HIGH BLOOD PRESSURE

The first suggestion that high blood pressure might have its origins before birth came from studies of Swedish military recruits, and from a continuing study of a group of British men and women who were all born during the same week in 1946. The pressure in a baby’s circulation is critically important to it, because its nourishment depends on its ability to maintain an adequate pressure so that its blood flows freely through the placenta. Babies with small placentas, which have narrower blood vessels, may need to have to have higher blood pressure to maintain this flow. After birth these babies, who tend to be at the lower end of the birthweight range, continue to have higher blood pressures.

People who had low birthweight are twice as likely as other people to need medicine to control their blood pressures towards the end of their lives.Like children, babies respond variously to undernutrition, depending on its intensity, nature and at what age it occurs. Newborn babies who were thin or short, babies with small placentas or large placentas, have all been found to have high blood pressure in later life. Yet until middle age their blood pressures are only a little higher than those of other people, insufficiently raised to be a source of concern either to themselves or their doctors. It seems that even though a baby may be born with raised blood pressure it can maintain pressures within the normal range, preserve its internal constancy, the marker of good health, for many years. Eventually, as the system begins to wear out with age, this becomes impossible and blood pressure begins to rise steeply. When blood pressure rises, it damages the control systems, which include the kidney. The gentle rise in pressure that accompanies normal aging becomes a steep rise, a climbing pathway that leads to hypertension, increased risk of heart disease or stroke and the need for treatment. People who had low birthweight are twice as likely as other people to need medicine to control their blood pressures towards the end of their lives.

Within the human kidney there are at least a million functional units called nephrons, through which blood circulates so that the waste in it can be extracted. People who had lower birth weights have up to three times fewer nephrons than people who were larger at birth. The kidney does not have high priority for growth because, in the womb, the excretion of waste is carried out by the mother’s kidney. The baby’s kidney is readily traded off. If, as a result, a kidney has fewer nephrons once the baby is born each nephron will have to process more blood than it otherwise would have. This increases the wear and tear on them, and hastens the death of nephrons that occurs with normal aging. As nephrons die, blood pressure climbs, accelerating further nephron death and, it is thought, setting in motion a self-perpetuating cycle of rising blood pressure and nephron loss.

Nephrons are made during a brief period towards the end of life in the womb. If it were possible to make more nephrons after birth, kidney transplants would not be necessary. A review of the US Kidney Transplant program showed that the worst results, with failure of the transplanted kidney after only a few months, occurred when the kidney from a small person was transplanted into a large person. A large body has more blood to be cleared of waste, and the demand on each nephron is increased beyond its capacity. The nephrons die and the kidney fails. This may explain why people who had low birthweight are more likely to develop high blood pressure if they put on weight rapidly in childhood. Their nephrons die sooner and their journey to premature death is accelerated.

Kidney failure is commoner in South Carolina than in any other state in the US.Kidney failure is commoner in South Carolina than in any other state in the US. It is usually preceded by high blood pressure or diabetes, but there are other causes. More men than women are affected, and people as young as 20 get it. To have kidney failure at so young an age is almost unheard off in many states. Many patients are poor, and the main burden falls on African-Americans in whom it is five times more common than it is among whites. We know all this because the costs of treatment, whether renal dialysis or kidney transplantation, are born by the Federal Government who keep accounts of what they spend and where they spend it. South Carolina is part of the so-called ‘Stroke Belt’, the cluster of states in the Deep South with high rates of stroke. Every baby born in the state since 1950 had its birthweight recorded on its birth certificate. It has therefore been relatively simple to show that people with kidney failure tend to have had lower birthweight. The high rates of kidney failure in the state may be the result of an unusually large number of people being born with below average numbers of nephrons. If their kidneys are damaged by diabetes or other disorders they fail rapidly.