Guyton and Hall Physiology Chapter 21 - Muscle Blood Flow and Cardiac Output During Exercise

Blood Flow in Skeletal Muscles

• During exercise, the skeletal muscles require a large amount of blood flow to meet their oxygen demands.

Local Control of Blood Flow

• Decreased oxygen levels in muscle tissues cause vasodilation, increasing blood flow to the muscles.
• This dilation happens primarily through the opening of dormant capillaries.
• The distance oxygen must diffuse from capillaries to muscle fibers decreases during exercise, improving oxygen delivery.

Nervous Control of Blood Flow

• Sympathetic vasoconstrictor nerves: Secrete norepinephrine, reducing blood flow to resting muscles.
• This mechanism can decrease blood flow to as little as one-half to one-third of normal levels.

Cardiac Output During Exercise

• Increased oxygen demand by skeletal muscles results in a need for an increased cardiac output.
• Cardiac output can increase to four to five times normal in non-athletes and six to seven times normal in trained athletes.

Coronary Circulation

• Coronary arteries supply the heart muscle with blood and oxygen.
• The left coronary artery nourishes the anterior and left lateral parts of the left ventricle.
• The right coronary artery supplies most of the right ventricle and the inferior part of the left ventricle in most people.
• Coronary blood flow is closely tied to the metabolic needs of the heart muscle, especially oxygen demand.

Cardiac Muscle Compression

• The heart muscle compresses coronary blood vessels during systole, reducing blood flow.

Factors Affecting Coronary Flow

• Heart rate, blood pressure, and contractility all contribute to changes in coronary blood flow.

Skeletal Muscle Blood Flow Rate

• At rest, skeletal muscle blood flow averages 3 to 4 ml/min/100 g of muscle.
• During intense exercise, this flow can increase 25 to 50-fold, reaching 100 to 200 ml/min/100 g of muscle.
• Peak blood flows as high as 400 ml/min/100 g of muscle have been reported in endurance athletes.

Blood Flow During Muscle Contractions

• Blood flow to the muscle fluctuates with each contraction, increasing and decreasing with the rhythm of the exercise.
• After exercise, blood flow to the muscles remains elevated for a few seconds before returning to normal.

Coronary Blood Flow Regulation in Heavy Exercise

• Despite increased cardiac output, coronary venouse flow decreases during heavy exercise
• Sympathetic stimulation of the heart increases blood flow back into the heart through the Thebesian veins

Normal Coronary Circulation

• Makes up about 5% of cardiac output.
• At rest, blood flow is 70 ml/min/100g of heart weight, or about 225 ml/min.
• During strenuous exercise, cardiac output increases by 4- to 7-fold.
• The heart works harder, pumping blood against higher arterial pressure.
• Coronary blood flow increases 3- to 4-fold to meet the increased metabolic demands of the heart muscle.

Cardiac Muscle Compression

• Compression of the heart muscle during systole reduces coronary blood flow.
• During diastole, coronary blood flow increases as the heart relaxes.

Oxygen Demand and Coronary Blood Flow

• Coronary blood flow is regulated to meet the oxygen demand of the heart muscle.
• About 70% of oxygen in the coronary arterial blood is removed as blood flows through the heart muscle.
• Coronary blood flow increases proportionate to the need for oxygen in the heart muscle.

Angina Pectoris

• Pain associated with insufficient coronary blood flow.
• Pain is described as hot, pressing, and constricting.
• This type of pain usually causes the patient to cease unnecessary activity.

Drug Treatment for Angina Pectoris

• Vasodilators like nitroglycerin can relieve pain during an acute anginal attack.
• Chronic stable angina can be treated with drugs like angiotensin-converting enzyme inhibitors, angiotensin receptor blockers, calcium channel blockers, and ranolazine.
• Beta blockers block sympathetic beta-adrenergic receptors, reducing heart rate and metabolic demand.

Surgical Treatment of Coronary Artery Disease

• Aortic-coronary bypass surgery involves grafting a vein from the aorta to a coronary artery beyond a blockage.
• Stents are placed inside dilated coronary arteries to prevent restenosis.
• Drug-eluting stents release drugs to prevent excessive scar tissue formation.
• Laser therapy can dissolve atherosclerotic lesions without damaging the arterial wall.

Blood Flow Regulation in Skeletal Muscle

• Strenuous exercise requires significant blood flow to skeletal muscles.
• Exercise increases blood flow to skeletal muscles by opening dormant capillaries
• This increases the diffusion surface area for oxygen and nutrients.

Control of Skeletal Muscle Blood Flow

• Increased metabolic demand in muscles during exercise reduces oxygen levels.
• Low oxygen levels activate vasodilators that increase local blood flow.

Coronary Circulation

• Coronary artery disease is a leading cause of death in developed countries.
• Coronary arteries deliver blood to the heart muscle.
• About 5% of cardiac output is dedicated to the coronary circulation.

