The pressure in the pulmonary artery decreases by approximately half (to ≈35 mm Hg) due to the decrease in pulmonary vascular resistance. This change in pressure, along with a slight increase in aortic pressure, causes the blood flow through the ductus arteriosus to reverse. However, within a few minutes, the large ductus arteriosus starts to constrict, resulting in turbulent flow that can be heard as a murmur in newborn babies. The constriction of the ductus arteriosus progresses and is usually complete within 1 to 2 days after birth. The closure of the ductus arteriosus seems to be triggered by the high oxygen levels in the arterial blood passing through it. Ventilation with oxygen leads to the closure of the ductus, while ventilation with low oxygen air opens this shunt vessel. The exact mechanism of how oxygen affects the ductus is still unknown.

At birth, the walls of both ventricles have similar thickness. Additionally, the muscle layer of the pulmonary arterioles is thick, contributing to the high pulmonary vascular resistance in fetuses. After birth, the thickness of the right ventricle walls decreases, as does the muscle layer of the pulmonary arterioles. In contrast, the walls of the left ventricle become thicker. These changes occur gradually over a few weeks after birth and are influenced by different hemodynamic forces, such as vascular resistance, acting on the two ventricles. Cardiac hypertrophy plays a role in the increase in heart weight during normal growth after birth. The developing cardiovascular system's demands, combined with increased levels of growth hormone and insulin-like growth factor-1, contribute to the physiological hypertrophy, where the left ventricular mass more than doubles from birth to early adulthood.

Sometimes, the ductus arteriosus fails to close after birth. This congenital cardiovascular abnormality, known as patent ductus arteriosus, can be corrected in newborns by administering nonsteroidal anti-inflammatory agents like ibuprofen. If this approach is unsuccessful in closing the ductus or if the child is older, surgical closure becomes necessary.

1. The vascular system comprises the systemic circulation and the pulmonary circulation, which are connected in series. These subdivisions consist of various types of vessels, including arteries, arterioles, and capillaries, arranged in parallel.
2. The mean velocity (v) of blood flow in a particular type of vessel is directly proportional to the total blood flow pumped by the heart and inversely proportional to the cross-sectional area of all parallel vessels of that type.
3. In larger blood vessels, blood flow that is steady and laminar follows Poiseuille's law. However, very small blood vessels exhibit non-Newtonian flow, rendering Poiseuille's law inapplicable.
4. Turbulent flow occurs when flow velocity is high, fluid viscosity is low, fluid density is high, vessel diameter is large, or the vessel wall is irregular.
5. Arteries not only transport blood from the heart to the capillaries but also store a portion of the ejected blood during each cardiac systole. Consequently, blood flow continues through the capillaries during cardiac diastole. Veins return blood from the capillaries to the heart and possess relatively low resistance and high capacitance, allowing them to function as blood reservoirs.
6. With aging, arterial and venous compliance decreases. Reduced arterial compliance necessitates increased cardiac work to achieve a given cardiac output. Decreased venous compliance impairs their ability to store blood.
7.
The decrease in pulmonary vascular resistance causes the pressure in the pulmonary artery to fall to approximately half its previous level (to ≈35 mm Hg). This change in pressure, coupled with a slight increase in aortic pressure, reverses the flow of blood through the ductus arteriosus. However, within several minutes, the large ductus arteriosus begins to constrict. This constriction produces turbulent flow, which is manifested as a murmur in newborn infants. Constriction of the ductus arteriosus is progressive and usually complete within 1 to 2 days after birth. Closure of the ductus arteriosus appears to be initiated by the high PO2 of the arterial blood passing through it; pulmonary ventilation with O2 closes the ductus, whereas ventilation with air low in O2 opens this shunt vessel. Whether O2 acts directly on the ductus or through the release of a vasoconstrictor substance is not known.
At birth, the walls of the two ventricles have approximately equal thickness. In addition, the muscle layer of the pulmonary arterioles is thick; this thickness is partly responsible for the high pulmonary vascular resistance of the fetus. After birth, the thickness of the walls of the right ventricle diminishes, as does the muscle layer of the pulmonary arterioles. In contrast, the left ventricular walls become thicker. These changes progress over a period of weeks after birth and reflect the effects of different hemodynamic forces (e.g., vascular resistance) on the two ventricles. Cardiac hypertrophy underlies the increase of heart weight during the normal growth period after birth. The physical demands imposed by the developing cardiovascular system, together with increased levels of soluble factors (e.g., growth hormone, insulin-like growth factor-1), account for physiological hypertrophy by which left ventricular mass more than doubles during the period from birth to early adulthood.
Theductusarteriosusoccasionallyfailstocloseafterbirth.Inthenewborn,thiscongenitalcardiovascularabnormality,calledpatent ductus arteriosus, cansometimesbecorrectedbytheadministrationofnonsteroidalanti-inflammatoryagentssuchasibuprofen.Ifthisdoesnotresultinclosureoftheductusorifthechildisolder,closuremustbeachievedsurgically.
1.
The vascular system is composed of two major subdivisions: the systemic circulation and the pulmonary circulation. These subdivisions are in series with each other and are composed of a number of vessel types (e.g., arteries, arterioles, capillaries) that are aligned in series with one another. In general, the vessels of a given type are arranged in parallel with each other.
2.
The mean velocity (v) of blood flow in a given type of vessel is directly proportional to the total blood flow being pumped by the heart, and it is inversely proportional to the cross-sectional area of all the parallel vessels of that type.
3.
Poiseuille’s law characterizes blood flow that is steady and laminar in vessels larger than arterioles. However, blood flow is nonnewtonian in very small blood vessels (i.e., Poiseuille’s law is not applicable).
4.
Flow tends to become turbulent when (1) flow velocity is high, (2) fluid viscosity is low, (3) fluid density is great, (4) vessel diameter is large, or (5) the wall of the vessel is irregular.
5.
Arteries not only conduct blood from the heart to the capillaries but also store some of the ejected blood during each cardiac systole. Hence, blood flow continues through the capillaries during cardiac diastole. Veins return blood to the heart from the capillaries and have a relatively low resistance and high capacitance that enables them to serve as reservoirs for blood.
6.
The aging process diminishes compliance of the arteries, as well as of the veins. The less compliant the arteries are, the more work the heart must do to achieve a given cardiac output. The less compliant the veins are, the poorer is their ability to store blood.
7.