Payhogenesis of HTN
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pathogenesis hypertension RAAS renin angiotensin sympathetic nervous system mechanism diagram
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Pathogenesis of Hypertension
Blood pressure (BP) = Cardiac Output (CO) × Systemic Vascular Resistance (SVR). Sustained hypertension arises when either — or both — are chronically elevated. Primary (essential) hypertension accounts for 90–95% of all cases and results from a complex interplay of genetic susceptibility and environmental factors; the remaining 5–10% are secondary to identifiable causes.
1. Fundamental Haemodynamic Framework
| Phase | Mechanism | Predominant Change |
|---|---|---|
| Early (young adults) | Sympathetic overactivity → ↑ CO | ↑ Cardiac output, labile BP, ↑ HR |
| Established | Total-body autoregulation → vasoconstriction | ↑ SVR, normal CO |
| Late/Elderly | Arterial stiffness → faster pulse-wave return | Isolated systolic HTN |
Increased cardiac output does not persist: each organ's intrinsic autoregulation translates excess flow into vasoconstriction over 5–15 days ("total-body autoregulation"), converting the haemodynamic abnormality into a structurally sustained rise in SVR.
2. Renal Sodium Handling — The Central Defect
The kidney is the pivotal long-term regulator of BP via pressure natriuresis: a rise of as little as 1–3 mmHg in mean arterial pressure normally triggers a sharp increase in urinary NaCl excretion, correcting volume and BP.
In hypertension this curve is reset/shifted rightward:
- The kidney retains more sodium at any given perfusion pressure
- Obligatory fluid retention → ↑ ECF volume → ↑ blood volume → ↑ CO → ↑ BP
- Eventually a new steady state is reached, but only at a higher BP
Two mechanisms of pressure natriuresis are impaired:
- Medullary pressure-flow pathway: ↑ perfusion pressure → ↑ medullary blood flow → ↑ interstitial hydrostatic pressure → ↓ proximal tubule reabsorption. Blunted in HTN.
- RAAS suppression: ↑ BP should suppress renin from the juxtaglomerular apparatus → ↓ Ang II → ↓ Na⁺ reabsorption. A normal or elevated PRA in a hypertensive patient is therefore "inappropriately" high and perpetuates the disorder.
Salt sensitivity (present in ~30% of normotensives, ~60% of hypertensives) is especially common in Black individuals, the elderly, and those with CKD. It correlates with low plasma renin activity.
3. Renin–Angiotensin–Aldosterone System (RAAS)
The RAAS is the master regulator of salt/volume homeostasis:
Renin (JGA) → cleaves Angiotensinogen → Angiotensin I
ACE (lung/endothelium) → Angiotensin II
Angiotensin II actions that raise BP:
- Potent vasoconstriction (AT₁ receptor → SVR ↑)
- Stimulates aldosterone → ENaC activation → Na⁺/water retention in collecting duct
- Stimulates ADH (vasopressin) release → water reabsorption
- Inhibits ANP → blunts natriuresis
- Stimulates thirst
- Promotes vascular smooth muscle cell (SMC) hypertrophy and fibrosis
Genetic polymorphisms in angiotensinogen, ACE, and AT₁ receptor genes contribute to interindividual variation in BP and population-level differences (e.g., lower-renin hypertension in Black individuals).
