Hypertension

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"hypertension"[MeSH Terms] AND "pathophysiology"

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Pathophysiology of Hypertension

Hypertension is not a single-mechanism disease. It arises from the dysregulation of multiple interacting systems that normally maintain blood pressure (BP) within a narrow range. The fundamental equation is:

BP = Cardiac Output (CO) × Total Peripheral Resistance (TPR)

But long-term BP regulation - and therefore hypertension - involves far more than just the heart and vessels. The kidneys, nervous system, hormones, immune cells, and vascular biology all interact.

1. The Renal-Pressure Natriuresis Mechanism

The kidney is the central long-term regulator of BP. Normally, a rise in mean arterial pressure (MAP) of as little as 1-3 mmHg triggers "pressure natriuresis" - the kidney excretes more sodium and water to normalize BP. This operates via two mechanisms:

Medullary blood flow: A rise in perfusion pressure selectively increases medullary (not cortical) blood flow, raising renal interstitial hydrostatic pressure, reducing proximal tubular reabsorption, and promoting NaCl excretion.
Tubular transport regulation: Changes in pressure directly modulate tubular reabsorption in the proximal tubule, loop of Henle, and distal segments.

In hypertension, this pressure-natriuresis curve is reset or blunted - the kidney retains salt at higher-than-normal BP levels, making the system defend a higher pressure set-point. This is the unified mechanism underlying essentially all forms of chronic hypertension.

Salt Sensitivity: About 30% of normotensive individuals are salt-sensitive (closer to 60% of hypertensives). It is more common in Black individuals, the elderly, and those with chronic kidney disease (CKD), and is typically associated with lower plasma renin activity (PRA). Salt sensitivity reflects an impaired ability to excrete sodium loads without a compensatory BP rise.

2. The Renin-Angiotensin-Aldosterone System (RAAS)

The RAAS is one of the most important pressor systems:

Renin is released from juxtaglomerular cells in response to decreased renal perfusion pressure, decreased NaCl delivery to the macula densa, or sympathetic nerve stimulation.
Renin cleaves angiotensinogen to angiotensin I, which is converted by ACE to angiotensin II (Ang II).
Ang II causes:
- Direct vasoconstriction (raising TPR)
- Aldosterone release from the adrenal cortex → sodium and water retention
- Impaired pressure natriuresis in the kidney
- Activation of the sympathetic nervous system
- Vascular remodeling and hypertrophy over time

In salt-sensitive hypertension, RAAS activity is often low (due to volume expansion suppressing renin). In salt-insensitive (renin-dependent) hypertension, RAAS activity is elevated or inappropriately normal.

3. The Sympathetic Nervous System (SNS)

The SNS regulates BP rapidly (within seconds) via:

Vasoconstriction in resistance and capacitance vessels
Increased cardiac output (via heart rate and contractility)
Renal sodium retention (via renal tubular adrenoceptors)
Renin secretion from the juxtaglomerular apparatus

In primary (essential) hypertension, renal sympathetic overactivity is frequently present. It promotes sodium retention, renin release, and impairs the normal pressure-natriuresis relationship. This is the basis for catheter-based renal denervation (RDN) as a treatment - though clinical trials show modest effects, partly because catheter-based RDN achieves <50% nerve ablation and patients are often already on RAAS-blocking drugs.

Baroreceptor resetting: In chronic hypertension, arterial baroreceptors in the carotid sinus and aortic arch reset their operating range upward, losing the ability to "correct" elevated BP back to normal. They continue to buffer moment-to-moment fluctuations but defend the new, higher set-point.

4. Vascular Mechanisms and Autoregulation

Long-term elevated BP causes structural vascular changes:

Wall thickening (hypertrophy and hyperplasia of vascular smooth muscle)
Reduced lumen diameter
Capillary rarefaction (decreased number of capillaries)

These changes increase TPR and perpetuate hypertension even if the original trigger resolves. This structural remodeling is driven by Ang II, endothelin-1, and mechanical stretch on vessel walls.

Autoregulation: Each tissue regulates its own blood flow according to metabolic needs. In chronic hypertension, this autoregulatory range shifts rightward - tissues tolerate higher pressures without vasodilating, further cementing the elevated BP state.

