The physiology of newborns is fundamentally different than the physiology of older children and adults. Perhaps the reason it is so different is that it constantly changes, with the biggest change from intrauterine to extrauterine life. While some aspects, such as cardiovascular alterations, change the moment the newborn takes its first breath, other aspects, such as modifications in hemoglobin, change within a few months. The purpose here is to discuss the physiology of newborns, particularly how it differs from than that of adults. Major organ systems that will be discussed include cardiovascular, pulmonary, blood, and lymph, with special considerations in energy metabolism and thermoregulation.[1][2][3]

As mentioned, the physiology of newborns is constantly evolving and adapting to extrauterine life. It is important to note these changes and ensure proper development at the appropriate times. For instance, it is important for the infant, while taking its first breath, to shut down and rewire the intrauterine cardiovascular shunts present in the infant's body. Failure to do so can cause physiological imbalances, such as not getting enough oxygen to the brain. Oxygenated blood, as opposed to deoxygenated blood, keeps getting oxygenated. Additionally, it is important to understand what the infant lacks in the newborn period that requires supplementation. For instance, a newborn infant is deficient in vitamin K, putting it at risk for a hemorrhagic disease. To prevent this, all infants born should be given vitamin K prophylaxis.[4]

Cardiovascular SystemTo understand the changes occurring in the cardiovascular physiology of the newborn, one must understand intrauterine fetal circulation. In the fetus, oxygenated blood comes from the mother’s umbilical cord. Oxygenated blood enters the fetus through the umbilical vein and then through the ductus venosus, the first of the three shunts to be discussed. This ductus venosus conducts the well-oxygenated blood from the umbilical vein to the inferior vena cava and right atrium. The reason it is considered a shunt is that it bypasses the hepatic circulation. In the fetus, oxygenated blood is essential for life and is preferentially delivered to the brain and heart myocardium.From the right atrium, the oxygenated blood travels through the foramen ovale – the second shunt – and to the left atrium, as opposed to the right ventricle in children and adults. Oxygenated blood is then subsequently delivered to the left ventricle, to the brain, and to the rest of the body via the aorta, similar to adult circulation.Deoxygenated blood from the liver, superior vena cava, and coronary sinus is preferentially directed from the right atrium to right ventricle to the pulmonary arteries. From there, instead of going to the lungs, the deoxygenated blood bypasses the pulmonary system via the ductus arteriosus, our third and final shunt. The ductus arteriosus shunts blood away from the lungs, due to high fetal pulmonary arterial resistance, and to the descending aorta. The primary mechanisms contributing to the high pulmonary vascular resistance is the low oxygen tension and lack of pulmonary arterial flow. These mechanisms allow the synthesis and release of prostaglandins from the endothelium located in the pulmonary vessels. It is due to these prostaglandins that the ductus arteriosus remains patent. It also is important to note that the placenta produces prostaglandins, contributing to the patency of the ductus arteriosus.

With the birth of the infant and removal of the low-resistance placenta, there are major cardiovascular responses related to pressures, blood flow, and pulmonary circulation. As the infant takes its first breath, this causes a marked decrease in pulmonary vasculature resistance. This causes an increase in left atrial pressure (due to blood flow from pulmonary vasculature), and this pressure is higher than the pressure in the right atrium, prompting the foramen ovale to close.Now that the newborn is breathing, functional closure of ductus arteriosus begins and can last several days. Due to the decrease in pulmonary arterial resistance and increase in oxygen, there is a decrease in prostaglandins, subsequently closing the ductus arteriosus. With the placenta now separated, there is also a decrease in prostaglandin synthesis, contributing to the closure of the ductus arteriosus.Lastly, and perhaps the longest to close (3 to 7 days), is the ductus venosus. The umbilical vessels now constrict in response to two things: (1) increased systemic vascular resistance due to clamping of the placenta and (2) increased oxygen content from the infant’s respirations. Now that blood flow through the ductus venosus is reduced, it starts to constrict and close, reducing blood to the inferior vena cava.Pulmonary SystemDuring intrauterine life, the fetal lungs are filled with amniotic fluid, so lung development requires the clearance of the lung amniotic fluid, consistent and automatic breathing, as well as secretion of surfactant. Infants that are born via vaginal deliveries are squeezed as they pass through the vaginal canal, allowing compression of the fluid in the lungs. Once the baby is out of the uterus, several external environmental factors, such as light, change in temperature, and noise, activate the nervous system and prompt the infant to take the first breath. Additionally, internal factors, such as central chemoreceptors, also play a role in driving respiration due to hypoxia. In the newborn, the work of breathing is usually labored (i.e., using accessory muscles, costal retractions, grunting) to overcome the high surface tension. As the fluid leaves the alveoli in the lungs, the effort of breathing is reduced. This is also one of the reasons why newborns have an increased respiratory rate (30 to 60 breaths per minute). Other reasons include compensation for high metabolic rate and perfusion-ventilation differences. More importantly, the presence of circulatory shunts forces the infant to increase the work of breathing.Due to immature central drive responses, newborns may have periods of apnea lasting less than 5 seconds. While this is considered abnormal in adults, it is normal for newborns to have apneic episodes.Hematological SystemThere are two things to consider when studying the hematology of a newborn: blood and clotting.Blood is made up of two different major components: plasma and cells (red blood cells, white blood cells, and platelets). In utero, blood is produced by the liver and then picked up by the bone marrow after birth. Red blood cells carry hemoglobin which transports oxygen and iron from the lungs to other tissues and organs of the body. There are many different types of hemoglobin, but those pertinent to this discussion are Hb F and Hb A.Hb F is the primary hemoglobin produced by the fetus. Its role is to transport oxygen adequately in low oxygen environments. It has a high affinity for oxygen, making it suitable for oxygen extraction from maternal hemoglobin across the placenta. Not only is Hb F important for intrauterine development, it is important in the newborn period due to impairment of oxygen delivery to the tissues. Around six months of age, Hb F is replaced with Hb A, also known as adult hemoglobin. It is the most common hemoglobin, encompassing 98% of the total red blood cell hemoglobin.Infants lack vitamin K due to immature hepatocyte function and lack of enteric bacteria that produce vitamin K. Vitamin K is used in the synthesis of clotting factors II, VII, IX, X and proteins C and S. Therefore, those who lack vitamin K have an increased risk of any form of hemorrhage, from any cause. As a result, due to the deficiency of vitamin K, a prophylactic shot of vitamin K is given to every newborn to protect against hemorrhagic disease.Metabolism and ThermoregulationIntrauterine temperature is that of the normal maternal temperature. Fetal body temperature is 0.5 C above the maternal temperature. At birth, the newborn loses its heat due to the dramatic drop in environmental temperature. The newborn’s heat is mostly lost via radiation, which can be reduced by raising the room temperature.For the newborn to be able to thermo-regulate, the newborn’s sympathetic system activates in response to the cold stimulus. The main mediators that aid the newborn’s transition to extrauterine life are cortisol and catecholamine. The sympathetic release activates thermogenesis via brown adipose tissue. Brown adipose tissue is present around the kidneys and muscles of the back. Brown adipose tissue generates heat via uncoupling oxidative phosphorylation in the mitochondria. The newborn also can produce heat by shivering thermogenesis, which is basically an increase in skeletal muscle activity and limb movements.

The high heart rate (120 to 160 beats per minute) seen in newborn infants can be attributed to the high metabolic rate of activity to main breathing, feeding, and thermogenesis.