Surface tension is the attraction of the molecules in a liquid to each other, pulling them together and minimising surface area. This is why, in zero gravity, water floats around in a ball rather than diffusing into a mist.
Alveoli are the small sacs where gas collects and diffuses into the blood during inhalation. These are lined with fluid. The molecules of this fluid pull together due to surface tension, in turn pulling the walls of the alveoli towards each-other, attempting to collapse the space in the alveoli.
Surfactant is a fluid produced by type II alveolar cells. It contains proteins and fats. It sits on top of the water in the lungs. It has a hydrophilic side, that faces the water, and a hydrophobic side, that faces the air. The surfactant reduces the surface tension of the fluid in the lungs, essentially providing a barrier that reduces the water molecules tendency to pull towards each other.
The result is that surfactant keeps the alveoli inflated and maximises the surface area of the alveoli. This reduces the force needed to expand the alveoli and therefore the lungs during inspiration. This is known as compliance. Therefore, surfactant increases lung compliance.
Additionally, as an alveolus expands, the surfactant becomes more thinly spread and therefore the surface tension increases, making it more difficult to expand that alveolus further. This stops one alveolus expanding massively whilst another alveolus only expands a little. Therefore, surfactant promotes equal expansion of all alveoli during inspiration.
Type II alveolar cells become mature enough to start producing surfactant between 24 and 34 weeks gestation. Therefore, pre-term babies have problems associated with reduced pulmonary surfactant.
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 previously collapsed alveoli for the first time. Adrenalin and cortisol are released in response to the stress of labour, stimulating respiratory effort.
The first breaths the baby takes expands the alveoli, decreasing the pulmonary vascular resistance. The decrease in pulmonary vascular resistance causes a fall in pressure in the right atrium. At this point the left atrial pressure is greater than the right atrial pressure, which squashes the atrial septum and causes functional closure of the foramen ovale. The foramen ovale then structurally closes and becomes the fossa ovalis.
Prostaglandins are required to keep the ductus arteriosus open. Increased blood oxygenation causes a drop in circulating prostaglandins. This causes closure of the ductus arteriosus, which becomes the ligamentum arteriosum.
Immediately after birth the ductus venosus stops functioning because the umbilical cord is clamped and there is no blood flow in the umbilical veins. The ductus venosus structurally closes a few days later and becomes the ligamentum venosum.
Last updated January 2020