8th February 2024

Blood pressure

The kidneys play an integral role in blood pressure control. 

The juxtaglomerular apparatus within the distal convoluted tubule detects periods of reduced blood volume (whether due to dehydration or haemorrhage) and thus reduces blood pressure and renal perfusion. Renin is then released from the kidney, which converts angiotensinogen to angiotensin I. Angiotensin I travels within the bloodstream to the lungs and is, in turn, converted to angiotensin II by angiotensin-converting enzyme (ACE). Angiotensin II results in vasoconstriction, increased thirst, and the release of aldosterone from the zona glomerulosa of the adrenal gland. This pathway can also initiated by hyponatraemia. 

Image Source: Wikimedia Commons (2019). Renal corpuscle-en, Licensed under a creative commons licence. Available for free at: https://upload.wikimedia.org/wikipedia/commons/6/69/Renal_corpuscle-en.svg  (Accessed: 4 January 2024).

Aldosterone promotes sodium reabsorption by activating sodium channels within the distal convoluted tubule and collecting duct. Water then follows by osmosis, increasing blood volume and reducing urine output. Spironolactone causes diuresis by acting competitively against aldosterone to prevent the activation of these sodium channels.

Hyperkalaemia can also trigger the release of aldosterone from the adrenal gland, which attempts to normalise potassium levels by increasing potassium and hydrogen ion loss in the urine.  

ACE inhibitors such as ramipril interrupt this chain, preventing the conversion of angiotensin I and reducing blood pressure.

Image Source: Wikimedia Commons (2018). Renin angiotensin aldosterone system, Licensed under a creative commons licence. Available for free at: https://upload.wikimedia.org/wikipedia/commons/3/36/Renin-angiotensin-aldosterone_system.svg  (Accessed: 4 January 2024).