Special Circulations: Coronary and Skeletal

Question 1

What is the dominant coronary artery?

Answer 1

It varies from individual to individual, but the right coronary is dominant in 50% of people.

Question 2

What are the determinants of coronary blood flow?

Answer 2

Aortic Pressure
Tissue Pressure
Neural
Metabolic

Question 3

What is the primary determinant of the coronary blood flow?

Answer 3

Aortic Pressure

Question 4

What is the primary regulator of the the coronary blood flow?

Answer 4

Metabolic Activity

Question 5

How does tissue pressure (mechanical) determine the coronary blood flow?

Answer 5

Tissue pressure from the LV influences the left coronary blood flow more than the right coronary and the highest tissue pressure occurs during early systole. This can cause the reversal of left coronary blood flow in early systole and it is not until early diastole when tissue pressure falls when the the maximal blood flow to the left coronary occurs.

Question 6

How much of the LV perfusion occurs during diastole?

Answer 6

60-65%

Question 7

Where is the LV EDP the greatest?

Answer 7

LV EDP is greatest near the endocardium.

Question 8

How does LV end diastolic pressure affect the endocardial and epicardial blood flow in normal conditions?

Answer 8

LV EDP is greatest near the endocardium and is the least near the epicardium so endocardial vessels are more compressed but blood flow remains equal due to the greater dilation of the endocardial vessels.

Question 9

How does LV end diastolic pressure affect the endocardial and epicardial blood flow in abnormal conditions?

Answer 9

In abnormal conditions, such as aortic stenosis, regurgitation or congestive heart failure, the greater endocardial pressure will restrict coronary blood flow and cause subendocardial ischemia.

Question 10

What happens to the endocardial tissue as the diastolic coronary blood pressure falls?

Answer 10

The endocardial blood flow will be restricted more that the epicardial blood flow because of the greater endocardial tissue pressure.

Question 11

Which tissue of the heart is most at risk for ischemia?

Answer 11

Endocardium

Question 12

Describe the neural regulation of coronary blood flow including the α, β1 and β2 receptors.

Answer 12

Neural regulation of coronary blood flow is weak.

α - activate weak vasoconstriction
β1 - cause strong vasodilation in response to increased metabolism
β2 - vasodilation but are less sensitive to NE stimulation

Question 13

What is the relationship between metabolism and coronary blood flow?

Answer 13

It is a linear relationship.

Question 14

What occurs with increased myocardial metabolism?

Answer 14

Decreased coronary resistance

Increased coronary blood flow

Question 15

What are the main metabolic substrates of the heart?

Answer 15

Fatty Acids

Carbohydrates

Question 16

What is the O2 supply of the heart limited by?

Answer 16

It is limited by flow. The heart cannot extract more O2 from blood like muscle can and so it can only increase blood flow.

Question 17

What is the O2 consumption of the heart related to?

Answer 17

It is directly related to the work of the heart.

Question 18

What kind of work consumes the most O2?

Answer 18

Pressure work consumes more O2 than volume work. Increasing the arterial pressure will increase the O2 consumption much more than increasing the stroke volume of the heart.

Question 19

What are some factors that determine the myocardial oxygen supply?

Answer 19

Diastolic Perfusion Pressure
Coronary Vascular Resistance
O2 Carrying Capacity of Blood

Question 20

What are some factors that contribute to the O2 demand of the heart?

Answer 20

Afterload
Heart Rate
Contractility

Question 21

What is never a primary cause of ischemia?

Answer 21

Excessive O2 demand

Question 22

What is the cause of ischemia?

Answer 22

It is caused by a relative lack of blood flow due to an imbalance in the ratio of O2 supply to demand.

Question 23

Can the collateral circulations of the heart prevent MI?

Answer 23

No, the vessels are insufficient to prevent sudden infarction, but in response to gradual obstruction, the collateral vessels can grow and restore adequate amounts of blood supply to the myocardium.

Question 24

What is coronary steal?

Answer 24

Coronary steal is where an increase in blood flow in one region of the heart can cause a decrease in blood flow in another region.

Question 25

Describe compensated coronary steal.

Answer 25

Compensated coronary steal occurs when one vessel is partially blocked by atherosclerosis and leads to ischemia in this region. Compensation occurs when the vessel is maximally dilated to increase the blood flow. The vessels in the same branch that are unaffected will be vasocontricted as normal.

Question 26

Describe uncompensated coronary steal.

Answer 26

Uncompensated coronary steal occurs when the cardiac work is increased like in exercise and the ischemic vessel that is normally compensated for cannot dilate any further, while the non-ischemic vessels can dilate. This will draw blood away from the ischemic region and make it more ischemic as a result.

Question 27

What is peripheral artery disease?

Answer 27

It is where that is a limping due to pain in the calf or thigh with walking because of the metabolites that have built up due to uncompensated blood steal.

Question 28

What is the largest vascular bed in the body?

Answer 28

Skeletal muscle

Question 29

How much of the cardiac output goes to skeletal muscle at rest?

Answer 29

20%

Question 30

What are the various control methods of skeletal muscle blood flow?

Answer 30

Autoregulation
Metabolic
Mechanical
Neurohormonal

Question 31

Describe metabolic control of skeletal muscle blood flow.

Answer 31

Skeletal muscle bloodflow is controlled with active hyperemia, where increased activity will increase blood flow and up to 80% of blood flow can go to muscle during exercise.

Question 32

Describe the mechanical control of skeletal muscle blood flow.

Answer 32

Tissue pressure increases with muscle contraction and compresses blood vessels and in exercise there is an alternation between compression and free flow of blood.

Question 33

On average, what is the mechanical result of blood flow in skeletal muscle.

Answer 33

There is a reduction in resistance overall due to vasodilation caused by metabolic factors.

Question 34

Why can isometric tension only be held for 1-2 mins?

Answer 34

At high isometric tension the blood flow through the contracting muscle will approach 0

Question 35

What is the difference in isometric exercise and cardiovascular exercise?

Answer 35

Isometric exercise will often increase vascular resistance and show a greater increase in arterial pressure.

Question 36

How does exercise affect venous circulation?

Answer 36

Muscle contraction pumps the blood in veins and will increase venous return.

Question 37

How does NE affect skeletal muscle blood flow?

Answer 37

NE binds to the alpha-adrenergic receptors and causes vasoconstriction -> most of the vascular tone is controlled in this way at rest

Question 38

What is the main innervation of skeletal muscle?

Answer 38

Sympathetic adrenergic fibers

Question 39

How does E affect skeletal muscle blood flow?

Answer 39

E is released from the adrenal medulla and it causes vasodilation at low concentrations by binding to the β2 receptors.

At high concentrations, it causes vasoconstriction via the α receptors.

Question 40

How does ACh affect skeletal muscle blood flow?

Answer 40

ACh is released by the sympathetic cholinergics that also innervate the skeletal muscle and it causes vasodilation by acting on the muscarinic receptors on endothelial cells that are tied to NO production.