Chapter 21: Muscle Blood Flow and Cardiac Output During Exercise; the Coronary Circulation and Ischemic Heart Disease

Written By: Razaullah Khattak (DPT Medical Professional)

Student Source Reference: Guyton and Hall Textbook of Medical Physiology

Chapter: 21

To read more about Cardiac Arrhythmias and Their Electrocardiographic Interpretation, check out Chapter 13🔗:

https://mednotesbyrazaullah.blogspot.com/2026/08/chapter-13-cardiac-arrhythmias-and.html

Muscle Blood Flow and Cardiac Output During Exercise

The quantitative regulation of skeletal muscle blood flow and coronary circulation represents a vital physiological response to physical stress and cardiac exertion. During strenuous physical exercise, local metabolic factors and mass sympathetic nervous system activation work in tandem to dramatically elevate tissue perfusion. Skeletal muscle blood flow increases up to 50-fold, driven by arteriolar vasodilation, dormant capillary recruitment, and a substantial rise in mean systemic arterial pressure. Concurrently, the systemic circulation undergoes powerful venoconstriction, raising mean systemic filling pressure to enhance venous return and support elevated cardiac output.

In the heart, coronary artery blood supply is dynamically adjusted to meet the high metabolic demands of the myocardium. While skeletal muscle perfusion increases during active contraction phases, left ventricular coronary capillary flow drops significantly during systole due to intramuscular vessel compression. Consequently, the left ventricle receives the vast majority of its nutrient blood supply during diastole when the myocardium relaxes. Understanding these dual mechanisms of blood flow regulation is essential for evaluating circulatory adaptation during extreme physiological stress and the development of ischemic heart conditions.

Overview of Muscle and Coronary Circulation

• Chapter covers skeletal muscle and coronary blood flow.
• Vascular resistance is controlled by local tissue metabolism.
• Discusses cardiac output control during strenuous exercise.
• Reviews characteristics of acute heart attacks.
• Explains mechanisms causing angina pectoris pain.

Skeletal Muscle Blood Flow Dynamics

• Exercise is a major stress on circulatory system.
• Large muscle mass requires immense blood volume.
• Cardiac output rises 4-to-5-fold in nonathletes.
• Cardiac output rises 6-to-7-fold in trained athletes.
• Resting muscle blood flow averages 3 to 4 ml/min/100g.
• Extreme exercise increases flow 25- to 50-fold.
• Exercise flow reaches 100 to 200 ml/min/100g.
• Peak thigh muscle flow reaches 400 ml/min/100g.

Phasic Flow and Capillary Recruitment

• Muscle contraction compresses intramuscular blood vessels.
• Blood flow drops during active muscle contraction.
• Flow rises significantly between rhythmic muscle contractions.
• Sustained tetanic contraction almost stops blood flow.
• Stopped flow causes rapid weakening of muscle contraction.
• Dormant capillaries remain closed during resting conditions.
• Strenuous exercise opens all resting muscle capillaries.
• Capillary opening decreases nutrient diffusion distance.
• Capillary surface area increases 2- to 3-fold.

Local Metabolic Control of Muscle Flow

• Local chemicals act on arterioles causing vasodilation.
• Tissue oxygen decrease strongly enhances muscle flow.
• Active muscles rapidly consume available tissue oxygen.
• Oxygen deficiency releases local tissue vasodilator substances.
• Hypoxia prevents arteriolar walls from maintaining contraction.
• Adenosine acts as an important local vasodilator.
• Adenosine infusion cannot sustain vasodilation past two hours.
• Potassium ions maintain sustained muscle capillary vasodilation.
• Adenosine triphosphate (ATP) contributes to active vasodilation.
• Lactic acid buildup promotes local arteriolar dilation.
• Carbon dioxide accumulation enhances local blood flow.

Nervous Regulation of Muscle Vessels

• Sympathetic vasoconstrictor nerves supply skeletal muscles.
• Nerve endings secrete norepinephrine to constrict vessels.
• Maximal sympathetic activation cuts resting flow by half.
• Vasoconstriction maintains arterial pressure during shock.
• Adrenal medullae secrete norepinephrine and epinephrine.
• Circulating norepinephrine causes systemic vascular constriction.
• Epinephrine stimulates beta-adrenergic vasodilator receptors.
• Alpha-adrenergic receptors mediate norepinephrine vasoconstriction.

