Fetal and placental physiology Flashcards
What does fetal growth depend on
Adequate transfer of nutrients and O2 across the placenta
What does placental development depend on
Adequate maternal nutrition
Uterine perfusion
What is the role of fetal hormones in fetal development
Affect metabolic rate, growth of tissues + maturation of organs
IGFs increase growth in late gestation
Insulin + thryoxine ensure normal growth in late gestation
Fetal hyperinsulinaemia results in macrosomia due to excessive fat deposition
What is the main determinant of fetal growth
Fetal genome
IGFs are important mediator
How does the placenta:fetus ratio change with pregnancy
Placenta grows steadily
Fetus grows faster
Ratio falls
How does the site of red cell production change in the fetus
Begins in yolk sac, then liver, then bone marrow
What is the normal fetal heart rate and blood pressure
120-140bpm
BP 66/35
What are the effects of deficient thyroid hormone production
Deficiency in skeletal and cerebral maturation
Delayed surfactant production
What is cortisol essential for
Lung compliance + surfactant release
Fetal liver- induces beta receptor + glycogen deposition
Gut- villus proliferation, induction of digestive enzymes
What physiological characteristics can be used to predict fetal growth potential
Pre-pregnancy weight, maternal booking weight Maternal + paternal height Maternal age + parity Ethnic group Fetal sex
How does fetal circulation differ to adult
Oxygenation in placenta instead of lung
Right and left ventricles in parallel rather than series
Heart, brain and upper body receive blood from left and right ventricles
What modifications in fetal vascularity ensure best oxygenated blood from placenta is delivered to fetal brain
Ductus venosus shunts blood away from liver
Foramen ovale shunts blood from right to left atrium
Ductus arteriosus shunts blood from pulmonary artery to aorta
Describe fetal circulation
Oxygenated blood from placenta passes through umbilical vein, divided into 2 branches
One to portal vein in liver (for glucose conversion to glycogen)
Ductus venosus joins inferior vena as it enters right atrium
50% blood each way.
Ductus venosus stream passes through foramen ovale to left atrium, then to left ventricle to aorta
50% left ventricle blood to head, remainder to aorta
Minimal amount to lungs- non functional
How does ductus venosus prevent mixing of well oxygenated blood with desaturated blood
Narrow vessel, high velocity
Streaming of blood + membranous valve in right atrium
What changes to circulation occur at birth
Prostaglandin + prostacyclin drop, ductus arteriosus closes within few days
No umbilical flow so ductus venosus closes
Fall in right atrium pressure closes forament ovale
Ventilation of lungs opens pulmonary circulation, rapid fall in pulmonary vascular resistance
Describe peristent fetal circulation
Delayes closure of ductus arteriosus as pulmonary vascular resistance does not fall
Left to right shunting from aorta to lung
Baby cyanosed, hypoxia
Mostly in prematurity
Reduction in blood flow to gastrointestinal tract, necrotising enterocolitis, invtraventricular haemorrhage
At what gestation does the respiratory system develop
Full differentiation of capillary + canicular elements of lung by 20 weeks
Alveoli development after 24 weeks
What is the role of surfactant
Group of phospholipids that prevent collapse of small alveoli during expiration by lowering surface tension
Produced by type 2 alveolar cells (10% lung parenchyma)
What is the predominant phospholipid in surfactant
Phosphatidylcholine
Production enhanced by cortisol
What can cause progressive respiratory failure
Oligohydramnios and reduced intrathoracic space
Chest wall deformities
Cause pulmonary hypoplasia
What is respiratory distress syndrome
Condition of premature babies associated with surfactant deficiency
Complicated by hypoxia, intraventriculat haemorrhage, necrotising enterocolitis
Incidence/severity reduced by administration of steroids
Describe fetal blood production
Begins on surface of yolk sac 14-19 days after conception until 3rd month
Begins in liver at 5th week of embryonic life
Bone marrow production by 7-8 weeks, predominates by 26 weeks
Describe fetal haemoglobin
2 alpha 2 gamma chains compared to HbA which has 2 alpha 2 delta
80:20 HbF:HbA at term
What are abnormalities of blood production
Beta thalassemia- severe anaemia, FGR, poor muculoskeletal development, skin pigmentation
Alpha thalassaemia- severe anaemia with cardiac failure, hepatosplenomegaly + oedema, stillborn or early death
When does the fetus produce lymphocytes
From 8 weeks
What immune elements are present by the second trimester
All phagocytic cells, T + B cells, complement
Does the fetus produce immunoglobulins
Yes- small amount IgM + IgA
IgG from mother
Presence of IgM and IgA without IgG indicates infection
What are the fetal immunological defences
Amniotic fluid- lysosyme + IgG
Placenta- lymphoid cells, phagocytes, barrier
Granulocytes from liver + bone marrow
