The RAAS (Renin-Angiotensin-Aldosterone System) is one of the most important hormone systems in the body. It acts as a "long-term thermostat" for blood pressure and fluid balance. While it is designed to protect the body during emergencies like dehydration, in Chronic Kidney Disease, this system often becomes "maladaptive," meaning it stays turned on and ends up damaging the kidneys further.
The Step-by-Step Biological Cascade
The RAAS is a chain reaction involving the kidneys, the liver, the lungs, and the adrenal glands.
1. The Trigger: Renin
When the kidney senses a drop in blood pressure, a decrease in sodium, or low blood volume, specialized cells (the juxtaglomerular apparatus) release an enzyme called Renin into the blood.
2. The Liver's Contribution: Angiotensinogen
The liver is constantly producing a protein called Angiotensinogen that floats in the blood. When it meets Renin, it is converted into Angiotensin I.
3. The Lungs' Role: ACE
As Angiotensin I passes through the lungs, it meets an enzyme called Angiotensin-Converting Enzyme (ACE). This converts it into the "active" form: Angiotensin II.
The Effects of Angiotensin II
Angiotensin II is a powerful biological "general" that gives three main orders to raise blood pressure:
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Vasoconstriction: It tells the blood vessels to narrow instantly. Narrower pipes mean higher pressure.
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Thirst: It tells the brain to make the cat thirsty so they drink more water to fill the pipes.
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Aldosterone Release: It tells the adrenal glands (sitting on top of the kidneys) to release Aldosterone.
The Final Step: Aldosterone
Aldosterone travels back to the kidney's nephrons and tells them to "save sodium." Because water follows salt, the kidneys reabsorb more water back into the blood instead of letting it become urine. This increases the total volume of blood in the body.
The Biological Link to CKD
In a healthy cat, the RAAS turns off once blood pressure is back to normal. In a CKD cat, the system gets stuck in a destructive loop:
Glomerular Hypertension
Angiotensin II specifically constricts the "exit" pipe (efferent arteriole) of the kidney's filter (the glomerulus).
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The Pressure Cooker: This increases the pressure inside the filter to try and force more waste out.
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The Result: This high pressure physically tears the delicate filter membrane, leading to Proteinuria (protein in the urine).
Fibrosis (Scarring)
Angiotensin II is "pro-inflammatory." When it is chronically high, it signals the body to create fibroblasts, which lay down scar tissue in the kidney. This turns functional nephrons into useless scars.
The Vicious Cycle
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Kidney damage leads to lower blood flow.
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The kidney "thinks" the body has low blood pressure and activates RAAS.
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RAAS raises blood pressure and causes glomerular scarring.
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The scarring leads to even lower blood flow, starting the cycle all over again.
Managing the RAAS
Because this system is so central to kidney decline, many renal treatments focus on "blocking" parts of this chain:
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ACE Inhibitors (like Benazepril): These stop the lungs from converting Angiotensin I into the active Angiotensin II.
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ARBs (like Telmisartan/Semintra): These let the body make Angiotensin II but "block the ears" of the blood vessels so they can't hear the command to constrict, specifically protecting the filters from high pressure.
Disclaimer: This content is for educational purposes only and does not constitute veterinary medical advice. Always consult with your primary care veterinarian or a veterinary internal medicine specialist regarding your cat’s specific condition.