Circulation System: Changes At Birth
Time period:
birth (38 weeks)
Dramatic and
clinically significant changes occur to the circulatory and respiratory systems
at birth. Here, we look at changes primarily of the circulatory system and how
these changes prepare the baby for life outside the uterus.
If we were
to follow the flow of oxygenated blood in the foetus from the placenta (Figure
31.1), we would start in the umbilical vein and track the blood moving towards
the liver. Here, half the blood enters the liver itself and half is redirected
by the ductus venosus directly into the inferior vena cava, bypassing
the liver.
The blood
remains well oxygenated and continues to the right atrium, from which it may
pass into the right ventricle in the expected manner or directly into the left
atrium via the foramen ovale (Figure 31.2). Blood within the left atrium
passes to the left ventricle and then into the aorta.
Blood
entering the right atrium from the superior vena cava and the coronary sinus is
relatively poorly oxygenated. The small amount of blood that returns from the
lungs to the left atrium is also poorly oxygenated. Mixing of this blood with
the well‐oxygenated blood from the ductus venosus reduces the oxygen saturation
somewhat.
Blood within
the right ventricle will leave the heart within the pulmonary artery, but most
of that blood will pass through the ductus arteriosus and into the
descending aorta. Almost all of the well‐oxygenated blood that entered the
right side of the heart has avoided entering the pulmonary circulation of the
lungs, and has instead passed to the developing brain and other parts of the
body (Figure 31.3).
The
umbilical arteries constrict after birth, preventing blood loss from the
neonate. The umbilical cord is not cut and clipped immediately after birth,
however, allowing blood to pass from the placenta back to the neonatal
circulation through the umbilical vein.
The ductus
venosus shunted blood from the umbilical vein to the inferior vena cava during
foetal life, bypassing the liver. After birth a sphincter at the umbilical vein
end of the ductus venosus closes (Figure 31.4). The ductus venosus will slowly
degenerate and become the ligamentum venosus.
Once the
umbilical circulation is terminated the umbilical vein will also degenerate and
become the round ligament (or ligamentum teres hepatis) of the liver.
This may be continuous with the ligamentum venosus. The umbilical arteries will
persist in part as the superior vesical arteries, supplying the bladder, and
the remain- der will degenerate and become the median umbilical ligaments.
The shunt
formed by the ductus arteriosus between the pulmonary trunk and the aorta in
foetal life causes blood rich in oxygen to bypass the lungs, which have a very
high vascular resistance during development. With birth, the first breath of
air and early use of the lungs the pulmonary vascular resistance drops and
blood flow to the lungs increases. An increase in oxygen saturation of the
blood, bradykinin produced by the lungs, and a reduction in circulating prostaglandins
cause the smooth muscle of the wall of the ductus arteriosus to contract,
restricting blood flow here and increasing blood flow through the pulmonary
arteries (Figure 31.4). Physiological closure is normally achieved within 15
hours of birth.
During the
first few months of life, the ductus arteriosus closes anatomically, leaving
the ligamentum arteriosum as a remnant. As this is a remnant of the
sixth aortic arch the left recurrent laryngeal nerve can be found here (see
Chapter 43).
The
direction in which blood flows into the right atrium from the inferior vena
cava and the crista dividens (the lower edge of the septum secundum, forming the
superior edge of the foramen ovale) preferentially direct the flow of blood
through the foramen ovale into the left atrium, reducing mixing with poorly
oxygenated blood entering the right atrium from the superior vena cava (Figures
31.2 and 31.3).
As the child
takes his or her first breath the reduction in pulmonary vascular resistance
and subsequent flow of blood through the pulmonary circulation increases the
pressure in the left atrium. As the pressure in the left atrium is now higher
than in the right atrium the septum primum is pushed up again the septum secundum,
thus functionally closing the foramen ovale (Figure 28.3). Anatomical closure
is usually completed within the next 6 months. In the adult heart a depression
called the fossa ovalis remains upon the interior of the right atrium.
Patent foramen ovale (PFO) is an atrial
septal defect. The foramen ovale fails to close anatomically although it is
held closed by the difference in interatrial pressure. A ‘backflow’ of blood
can occur from left to right under certain circumstances which increases
pressure in the thorax. These circumstances include sneezing or coughing, and
even straining during a bowel movement. Autopsy studies have shown a PFO
incidence of 27% in the US population but those with this defect
generally do not have symptoms. Treatment varies depending
upon age and associated problems, but often no treatment is necessary.
If the
ductus arteriosus fails to close at birth it is termed a patent ductus
arteriosus (PDA). Well‐oxygenated blood from the aorta mixes with poorly
oxygenated blood from the pulmonary arteries, causing tachypnoea, tachycardia,
cyanosis, a widened pulse pressure and other symptoms. Longer term symptoms
seen during the first year of life include poor weight gain and continued
laboured breathing. Premature infants are more likely to develop a PDA.
Treatment can be surgical or pharmacological.
A portosystemic
shunt is less common and occurs when the ductus venosus fails to close at birth,
allowing blood to continue to bypassic acid and ammonia in the blood can lead a
failure to gain weight, vomiting and impaired brain function.
