Adapting to Life Outside the Womb

Question 1

Describe the mechanism of lung expansion at birth.

Answer 1

• At birth, the walls of the alveoli are at first collapsed because of the surface tension of the viscid fluid that fills them.
• More than 25 mmHg of negative inspiratory pressure in the lungs is usually required to oppose the effects of this surface tension and to open the alveoli for the first time.
• Once the alveoli open, however, further respiration can be effected with relatively weak respiratory movements.
• Fortunately, the first inspirations of the normal neonate are extremely powerful; they are usually capable of creat- ing as much as 60 mm Hg negative pressure in the intra- pleural space.

Question 2

What causes respiratory distress syndrome?

Answer 2

• Insuficient surfactant secretion.
• Surfactant secretion by type 2 alveolar cells does not begin until the last 1-3 months of gestation.

Question 3

Describe how blood is shunted in the foetus.

Answer 3

• Blood returning from the placenta through the umbilical vein passes through the ductus venosus, mainly bypassing the liver.
• Most of the blood entering the right atrium from the inferior vena cava is directed in a straight pathway across the posterior aspect of the right atrium and through the foramen ovale directly into the left atrium.
  
  • Thus, well-oxygenated blood from the placenta enters mainly the left side of the heart, rather than the right side, and is pumped by the left ventricle mainly into the arteries of the head and forelimbs.
• The blood entering the right atrium from the superior vena cava is directed downward through the tricuspid valve into the right ventricle. This blood is mainly deoxygenated blood from the head region of the fetus. It is pumped by the right ventricle into the pulmonary artery and then mainly through the ductus arteriosus into the descending aorta, then through the two umbilical arteries into the placenta, where the deoxygenated blood becomes oxygenated.

Question 4

How does blood flow through the lungs change in the foetus after birth?

Answer 4

• In the foetus, only 12% of the blood flows through the lungs.
• Immediately after birth, virtually all of the blood flows through the lungs.

Question 5

What are the primary changes to the circulation at birth?

Answer 5

• Loss of the tremendous blood flow through the placenta, which ~doubles the systemic vascular resistance at birth.
  
  • This increases the aortic pressure, as well as the pressure in the left ventricle and left atrium.
• The pulmonary vascular resistance greatly decreases due to expansion of the lungs.
  
  • In the unexpanded foetal lungs, the blood vessels are compressed because of the small volume of the lungs.
  • Immediately on expansion, these vessels are no longer compressed and the resistance to blood flow decreases massively.
• Also, in foetal life, the hypoxia of the lungs causes considerable tonic vasoconstriction of the lung blood vessels, but vasodilation takes place when aeration of the lungs eliminates the hypoxia.
• All these changes together reduce the resistance to blood flow through the lungs as much as fivefold, which reduces the pulmonary arterial pressure, right ventricular pressure, and right atrial pressure.

Question 6

Describe the closure of the foramen ovale.

Answer 6

• The low right atrial pressure and the high left atrial pressure that occur secondarily to the changes in pulmonary and systemic resistances at birth cause blood to now attempt to flow backward through the foramen ovale, that is, from the left atrium into the right atrium, rather than in the other direction, as occurred during fetal life.
• Consequently, the small valve that lies over the foramen ovale on the left side of the atrial septum closes over this opening, thereby preventing further flow through the foramen ovale.

Question 7

Describe the closure of the ductus arteriosus.

Answer 7

• First, the increased systemic resistance elevates the aortic pressure while the decreased pulmonary resistance reduces the pulmonary arterial pressure.
• As a consequence, after birth, blood begins to flow backward from the aorta into the pulmonary artery through the ductus arteriosus, rather than in the other direction, as in foetal life.
• However, after only a few hours, the muscle wall of the ductus arteriosus constricts markedly and within 1 - 8 days, the constriction is usually sufficient to stop all blood flow.
• This is called functional closure of the ductus arteriosus. Then, during the next 1 to 4 months, the ductus arteriosus ordinarily becomes anatomically occluded by growth of fibrous tissue into its lumen.
• The cause of ductus arteriosus closure relates to the increased oxygenation of the blood flowing through the ductus, as well as loss of the vascular relaxing effects of prostaglandin E2 (PGE₂).

Question 8

Describe the closure of the ductus venosus.

Answer 8

• In foetal life the portal blood from the fetus’s abdomen joins the blood from the umbilical vein, and these together pass by way of the ductus venosus directly into the vena cava immediately below the heart but above the liver, thus bypassing the liver.
• Immediately after birth, blood flow through the umbilical vein ceases, but most of the portal blood still flows through the ductus venosus, with only a small amount passing through the channels of the liver.
• However, within 1 - 3 hours the muscle wall of the ductus venosus contracts strongly and closes this avenue of flow.
• As a consequence, the portal venous pressure rises from near 0 to 6 - 10 mm Hg, which is enough to force portal venous blood flow through the liver sinuses.
• Although the ductus venosus rarely fails to close, we know little about what causes the closure.

