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The commonest cause of secondary hypertension is kidney disease, which accounts for 8 out of 10 cases. High blood pressure can also be the cause of kidney disease, however, and it can be difficult for doctors to tell which condition developed first. When high blood pressure is the cause of kidney disease, this occurs because essential hypertension leads to hardening and furring up of the large renal arteries and also damages small blood vessels in the kidney. As a result, blood supply to the kidneys is reduced and they may start to shrink. At the same time, poor blood supply to the kidney filtering units (nephrons) means less urine is produced. Kidney function progressively deteriorates and fluid builds up in the circulation, contributing to hypertension. Poor blood supply to the kidneys also stimulates the special pressure receptors (baroreceptors) in the kidneys that are responsible for monitoring blood pressure. If they detect blood pressure has fallen within the kidneys, they trigger increased production of renin hormone, which raises blood pressure in an attempt to increase blood supply to the kidneys. This puts BP up even more, so another vicious cycle develops.

Where kidney disease comes first, and high blood pressure develops as a consequence, the usual kidney diseases involved are:

chronic glomerulonephritis (inflammation of the kidney filtration units)

chronic atrophic pyelonephritis (shrinking of kidney tissue due to chronic infection or inflammation)

congenital polycystic kidneys (abnormal kidney structure due to the formation of multiple cysts during embryonic development).

Kidney problems are thought to cause high blood pressure by reducing filtration of excess fluid and salts from the body, which build up in the circulation to raise blood pressure. Increased secretion of renin hormone is also involved.