Placental&fetal physiology Flashcards
Describe when plugging of the spiral arteries takes place.
- 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
Which hormone is the basis of the pregnancy test
hCG
What do elevated levels of oxidative stress in the peripheral regions of normal pregnancies cause?
villous regression and formation of the smooth chorion laeve
What are the functions of the placenta?
- respiratory organ
- nutrient transfer
- excretion of fetal waste products
- hormone synthesis
Is the fetal umbilical vein deoxygenated?
no. It is the only venous circulation in man that is oxygenated
How are amino acids transported across the placenta
- 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.
Outline FA transport in the placenta
- Lipoprotein lipase releases NEFAs from the triglycerides in lipoproteins
- NEFAs are then transported across the trophoblast cells by fatty acids transport proteins( FATP)
When does the fetus start swallowing amniotic fluid?
-From 12 weeks gestation
Outline the development of the fetal alimentary track
- 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
Describe glucose homeostasis in the fetus
- 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
Describe macrosomia & the complications associated with it
- Macrosomnia is being overweight at birth
- associated with maternal obesity& diabetes
- Increased risk of still
- C-section often needed
Describe fetal fluid homeostasis
- Fluid& electrolyte balance maintained primarily by placenta, but also fetal membranes
- Fetal urine important component of amniotic fluid (0.5l/day)
Outline Fetal urine production
- 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
How much amniotic fluid exchange occurs in the fetus within 24hours
500ml, most swallow membranes
Define oligohydramnios
too little amniotic fluid
Define polyhydramnios
Too much amniotic fluid
What are the contents of the amniotic fluid?
- Urine
- Amniotic membrane secretions
- Salivary secretions
- Fetal epithelial cells,amniotic cells, dermal fibroblasts
Outline control of fetal HR
- 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)
Describe the fetal circulatory response to hypoxia
- 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
What is responsible for the differences between the pre- and postnatal circulations?
-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
Describe the changes that occur at delivery
- 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
Which cells secrete surfactant?
Type-II alveolar epithelial cells
What is the composition of pulmonary surfactant?
- 70-80% phospholipids
- 10% protein
- 10% cholesterol
What is the function of pulmonary surfactant?
- Form a lattice-like structure
- Decrease surface tension
- Stabilise the lung
- Secreted from 30 weeks
Outline the link between surfactant and surface tension
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
Explain surfactant deficiency& the complications it may bring
- 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
How can surfactant deficiency be overcome?
Exogenous surfactant:
- synthetic
- Modified natural surfactant( bovine or porcine)
What is the role of cortisol in pregnancy
-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
Outline fetal haemoglobin
- 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
What is the role of lung liquid clearance at delivery
-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
What stimuli can be used for the first breath?
- Asphyxia of normal breath
- Physical manipulation& compression
- Cold shock
- Visual stimulus
- Gravity
