Piling on The Pressure

ENaC's (epithelial sodium channels) tasks do not end at birth.  It plays a vital role in regulating the amount of sodium in your blood and this, in turn, determines your blood pressure. If ENaC channels malfunction, your blood pressure can skyrocket, putting you at risk of a stroke.

Your kidneys are sophisticated organs that clean the blood, continuously filtering out toxins and waste products and flushing away excess water. Waste processing takes place in about a million individual units known as nephrons, where tufts of fine blood vessels, know as capillaries, are entwined with tiny tubules that act as urine-collecting devices.
Amazingly, the whole of your blood passes through the kidney twice every hour. The red blood cells and plasma proteins are retained in the capillary,  but the salts and water are forced out into the kidney tubule. Almost all of the sodium and much of the water that is filtered are subsequently reabsorbed as the fluid passes down the kidney tubules. What remains is stored in the bladder and excreted as urine.

ENaC channels in the membranes of the kidney tubule cells are responsible for reabsorption of sodium. As in the lung , sodium uptake is accompanied by water, which leads to an increase in blood volume and, because the circulation is a closed system, raises the blood pressure. A diet high in salt (sodium chloride) is bad for you because more sodium is taken up, which drags more water with it, increasing your blood volume and therefore your blood pressure.
Conversely, if blood sodium levels are low, insufficient water is retained by the body, leading to a fall in blood pressure. This is why it is important to ensure that you eat enough unrefined sea salt in a hot climate , where a lot of salt is lost through sweating.

Mutations in any of the three genes that make up the ENaC channel affect blood pressure.  Those that lead to increased ENaC activity cause a hereditary form of hypertension known as Liddle's disease, whereas those that reduce ENaC activity result in low blood pressure. The latter are particularly dangerous as they lead to a life-threatening salt-losing syndrome in newborns and infants.  Because sodium uptake is reduced, less water is reabsorbed, so that the child quickly becomes dehydrated and the blood concentration of other ions ( especially potassium ) becomes unbalanced.  The disease is fatal unless it is quickly recognised and treated.

Fortunately, mutations in ENaC are rare. However, it is thought that one reason for the greater incidence of high blood pressure and its attendant complications in black people than in Caucasians is because they have relatively common variants in their ENaC channels genes that predispose them to increased sodium uptake.
Why this is the case is uncertain, but one suggestion is that people living near the Sahara evolved very efficient mechanisms for absorbing salt as it was in such short supply. While this is advantage when salt is only rarely obtainable, it becomes a handicap in our present world where much processed food is very high in salt.

Drawing Life from Death

(except from  The Spark Of Life , Electricity in The Human Body, by Frances Ashcroft) [Time 8:23AM, 24/08/2016, counter @ 2088 3098]

Aya Soliman had a most unusual start in life, being born by Caesarean section two days after her mother Jayne was declared brain death.
Jayne, a champion ice skater, had a fatal brain haemorrhage when she was twenty-five weeks pregnant. She was flown by air ambulance to hospital in Oxford, but died shortly after arrival .
Although Jayne's brain was dead, doctors decided to keep her body alive to provide vital time for her daughter's lungs to mature.

Within the womb, the fetus floats in a cushioning sac of water. Its developing lungs are filled with fluid and it does not breath air, but obtains all the oxygen it needs via the umbilical cord that links it to the placenta. At birth, the water withing the lungs must be rapidly removed as the newborn child switches over to breathing air.This is achieved with the help of specialized epithelial sodium channels (ENaC channels) that are present in the cells that line the lung.
At birth the ENaC channels open, allowing sodium ions in the lung fluid to flow down their concentration gradient into the lung cells. because sodium ions  drag water with them, the lungs quickly dry out and so long as ENaC channels are present and functional, the lungs are rapidly cleared of fluid. Without ENaC, however, babies are at risk of drowning in their own fluid at birth, and may suffer from 'wet' lung.

During normal development ,  a rise in  steroid hormones switches on ENaC production a few weeks prior to birth, ensuring the lungs are fully mature when the baby  is delivered.
At twenty-five weeks of pregnancy, however, lung development is incomplete and the number of ENaC channels in the cells lining the lung is still very small. A chemical called surfactant that reduces the surface tension of the tiny air sacs in the lungs and so prevents their collapse is also low. Thus if a baby must be delivered early, and conditions permit, steroids are administered to the mother before birth. These cross over the placenta and help her premature baby's lungs mature. As a mother's womb is the optimal incubator for a baby, Jayne's body was kept alive (but brain death) on a life-support machine while steroids were given to provide her daughter with the best possible chance of life.

There is a further twist to this story. It turns out that at birth ENaC channels are stimulated to open more completely by stress hormone adrenaline, which rises dramatically in the mother's blood during the trauma of labour. This may explain why babies born by Cesarean section, where this stimulus is lacking, may have more difficulty clearing their lungs than those born naturally, and why they experience a higher incidence of respiratory complications in the postnatal period.