Question 9

How much time does the glucose in the infant’s body supply its needs for after birth?

What happens to the infant’s blood glucose?

Answer 9

• Before birth, the foetus derives almost all its energy from glucose obtained from the mother’s blood.
• After birth, the amount of glucose stored in the infant’s body in the form of liver and muscle glycogen is sufficient to supply the infant’s needs for only a few hours.
• The liver of the neonate is still far from functionally adequate at birth, which prevents significant gluconeogenesis.
• Therefore, the infant’s blood glucose concentration frequently falls the first day to as low as 30 to 40 mg/dl of plasma, which is less than half the normal value.

Question 10

What happens to the weight of an infnant in the first 2-3 days of life?

Answer 10

• Ordinarily, the infant’s weight decreases 5-10% and sometimes as much as 20% within the first 2-3 days of life.
• Most of this weight loss is loss of fluid rather than of body solids.

Question 11

Describe the normal respiration of a neonate.

Answer 11

• The normal rate of respiration in a neonate is about 40 breaths / minute, and tidal air with each breath averages 16mL, which results in a total minute respiratory volume of 640 mL/min—about twice as great in relation to the body weight as that of an adult.
• The functional residual capacity of the infant’s lungs is only 1/2 that of an adult in relation to body weight.
• This difference causes excessive cyclical increases and decreases in the newborn baby’s blood gas concentrations if the respiratory rate becomes slowed, because it is the residual air in the lungs that smoothes out the blood gas variations.

Question 12

What is the normal blood volume of a neonate?

Answer 12

• The blood volume of a neonate immediately after birth averages about 300mL, but if the infant is left attached to the placenta for a few minutes after birth or if the umbilical cord is stripped to force blood out of its vessels into the baby, an additional 75mL of blood enters the infant, to make a total of 375mL.
• Then, during the ensuing few hours, fluid is lost into the neonate’s tissue spaces from this blood, which increases the hematocrit but returns the blood volume once again to the normal value of about 300mL.

Question 13

Describe the cardiac output of the neonate.

Answer 13

• The cardiac output of the neonate averages 500 mL/min, which, like respiration and body metabolism, is about twice as much in relation to body weight as in the adult.
• Occasionally a child is born with an especially low cardiac output caused by hemorrhage of much of its blood volume from the placenta at birth.

Question 14

What happens to arterial pressure of the neonate after birth?

Answer 14

• The arterial pressure during the first day after birth averages about 70mmHg systolic and 50mmHg diastolic and increases slowly during the next several months to about 90/60.
• A much slower rise then occurs during the subsequent years until the adult pressure of 115/70 is attained at adolescence.

Question 15

Describe physiological hyperbilirubinaemia in the neonate.

Answer 15

• Bilirubin formed in the foetus can cross the placenta into the mother and be excreted through the liver of the mother, but immediately after birth, the only means for ridding the neonate of bilirubin is through the neonate’s own liver, which for the first week or so of life functions poorly and is incapable of conjugating significant quantities of bilirubin with glucuronic acid for excretion into the bile.
• Consequently, the plasma bilirubin concentration rises from a normal value of less than 1 mg/dL to an average of 5 mg/dL during the first 3 days of life and then gradually falls back to normal as the liver becomes functional.
• This effect, called physiological hyperbilirubinemia, is associated with mild jaundice of the infant’s skin and especially of the sclerae of its eyes for a week or two.

Question 16

How does erythroblastosis fetalis cause neonatal jaundice?

Answer 16

• By far the most important abnormal cause of serious neonatal jaundice is erythroblastosis fetalis.
• Briefly, the erythroblastotic baby inherits Rh+ RBCs from the father, while the mother is Rh-.
• The mother then becomes immunised against the Rh+ factor (a protein) in the foetus’s blood cells, and her antibodies destroy foetal RBCs, releasing extreme quantities of bilirubin into the foetus’s plasma and often causing foetal death because of a lack of adequate RBCs.

Question 17

How does the rate of fluid intake and excretion in the neonate compare to that of the adult?

Answer 17

• The rate of fluid intake and fluid excretion in the newborn infant is seven times as great in relation to weight as in the adult, which means that alteration of even a slight percentage in fluid intake or fluid output can cause rapidly developing abnormalities.

Question 18

Describe the metabolic rate of the infant compared to that of the adult.

