Cardio-Respiratory Changes at Birth
During birth, the thorax is squeezed as the body passes through the vagina, helping to clear fluid from the lungs. Birth, temperature change, sound and physical touch stimulate the baby to promote the first breath. A strong first breath is required to expand the alveoli for the first time. Adrenalin and cortisol are released in response to the stress of labour, stimulating respiratory effort.
In the fetus, the foramen ovale connects the right atrium with the left atrium, allowing blood to flow from the right atrium to the left atrium, bypassing the right ventricle and lungs. As the alveoli expand, the pulmonary vascular resistance decreases, causing a fall in right atrial pressure. The left atrial pressure is now higher than the right atrial pressure, squashing the atrial septum and causing functional closure of the foramen ovale. Blood now flows to the right ventricle and lungs. The foramen ovale later closes structurally and becomes the fossa ovalis.
In the fetus, the ductus arteriosus connects the pulmonary artery with the aorta, allowing blood to bypass the lungs. Prostaglandins are required to keep the ductus arteriosus open. Increased blood oxygenation causes a drop in circulating prostaglandins. The ductus arteriosus closes and becomes the ligamentum arteriosum.
In the fetus, the ductus venosus connects the umbilical vein to the inferior vena cava, allowing blood to bypass the liver. Shortly after birth, blood stops flowing through the umbilical veins, and the ductus venosus closes. It later becomes the ligamentum venosum.
Surfactant
Alveoli are small sacs in the lung where gas diffuses into the blood. They are lined with fluid. Surface tension is the attraction of the molecules in a liquid to each other, pulling them together and minimising surface area. In the alveoli, the fluid molecules attempt to pull together due to surface tension, which encourages the alveoli to collapse.
Surfactant is a fluid produced by type II alveolar cells. It contains proteins and fats. It sits on top of the fluid in the alveoli. It has a hydrophilic side that faces the fluid, and a hydrophobic side that faces the gas. Surfactant reduces the surface tension of the fluid in the lungs. It helps the alveoli inflate and maximises the surface area for gas exchange.
Surfactant also:
- Increases lung compliance (the lung’s ability to expand and inflate)
- Promotes equal expansion of the alveoli
Type II alveolar cells mature enough to produce surfactant between 24 and 34 weeks gestation. Preterm babies have problems associated with reduced pulmonary surfactant.
Thermoregulation
Neonates are particularly prone to losing body heat and developing hypothermia. This occurs due:
- Large surface-area-to-volume ratio
- Reduced subcutaneous fat
- Thin skin
- Reduce shivering ability
Energy Requirements
Neonates have increased energy requirements and are prone to developing hypoglycaemia. This occurs due to:
- Reduced glycogen stores
- Reduced fat stores
- Immature gluconeogenesis
- Increased metabolic rate
Haemoglobin
Haemoglobin is the molecule responsible for transporting oxygen around the body. It is found in red blood cells. Haemoglobin is formed of four protein subunits. Fetal haemoglobin (HbF) has two alpha (α) and two gamma (γ) subunits. Adult haemoglobin (HbA) has two alpha (α) and two beta (β) subunits.
From 32 to 36 weeks gestation, production of HbF decreases. At the same time, HbA is produced in greater quantities. Over time, there is a gradual transition from HbF to HbA.
Physiological jaundice can occur due to the increased breakdown of red blood cells (haemolysis), releasing bilirubin into the blood.
Physiological anaemia occurs due to reduced red blood cell production, haemolysis and increased blood volume (diluting the haemoglobin). The lowest haemoglobin concentration typically occurs at around 8-12 weeks of age (earlier in preterm infants).
Last updated May 2025
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