The Leaky Kidney

Each kidney contains about a million nephrons, and each nephron begins with a glomerulus, a knot of capillaries inside a capsule. Normally the glomerular capillary wall and the capsule act as a filter that keeps large proteins such as albumin in the blood while allowing water, salts, glucose and urea into the tubule. Long-standing high blood glucose damages the small blood vessels and the filtering membrane, making it leaky. Albumin then passes into the tubule and is lost in the urine (proteinuria), which makes the urine frothy. Because albumin is lost, blood albumin falls, and since albumin exerts the osmotic pull that holds water in the blood, water moves out of the blood into the tissues, causing oedema - puffiness around the eyes in the morning and swelling of the ankles later in the day. Less salt in the diet reduces water retention and helps limit the swelling.

The kidneys and blood pressure are linked in a damaging cycle. Damaged kidneys are less able to regulate salt and water, so fluid retention raises blood pressure. High pressure in the glomerular capillaries then forces more protein through the damaged filter and stretches and scars the small vessels further, so kidney damage gets worse. Lowering blood pressure reduces the pressure inside the glomeruli, which reduces protein leakage and slows the decline in kidney function. It also protects the heart and brain from the effects of hypertension. So the medicine treats the kidneys indirectly by protecting the filter, and it reduces the risk of heart attack and stroke. Blood pressure control works best when combined with tight blood glucose control, because high glucose is the original cause of the vessel damage.

Drinking extra water will not remove the protein, because the protein is leaking through a damaged filter, not sitting in the urine. The amount of protein lost depends on the pressure and leakiness of the glomerulus, not on how much water passes through. In fact, drinking very large amounts of water can be harmful if the kidneys cannot excrete it efficiently: it dilutes the blood sodium and can cause confusion and, rarely, dangerous brain swelling. A sensible fluid intake of about 1.5–2 litres a day is fine unless a doctor advises otherwise, but it must not be used instead of the prescribed medicine, glucose control and reduced salt. The most effective steps are controlling blood glucose and blood pressure, which reduce the leakage at its source.