Blood pressure
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blood pressure physiology regulation hypertension overview 2024
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Blood Pressure: A Comprehensive Overview
1. Definition & Units
Blood pressure (BP) is the force exerted by blood against any unit area of the vessel wall. It is measured in millimeters of mercury (mm Hg) - a standard introduced by Poiseuille in 1846. A pressure of 100 mm Hg means the force is sufficient to push a column of mercury 100 mm high against gravity.
- 1 mm Hg = 1.36 cm H₂O (because mercury's specific gravity is 13.6)
The key pressures are:
| Parameter | Value | Meaning |
|---|---|---|
| Systolic BP (SBP) | ~120 mm Hg | Peak pressure during ventricular contraction |
| Diastolic BP (DBP) | ~80 mm Hg | Minimum pressure during ventricular relaxation |
| Mean Arterial Pressure (MAP) | ~100 mm Hg | Driving force for tissue blood flow |
| Pulse Pressure | ~40 mm Hg | SBP minus DBP |
- Guyton and Hall Textbook of Medical Physiology, p. 2075
2. Fundamental Equation
Mean Arterial Pressure (MAP) = Cardiac Output (CO) × Total Peripheral Resistance (TPR)
This is the single most important equation in cardiovascular physiology:
- CO = Heart rate × Stroke volume
- TPR = Resistance in all systemic vessels combined (mainly arterioles)
MAP can be altered by changing either CO, TPR, or both. Importantly, CO and TPR are not independent - if TPR doubles, CO will simultaneously decrease by nearly half, so MAP rises only modestly.
- Costanzo Physiology 7th Edition, p. 550
3. Regulation of Blood Pressure
The body maintains MAP at approximately 100 mm Hg through three overlapping systems operating at different time scales:
A. Short-Term: Baroreceptor Reflex (seconds)
This is the fastest blood pressure regulator - neurally mediated and active within seconds.
Baroreceptors are stretch-sensitive mechanoreceptors located in:
- The carotid sinus (wall of the internal carotid artery)
- The aortic arch
How it works when BP rises:
- Baroreceptors detect stretch → increased firing rate in CN IX (glossopharyngeal) and CN X (vagus)
- Signals travel to the nucleus tractus solitarius (NTS) in the medulla
- NTS directs:
- Increased parasympathetic outflow → slows SA node → ↓ heart rate
- Decreased sympathetic outflow → ↓ HR, ↓ contractility, arteriolar vasodilation (↓ TPR), venodilation (↑ venous capacitance)
- CO ↓ + TPR ↓ → MAP returns toward 100 mm Hg
How it works when BP falls (e.g., hemorrhage):
- Opposite response: ↑ sympathetic, ↓ parasympathetic → ↑ HR, ↑ contractility, vasoconstriction → ↑ MAP
The baroreceptor reflex can be clinically tested with the Valsalva maneuver (forced expiration against a closed glottis).
Baroreceptor reflex pathway - Costanzo Physiology 7th Edition
B. Medium-to-Long-Term: Renin-Angiotensin-Aldosterone System (RAAS) (hours-days)
This hormonal system is slower but more sustained than the baroreceptor reflex. Activated when MAP falls:
-
↓ Renal perfusion pressure → mechanoreceptors in afferent arterioles → juxtaglomerular cells convert prorenin to renin
- Also stimulated by: renal sympathetic nerve activity, β₁ agonists
- Inhibited by: β₁ antagonists (e.g., propranolol)
-
Renin cleaves angiotensinogen (from liver) → Angiotensin I (inactive decapeptide)
-
Angiotensin-Converting Enzyme (ACE) in lungs/kidneys converts Angiotensin I → Angiotensin II (active octapeptide)
-
Angiotensin II acts on multiple targets via AT1 receptors:
- Adrenal cortex (zona glomerulosa) → secretes aldosterone → ↑ Na⁺ reabsorption in distal tubule/collecting duct → ↑ ECF volume → ↑ blood volume → ↑ CO → ↑ MAP
- Arterioles (via IP3/Ca²⁺) → vasoconstriction → ↑ TPR → ↑ MAP
- Hypothalamus → ↑ thirst + stimulates ADH → ↑ water reabsorption → ↑ blood volume
- Renal proximal tubule → stimulates Na⁺-H⁺ exchange → ↑ Na⁺ + HCO₃⁻ reabsorption
- Costanzo Physiology 7th Edition, p. 680-700
C. Long-Term: Renal-Body Fluid System (days-weeks)
The most powerful long-term control mechanism. If blood volume rises, arterial pressure rises, which causes:
- Pressure diuresis - kidneys excrete more water
- Pressure natriuresis - kidneys excrete more sodium
An increase in arterial pressure of just a few mm Hg can double renal output of water and salt. This system is fundamental for long-term BP homeostasis and is the primary reason the kidney is central to hypertension.
