CARDIOVASCULAR PHYSIOLOGY:
Regional Circulation
Oluwadare Ogunlade
Department of Physiological Sciences,
Obafemi Awolowo University, Ile-Ife.
1
Learning Objectives
1. Define regional circulation – How blood is distributed to
organs based on metabolic demand.
2. Understand autoregulation – Myogenic vs. metabolic
mechanisms.
3. Compare organ-specific circulations – Brain (rigid
autoregulation) vs. skin (thermoregulatory shifts).
4. Link to pathology – How dysfunction leads to ischemia,
stroke, or hypertension.
2
Introduction
• Blood flow is not uniform—it prioritizes organs with
high metabolic demand (brain, heart).
• Total CO ≈ 5 L/min, distributed as:
• Liver + GI: 25%
• Kidneys: 20%
• Brain: 15%
• Heart: 5%
• Skeletal muscle: 15% (↑ to 80% during exercise).
• Regulation tiers:
• Local (metabolic, autoregulation).
• Systemic (sympathetic, hormones).
3
General Principles of Blood Flow
General Principles of Blood Flow
Ohm’s Law (Q = ΔP/R):
• ΔP = Mean arterial pressure (MAP) – Venous pressure.
• Resistance (R) depends on:
• Vessel radius (most critical) – Small change → large
resistance change (r⁴ effect).
• Blood viscosity (e.g., polycythemia ↑ resistance).
• Vessel length (constant in adults).
Poiseuille’s Law:
• Explains why arterioles (small radius) are major
resistance regulators.
4
Poiseuille’s Law & Blood Flow
• Flow ∝ r⁴ → Small radius changes dramatically alter
flow.
• Example:
• 2× wider arteriole → 16× more flow (2⁴ = 16).
• 50% constriction → 94% less flow (0.5⁴ = 0.0625).
5
Autoregulation of Blood Flow
▪ Definition: Organs maintain constant flow despite BP
changes (e.g., MAP 60–140 mmHg in brain).
Mechanisms:
1. Myogenic Response:
1. Vascular smooth muscle contracts when stretched (↑ BP
→ vasoconstriction).
2. Example: Renal afferent arteriole.
2. Metabolic Theory:
1. ↓ O₂, ↑ CO₂, ↑ H⁺, ↑ adenosine → vasodilation.
2. Example: Coronary arteries during exercise.
Clinical: Hypertension damages autoregulation → Organ
ischemia.
6
Neural Regulation(Autonomic Tone)
Sympathetic NS (Vasoconstriction):
• α1 receptors on arterioles (skin, kidneys, GI tract).
• Exceptions:
• β2 receptors in skeletal/heart muscles → vasodilation during
stress.
• Parasympathetic NS: Minimal role (vasodilation in salivary
glands, penis/clitoris).
Baroreceptor Reflex:
• Acute BP drop → ↑ Sympathetic tone → Vasoconstriction
(except in brain/heart).
7
Hormonal Regulation
Vasoconstrictors:
• Norepinephrine (α1), Angiotensin II (renal efferent
arteriole), ADH (V1 receptors).
Vasodilators:
• ANP (opposes angiotensin II), Bradykinin (inflammatory
response), NO (shear stress).
Example:
• Hemorrhage → ↑ angiotensin II → vasoconstriction +
aldosterone → BP maintenance.
8
Coronary Circulation
• High O₂ extraction (70–80% at rest → little reserve).
• Phasic Flow:
• Left ventricle: Flow ↓ during systole (compression).
• Right ventricle: Some flow in systole (lower pressure).
Regulation:
• Adenosine (↑ during hypoxia → potent vasodilation).
• Sympathetic: β2 dilation > α1 constriction during exercise.
Clinical: Atherosclerosis- Coronary artery disease;
Myocardial Ischemia and Heart Attack
9
Cerebral Circulation
Autoregulation (60–140 mmHg MAP):
• CO₂: Most potent regulator (↑CO₂ → vasodilation →
hyperemia in stroke).
