Fetal Physiology

Consists of :                            

Fetal blood

Pulmonary system

Immune system

Skin 

Nervous system

Gastro-intestinal  system

Urinary system

Endocrine glands

Cardiovascular

Fetal blood

Haematopoesis:

1st fetal blood cells  : Day 14 embryo : in yolk sac

Next, major site is liver: 6th wk embryo  : (hence, enlarged liver) & spleen (lesser extent)

Finally, bone marrow starts to produce RBC:16 wks

By 26 wks :  bone marrow is the predominant source of RBC

As the fetus grows, volume of blood in fetoplacental circulation increases.

       At term, 125 ml/kg fetal wt.

Hb in the fetal blood also rises.

       At term, Hb  18 g/dl. Why ? Enhances    the transfer of O2 across placenta.

Fetal RBC has shorter lifespan.

       At term, 90 days.

Hb in fetus:

Mostly fetal haemoglobin (HbF):    α2 γ2  instead of adult Hb A (α2 β2 ) and Hb A2 (α2 δ2 )

   

     10 – 28 wks    90 % of fetal Hb is HbF

     28 – 34 wks    HbF: HbA= 80:20

     6 mths age     HbF = 1 %

HbF vs HbA

     HbF has greater O2 binding capacity and it is resistant to denaturation by acid/alkali.

Fetal Haemoglobin

HbF  has higher affinity for O2 than adult in vivo due to a lower sensitivity to DPG (2,3 diphosphoglyceric acid)

Partial pressure of O2 in fetal circulation is low (92.7KPa) but this is compensated for by high Hb concentration and greater oxygen affinity

CVS

Differences (fetal vs adult circulation):

Major portion of Rt ventricular output bypasses the lungs coz fetal blood does not need to enter pulmonary vasculature

Oxygenation occurs in the placenta

Right & Left ventricles work in parallel ,rather than in series

Heart, brain & upper body receive blood from left ventricle ; placenta & lower body receive blood from both right & left ventricles

Presence of shunts:

         1. Ductus Venosus

         2. Foramen ovale

         3. ductus arteriosus

Fetal Circulation

Fetal blood pathway

Placenta  ----   umbilical vein 

----

      ductus      venosus  

----

        IVC 

----

      right atrium  

----

      foramen ovale  

----

      left atrium  

----

       left      ventricle   

----

       ascending aorta

•    coronary/cerebral arteries       SVC      right atrium       right ventricle       pulmonary artery        ductus arteriosus       descending aorta        umbilical artery      placenta

After birth, constriction/collapse of:

 umbilical arteries: instantaneous functional closure / actual obliteration takes 2-3 mths. Distal part – form lateral umbilical ligament. Proximal part – remain as superior vesical arteries

Umbilical veins: closure a little later than arteries. Forms ligamentum teres.

ductus arteriosus: functional closure soon after pulmonary circn is established. Anatomical obliteration takes 1-3 mths. Forms ligamentum arteriosum.

foramen ovale: functional closure soon after birth, anatomical closure in 1 yr.

ductus venosum: forms ligamentum venosum

Incidence of closure of fetal channels

Fetal circulatory response to hypoxia

• Heart rate 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 (reduced volume of amniotic fluid)

Control of Fetal Heart Rate

Control of FHR is complex.

It is subject to modulating influences such as catecholamines and baroreceptors.

These influences generally act on FHR via the autonomic nervous system.

Parasympathetic tone dominates

Fetal Heart rates (FHR):

-20 weeks 155/minute

-30 weeks 144minute

-Term 140/min

FHR decreases in response to hypoxia

Fetal  Lung and Breathing

Development consist of

Pseudoglandular stage: 5-16 w

Canalicular stage: 16-25 w

Terminal sac stage: type II cells begin to produce surfactant 

At birth only 15% of adult alveoli are present in lungs       continue to grow up to 8 years

20 wks: full differentiation of capillary & canalicular elements of fetal lung

24 wks: alveoli develop, surfactant appear.

   Lung alveoli lined by surfactant (a group of phospholipids). 

   Prevents collapse of small alveoli during expiration by lowering surface tension

   Synthesized by type 2 alveolar cells ( 10 % of lung parenchyma)

   Lecithin ( phosphatidyl choline): main (80%)

Increased lecithin production :

Cortisol

Growth restriction

Prolonged rupture of membranes

Delayed lecithin formation in:

diabetes 

Phosphatidyl glycerol is more predictive of RDS esp in diabetic fetus.

Surfactant:

Composition: 90% lipids, 10% proteins

Phosphatidylcholine (lecithins):

                  DPPC: active component(50%)

                  PG: second(8-15%)

At birth,with first breath, an air to tissue interface is produced in the alveolus. Surfactant spreads to line alveolus to prevent collapse during expiration

Glucocorticoids and Surfactant :

Accelerate type 2 alveolar cell development

Accelerate surfactant synthesis

Accelerate lung structural maturation

Glucocorticoids play essential role in maturation of many fetal organs but excess amounts reduce fetal growth

Fetal breathing: 

Numerous but intermittent fetal breathing

   movements occur in utero especially during 

   sleep ( lung maturation).

Immune system

8 wks:  lymphocytes appear

Mid 2nd trimester: all phagocytic cells, T & B cells, complement

 Fetus consists almost totally of maternal IgG (transferred across placenta).

