Blood Pressure and the Kidney

Question 1

How is Sodium linked to blood pressure?

Answer 1

1. ↑Na+ balance = ↑ECF Osmolality → ↑ADH release.
2. ADH = ↑Water reabsorption (in CD) = ↑BV
3. ↑BV = ↑SV = ↑BP - due to CO = HR X SV and BP = CO X TPR.

Question 2

How does the Cardiovascular system control Sodium levels?

Why do we control Na balance through output?

Answer 2

Change in Na+ intake → change in BV - stimulates 2 different pathways:

• Afferent pathways: Cardiac volume receptors, Baroreceptors, Renal arterial pressure.
• Efferent pathways: Neuronal (SNS), Hormonal (RAAS, ANP), Haemodynamic (GFR, Pressure Natriuresis).

Efferent pathways → change in renal Na+ output

• We have a low Na diet, so we conserve Na and control its output.

Question 3

Outline the mechanism used to conserve sodium?

What stimulates Renin secretion from the juxtaglomerular cells in the kidney?

What’s the effect of Aldosterone on the kidneys, in terms of Na and K?

Answer 3

• RAAS: Renin release leads to Ang II production, which causes vasoconstriction and aldosterone secretion.
• 1. ↓Renal blood flow - due to ↓BP & ↓BV
     
     2. ↓Na load at Macula Densa
     3. Sympathetic activation of β1-receptors
• Increases Na+ reabsorption (↑ENaC) and K+ secretion (↑Na/K pump) - Excess aldosterone → Hypokalaemia

Question 4

What are the mechanisms used to excrete sodium?

What are the types of cardiac natriuretic peptides? Where are they found? What stimulates their release?

Answer 4

• Cardiac Natriuretic Peptides and Pressure Natriuresis
• Atrial and Brain Natriuretic Peptides (ANP and BNP) found in the specialised cardiomyocytes - released in response to ↑Cardiac Filling Pressure (↑ECFV)

Question 5

What are the effects of ANP on the kidneys?

Answer 5

ANP INHIBITS RAAS!

• ↑Na+/H2O excretion - Natriuresis/Diuresis
• Vasodilation of AFFERENT arteriole = ↓BP = ↑GFR
• ↓Aldosterone and Renin secretion

Question 6

What are the effects of Pressure Natriuresis (Na excretion) on the kidneys?

What is it due to?

Answer 6

• ↑Na+ excretion due to ↑BP - no change in GFR due to autoregulation
• Due to ↑Medullary capillary pressure = ↑Fluid filtration and Interstitial pressure to prevent Na+ reabsorption - will ↓BV, which will then ↓BP as a result.

Question 7

What is the clinical importance of the kidneys controlling blood pressure?

What are the 2 types of Hypertension and how can the kidney cause them?

Answer 7

• Can lead to Hypertension - Systolic > 140mmHg and/or Diastolic > 90mmHg
• 1. Essential HT - due to abnormal handling of Na+ balance
     
     2. Secondary HT - due to excess renal Na+ reabsorption and abnormalities in hormone secretion - examples of conditions are Liddle’s syndrome, Conn’s syndrome, Renal Artery Stenosis.

Question 8

How does Liddle’s Syndrome lead to ↑[Na+] and ↑BP?

How does Conn’s Syndrome lead to ↑[Na+] and ↑BP?

How does Renal Artery Stenosis lead to ↑[Na+] and ↑BP?

Answer 8

• Genetic mutation of ENaC (in CD) = ↑ENaC activity. This will ↑Na+ reabsorption = ↑ECFV and BP - this action suppresses Renin/Aldosterone.
• Adrenal Adenoma of Zona Glomerulosa excessively producing Aldosterone. This will ↑ENaC and Na/K pumps = ↑Na+ reabsorption and K+ secretion. Results in ↓[K+] and ↑ECFV - ↑ECFV will decrease Renin secretion.
• Stenosis (narrowing) = ↓Renal blood flow = Renin secretion. This will activate the RAAS = ↑Ang II and Aldosterone = ↑Vasoconstriction and Na+ reabsorption (and BP).