- Guyton and Hall Textbook of Medical Physiology, p. 3892-3914
D. Other Regulatory Mechanisms
| Mechanism | Effect |
|---|---|
| ANP/BNP (atrial/brain natriuretic peptides) | Released by cardiac stretch → natriuresis, vasodilation → ↓ BP |
| Endothelin | Potent vasoconstrictor from endothelium |
| Nitric oxide (NO) | Vasodilator from endothelium → ↓ TPR |
| ADH (vasopressin) | Water retention + vasoconstriction |
| Local autoregulation | Organs regulate their own blood flow independently |
4. Measurement of Blood Pressure
Clinical Methods
| Method | Details |
|---|---|
| Sphygmomanometer + Korotkoff sounds | Standard indirect method; cuff inflated above SBP, slowly deflated; first sound = SBP, disappearance = DBP |
| Electronic oscillometric | Used in automated/ambulatory devices |
| Intra-arterial catheter | Gold standard; continuous waveform; used in ICU |
High-fidelity electronic transducers convert BP to electrical signals using:
- Capacitance transducers (metal membrane)
- Inductance transducers (iron slug in coil)
- Resistance transducers (strain gauge wire)
These can record up to 500 pressure cycles/sec.
- Guyton and Hall Textbook of Medical Physiology, p. 2079-2104
5. Blood Pressure Classification (ACC/AHA 2017)
| Category | Systolic (mm Hg) | Diastolic (mm Hg) | |
|---|---|---|---|
| Normal | < 120 | AND | < 80 |
| Elevated | 120-129 | AND | < 80 |
| Stage 1 Hypertension | 130-139 | OR | 80-89 |
| Stage 2 Hypertension | ≥ 140 | OR | ≥ 90 |
| Hypertensive Crisis | > 180 | OR | > 120 |
Note: The 2024 ESC Guidelines define hypertension as SBP ≥ 140 and/or DBP ≥ 90 mm Hg - a somewhat higher threshold than ACC/AHA.
6. Hypertension
Types
- Primary (Essential) hypertension (~90-95%): No identifiable cause; involves genetic, environmental (salt intake, obesity, stress), and renal factors
- Secondary hypertension (~5-10%): Identifiable causes including renal artery stenosis, primary hyperaldosteronism, pheochromocytoma, Cushing syndrome, obstructive sleep apnea, medications (NSAIDs, OCP, steroids)
Pathophysiology of Essential Hypertension
- Increased sympathetic nervous system activity
- Dysregulation of RAAS
- Impaired renal sodium excretion (altered pressure-natriuresis curve)
- Endothelial dysfunction (reduced NO)
- Vascular remodeling (increased TPR)
Complications (End-Organ Damage)
| Organ | Complication |
|---|---|
| Heart | LV hypertrophy, heart failure, CAD, MI |
| Brain | Stroke, hypertensive encephalopathy |
| Kidneys | Hypertensive nephrosclerosis, CKD |
| Eyes | Hypertensive retinopathy |
| Peripheral vessels | PAD, aortic aneurysm |
Hypertension carries a 2.5-fold age-adjusted risk for peripheral arterial disease in men and 3.9-fold in women.
