Functions of the Kidneys 4: Renin and Blood Pressure Control

Functions of the Kidneys 4: Renin and Blood Pressure Control

How the kidneys regulate long-term blood pressure

Blood pressure is controlled over the long term by the kidneys.

By adjusting how much sodium and water the body keeps, the kidneys regulate blood volume and therefore blood pressure.

A key hormone in this process is renin, which helps raise blood pressure when the body senses reduced circulation or blood volume.

In Chronic Kidney Disease (CKD), renin levels vary but are often elevated due to the kidney’s response to reduced blood flow.

Why the kidneys are central to blood pressure control

Blood pressure depends on:

• The amount of blood in circulation
• How narrow or relaxed blood vessels are

The kidneys influence both by controlling:

• Sodium and water excretion, which affects ..
• Blood volume
• Hormones that affect blood vessel tone

This makes the kidneys the main long-term regulators of blood pressure.

Sodium balance and blood volume

Sodium determines how much water the body retains.

• More sodium → more water → higher blood pressure (and higher blood volume)
• Less sodium → less water → lower blood pressure (and lower blood volume)

Even small long-term changes in kidney sodium handling can cause sustained high blood pressure.

Renin: the kidney’s pressure signal

Renin is released when the kidneys sense:

• Reduced blood flow or low blood pressure
• Reduced sodium levels
• Sympathetic nervous system activation (‘fight or flight’)

Renin release signals that the body perceives low effective blood volume and is reacting to it.

The renin–angiotensin–aldosterone system (RAAS)

When BP is low, renin activates a hormone system that raises blood pressure:

• Leads to formation of angiotensin II
• Angiotensin II:
  • Narrows blood vessels
  • Increases aldosterone release, which promotes sodium and water retention

This increases blood volume and vascular resistance – both of which raise the BP.

Chronic Kidney Disease (CKD) and hypertension

Kidney disease and high blood pressure reinforce each other:

• Impaired sodium removal raises blood pressure
• Increased renin production (causing RAAS activation) raises BP as well
• High blood pressure worsens kidney damage, which accelerates kidney decline

BP treatment in CKD

• Hence CKD commonly causes hypertension, and high BP accelerates kidney function decline
• Many effective treatments act on kidney function – either by reducing blood volume or de-activating the RAAS
• Breaking this cycle is one of the key parts of the trearment of Chronic Kidney Disease (CKD).

Why many blood pressure drugs (including ACE/ARBs) target the kidneys

Common treatments lower blood pressure by acting on kidney pathways:

• ACE inhibitors (‘ACEs’) and Angiotensin Receptor Blockers (‘ARBs’) reduce angiotensin II levels
• Diuretics increase sodium and water loss

These drugs lower blood pressure and protect the kidneys.

Key points

• The kidneys control long-term blood pressure
• In CKD
  • Sodium excess increases blood volume, and hence BP
  • Renin activates hormones (angiotensiin II and aldosterone) that raise blood pressure. In CKD renin levels vary but are often elevated
• Hence CKD commonly causes hypertension, and high BP accelerates kidney function decline
• Many effective BP treatments act on the kidneys – either by reducing blood volume or de-activating the RAAS – to break this cycle.