4. Sympathetic Nervous System (SNS)
- Many hypertensive patients show elevated plasma catecholamines, increased sympathetic nerve discharge, and elevated heart rate — despite the stimulus that should suppress sympathetic tone
- Baroreflex resetting: after 2–5 days of elevated BP, baroreceptors adapt to the new set-point and defend the higher BP rather than correcting it
- With aging and atherosclerosis, carotid sinus walls stiffen → reduced baroreceptor stretch sensitivity → further SNS over-activation
- SNS activation increases: CO (via ↑ HR, ↑ contractility), SVR (vasoconstriction), and renal Na⁺ retention (via direct tubular effects + renin release)
5. Endothelial Dysfunction and Vasomotor Imbalance
A balance between vasodilators and vasoconstrictors normally maintains vascular tone:
| Vasodilators | Vasoconstrictors |
|---|---|
| Nitric oxide (NO) | Angiotensin II |
| Prostacyclin (PGI₂) | Endothelin-1 (ET-1) |
| Bradykinin | Thromboxane A₂ |
| ANP/BNP | ROS-generated isoprostanes |
In hypertension:
- Nitric oxide (NO) synthesis (via eNOS) is reduced → loss of vasodilation + reduced renal Na⁺ excretion
- Reactive oxygen species (ROS), particularly superoxide (O₂⁻), scavenge NO, forming peroxynitrite (ONOO⁻) → oxidative stress → vasoconstriction, salt retention, platelet aggregation, LDL oxidation → accelerated atherosclerosis
- Endothelin-1 released by endothelial cells acts on ETA receptors → vasoconstriction; potentiates Ang II-mediated vasoconstriction
- NADPH oxidase–derived ROS are amplified by Ang II, further coupling the RAAS to endothelial dysfunction
6. Vascular Remodeling — Structural Perpetuation
Chronic vasoconstriction induces irreversible structural changes:
- Wall hypertrophy of resistance arterioles → fixed reduction in lumen-to-wall ratio → permanently elevated SVR
- Enhanced vascular reactivity: thickened walls produce greater luminal narrowing for any given vasoconstrictor stimulus
- Microvascular rarefaction: loss of arterioles and capillaries reduces the cross-sectional area available for flow
- Impaired myogenic response: blunts autoregulation of renal blood flow → barotrauma to glomeruli
- These changes blunt pressure natriuresis (juxtaglomerular apparatus hypertrophies → renin sensing impaired) → salt sensitivity worsens → hypertension self-perpetuates
This explains why antihypertensive therapy often takes weeks to reach maximal effect, and why vascular hypertrophy is only partially reversible.
7. Genetic Factors
- Familial clustering and twin studies confirm a strong heritable component (estimated heritability ~30–60%)
- Most causative alleles are polygenic and small-effect (GWAS identifies >1000 loci)
- Known candidates: angiotensinogen gene variants, AT₁R polymorphisms, ENaC subunit mutations (Liddle syndrome — rare monogenic form), aldosterone synthase
- Monogenic forms (e.g., Liddle, Gordon, glucocorticoid-remediable aldosteronism) provide proof-of-concept for specific pathway derangements
8. Environmental / Modifiable Factors
| Factor | Mechanism |
|---|---|
| High dietary sodium | Overwhelms pressure natriuresis in salt-sensitive individuals |
| Obesity | Adipose-derived Ang II, leptin → SNS activation, insulin resistance → Na⁺ retention |
| Physical inactivity | Reduced vascular compliance, SNS over-activity |
| Smoking | ROS generation, endothelial injury, vasoconstriction |
| Chronic stress | Sustained SNS activation, cortisol |
| Alcohol excess | SNS activation, renin release |
9. Vascular Pathology (End-Organ Consequence)
| Lesion | Setting | Mechanism |
|---|---|---|
| Hyaline arteriolosclerosis | Benign/primary HTN | Plasma leakage across injured endothelium; SMC ECM deposition; → nephrosclerosis |
| Hyperplastic arteriolosclerosis | Malignant HTN (DBP >120 mmHg) | "Onion-skin" laminated SMC proliferation; fibrinoid necrosis → acute ischemia |
Integrated Pathogenetic Model
The pathogenesis can be synthesized as a self-reinforcing cycle:
Genetic predisposition + environmental triggers → impaired renal sodium handling → ↑ volume/CO → baroreceptor resetting → inappropriate SNS & RAAS activation → endothelial dysfunction + ↓ NO → vasoconstriction → ↑ SVR → vascular remodeling → structural HTN → further impairment of pressure natriuresis → cycle perpetuates
Sources:
- Robbins & Kumar Basic Pathology, Pathogenesis of Primary Hypertension, p. 278–311
- National Kidney Foundation Primer on Kidney Diseases, 8e, Pathophysiology of Hypertension, Ch. 63
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