Proteinuria and High Blood Pressure

The kidney is essentially a filter that functions to remove certain waste products from the blood. In its simplest form, the kidney functions much like a simple kitchen strainer. Blood passes over a series of holes in the kidney blood vessels, and a small amount of it is allowed to flow through the more complicated regions of the kidney, where more detailed filtering occurs. Though later filtering steps are more complex, this initial filtering functions to exclude certain blood components based solely on size. Proteinuria represents a breakdown in early filtering apparatus of the kidney.

The intricate web of blood vessels and other tissues that form the physical filter of the kidney is quite delicate and functions appropriately only within a narrow range of parameters.

The Importance of Blood Pressure in the Body

When you pour a kettle of noodles into a kitchen strainer, gravity pulls both noodles and water downwards through the strainer. Inside the body, the force that causes blood to move through the kidney’s filter is the blood pressure. Make the ​blood pressure too low and there isn’t enough force to push adequate amounts of blood through the filter, causing a decrease in the amount of blood filtered and lowering the amount of urine produced. In much the same way, one would expect that increasing the blood pressure would cause a rise in the amount of blood filtered and urine produced. However, this does not always happen. The kidney has a sort of built-in gate system that allows it to resist increased blood pressures and keep the filtration rate fairly constant.

In situations where the blood pressure is chronically elevated, this gating mechanism starts to break down. As this happens, the delicate filtering structures of the kidneys are exposed to abnormally high pressures, which they are not designed to handle. One result of this increased pressure is a gradual deterioration of the vessel structures that form the actual filter. This deterioration is much like making the holes on a kitchen strainer larger. As the size of the holes increases, progressively larger substances are able to pass through this initial barrier into the kidney, where they eventually become part of the urine.

What Happens If Proteins Are Found in Urine?

In spite of these larger holes in the initial filter, most blood proteins are still kept out of the kidney by other mechanisms. Both the filter and most blood proteins carry a small charge, like magnets. Just as placing the same end of two magnets together causes repulsion, most blood proteins are repelled by the kidney. However, albumin — the most abundant blood protein — is not charged and is the protein most likely to pass through the damaged filter. It is this protein that urine tests look for when assessing proteinuria. Some disease states cause the number of other proteins in the urine — besides albumin — to increase, and special tests are needed to screen for these.

In general, damage caused to the kidney by high blood pressure is not reversible, so it is important to control the blood pressure to avoid such damage. While other diseases besides high blood pressure can cause proteinuria, high blood pressure is the most common and preventable cause of kidney damage.