Answer 18

• The rate of metabolism in the infant is 2x as great in relation to body mass as in the adult, which means that twice as much acid is normally formed, creating a tendency toward acidosis in the infant.
• Functional development of the kidneys is not complete until the end of about the first month of life.
  
  • For instance, the kidneys of the neonate can concentrate urine to only 1.5x the osmolality of the plasma, whereas the adult can concentrate the urine to three to 4x the plasma osmolarity.
  • Therefore, considering the immaturity of the kidneys, together with the marked fluid turnover in the infant and the rapid formation of acid, one can readily understand that among the most important problems of infancy are acidosis, dehydration, and, more rarely, overhydration.

Question 19

Describe the liver function of the neonate.

Answer 19

• During the first few days of life, liver function in the neonate may be quite deficient, as evidenced by the following effects:
  1. The liver of the neonate conjugates bilirubin with glucuronic acid poorly and therefore excretes only a slight amount of bilirubin during the first few days of life.
  2. Because the liver of the neonate is deficient in forming plasma proteins, the plasma protein concentration falls during the first weeks of life to 15-20% less than that for older children. Occasionally the protein concentration falls so low that hypoproteinemic edema develops.
  3. The gluconeogenesis function of the neonate’s liver is particularly deficient. As a result, the blood glucose level of the unfed neonate falls to about 30 to 40 mg/dL (about 40% of normal), and the infant must depend mainly on its stored fats for energy until sufficient feeding can occur.
  4. The liver of the neonate usually also forms too little of the blood factors needed for normal blood coagulation.

Question 20

Describe the digestion, absorption and metabolism of nutrition in the neonate.

Answer 20

• In general, the ability of the neonate to digest, absorb, and metabolise foods is no different from that of the older child, with the following three exceptions:
  1. Secretion of pancreatic amylase in the neonate is deficient, so the neonate uses starches less adequately than do older children.
  2. Absorption of fats from the GIT is somewhat less than that in the older child. Consequently, milk with a high fat content, such as cow’s milk, is often inadequately absorbed.
  3. Because the liver functions imperfectly during at least the first week of life, the glucose concentration in the blood is unstable and low.

Question 21

What are the specific problems which occur in the early nutrition of the infant?

Answer 21

• Need for Calcium and Vitamin D.
  
  • Because the neonate is in a stage of rapid ossification of its bones at birth, a ready supply of calcium throughout infancy is necessary.
  • This is ordinarily supplied adequately by the usual diet of milk. Yet, absorption of calcium by the GIT is poor in the absence of vitamin D. Therefore, within only a few weeks, severe rickets can develop in infants who have vitamin D deficiency.
  • This is particularly true in premature babies because their gastrointestinal tracts absorb calcium even less effectively than do those of normal infants.
• Necessity for iron in the diet.
  
  • If the mother has had adequate amounts of iron in her diet, the infant’s liver usually has stored enough iron to keep forming blood cells for 4 to 6 months after birth.
  • However, if the mother has had insufficient iron in her diet, severe anemia is likely to occur in the infant after about 3 months of life.
  • To prevent this possibility, early feeding of the infant with egg yolk, which contains reasonably large quantities of iron, or the administration of iron in some other form is desirable by the second or third month of life.
• Vitamin C deficiency in infants.
  
  • ​Ascorbic acid (vitamin C) is not stored in significant quantities in the fetal tissues, yet it is required for proper formation of cartilage, bone, and other intercellular structures of the infant.
  • However, adequate amounts of vitamin C are normally provided in the mother’s breast milk unless the mother has severe vitamin C deficiency.
  • Cow’s milk has 1/4 as much vitamin C as human milk. In some cases orange juice or other sources of ascorbic acid are prescribed for infants with vitamin C deficiency.

Question 22

Describe the immunity of a newborn baby.

Answer 22

• The neonate inherits a great degree of immunity from the mother because many protein antibodies diffuse from the mother’s blood through the placenta into the foetus.
• However, the neonate does not form antibodies of its own to a significant extent.
• By the end of the first month, the baby’s gamma globulins, which contain the antibodies, have decreased to less than half the original level, with a corresponding decrease in immunity.
• Thereafter, the baby’s own immune system begins to form antibodies and the gamma globulin concentration returns essentially to normal by the age of 12 to 20 months.
• Despite the decrease in gamma globulins soon after birth, the antibodies inherited from the mother protect the infant for about 6 months against most major childhood infectious diseases, including diphtheria, measles, and polio.
• Therefore, immunisation against these diseases before 6 months is usually not necessary.
• However, the inherited antibodies against whooping cough are normally insufficient to protect the neonate; therefore, for full safety, the infant requires immunisation against this disease within the first month or so of life.