• O₂: Severe hypoxia (<50 mmHg) → vasodilation.
Blood-Brain Barrier (BBB):
• Tight junctions protect brain from toxins.
• Disrupted in: Hypertension, trauma, infection.
Clinical:
• Stroke: Loss of autoregulation → Ischemic penumbra.
• Cushing’s Reflex: ↑ ICP → hypertension +
bradycardia.
10
Skeletal Muscle Circulation
At Rest: Sympathetic α1 tone (vasoconstriction).
Exercise:
• Metabolic hyperemia: ↑ K⁺, lactate, adenosine →
vasodilation.
• Sympathetic β2 activation: Overrides α1 → ↑ flow 20x.
Example:
• Cold hands in stress (α1 constriction) vs. warm muscles (β2
dilation).
11
Cutaneous Circulation
Thermoregulation:
• Heat loss: AV shunts open → ↑ skin flow →
radiant cooling.
• Cold: Sympathetic α1 → vasoconstriction
(frostbite risk).
Blushing: Emotion → NO release → vasodilation.
12
Renal Circulation
Autoregulation (80–180 mmHg):
• Myogenic + TGF: Macula densa senses NaCl → adjusts afferent
resistance.
•
Angiotensin II Constricts efferent arteriole → maintains GFR
during hypotension.
Renal circulation plays a role in glomerular filtration and
urine formation.
Clinical:
• ACE inhibitors → dilate efferent arteriole → ↓ GFR in renal
artery stenosis.
• ACE 2 impairment- Unopposed angiotensin II activities
13
Renal Circulation: Glomerular
Filtration and Urine Formation
1. Autoregulation (80–180 mmHg):
2. Renal Blood Flow (RBF) & Filtration:
1. RBF = 20–25% of CO (~1.1 L/min).
2. Glomerular Filtration Rate (GFR) = 125 mL/min (180
L/day).
3. Filtration Fraction (FF) = GFR/RPF ≈ 20% (RPF = Renal
Plasma Flow).
3. Pressure Gradients Driving Filtration:
1. Forces (Starling’s Equation):
1. PGC (Glomerular Hydrostatic Pressure) = 55 mmHg (main driver).
2. PBS (Bowman’s Capsule Pressure) = 15 mmHg (opposes).
3. πGC (Colloid Osmotic Pressure) = 30 mmHg (opposes).
2. Net Filtration Pressure (NFP) = PGC – PBS – πGC ≈ 10
14 mmHg.
Renal Circulation Regulation
1. Autoregulation of GFR & RBF:
1. Myogenic Mechanism: Stretch-induced afferent arteriole
constriction (↑BP → ↓RBF).
2. Tubuloglomerular Feedback (TGF):
1. Macula densa senses ↑NaCl → signals afferent constriction (↓GFR
if flow too high).
2. Hormonal Control:
1. Angiotensin II: Constricts efferent arteriole → maintains
GFR during hypotension.
2. ANP: Dilates afferent arteriole → ↑GFR (volume overload
response).
3. Prostaglandins: Vasodilate afferent arteriole (protects RBF
in shock).
15
Renin-Angiotensin-Aldosterone Pathways:
Classic and Alternate Pathways
16
Pulmonary Circulation
Low Pressure (15/8 mmHg):
• Prevents pulmonary edema.
•
Hypoxic Vasoconstriction:
• Unique to lungs → Diverts blood to ventilated areas (opposite
systemic response).
Example: High altitude → pulmonary hypertension.
17
Hepatic Circulation
Dual Supply:
• Portal vein (75%) – Nutrient-rich, low O₂.
• Hepatic artery (25%) – High O₂.
Postprandial Hyperemia: ↑ GI flow → ↑ portal vein flow.
Clinical: Cirrhosis → portal hypertension → varices.
18
Clinical Correlations
1. Stroke: Autoregulation failure →
hemorrhage/infarction.
2. MI: Coronary blockage → demand ischemia.
3. Renal Disease: Renal failure, hypertension
4. Sepsis: Pathologic vasodilation → distributive shock.
19