Coronary Blood Flow

• During exercise, the heart pumps 4-7 times more blood than at rest.
• Coronary blood flow increases by 3-4 times during exercise.

Physiology of Cardiac Muscle Metabolism

• Cardiac muscle metabolism is similar to other muscle tissue.
• Sympathetic nervous system stimulation generally constricts coronary vessels but can also cause dilation.
• Metabolic factors override direct nervous system stimulation.

Ischemic Heart Disease

• Ischemic heart disease is the most common cause of death in developed countries.
• Atherosclerosis is a major cause of ischemic heart disease.
• Atherosclerosis reduces coronary blood flow, leading to heart attacks.

Myocardial Infarction

• A large ischemic area can lead to the death of heart muscle cells (necrosis)
• The area of myocardial infarction is surrounded by a zone of nonfunctional muscle and a zone of weakly contracting muscle.
• Cardiac muscle cells in the infarcted area are replaced by scar tissue.

Coronary Ischemia

• A significant portion of the coronary arteries may become blocked due to atherosclerosis.
• Blockage of the coronary arteries causes pain, known as angina pectoris.
• Ischemia can lead to abnormal conduction pathways within the heart.

Cardiac Output Control During Exercise

• Exercise places a considerable strain on the circulatory system.
• Increased blood flow in skeletal muscle requires a significant increase in cardiac output.
• During exercise, cardiac output increases by 4- to 7-fold.

Effects of Coronary Ischemia

• Loss of adenosine, an important vasodilator, can exacerbate ischemia.
• Myocardial ischemia damages and kills cardiac muscle cells.
• Ischemia can lead to arrhythmias and heart failure.

Blood Flow and Cardiac Output During Exercise

• Venous return increases with strenuous exercise resulting in rise in cardiac output.
• Increase in venous return during strenuous exercise leads to a rise in right atrial pressure.
• Sympathetic nervous response is activated during exercise resulting in vasodilation in active muscles.
• During tense conditions vasodilation occurs in a few active muscles but vasoconstriction occurs in other areas of the body.
• Increase in mean arterial pressure is minimal during massive whole-body exercise due to vasodilation in active muscle masses.
• Mean arterial pressure can increase to as high as 170 mm Hg during tense conditions with limited muscle activity.

Coronary Blood Flow and Regulation

• The heart receives its blood supply through coronary arteries.
• Only the inner one-tenth millimeter of the endocardial surface receives nutrition directly from the blood inside the cardiac chambers.
• The left coronary artery supplies blood to the anterior and left lateral portions of the left ventricle.
• The right coronary artery supplies most of the right ventricle as well as the posterior part of the left ventricle in 80-90% of people.
• Coronary blood flow is regulated in proportion to the need of the cardiac musculature for oxygen.
• During systole, capillary blood flow in the left ventricle muscle falls to a low level due to cardiac muscle compression.
• About 70% of the oxygen in the coronary arterial blood is removed as blood flows through the heart muscle.

Nervous Stimuli & Coronary Vasculature

• Parasympathetic stimulation releases acetylcholine which dilates coronary arteries.
• Sympathetic innervation of coronary vessels is more extensive than parasympathetic innervation.
• Norepinephrine and epinephrine can dilate coronary vessels depending on the presence of receptor types.
• Alpha receptors are more prevalent in epicardial coronary vessels and cause vasoconstriction.
• Beta receptors are more prevalent in intramuscular arteries and cause vasodilation.
• Anastomoses among the smaller arteries (20-250 micrometers in diameter) provide collateral flow during coronary artery occlusion.
• Collateral flow can reach normal or almost normal coronary flow within a month following coronary artery occlusion.

Ischemic Heart Disease

• Myocardial infarction occurs when blood flow to an area of cardiac muscle becomes insufficient.
• Infarcted areas are characterized by engorged blood vessels, bluish-brown hue, and edema.
• Cardiac muscle cells die within a few hours of almost no blood supply.
• Cardiac muscle requires 1.3 ml of oxygen/100 g of muscle tissue/min to remain alive.
• 8 ml oxygen/100 g are delivered to the normal resting left ventricle each minute.
• 15-30% of normal resting coronary blood flow is sufficient to prevent cardiac muscle death.
• Coronary steal syndrome occurs when increased workload of the heart during exercise diverts blood flow away from ischemic areas, worsening the condition.
• The recovery process for myocardial infarction requires absolute body rest to prevent further damage.

Angina Pectoris

• Angina pectoris is characterized by severe pain typically felt beneath the upper sternum.
• Angina pectoris pain is often referred to other areas of the body, such as the left arm, shoulder, neck, and face.
• Episodes of angina pectoris usually last for a few minutes, but can be constant in severe cases.
• The pain is intensified by exercise, emotional strain, cold temperatures, and a full stomach.