Regulation of Blood Pressure & Blood Flow – Key Vocabulary

Short-Term Neural Control of Blood Pressure

• Core idea: moment-to-moment adjustments in arterial pressure are achieved by altering peripheral resistance through changes in vessel diameter (vasoconstriction ↔ vasodilation).

• Reflex Arc Components
– Cardiovascular centers in the medulla oblongata (vasomotor, cardioacceleratory, cardioinhibitory).
– Receptor inputs:
• Baroreceptors in carotid sinuses, aortic arch, and other large arteries (pressure-sensitive).
• Chemoreceptors that respond to ↑ $CO<em>2$ , ↓ $pH$ , or ↓ $O</em>2$ in aortic arch & neck arteries.
– Higher brain centers (hypothalamus & cortex) can override medulla to redistribute flow during exercise or thermoregulation.

• Baroreceptor Reflex (classic sequence)

Stimulus: ↑ arterial pressure (above normal range).
Baroreceptors stretch → ↑ firing to medulla.
Medulla: stimulates cardioinhibitory & inhibits vasomotor + cardioacceleratory centers.
Effects: ↓ sympathetic drive → ↓ HR, ↓ contractility, generalized vasodilation.
Result: ↓ $CO$ & ↓ $TPR$ → BP returns to set point.
• Reverse sequence occurs when BP falls (↓ firing → sympathetic activation → vasoconstriction + ↑ HR/SV).

• Chemoreceptor Reflex
– Triggered by hypercapnia, acidosis, or hypoxia.
– Medulla response: ↑ sympathetic outflow → peripheral vasoconstriction; cardioacceleratory centers ↑ HR/SV → restores gas homeostasis & supports cerebral/respiratory perfusion.

• Characteristics
– Very rapid (seconds).
– Adapt quickly to sustained changes; therefore NOT effective for chronic hypertension/hypotension.

• Example/Scenario
– Standing up quickly: transient ↓ venous return → ↓ BP → baroreflex raises HR & constricts vessels to prevent fainting.

Short-Term Hormonal Control

• Hormones modulate BP either by altering peripheral resistance (fast) or blood volume (slower interface with renal system).

• Key agents & actions (Table 19.2 summarized):
– Adrenal medulla catecholamines (epinephrine, norepinephrine)
• Sites: heart ( $\beta<em>1$ ) & arterioles ( $\alpha</em>1$ ).
• ↑ $CO$ (↑ HR & contractility); ↑ $TPR$ (vasoconstriction).

– Angiotensin II
• Potent systemic vasoconstrictor → ↑ $TPR$ .

– Antidiuretic hormone (ADH / vasopressin)
• High levels cause vasoconstriction; also promotes water re-absorption → ↑ blood volume.

– Aldosterone (mineralocorticoid)
• Promotes renal $Na^+$ & water retention → ↑ blood volume.

– Atrial natriuretic peptide (ANP)
• Antagonizes aldosterone → ↑ $Na^+$ /water loss, vasodilation → ↓ blood volume & $TPR$ .

• Integration: Hormonal responses complement neural reflexes—e.g., catecholamines reinforce sympathetic discharge during “fight-or-flight.”

Long-Term Renal Regulation

• Baroreceptors adapt to sustained pressure; chronic control shifts to the kidneys.

• Mechanisms
– Direct renal: pressure diuresis & natriuresis—↑ BP → ↑ renal filtration → ↑ urine output → ↓ blood volume.
– Indirect renal (Renin-Angiotensin-Aldosterone System, RAAS)
1. ↓ BP → kidneys release renin.
2. Renin converts angiotensinogen → angiotensin I; ACE converts → angiotensin II.
3. Angiotensin II: vasoconstriction + stimulates aldosterone & ADH, thirst → collectively ↑ volume & $TPR$ .

• Outcome: Maintains arterial pressure over hours→days.

Clinical Applications

• Transient BP Variations
– Normal during posture change, exercise, emotional stress, fever.
– Influenced by age, sex, weight, race, mood, posture.

• Hypertension
– Defined: sustained BP ≥ $140/90\ \text{mmHg}$ .
– “Pre-hypertension” = values between normal & hypertensive thresholds.
– Consequences: heart failure, vascular disease, renal failure, stroke.
– Primary (essential) ≈ 90 %: multifactorial (genetics, diet, obesity, age, DM, stress, smoking).
• Management: ↓ salt/fat, weight loss, exercise, smoking cessation, antihypertensive drugs.
– Secondary: identifiable cause (renal artery stenosis, kidney disease, endocrine disorders such as hyperthyroidism, Cushing’s) → treat underlying issue.

• Hypotension
– Defined: BP < $90/60\ \text{mmHg}$ .
– Usually benign; often correlated with longevity & cardiovascular fitness.
– Variants:
• Orthostatic: transient dizziness when rising quickly (baroreflex lag).
• Chronic: may signal malnutrition, Addison’s disease, hypothyroidism.
• Acute: key warning sign for circulatory shock.

• Circulatory Shock (inadequate tissue perfusion)
– Hypovolemic: massive blood/fluid loss.
– Vascular: extreme vasodilation → ↓ $TPR$ (e.g., anaphylaxis, septic shock).
– Cardiogenic: pump failure; heart can’t maintain $CO$ .

Control of Blood Flow (Tissue Perfusion)

• Functions supported by regional blood flow

Delivery of $O_2$ & nutrients / removal of wastes.
Gas exchange in lungs.
Nutrient absorption in GI tract.
Urine formation in kidneys.

• Regulation of Flow
– Extrinsic (systemic): sympathetic nerves & circulating hormones distribute total blood flow—constrict arterioles in low-priority regions to maintain MAP.
– Intrinsic (local / autoregulation): tissues adjust their own arteriolar diameter to match metabolic demand (paracrine factors, myogenic responses).

• Comparison
– Extrinsic: “outside” the organ; neural or endocrine signals.
– Intrinsic: “within” the organ; relies on local chemical milieu and smooth-muscle properties.

• Goal: ensure each organ receives “just right” flow for current activity without compromising systemic pressure.

Key Terminology & Relationships

• Cardiac Output: $CO = HR \times SV$
• Mean Arterial Pressure: $MAP = CO \times TPR$
• Total Peripheral Resistance (TPR): cumulative friction in systemic circulation; mainly governed by arteriolar radius ( $r$ ). Poiseuille relationship $TPR \propto \dfrac{1}{r^4}$ underscores potency of vasoconstriction.

Practical / Ethical Notes

• Patient education on lifestyle modification is ethically important—non-pharmacologic measures can markedly reduce primary hypertension risk.
• Recognition of orthostatic hypotension in elderly prevents falls & injury—clinical vigilance has direct quality-of-life implications.
• Appropriate triage of acute hypotension or shock is life-saving; rapid identification of type (hypovolemic vs. cardiogenic, etc.) guides therapy.