Placental&fetal physiology

Question 1

Describe when plugging of the spiral arteries takes place.

Answer 1

• During early pregnancy the volume of endovascular trophoblast is such that it plugs the mouths of the spiral arteries, preventing maternal blood flow into the placenta
• The plugging coincides with the period of histiotrophic nutrition
• up to 10-12 weeks, the trophoblast cells stop maternal blood coming in through the maternal circulation& surrounding the placenta

Question 2

Which hormone is the basis of the pregnancy test

Answer 2

hCG

Question 3

What do elevated levels of oxidative stress in the peripheral regions of normal pregnancies cause?

Answer 3

villous regression and formation of the smooth chorion laeve

Question 4

What are the functions of the placenta?

Answer 4

• respiratory organ
• nutrient transfer
• excretion of fetal waste products
• hormone synthesis

Question 5

Is the fetal umbilical vein deoxygenated?

Answer 5

no. It is the only venous circulation in man that is oxygenated

Question 6

How are amino acids transported across the placenta

Answer 6

• System A: sodium dependent; transports small non-essential AAs e.g alanine, glycine and serine; 3 isoforms= SNAT1,SNAT2 &SNAT4. SNAT1 is a major contributor to system A activity; regulated by cytokines and hormones (insulin, IL-6, leptin & TNF alpha)
• System L: sodium independent, non-essential AAs exchanged for essential AAs e.g leucine, phenylalanine-enabling transport against conc. gradient.Regulated by mTOR nutrient sensing pathway
• Taurine transporter- transport taurine against conc., gradient- co transport with sodium& chloride.

Question 7

Outline FA transport in the placenta

Answer 7

• Lipoprotein lipase releases NEFAs from the triglycerides in lipoproteins
• NEFAs are then transported across the trophoblast cells by fatty acids transport proteins( FATP)

Question 8

When does the fetus start swallowing amniotic fluid?

Answer 8

-From 12 weeks gestation

Question 9

Outline the development of the fetal alimentary track

Answer 9

• Intestinal villi formed by 16 weeks & well developed by 19weeks gestation
• Gut development important for amniotic fluid homeostatis
• Gastrin, motilin& somatostatin regulate growth and development- present in gut by 13 weeks, maturity by 24weeks
• Digestive enzymes e.g disaccharides present by 9-10 weeks, maturity at term

Question 10

Describe glucose homeostasis in the fetus

Answer 10

• Fetus is dependent on placental transfer of glucose from mother; fetus has little capacity for gluconeogenesis; necessary enzymes do not function at ambient low pO2
• Fetus syntesises insulin from 9-11 weeks. Not derived from mother. Fetal insulin determines glucose metabolism. Excess glucose leads to excess growth & fat deposition. Inadequate glucose leads to emaciation

Question 11

Describe macrosomia & the complications associated with it

Answer 11

• Macrosomnia is being overweight at birth
• associated with maternal obesity& diabetes
• Increased risk of still
• C-section often needed

Question 12

Describe fetal fluid homeostasis

Answer 12

• Fluid& electrolyte balance maintained primarily by placenta, but also fetal membranes
• Fetal urine important component of amniotic fluid (0.5l/day)

Question 13

Outline Fetal urine production

Answer 13

• 3% CO goes to the kidney
• Fetal GFR is 50% of that of an adult
• Urine: fetal bladder fills & empties every 20-30 mins
• 10-15ml/kg/h
• 500-700ml/day at term
• hypotonic due to immature ADH

Question 14

How much amniotic fluid exchange occurs in the fetus within 24hours

Answer 14

500ml, most swallow membranes

Question 15

Define oligohydramnios

Answer 15

too little amniotic fluid

Question 16

Define polyhydramnios

Answer 16

Too much amniotic fluid

Question 17

What are the contents of the amniotic fluid?

