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Flashcards in SPR L9 Neonatal Physiology Deck (13):

Neonatal Physiology

 Learning Outcomes for general perusal

  • To elucidate the importance of Reproduction to the human
  • To show how the fetus adapts its respiratory and circulatory systems to go from the uterine to the outside environment
  • To outline the other challenges to neonatal nutrition, temperature regulation, liver and immune functions that exist.

Concepts and Learning Objectives

For general perusal

  • Rounding off the story of reproduction is a description of how the neonate adapts its respiration and circulation to go from the uterine environment to the world outside the uterus.
  • For this to happen changes must occur in the circulation which had hitherto been serving gas exchange and supply needs through the umbilicus, but now has 2 separate circulations (pulmonary and systemic) serving these functions.
  • The respiratory adjustments after birth will be described here too.
  • Other challenges to the neonate include weight maintenance, temperature regulation, liver immaturity and the need to develop immune function.  These are met by various means.


Importance of the Topic

For general perusal

It is evident that the world inside the uterus is very different to the world outside it.  In the former, respiratory, nutrition, immune, waste transport and detoxification functions are largely carried out by the umbilical and placental circulations, and ultimately by the mother.  Once out in the big bad world, the neonate must take on all of these tasks by itself.  There are many physiologic changes occuring at birth and in the month after this  that reflect that huge change.  These can break down, and to understand and treat what happens when they go wrong, it’s important to appreciate basic neonatal physiology.


Fetal - Neonatal Circulations

The Fetal Circulation

  1. What does the foetal circulation revieve? Via what?
  2. This is circulated round the body, and then what returns it to resupplied?
  3. Why would excess circulation to the fetal liver and lungs be wasted?
    1. What happens as a result?

  1. oxygenated purified, detoxified blood via the placental (and maternal) circulation in the umbilical veins.
  2. the umbilical arteries.
  3. the function of these organs is taken by the mothers circulation (via the placenta). 
    1. For this reason the circulation to the liver and lungs is partially bypassed in the neonate with anatomic shunts.


Fetal - Neonatal Circulations

The Fetal Circulation

Describe the fetal circulation

  • oxygenated blood comes from the umbilical vein.
  • Some passes through the liver and to the inferior vena cava, but 50% of it bypasses the liver and goes straight through the ductus venosus (linking the hepatic, portal and umbilical circulations) to the inferior vena cava (IVC). 
  • The IVC also returns deoxygenated blood from the lower body, so mixed oxygenated blood goes to the right atrium. 
  • This blood flow is divided into 2 streams, but most of it goes straight across to the left atrium via the foramen ovale
    •  2 streams: proposed that a primitive anatomic structure (the crista dividens) helps divide flow thus, but debated.  
  • From the left atrium, the partially oxygenated blood flows to the left ventricle where it is pumped to supply the head of the fetus. 
  • Blood returning from the fetal head arrives at the right atrium where it goes mainly (as described above) to the right ventricle and thence the pulmonary trunk. 
  • Blood to primitive lungs would be wasted circulation (gas exchange takes place at the placenta), so the majority of this blood travels to the descending aorta via the ductus arteriosus
  • Only then (after being twice denuded of its oxygen and nutrients) does the blood return to the placenta via the umbilical arteries


Fetal - Neonatal Circulations

The Fetal Circulation

Describe the fetal circulation

ductus venosus - between umbilical vein and (linking the hepatic, portal and umbilical circulations) to the inferior vena cava (IVC). 

foramen ovale - between R Atrium and L Atrium

ductus arteriosus - pulmonary trunk to aorta


Changes at Birth

  1. Generally, what must happen at birth?
  2. What does clamping of the umbilical veins and arteries at birth cause?
  3. What do the expanding lungs and increased oxygen tensions in the newborn cause?
  4. What happens then to pressures in the R atrium, R Ventricle and Pulmonary trunk?
  5. What happens to the pressure differences between the R and L that had been useful in utero?
  6. How does the foramen ovale close?
  7. When does the ductus arteriosus close? Why?
  8. When does the ductus venosus close?
  9. What can present a problem?
    1. Why is this dangerous?

  1. gas exchange functions must be taken over by the neonatal lung with detoxification taken over by its own liver, so the circulations must fully divide into systemic and pulmonary circulations - the shunts must now close over. 
  2. a rise in peripheral resistance in the infant systemic circulation which causes BP to increase there. 
  3. a huge reduction in pulmonary vascular resistance. 
  4. these all fall
  5. reversed
  6. closed by a flap that acts like a valve preventing reverse flow - the hole then fibroses in the few months after birth. 
  7. within the first few days of life, probably due to high oxygen tension. 
  8. 1 – 3 hours after birth with subsequent fibrosis.
  9. When persistent left to right shunts which may occur when the foramen ovale or ductus arteriosis remain open after birth. 
    1. This is because, although there is no effect on blood oxygenation (blood has already passed through the pulmonary circulation prior to shunting), they do increase the work of the heart, and may result in cardiac failure if not treated by surgery.


