As the fetus develops from a single dependent cell into a fully formed neonate capable of sustained life outside the womb, the physiology of individual organs and the integrated systems of the body undergo substantial developmental changes. Although physiology in the early stages may be crude, and significantly different from that observed in maturity, it usually reflects functional differences that allow the fetus to cope with the challenges of the intrauterine environment, and also with the sudden, extreme changes needed for adaptation to extrauterine life. For example, the presence of fetal haemoglobin in utero allows oxygen to be extracted from the placenta in a very low-oxygen environment compared to after birth. In considering developmental physiology (ontogeny) it is necessary to see fetus, preterm newborn, neonate, infant, child and adolescent as stages of development that merge into each other.