7. Treatment of Hypertension
Lifestyle Modifications (First-line for all stages)
- DASH diet (↓ sodium, ↑ potassium)
- Weight loss
- Regular aerobic exercise
- Limiting alcohol
- Smoking cessation
Pharmacological Treatment
Treatment Goals:
- Most patients: target < 130/80 mm Hg (ACC/AHA 2017)
- 2024 ESC Guidelines: target SBP 120-129 mm Hg for patients on medication
- CKD (KDIGO 2020): SBP < 120 mm Hg
First-line Drug Classes:
| Drug Class | Examples | Mechanism |
|---|---|---|
| Thiazide diuretics | Hydrochlorothiazide, chlorthalidone | ↓ Na⁺ reabsorption → ↓ blood volume |
| ACE Inhibitors (ACEi) | Lisinopril, enalapril, captopril | Block ACE → ↓ Angiotensin II → vasodilation + ↓ aldosterone |
| Angiotensin Receptor Blockers (ARBs) | Losartan, valsartan | Block AT1 receptor → same downstream effect as ACEi |
| Calcium Channel Blockers (CCBs) | Amlodipine, nifedipine (DHP), verapamil | Block Ca²⁺ entry in vascular smooth muscle → vasodilation |
| Beta-blockers | Metoprolol, carvedilol | Block β₁ → ↓ HR, ↓ contractility, ↓ CO; also ↓ renin release |
| Alpha-blockers | Doxazosin | Block α₁ → vasodilation |
| Aldosterone antagonists | Spironolactone | Block aldosterone → natriuresis |
Population-Specific First-line Choices:
- Non-Black patients: Thiazide, ACEi, ARB, or CCB
- Black patients: Thiazide or CCB preferred (ACEi less effective as monotherapy)
- Diabetes: ACEi or ARB (renoprotective)
- CKD: ACEi or ARB
- Heart failure with reduced EF: ACEi/ARB + beta-blocker + aldosterone antagonist
- Post-MI: Beta-blocker + ACEi
Combination Therapy:
-
If SBP > 20 mm Hg above goal OR DBP > 10 mm Hg above goal: start two drugs simultaneously
-
Preferred combinations: ACEi/ARB + CCB, or ACEi/ARB + thiazide
-
Avoid: Non-dihydropyridine CCB (e.g., verapamil) + beta-blocker (both cause bradycardia)
-
Avoid: ACEi + ARB combination (risk of hypotension, hyperkalemia, renal failure)
-
Lippincott Illustrated Reviews Pharmacology, p. 4086-4089
8. Hypotension & Low Blood Pressure
Hypotension is generally defined as SBP < 90 mm Hg or MAP < 65 mm Hg.
Common causes:
- Hypovolemia (dehydration, hemorrhage)
- Cardiogenic (MI, heart failure, arrhythmia)
- Distributive (sepsis, anaphylaxis, neurogenic)
- Obstructive (PE, tension pneumothorax, cardiac tamponade)
- Orthostatic (autonomic dysfunction, medications)
The baroreceptor reflex is the first response to acute hypotension: ↑ sympathetic output → ↑ HR + vasoconstriction → attempt to restore BP.
9. Blood Pressure in Special Populations
| Population | Key Consideration |
|---|---|
| Elderly (>65) | Higher SBP targets acceptable if frailty or orthostatic symptoms; SBP > 85 years → individualize |
| Pregnancy | Hypertension in pregnancy = pre-eclampsia risk; severe HTN treated with labetalol, hydralazine, nifedipine; ACEi/ARBs contraindicated |
| Diabetes | ACEi/ARB preferred to slow progression of diabetic nephropathy |
| CKD | ACEi/ARB first-line; target SBP < 120 mm Hg |
| Children/Adolescents | BP classified as percentile-based for age, sex, and height |
10. Summary: The BP Control Network
SECONDS: Baroreceptor reflex → neural adjustments to HR, TPR
↓
HOURS: RAAS → Angiotensin II → aldosterone → Na⁺ retention → ↑ blood volume
↓
DAYS-WEEKS: Renal pressure-natriuresis → long-term volume & pressure equilibrium
All three systems work in concert. Hypertension develops when one or more of these mechanisms is dysregulated - most commonly through impaired renal sodium excretion, overactive RAAS, or excessive sympathetic tone.
Sources:
- Costanzo Physiology 7th Edition - Regulation of Arterial Pressure (baroreceptor reflex, RAAS)
- Guyton and Hall Textbook of Medical Physiology - Nervous regulation, renal-body fluid system
- Lippincott Illustrated Reviews Pharmacology - Antihypertensive treatment strategies
- 2024 ESC Guidelines for Hypertension - Target BP 120-129 mm Hg for medicated patients
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