum arteriosum.
• Foramen ovale $\rightarrow$ fossa ovalis.
• Ductus venosus $\rightarrow$ ligamentum venosum/teres.
• Umbilical arteries $\rightarrow$ medial umbilical ligaments.
• See the image for a quick review.

Lymphatic System

• A secondary circulatory system that complements the cardiovascular system.
• Returns fluid to veins, transports lipids, and supports the immune system.
• Lymphatic system has three functions:
  • Return fluid to our veins.
  • Transport lipids.
  • Immune system.

Functions of the Lymphatic System

• Returns fluid to the veins to maintain blood volume.
• Transports lipids, including fat-soluble vitamins, via lacteals in the small intestine.

Flow Pattern

• Lymphatic capillaries $\rightarrow$ lymphatic vessels $\rightarrow$ lymph nodes $\rightarrow$ lymphatic trunks $\rightarrow$ lymphatic ducts.
• Lymphatic ducts drain into the right and left subclavian areas near the heart.

Lymphatic Capillaries

• Collect excess substances around blood capillary beds.
• Have a tunica intima but are modified to allow fluid entry but not exit.
• End points with finger-like projections.
• Epithelial cells overlap, forming mini-valves that open with increased interstitial fluid pressure.
• Mini-valves allow large fats, viruses, bacteria, and white blood cells to enter.
• Create lymph, which is similar to blood but without erythrocytes.

Lacteals

• Specialized capillaries in the small intestine that absorb chylomicrons.
• Large mini-valves facilitate chylomicron uptake.

Lymphatic Vessels

• Run parallel to veins and arteries, primarily veins.
• Similar in structure to veins (three layers, valves).
• Lymphatic vessels have lymph nodes along the pathway to filter lymph.
• Function in low pressure environments.
  Lymphatic Vessels:
• Three Layers:
  • Tunica Intima
  • Tunica Media
  • Tunica Externa
• Wider Lumen.

Lymphatic Trunks

• Lymphatic vessels merge to form lymphatic trunks that drain specific body regions.
• Named after the region they drain (e.g., intestinal, lumbar, jugular).

Lymphatic Ducts

• Trunks merge to form two lymphatic ducts: the right lymphatic duct and the thoracic duct.
• Location of Lymphatic Ducts:
  * The midline.
  * Diaphragm.
  * Cut the right side from the left side.

Right Lymphatic Duct

• Drains the right head and neck, right arm, and right thorax.
• Short and empties into the junction of the subclavian and internal jugular veins.

Thoracic Duct

• Drains the lower limbs, abdominal-pelvic area, left thorax, left arm, and left head and neck.
• Longer and originates at the cisterna chyli.
• Travels retroperitoneally along the vertebral bodies and empties into the junction of the internal jugular and subclavian veins on the left side.