Answer 17

• Urine
• Amniotic membrane secretions
• Salivary secretions
• Fetal epithelial cells,amniotic cells, dermal fibroblasts

Question 18

Outline control of fetal HR

Answer 18

• complex
• subject to modulating influences such as catecholamines, chemoreceptors& baroreceptors
• These influences generally act on FHR via the autonomic nervous system
• Parasympathetic tone dominates (vagal)

Question 19

Describe the fetal circulatory response to hypoxia

Answer 19

• HR falls
• Resistance in the umbilical artery increases
• Resistance in the middle cerebral artery decreases thus protecting flow to the fetal brain
• Blood flow increased to heart and adrenals
• Blood flow reduced to kidneys producing oligohydramnios

Question 20

What is responsible for the differences between the pre- and postnatal circulations?

Answer 20

-Presence of the placental circulation
-Lack of circulation to the lungs
Adaptations which allow this are:
-Umbilical vein & artery
-Ductus venosus
-Foramen ovale
-Ducturs arteriosus

Question 21

Describe the changes that occur at delivery

Answer 21

• Cord occlusion decreases right atrial pressure so foramen ovale closes
• Inspiration causes vasodilation of pulmonary artery & decreased resistance in pulmonary circulation reducing flow through foramen ovale & ductus arteriosus
• Increased arterial po2, leads to closure of ductus arteriosus
• Prostaglandin E2, and prostacyclin delay duct closure
• NSAIDs accelerate duct closure; may be used therapeutically after birth
• Pulmonary vascular resistance drops 8-fold partly due to increased arterial po2
• Gas exchange commences
• Liquid secretion stops& liquid cleared
• surfactant secretion continues

Question 22

Which cells secrete surfactant?

Answer 22

Type-II alveolar epithelial cells

Question 23

What is the composition of pulmonary surfactant?

Answer 23

• 70-80% phospholipids
• 10% protein
• 10% cholesterol

Question 24

What is the function of pulmonary surfactant?

Answer 24

• Form a lattice-like structure
• Decrease surface tension
• Stabilise the lung
• Secreted from 30 weeks

Question 25

Outline the link between surfactant and surface tension

Answer 25

Surface tension:
-is the collapsing pressure exerted upon the alveoli
-LaPlace’s law P=2T/r, where P is the collapsing pressure, T is the surface tension and r is the radius of the alveolus
Surfactant:
-decreases surface tension
-Prevents alveoli from collapsing in
-Increases compliance

Question 26

Explain surfactant deficiency& the complications it may bring

Answer 26

• Premature infants may be born with surfactant deficient lungs may cause e.g:
  1. ) Neonatal respiratory distress syndrome
  2. ) Increased work of breathing, decreased lung compliance, alveolar collapse

Question 27

How can surfactant deficiency be overcome?

Answer 27

Exogenous surfactant:

• synthetic
• Modified natural surfactant( bovine or porcine)

Question 28

What is the role of cortisol in pregnancy

Answer 28

-Evidence that cortisol plays a role in maturation
-There’s a LATE pregnancy rise in cortisol
Cortisol:
-stimulates surfactant synthesis& secretion
-Epithelial cell differentiation
-Lung liquid reabsorption
-Increases activity of anti-oxidants

Question 29

Outline fetal haemoglobin

Answer 29

• Predominantly HbF
• Gradual switch to HbA starts from 28weeks with HbF: HbA 80:20 at birth
• HbF has higher sensitivity to DPG( 2,3 diphosphoglyceric acid)
• Partial pressure of o2 in fetal circulation is low (30mmHg) but this is compensated for by high Hb conc. and a greater o2 affinity

Question 30

What is the role of lung liquid clearance at delivery

Answer 30

-The physical force during labour; physically forces liquid from the lungs- so there is a faster clearance of lung fluid with those vaginally delivered
-Activation by ENaCs:
reversal of osmotic gradient
adrenaline and vasopressin
-Transpulmonary hydrostatic pressure gradient:
pressure difference between the lung interstitial tissue& alveoli

Question 31

What stimuli can be used for the first breath?

Answer 31

• Asphyxia of normal breath
• Physical manipulation& compression
• Cold shock
• Visual stimulus
• Gravity