5. Endothelial Dysfunction and Nitric Oxide

Nitric oxide (NO) - produced by endothelial nitric oxide synthase (eNOS) - is a powerful vasodilator and inhibits tubular NaCl reabsorption in the kidney. In hypertension:

NOS activity is reduced in both vascular endothelium and kidney.
Less NO → increased vasoconstriction → higher TPR.
Reduced renal NO → impaired natriuresis → salt sensitivity.

This endothelial dysfunction also promotes atherosclerosis and vascular stiffness.

6. Reactive Oxygen Species (Oxidative Stress)

Superoxide (O₂⁻) and peroxynitrite (ONOO⁻) generated by NADPH oxidase and mitochondria:

Scavenge NO, reducing its vasodilatory effect
React with lipids to form oxidized LDL (pro-atherogenic) and isoprostanes (vasoconstrictors, promote salt retention)
Drive vascular inflammation and remodeling

Hypertension - especially in the setting of CKD - is a state of chronic oxidative stress. Ang II is a potent activator of vascular NADPH oxidase, linking RAAS activation directly to oxidative injury.

7. Endothelins

Endothelin-1 (ET-1), produced by vascular endothelial cells and collecting duct cells:

Acts on ETA receptors → vasoconstriction and aldosterone release
Acts on ETB receptors → NO release and natriuresis (counterbalancing effect)

In hypertension and CKD, ET-1 levels are elevated, and the balance shifts toward ETA-mediated vasoconstriction and sodium retention.

8. Atrial Natriuretic Peptide (ANP)

ANP is released from atrial cardiomyocytes in response to stretch (volume overload). It opposes hypertension by:

Increasing GFR and sodium excretion
Suppressing RAAS
Enhancing pressure natriuresis

ANP deficiency (as seen in obesity, where adipocyte-derived neprilysin degrades ANP) contributes to salt-sensitive hypertension. Genetic knockout of ANP or its NPR-A receptor causes hypertension in animal models.

9. Immune and Inflammatory Mechanisms

A newer, well-supported concept: immune cells - particularly T lymphocytes and macrophages - infiltrate the kidney and blood vessels in hypertension, promoting:

Renal inflammation and impaired natriuresis
Vascular inflammation and stiffness
Oxidative stress

This mechanism is seen in Ang II-induced hypertension, aldosterone-mediated hypertension, salt-sensitive models, and in conditions like lupus and rheumatoid arthritis where immunosuppression also lowers BP. A 2024 review in Nature Reviews Nephrology (PMID 38658669) provides an updated account of these immune mechanisms.

10. Salt-Sensitive vs. Salt-Insensitive (Renin-Dependent) Hypertension

Feature	Salt-Sensitive	Salt-Insensitive (Renin-Dependent)
Pressure-natriuresis curve	Decreased slope (blunted)	Parallel right-shift
Plasma renin activity	Low	High or normal
Renal blood flow	Decreased	Decreased or normal
Glomerular injury risk	Yes	Protected (increased pre-glomerular resistance)
Examples	CKD, primary aldosteronism, low nephron mass	Renal artery stenosis, Goldblatt model

Summary: The Unifying Concept

All forms of chronic hypertension ultimately require the kidney to defend a higher-than-normal BP - either because:

The pressure-natriuresis curve is blunted (kidney retains salt at elevated pressures), or
The curve is shifted rightward (kidney requires a higher pressure to achieve normal sodium balance).

The underlying drivers of this renal set-point shift include RAAS activation, sympathetic overactivity, reduced NO bioavailability, structural vascular changes, immune/inflammatory infiltration, and genetic factors governing nephron number and tubular transport.

Sources: Fuster and Hurst's The Heart, 15th Edition; National Kidney Foundation Primer on Kidney Diseases, 8th Edition

Recent evidence note: A 2024 review (PMID 38658669, Nat Rev Nephrol) highlights emerging evidence for T-cell and innate immune mechanisms as active contributors to hypertension pathogenesis, not merely bystanders - potentially opening new therapeutic targets beyond RAAS and SNS blockade.

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