           16 wks: Maternal IgG transfer begins & increases

            Last 4 wks: bulk of IgG acquired.

            Hence, preterm :  less IgG

            Newborns produce IgG slowly & adult values are   reached only after 3 yrs.

IgM & IgA: very little is produced by fetus.

           not transferred across placenta.

          

General immunological defences:

1.Amniotic fluid (lysosomes, IgG)

2.Placenta (lymphoid cells, phagocytes, barrier)

3.Liver ( granulocytes)

Skin

16 wks: lanugo appears but disappears near term

1 month gestation till birth: skin thickness progressively increases

20 wks:  stratum corneum  distinct

Last wks: skin covered by vernix caseosa

                   desquamated skin cells

                   rich in cholesterol and glycogen

Preterm babies

• No vernix & thin skin ie, larger insensible loss

• Deficient brown fat

• Delayed development of sweat glands

Alimentary system & energy stores

• 10 wks: Swallowing reflex develops & gradually matures. Continuously and increasingly swallows amniotic fluid

• 2nd trimester:  Peristalsis in intestines

• Term : Large bowel is filled with meconium

    

• But defaecation in utero (meconium in amniotic fluid) unusual unless fetal anoxia

Intestinal villi well developed by 19weeks

• Gut development important for amniotic fluid

    homeostasis; (fetus swallows amniotic fluid

    from 12 weeks and amniotic fluid contains

    hormones and growth factors that stimulate

    gut development)

• Gastrin, motilin and somatostatin regulate

    growth and development      present in gut by 13

    weeks         matures by 24 weeks

• Digestive enzymes eg disaccharidases

    present by 9-10 weeks maturity at term

Meconium: 

Lanugo, vernix, scalp hair, epithelial cells from skin, mucus, exfoliated intestinal epithelium, intestinal juices

    

Liver:

Reduced capacity for conjugation of bilirubin esp. in preterm

Preterm infants:

Virtually no fat, so severely reduced ability to withstand starvation

Incompletely developed alimentary system

Poor and unsustained sucking ability

Uncoordinated swallowing mechanism

Delayed gastric emptying

Poor absorption of carbohydrates/fat/other nutrients

Kidney & urinary tract

• Urine usu found in UB of even small fetuses

• Fetal urine  gives rise to much of amniotic fluid. 

   (protein-free & sugar-free hypotonic ultrafiltrate of fetal plasma)

       12 wks:  urine production starts

       32 wks:  12 ml per hour

       40 wks:   38 ml per hour 

• 36 wks: nephrogenesis complete

   But maturation of excretory & concentrating ability of fetal kidneys is gradual

   Preterm: immature, so abnormal

Amniotic fluid

Source:

• Early pregnancy: ultrafiltrate of maternal plasma
• 10 wks: transudate of fetal serum via skin & umbilical cord
•  After 20 wks: mainly fetal urine ( as cornification of fetal skin, impermeable to water).

    Pulmonary fluid and fluid filtering through  placenta contribute little.

• AF volume:

        Varies at each week of gestation. 

        Volume increases with increasing    gestation.

        10 wks: 30 ml

        20 wks: 300 ml

        30 wks: 600 ml

        38 wks: 1000 ml

        40 wks:  800 ml

        42 wks:  350 ml

Functions of AF:

• Cushions fetus: Protect fetus from mechanical injury
• Permits movements of fetus while preventing limb contracture
• Prevents adhesions between fetus & amnion
• Maintains temperature
• Minimal nutritive function
• Permit fetal lung development--  2 way movements of fluid into fetal bronchioles
•    Absence of AF in 2nd trimester:  pulmonary hypoplasia
• Promotes growth & differentiation of GI tract 

• Renal agenesis, cystic kidney, IUGR ----oligohydramnios----reduced contribution of fluid into amniotic sac ----- major alterations in AF volume.

• Anencephaly, oesophageal atresia, duodenal atresia ----- polyhydramnios----- reduced removal of fluid

Fetal behaviour

• 18 wk: first fetal movements (quickening) perceived in primipara

    Several wks earlier in multipara

• Formal counting of fetal movements (screening)

Reduced or Absent in 

• Chronic hypoxia & growth failure
• fetal death

• Spinal cord extends along entire length of vertebral column in embryo, but after that, it grows more slowly.

    By 24 wks:  spinal cord extends to S1

    At birth:  L3

    In adults:  L1

• Myelination of spinal cord:  starts mid-preg
• 10 wks: swallowing
• 14-16 wks: respiration evident
• After 24 wks: ability to suck
• 24-25 wks: can hear some sounds in utero
• 28 wks: sensitive to light

Fetal endocrinology

• 10 wks: GH, ACTH, PRL , TSH produced by fetal pituitary
• 1st trimester: Vasopressin, oxytocin from posterior pituitary
• Fetal adrenal: hypertrophy of reticular zone (site of synthesis of DHEA-dehydroepiandrosterone). This zone is absent in anencephaly. Adrenal medulla produces catecholamines.
• 11 wks: Fetal thyroid produces small amount thyroxine

• Fetal ovaries remain inactive

• Fetal testicles mediate development of male reproductive structures

• 12 wks: fetal pancreas secrete insulin