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.