Interferon from lymphocytes
What are the problems of prematurity with skin + homeostasis
Preterm have no vernix + thin skin, large amount of water loss
Preterm babies deficient for brown fat which can produce heat
What are 2 methods for heat conservation in newborn
Peripheral vasoconstriction
Brown fat catabolism
How does the alimentary system develop
Midgut becomes herniated during 6th week as abdominal cavity is too small
Re-enters abdominal vavity by 12th week after rotation
What happens if the midgut fails to re-enter the abdominal cavity
Gastroshisis- can be delivered normally at full term, gut can be returned and stitched
Omphalocoele- more serious, indicative of chromosomal abnormality, associated with other problems
What is anencephaly
Failure of the cranial bones to form
Brain not protected so broken down over time
No functioning brain, not compatible with life
Describe liver development
Appears 18th day- diverticulum from duodenum
T shaped outgrowth by 25th day, invaded by blood vessels
How is the fetal liver different to adult
Reduced ability to conjugate bilirubin due to relative deficiencies in enzymes eg. glucuronyl transferase
Describe kidney development
Nephrogensis complete by 36 weeks
Maturation + concentrating ability gradual
What are the fetal behavioural states
1F- quiet sleep, absence of eye + body movements
2F- REM sleep, periodic eye + body movements
3F- quiet wakefulness, eye but no body movements
4F- active ongoing eye + body movements
Describe amniotic fluid development
By 12 weeks amnion contacts inner surface of chorion, obliterates extraembryonic space
Initially secreted by amnion
Volume increases progressively
What is the function of amniotic fluid
Protects fetus from mechnical injury
Permits fetal movements, prevents limb contracture
Prevents adhesion between fetus and amnion
Permits fetal lung development- absence leads to pulmonary hypoplasia
What are the main functions of the placenta
Protection, nutrition, respiration, excretion, hormone production
How is the placenta composed
Fetal component derived from chorion
Maternal component derived from mofidications of uterine endometrium
What does the placenta transport
Gases, water, minerals + vitamins, glucose + amino acids, proteins, lipids, large peptide molecules (insulin), smaller hormones (steroids), toxic substrates, bacteria + viruses
Describe water + nutrient exchange
Water exchange at placenta + non-placental chorion
Simple diffusion due to hydrostatic pressure difference
Describe electrolyte exchange
Fetus pumps Na+ into mother making fetus electronegative, creates gradient to drive other exchanges
Potassium- simple diffusion
Chloride- active transport
Iodide- active trapping
Describe waste product exchange
Unconjugated bilirubin passes to mother for excretion
Gut + urinary tract opens into amniotic fluid
Describe iron transport
Active transport
Maternal intestinal absorption enhanced
What is the additional requirement of iron in pregnancy
550mg- 300 fetus, 50 placenta, 200 postpartum blood loss
Describe protein + amino acid transport
Maternal metabolism more efficient due to progesterone
Active transport
Fetal urea diffuses back into mother
Describe calcium transport
Actively transported for bone formation
What are the fetal energy substrates
Predominantly carbohydrate- 1/2 glucose, remainder amino acids + lactate
How does the fetus obtain glucose
Progesterone stimulates maternal apetite
Maternal metabolism switched via GH-like actions, mobilises fatty acid stores, maternal tissues less sensitive to insulin
Rise in maternal glucose fetus can capture
Transport by facilitated diffusion
What causes the large gradients for O2 and CO2
Placental O2 consumption
Long diffusion path
Pattern of blood flow
How does pH affect O2 transport
Fall in pH of maternal blood through placenta causes release of O2
Rise in pH of fetal blood through placenta causes uptake of O2
Why does HbF have higher oxygen affinity
Makes less 2,3-DPG, fewer binding sites- 2,3-DPG reduces oxygen binding
What are the functions of placental hormones
Stimulate ovarian function Maintain pregnancy Influence fetal growth Stimulate mammary function Assist in parturition
What hormones does the placenta produce
Progesterone
Oestrogens (E3, Oestriol)
Placental lactogen/Somatomammotropin
Relaxin
What is the role of placental progesterone
Stimulus for elevated secretion by endometrial glands
inhibits myometrial contraction
What is the role of placental oestrogen
Important during last part of gestation, preparturient period
What is the role of placental lactogen
Growth of fetus
Stimulates mammary gland
What is the role of placental relaxin
Softens the connective tissue in the cervix
Promotes elasticity of pelvic ligaments
Describe the fetal physiology of parturition
Limited space for fetus alerts fetal hypothalamus which produces ACTH which stimulates cortisol, which stimulates placental oestrogen, which stimulates uterine PGF2a + progesterone which induces parturition
