MPP II 4.5 - REPRODUCTIVE PHYS. (PREGNANCY AND LABOR)

Adaptations for Pregnancy

• The respiratory, cardiovascular, and renal systems adapt to the physiological demands
• The uterus and cervix undergo morphological changes
• Basic steps of labor, and identify the roles of the main hormones
• Functions of hormones during gestation compared to during labor
• Central fetal adaptations at birth and compare them to their functions during gestation
• An APGAR score can be determined for newborns

Successful Pregnancy Factors

• The embryo must implant in the uterine endometrium after fertilization
• An interface must form between maternal and fetal circulations
• The body adapts to meet the needs of the growing fetus, involving:
  • Uterine blood flow
  • Cardiac function
  • Respiratory function
  • Renal function
• At birth, the link between mother and fetus must be broken, allowing both to survive and thrive

Uterine Blood Flow

• The main source of uterine vascular resistance comes from the spiral arteries
  • These resistance vessels contract and relax to modulate blood flow as uterine metabolic changes
• The developing fetal placenta erodes and invades the spiral arteries
  • Arterial walls are remodeled, replacing smooth muscle with fibrous material, creating wide vessels with very high flow rates
• CV consequences for the mother include increased flow and inability to control flow through these vessels

Cardio Changes During Pregnancy

• Cardiac output and blood volume increases
• Full-term sees an approximate 40-50% increase
• The increase is needed to supply the maternal placenta and other organs, including skin, kidneys, liver, and the GI tract

Symptoms of Pre-Eclampsia

• High blood pressure, over 140 systolic or 90 diastolic
• Swelling of hands and feet
• Protein in urine

Renal Changes During Pregnancy

• GFR increases by 50%
  • Increase caused by relaxin and extracellular fluid volume
• Relaxin is secreted by the corpus luteum
  • Acts as a vasodilator
  • Relaxes ligaments
• There is a 50% increase in renal reabsorptive capacity for sodium, chloride, and water

Respiratory Changes During Pregnancy

• The O₂ demands of the mother and growing fetus increase rapidly
• O₂ consumption at term increases by ~30% due to increased tidal volume
  • There is a small rise in respiratory rate of 2-3 breaths/min
• There is a 20% decrease in functional reserve capacity (FRC), expiratory reserve capacity (ERC), and residual volume (RV)

Reproductive Tract Changes

• Uterus undergoes these changes
  • Weight and volume Increases significantly
  • Fibrous connective tissue and blood vessel Increase
• Cervix undergoes four main stages
  • Softening
  • Ripening
  • Dilation
  • Postpartum repair

Hormonal Control of Labor

• Labor is regulated by a series of paracrine and autocrine hormones
• These hormones act in a specific cascade during parturition
• During gestation, progesterone maintains uterine quiescence
• Initiating labor does not rely on any single signaling mechanism

Hormonal Events Prior to Labor - CAPS

• Hormonal events lead up to labor and involve Contraction-Associated Proteins (CAPs)
• There is increased prostaglandin synthesis and release from the uterus
• Increased gap junction formation
• Upregulation of oxytocin receptors
• The body switches from irregular contractions to regular contractions

Hormonal Events During Labor

• The fetus, during labor, experiences the activation of the HPA Axis
  • CRH increases
  • POMC Increases
  • ACTH receptor increases
  • Adrenal steroidogenic enzymes increase
• SROM occurs
  • Spontaneous rupture of membranes

Labor Basics

• Ratio of Estrogen/Progesterone is a key hormonal control factor during labor
• During pregnancy, Progesterone suppresses uterine contraction
• Estrogens promote uterine excitability by increasing expression of:
  • Na+ channels
  • Ca++ channels
  • Gap junctions
• During parturition:
  • Ratio of E/P increases greatly
  • Uterus begins contracting regularly

Labor Hormones - Prostaglandins

• Hormones driving labor include prostaglandins
  • The hormones are produced by the fetus, placenta, and uterus
  • Hormones stimulate uterine contractions
  • Increased estrogen levels promote increase in prostaglandins

Labor Hormones - Oxytocin

• Oxytocin is a hormone involved in labor
  • It is a potent uterine stimulant
  • Released from the posterior pituitary in response to cervical distension

Labor Hormones - Cortisol

• Cortisol is a driving hormone during labor
  • Released by the fetus
  • Aids in increasing the Estrogen/Progesterone ratio

Phases of Labor

• Labor occurs in distinct stages
  • Dilation
  • Fetal expulsion
  • Placental expulsion

Labor Stages - Dilation

• Cervix progresses to fully dilated state
• Contractions initially begin rhythmically then increase in intensity The stage typically lasts 8-15 hours

Labor Stages - Fetal Expulsion

• Stage typically lasts 45-100 minutes
• Umbilical cord is clamped after birth
  • Sometimes delayed clamping may occur
  • Cord "milking" also takes place

Labor Stages - Placental

• It is the final stage, when the placenta is expelled
• The expulsion occurs minutes after infant delivery
• The uterus continues to contract and shrink
• The placenta is sheared from the uterine wall

Fetal Adaptations at Birth

• Critical adaptations are required by the newborn
• Functions once carried out by the placenta must be taken over
• This concerns several organ/system functions
  • Gas exchange through lungs
  • Nutrition through GI tract
  • Waste Removal and Nutrition via Liver
  • Waste Removal, Fluid & Electrolyte balance using kidneys

Fetal Shunts

• Three shunts limit blood flow to organs underutilized during gestation
  • Ductus Venosus, which bypasses the liver
  • Foramen Ovale, where blood entering the RA crosses to enter the LA through a special 'Right-to-Left Shunt'
  • Ductus Arteriosus, where blood is directed from the pulmonary artery to the Aorta

Key Cardiopulmonary Adjustments

• SVR increase
• Lung inflation
• PVR decrease
• Closure of the three shunts
• A shift from a "Right-to-Left sided" circulatory dominance occurs

Cardiopulmonary Adjustments - SVR

• Increased fetal SVR if umbilical cord is severed

Cardiopulmonary Adjustments - Lung Inflation

• Umbilical cord removal causes asphyxia
• Sudden body temp cooling occurs at birth
  • This triggers the respiratory control center of the brainstem
• Intraalveolar pressure drops below atmospheric pressure, creating a pressure gradient for air

PVR Decreases - Lungs

• Lungs typically collapsed during development, there for pulmonary arteries are compressed
• Air causes alveolar and pulmonary arterial PO₂ levels to rise significantly due to vasodilation
• Lung inflation stretches the pulmonary vessels and increasing their internal diameter, also thinning walls
• PVR drops dramatically, there is a coincident increase in pulmonary blood flow

Cardiopulmonary Adjustments - Shunts Close

• Ductus Arteriosus reverses blood flow, taking months to completely close
• Foramen Ovale experiences right-left pressure inversion, and increasing LA pressure hold flap closed
• Interatrial septum fuses
• Ductus Venosus closes by a sphincter-like mechanism
  • Mechanism on how is currently unknown

Fetal Circulation

• Shift from "Right-to-Left sided" circulation occurs
• LV slowly hypertrophies to an increase in SVR
• Right heart pumps against lower PVR
  • Muscle mass slowly decreases relative to the left heart.

Apgar Scores

• Test performed at 1 and 5 minutes after birth
• Scale is from 0-10
  • Severely depressed gives a score of: 0-3
  • Moderately depressed scores: 4-6
  • Excellent condition scores: 7-10
• The 1 min test is an assessment of how well the baby tolerated birth
• The 5 min test is another assessment of how well the baby adapts outside