Guyton Chapter 21 Muscle Blood Flow

Question 1

What is the tremendous increase in muscle blood flow that occurs during skeletal muscle activity caused by?

Answer 1

Mainly by chemicals acting directly on the muscle arterioles to cause dilation. One of the most important chemical effects is reduction of oxygen in the muscle tissue. When muscles are active, they use oxygen rapidly thereby decreasing oxygen concentration in the tissue fluids. This in turn causes local arteriolar vasodilation because the arteriolar walls cannot maintain contraction in the absence of oxygen and because oxygen deficiency causes release of vasodilator substance.

Question 2

Even after the muscle blood vessels have become insensitive to the vasodilator effects of adenosine, still other vasodilator factors continue to maintain increased capillary blood flow as long as the exercise continues. Which are these factors?

Answer 2

1. Potassium ions
2. ATP
3. Lactic acid
4. Carbon dioxide

Question 3

What does the left coronary artery supply?

Answer 3

Mainly the anterior and left lateral portions of the left ventricle.

Question 4

What does the right coronary artery supply?

Answer 4

Most of the right ventricle as well as the posterior part of the left ventricle.

Question 5

By which way does most of the coronary venous blood flow from the left ventricular muscle return to the right atrium of the heart?

Answer 5

By way of the coronary sinus

Question 6

By which way does most of the coronary venous blood from the right ventricular muscle return to the heart?

Answer 6

Through small anterior cardiac veins that flow directly into the right atrium not by way of the coronary sinus.

Question 7

What happens to the coronary capillary blood flow in the left ventricle during systole and what is the reason for this?

Answer 7

It falls to a low value because of strong compression of the intramuscular blood vessels during systolic contraction.

Question 8

Describe the special arrangement of the coronary vessels at different depths in the heart muscle?

Answer 8

Outer surface: epicardial coronary arteries which supply most of the muscle

Smaller intramuscular arteries derived from the epicardial arteries penetrate the muscle supplying the needed nutrients

Lying immediately beneath the endocardium is a plexus of subendocardial arteries

Question 9

What happens with the coronary blood flow in systole?

Answer 9

During systole, blood flow through the subendocardial plexus of the left ventricle where the intramuscular coronary vessels are compressed greatly by ventricular muscle contraction, tends to be reduced. The extra vessels of the subendocardial plexus normally compensate for this reduction.

Question 10

What is blood flow through the coronary system mostly regulated by?

Answer 10

By local arteriolar vasodilation in response to the nutritional needs of cardiac muscle.

Question 11

Name a substance with great vasodilator propensity?

Answer 11

Adenosine

Question 12

What happens with ATP in the presence of very low concentrations of oxygen in the muscle cells?

Answer 12

A large proportion of the cells ATP degraded to AMP. Small portions of this substance are then further degraded and release adenosine to the tissue fluids of the heart muscle with a resultant increase in local coronary blood flow. After adenosine causes vasodilation, much of it is reabsorbed into the cardiac cells to be reused for production of ATP.

Question 13

What effect does adenosine have in the heart?

Answer 13

Vasodilation

Question 14

What are additional vasodilating products in the heart than Adenosine?

Answer 14

Adenosine phosphate compounds
Potassium ions
Hydrogen ions
Carbon dioxide
Prostaglandins
Nitric oxide

Question 15

What are the direct and indirect nervous control of coronary blood flow?

Answer 15

The direct effects result from action of the nervous transmitter substance ms acetylcholine from the vagus nerves and norepinephrine from the sympathetic nerves in the coronary vessels.

The indirect effects result from secondary changes in coronary blood flow caused by increased or decreased activity of the heart.

Question 16

What effect does acetylcholine have on the coronary arteries?

Answer 16

It dilates the coronary arteries

Question 17

Sympathetic transmitter substances norepinephrine and epinephrine can have vascular constrictor or vascular dilator effects depending on the presence of dilator receptors in the blood vessel walls. What are the constrictor and dilator receptors called?

Answer 17

Constrictor receptors are called alpha receptors and dilator receptors are called beta receptors

Question 18

What is the distribution of alpha and beta receptors in the coronary vessels?

Answer 18

In general the epicardial coronary vessels have a preponderance of alpha receptors whereas the intramuscular arteries may have a preponderance of beta receptors.

Question 19

Under resting conditions, does the cardiac muscle consume mostly fatty acids or carbohydrates?

Answer 19

It normally consumes more fatty acids than carbohydrates.