The kidney is the main organ involved in controlling blood pressure. Blood pressure can be described as the resistance of the blood vessels acting against the blood flow generated by a heartbeat. The kidneys are able to regulate blood pressure through two main actions. The first action by the kidneys allows the organ to control the diameter of blood vessels. With a wider diameter, the pressure is lower but a smaller diameter causes pressure to be higher. The second action affecting blood pressure is through the retention or excretion of water from the body initiated by hormones released from the kidney. The mechanism that balances blood pressure is called the renin-angiotensin-aldosterone system (RAAS). In people with kidney disease, this system however fails to regulate blood pressure. The gradual loss of kidney function results in the retention of water and tightening of the blood vessel, which causes high blood pressure. As the blood pressure rises, patients may need to start taking various medications to maintain and control their blood pressure at the appropriate level.