Mass Sympathetic Exercise Discharge

• Brain sends motor signals to contracting muscles.
• Brain simultaneously stimulates medullary vasomotor center.
• Mass sympathetic discharge activates entire circulatory system.
• Parasympathetic signals to heart are attenuated.
• Sympathetic stimulation increases heart rate significantly.
• Sympathetic activation doubles myocardial contraction strength.
• Peripheral arterioles constrict except in active muscles.
• Nonmuscular blood flow decreases to lend flow.
• Nonmuscular vasoconstriction redirects 2 L/min to muscles.
• Coronary and cerebral vessels spare vasoconstrictor effects.
• Heart and brain have poor sympathetic vasoconstrictor innervation.
• Veins contract to elevate mean systemic filling pressure.
• Venous contraction increases venous return to heart.

Arterial Pressure Rise During Exercise

• Sympathetic stimulation increases systemic arterial pressure.
• Arteriolar constriction in inactive tissues raises pressure.
• Increased cardiac pumping drives arterial pressure upward.
• Elevated mean systemic filling pressure increases pressure.
• Arterial pressure increases by 20 to 80 mm Hg.
• Tense exercise using few muscles causes high pressure.
• Localized exercise can raise mean pressure to 170 mm Hg.
• Massive whole-body exercise increases pressure only 20–40 mm Hg.
• Massive active muscle vasodilation prevents extreme pressure rise.
• Pressure elevation pushes blood through active muscle tissue.
• Increased pressure stretches vessel walls to increase flow.
• Pressure rise allows flow increases over 20-fold.

Cardiac Output Limits and Exercise

• Circulatory system limits maximum continuous muscle work.
• Increased cardiac output supplies essential muscle oxygen.
• Marathon runners achieve greatest cardiac output increases.
• Sympathetic heart stimulation shifts output curve upward.
• Maximum exercise heart rate reaches 170 to 190 bpm.
• Unstimulated cardiac output limits maximum increase to 2.5-fold.
• Untrained runners reach 4-fold cardiac output increase.
• Marathon runners reach 7-fold cardiac output increase.

Venous Return Adjustments in Exercise

• Normal venous return curve caps output at 6 L/min.
• Sympathetic venoconstriction increases mean systemic filling pressure.
• Abdominal muscle tensing compresses internal vascular beds.
• Mean systemic filling pressure rises from 7 to 30 mm Hg.
• Active muscle vasodilation rotates return curve upward.
• Muscle vasodilation decreases overall resistance to venous return.
• Combined factors shift total venous return curve upward.
• New equilibrium point B establishes high cardiac output.
• Right atrial pressure shifts slightly by only 1.5 mm Hg.
• Strong hearts lower right atrial pressure below normal.

Coronary Anatomy and Baseline Flow

• Coronary artery disease causes one-third Western deaths.
• Main coronary arteries lie on cardiac outer surface.
• Small arteries penetrate surface to nourish cardiac muscle.
• Endocardium inner 1/10 millimeter absorbs nutrition directly.
• Left coronary artery supplies anterior left ventricle.
• Left coronary artery supplies left lateral left ventricle.
• Right coronary artery supplies most of right ventricle.
• Right coronary artery supplies posterior left ventricle in 80-90%.
• Coronary sinus drains 75% left ventricular blood.
• Anterior cardiac veins drain right ventricular blood.
• Minute thebesian veins empty directly into heart chambers.
• Resting coronary flow averages 70 ml/min/100g heart weight.
• Resting coronary flow equals 225 ml/min total volume.
• Resting coronary flow equals 4 to 5 percent total output.

Coronary Dynamics During Work and Systole

• Cardiac work output increases 6-to-9-fold during exercise.
• Coronary blood flow increases 3-to-4-fold during exercise.
• Cardiac energy utilization efficiency increases during stress.
• Left ventricular capillary flow drops during cardiac systole.
• Systolic contraction compresses intramuscular ventricular vessels.
• Left ventricular muscle relaxes during cardiac diastole.
• Rapid coronary blood flow occurs during all of diastole.
• Right ventricular contraction force is much less severe.
• Right ventricular phasic flow changes are only partial.

Disclaimer: This content is for educational purposes only and not a substitute for professional medical advice.