We are warmblooded (homeothermic) creatures who have to expend a fair amount of energy to maintain our temperatures at about 37°C. When we are slightly cold the smooth muscles of our arterioles contract, preventing heat loss from the blood; if that doesn’t help we start shivering and when all else fails we produce thyroid hormones which cause food to be burned. We also have layers of fat (more in some of us than others) which insulate us. When we are too warm our arterioles dilate releasing heat and we also sweat.
Staying warm is no problem for the fetus; its mother’s body provides the necessary heat. In fact because of the high metabolic rate of the fetus and the placenta, the fetus tends to be about 0.5°C warmer than its mother so that heat is released to her. As a result pregnant women often feel warmer than usual.
Once the fetus is born its thermal free ride is over. It is forced, wet and naked, out of its warm intrauterine environment into a cold delivery room. Its body temperature may quickly drop a few degrees because it is lacking some of the adult devices for keeping warm. The newborn does not have a well-developed shivering mechanism and its layer of fat may be quite thin. It also has another handicap: its volume is about five percent of that of an adult but its surface area is fifteen percent (Niswander, 1976).
This means that it has relatively more surface area than an adult, from which heat can escape into the air.
the newborn counteract the loss of heat? Not very successfully at first. It can cry, thrash around and flex its limbs (to reduce surface area) but this is not effective. Its real weapon against cold is brown fat.
Brown fat is a tissue that actively produces heat in contrast to normal white fat which just serves as insulation. We adults do not have this type of fat but strangely enough, hibernating animals do, and it warms them fast as they wake up from a winter’s sleep( Dawkins and Hull, 1965). During most of its development the human fetus is making the normal white fat used for insulation and as an energy source after birth but around the ninth month it starts producing brown fat between the shoulder blades, around the neck, down the breast bone and around the kidneys. (See diagram on this page.)
What is so special about brown fat? Unlike white fat its cells have a large number of mitochondria which metabolize the fat to produce heat, which is then carried to other parts of the body by the rich blood supply. Thus the neonate can produce heat by other means than shivering (this is called nonshivering thermo genesis). If a newborn is cold the nape of its neck where the brown fat is located will always be a few degrees higher than the rest of its body.
What a shame that we adults weren’t able to retain this method of heat production. It would have been useful, but unfortunately the brown fat of the newborn soon turns into white fat.
(figure available in print form)
Newborn infant showing areas of brown fat (cross hatching)