Aldosterone is a steroid hormone produced by the adrenal glands, two small glands located on top of the kidneys. It belongs to a class of hormones called mineralocorticoids. The primary purpose of aldosterone is to regulate the balance of water and salts within the body. This hormone circulates through the bloodstream to act on various tissues.
Regulating Blood Pressure and Electrolytes
Aldosterone’s main role is to maintain blood pressure, blood volume, and the proper balance of electrolytes. It achieves this primarily by acting on the kidneys. The hormone travels to the kidneys and binds to mineralocorticoid receptors located in the distal tubules and collecting ducts, which are the final segments of the tiny tubes, or nephrons, that make up the kidneys. This binding prompts specific actions related to two electrolytes: sodium and potassium.
Upon receiving the signal from aldosterone, the kidney cells actively increase the reabsorption of sodium from the urine back into the bloodstream. Because water naturally follows sodium due to osmotic pressure, this increased sodium retention causes more water to be held in the blood vessels. The increase in blood volume subsequently raises blood pressure, similar to how adding more water to a hose increases the pressure inside it.
At the same time it promotes sodium reabsorption, aldosterone instructs the kidneys to excrete potassium. It signals the cells in the nephron to move potassium from the blood into the urine for removal from the body.
The Renin-Angiotensin-Aldosterone System (RAAS)
The body uses a signaling pathway called the Renin-Angiotensin-Aldosterone System (RAAS) to control when aldosterone is released. This system constantly monitors blood pressure and the concentration of sodium in the blood. When specialized cells in the kidneys detect a drop in blood pressure or low sodium levels, they release an enzyme called renin into the bloodstream.
Renin acts on a protein produced by the liver called angiotensinogen, converting it into angiotensin I. As angiotensin I circulates through the body, particularly through the lungs, an enzyme called Angiotensin-Converting Enzyme (ACE) transforms it into angiotensin II. Angiotensin II is a substance with two effects that work to raise blood pressure.
First, angiotensin II causes the muscular walls of small arteries throughout the body to constrict, which narrows the vessels and immediately increases blood pressure. Second, it travels to the adrenal glands and directly stimulates the zona glomerulosa to produce and secrete aldosterone. The resulting aldosterone release then restores blood volume and pressure, completing the feedback loop.
Consequences of Aldosterone Imbalance
Disruptions in the RAAS or the adrenal glands can lead to either an excess or a deficiency of aldosterone, each with distinct consequences. When the body produces too much aldosterone, a condition known as hyperaldosteronism, it leads to the excessive retention of sodium and water. This causes persistent high blood pressure, or hypertension, which can be difficult to control with standard medications. A hallmark of this condition is the concurrent loss of too much potassium.
This resulting low potassium level, or hypokalemia, can cause symptoms such as muscle weakness, cramping, fatigue, and even heart palpitations. In some cases, hyperaldosteronism is caused by a benign tumor on one of the adrenal glands, a condition known as Conn’s syndrome. The overproduction of aldosterone can also promote inflammation and the formation of scar tissue in the heart and blood vessels.
Conversely, when the body produces too little aldosterone, it results in a state called hypoaldosteronism. Without enough of this hormone to signal for sodium retention, the body loses excessive amounts of sodium and, consequently, water in the urine. This leads to low blood pressure, or hypotension, which can cause dizziness and fainting. It also results in the retention of potassium, leading to high potassium levels, or hyperkalemia, which can affect heart rhythm. People with this condition may also experience salt cravings, dehydration, and fatigue.
Medical Evaluation and Management
Diagnosing an aldosterone imbalance begins with blood tests to measure the levels of aldosterone, renin, and electrolytes like potassium and sodium. A urine test may also be used to measure the amount of aldosterone and electrolytes being excreted. The ratio of aldosterone to renin in the blood is a useful indicator that helps distinguish between different causes of imbalance.
For hyperaldosteronism, treatment may involve medications called mineralocorticoid receptor antagonists, such as spironolactone, which block the effects of aldosterone at the kidney level. If a tumor on an adrenal gland is the cause, surgery to remove it may be recommended. For hypoaldosteronism, the approach is hormone replacement therapy. This involves taking a medication that mimics the action of aldosterone to help the body retain the proper amount of sodium and water.