Guyton Chapter 20 - Cardiac Output

Question 1

What is cardiac output?

Answer 1

The quantity of blood pumped into the aorta each minute by the heart.

Question 2

What is venous return?

Answer 2

The quantity of blood flowing from the veins into the right atrium each minute.

Question 3

Name 4 factors that affect cardiac output?

Answer 3

1. The basic level of body metabolism
2. Level of exercise
3. Age
4. Size of the body

Question 4

How does the stretch of the sinus node in the wall of the right atrium affect rhythmicity?

Answer 4

It has a direct effect on the rhytmicity of the node itself to increase heart rate as much as 10-15%.

Question 5

What is the Bainbridge reflex?

Answer 5

A nervous reflex that is initiated by a stretched right atrium passing first to the vasomotor centre of the brain and then back to the heart by way of the sympathetic nerves and vagi also to increase the heart rate.

Question 6

What is the formula of cardiac output?

Answer 6

Cardiac output = arterial pressure / total peripheral resistance

Question 7

Name two types of factors that can make the heart a better pump than normal?

Answer 7

1. Nervous stimulation
2. Hypertrophy of the heart muscle

Question 8

A combination of sympathetic stimulation and parasympathetic inhibition does two things to increase the pumping effectiveness of the heart - which are they?

Answer 8

1. It greatly increases the heart rate
2. It increases the heart contractility

Question 9

What prevents the blood pressure from dropping when local tissue blood vessels dilate thereby increasing venous return and cardiac output above normal?

Answer 9

The nervous system plays an exceedingly important role in preventing the arterial pressure from falling to disastrously low levels.

Question 10

Name some factors that can cause a hypoeffective heart?

Answer 10

Increased arterial pressure again which the heart must pump such as in severe hypertension

Inhibition of nervous excitation of the heart

Pathological factors that cause abnormal heart rhythm or rate of heartbeat

Valvular heart disease

Congenital heart disease

Cardiac hypoxia

Myocarditis

Question 11

Name some non cardiac peripheral factors that can decrease cardiac output:

Answer 11

1. Decreased blood volume
2. Acute venous dilation
3. Obstruction of the large veins
4. Decreased tissue mass
5. Decreased metabolic rate of the tissues

Question 12

What are some factors that can alter the external pressure on the heart and thereby shift the cardiac output curve?

Answer 12

1. Cyclical changes of intrapleural pressure during respiration
   2 breathing against a negative pressure
2. Positive pressure breathing
3. Opening of the thoracic cage
4. Cardiac tamponade

Question 13

What are 3 fastors that affect venous return?

Answer 13

1. Right atrial pressure
2. Degree of filling of the systemic circulation (measured by the mean systemic filling pressure)
3. Resistance to blood flow

Question 14

What is the mean circulatory fillin pressure?

Answer 14

When there is no blood flow, the pressure everywhere in the circulation becomes equal. This equilibrated pressure level is called the mean circulatory filling pressure.

Question 15

At normal blood volume, how much does the maximal sympathetic stimulation increase the mean circulatory filling pressure?

Answer 15

From 7 to about 2,5 times that value or about 17 mmHg

Question 16

How much does complete inhibition of the sympathetic nervous system relax the blood vessels and the heart and how much is the mean circulatory filling pressure decreased?

Answer 16

From 7 mmHg to 4 mmHg

Question 17

What is the mean systemic filling pressure and what is its relation to the mean circulatory filling pressure?

Answer 17

It’s the pressure measured everywhere in the systemic circulation after blood flow has been stopped by clamping the large blood vessels at the heart so the pressures in the systemic circulation can be measured independently from those in the pulmonary circulation.

The mean systemic filling pressure is almost always nearly equal to the mean circulatory filling pressure.

Question 18

What is the pressure gradient for venous return?

Answer 18

The difference between the mean systemic filling pressure and the right atrial pressure. The greater the difference is between these, the greater becomes the venous return.

Question 19

How much of the resistance to venous return is determined by venous resistance and how much is determined by the arteriolar and small artery resistance?

Answer 19

2/3 of the venous resistance
1/3 arteriolar and small artery resistance

Question 20

How can venous return be calculated?

Answer 20

Venous return = (mean systemic filling pressure - right atrial pressure) / resistance to venous return

Question 21

What is the highest level to which the right atrial pressure can rise?

Answer 21

Equal to the mean systemic filling pressure