Repiratory Adjustments at Birth (1)

  1. Why is it vital that the lungs begin to function immediately?
  2. What does the initial breath require and why?
    1. What happens to this afterwards?
  3. Describe neonatal compliance in comparison with an adult?
  4. Describe expiration of the newborn, why is it like this?

  1. the lungs take over gas exchange from the placenta at birth - necessary to avoid brain anoxia and damage.
  2. a huge inspiratory intrapleural pressure to be developed to overcome the surface tension of the fluid that fills the alveoli
    1. However, this rapidly reduces.
  3. remains less than that of an adult. 
  4. expiration is active rather than passive in the newborn to overcome increased resistance from fluid in the airways. 


Respiratory Adjustments at Birth (2)

  1. What is the respiratory rate of a neonate?
  2. What is the minute volume of a neonate?
  3. Compare the ventilation rate of an adult and a neonate when body sizes are taken into account.
  4. What is Respiratory Distress Syndrome due to?
    1. Who is it more likely to be seen in?


  1. 40 breaths per minute (adult = 12)
  2. 650ml.min-1 (adult = 6 litres.min-1) 
  3. the neonate has about twice the adult ventilation
  4. due to surfactant deficiency increasing the work of breathing.
    1. preterm infants


Weight Change and Nutrition

  1. ​​What happens to the weight of the neonate over the first few days of life?
  2. What changes have occured by day 10?
  3. What happens to weight within the first year of life?
  4. What vitamins and minerals are very much in demand during this period?
    1. Who often needs supplementation therefore?

  1. usually drops in weight (by as much as 10%, mostly due to fluid loss) due to difficulties in breastfeeding both from the mother initiating the supply and the infant suckling
  2. The drop in weight resolves. 
  3. Triples
  4. Vitamin D, calcium (developing skeleton) and Iron (developing respiratory system)
  5. babies who are being breastfed 




  1. Describe the metabolic rate of an infant, what implicationds does this have?
  2. Describe changes in temperature after birth.
  3. Why do fluctuations in core temperature still occur for the next few weeks of life?
  4. Why does the neonate lose head quicker?
    1. What attempts to counter this?

  1. about twice the metabolic rate of the adult, so generates more heat. 
  2. an initial drop in temperature in the newborn infant of about 1 – 2 degrees celcius which only resolves after about 12 hours.
  3. due to immaturity of thermoregulatory mechanisms.  
  4. has a lot of surface area to body volume and so loses heat quicker. 
    1. This may be countered by brown fat in the infant which uncouples ADP phosphorylation from mitochondrial function, with all of the energy going to heat.  This extra heat generated may be vital to infants.


Liver Function

  1. The liver is poorly formed at birth - what does this lead to (particularly in prematures)?
  2. What else is compromised in the first day after birth, necessitating frequent feeding?
  3. What increases the risk of bleeding?

  1. leading to a rise in plasma bilirubin which reaches a maximum about 5 times greater than normal after 1 – 2 weeks (neonatal jaundice).  As the liver matures, however, levels fall back to normal over the next few months. 
  2. Glycogen storage and manufacture are also compromised (glucose can drop to 2mmol.l-1 in the first day after birth) 
  3. Plasma protein levels are also low (including clotting factors).



  1. What must the newborn rely upon and why?
  2. What is another source of immunoglobulins?
  3. What happens to passive immunity over the first few months of life?
  4. When do infant immunisation programmes start and why?
  5. What might compromise immunocompetence?

  1. must rely on the mothers immunoglobulins (IgG’s) absorbed from the placenta, becasue active acquired immunity is poorly devloped. 
  2. More may be absorbed (IgA’s) in the colostrum
  3. This passive immunity does decline, being slowly taken over by immunity acquired by infant exposure to pathogens
  4. after 2 - 3 months, when the infant has a capable acquired immunity.
  5. over-clean environments, which may explain the increased incidence of allergies over the past few years  



Summing Up 

For general perusal 

  • The circulation of the fetus adjusts to birth by closing off links between pulmonary and systemic cirulations and closing the link between the hepatic and portal circulations
  • Respiratory adjustments are made in the first days of life that are essential to initiate and sustain tissue oxygenation
  • The neonatal metabolic, thermoregulatory, liver and immune functions are all immature at birth, and take some time to adapt

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