The kidneys regulate the body’s fluid volume, maintain electrolyte concentrations and acid-base balance, filter the blood and remove wastes. The kidneys directly influence blood volume and blood pressure by controlling sodium levels in the body and by activating a hormone in the blood that stimulates the constriction of blood vessels. Additionally, the kidneys secrete a hormone that increases the production of red blood cells.
Renin Angiotensin System
The renin-angiotensin system regulates salt balance and blood volume and pressure. Specialized cells in the kidney detect the concentration of sodium flowing through the kidney and changes in blood pressure. In response to low sodium levels or a decrease in blood pressure granular cells in the kidney release renin. Renin is an enzyme that circulates in the blood and breaks down angiotensinogen to angiotensin I.
Another enzyme in the lungs breaks down angiotensin I to the active hormone, angiotensin II. Angiotensin II stimulates the constriction of blood vessels, which consequently increases blood pressure. It also acts on the central nervous system to increase thirst and increase the release of vasopressin, which is another hormone that acts on the kidney stimulating the reabsorption of water from the urine.
Aldosterone
In addition to the aforementioned effects of angiotensin II, it stimulates cells in the adrenal glands to release the hormone, aldosterone. Aldosterone acts on the kidney to increase the amount of sodium reabsorption. An increase in sodium reabsorption also increases water reabsorption. Therefore, aldosterone increases blood volume and blood pressure.
Erythropoietin
Another hormone that is secreted by the kidneys is erythropoietin. This hormone acts on cells in the bone marrow and stimulates them to increase the production of red blood cells.
Secondary Hypertension
The American Heart Association reports kidney dysfunction is the primary cause of secondary hypertension, which accounts for between five and ten percent of hypertension cases. The factors that cause hypertension are an increase in cardiac output and increased vascular resistance. When the kidneys are functioning normally, they will increase their excretion of salt and water in response to an increase in blood pressure. This reduces blood volume and blood pressure returns to normal. If the kidneys are not functioning adequately more sodium and water will be retained causing a chronic increase in blood volume. The increase in blood volume increases the force of heart contraction and increases blood pressure causing secondary hypertension.
Cardiovascular Disease
A 2009 review study in Vascular Health and Risk Management reported patients with chronic kidney disease have a markedly higher cardiovascular mortality. Several factors contribute to this increased risk. One factor being, as kidney impairment progresses the renin-angiotensin system is upregulated leading to an increase in blood pressure. The authors of the 2009 review study state that a "vicious circle" arises in patients with decreasing kidney function, because this increases hypertension, which causes further worsening of kidney function.
References
- Cardiovascular Physiology Concepts: Secondary Hypertension
- Cardiovascular Physiology Concepts: Renin-Angiotensin-Aldosterone System
- American Heart Association: Kidneys and Kidney Function
- “Vascular Health and Risk Management”; Cardiovascular Disease and Kidney Disease
- The Merck Manuals Online Medical Library: High Blood Pressure
