Chapter 20 - Guyton

Question 1

Define cardiac output.

Answer 1

Cardiac output is the quantity of blood pumped into the aorta each minute by the heart. This is also the quantity of blood that flows through the circulation. (usually about 5 L/min)

Question 2

Define venous return.

Answer 2

Venous return is the quantity of blood flowing from the veins into the right atrium each minute.

Question 3

Factors that directly affect cardiac output:

Answer 3

basic level of body metabolism, exercise status, age, size of the body

Question 4

What is the Frank-Starling law of the heart?

Answer 4

when increased quantities of blood flow into the heart, the heart chambers stretch and the cardiac muscle contracts with more force

Question 5

Why will the stretching of the heart that occurs with increased blood flow into the heart also result in an increased heart rate?

Answer 5

stretching of the sinus node, also initiates a nervous reflex called the Bainbridge reflex, passing first to the vasomotor center 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 primarily controls cardiac output?

Answer 6

peripheral factors that determine venous return, heart will only become limiting factor when the returning blood is more than the heart can pump

Question 7

As total peripheral resistance decreases, cardiac output will ________.

Answer 7

increase, and vice versa

Question 8

About how much can cardiac output increase?

Answer 8

2.5x normal

Question 9

Factors that can cause hypereffective heart.

Answer 9

nervous stimulation, hypertrophy of the heart muscle

Question 10

What two nervous system events occur to increase heart pumping?

Answer 10

sympathetic stimulation and parasympathetic inhibition (increase heart rate and contractility)

Question 11

Factors that cause a hypoeffective heart.

Answer 11

coronary artery blockage, inhibition of nervous excitation of the heart, pathological factors that cause abnormal heart rhythm or rate of heartbeat, valvular heart disease, increased arterial pressure against which the heart must pump, such as in hypertension, congenital heart disease, myocarditis, cardiac hypoxia

Question 12

During exercise, why does the nervous system function to increase arterial pressure?

Answer 12

The same brain activity that sends motor signals to the muscles sends simultaneous signals into the autonomic nervous centers of the brain to excite circulatory activity, causing large vein constriction, increased heart rate, and increased contractility of the heart.

Question 13

Name some conditions that can increase cardiac output (decrease peripheral resistance rather than excessive excitation of the heart).

Answer 13

Beriberi disease, arteriorvenous fistula (shunt), hyperthyroidism, anemia

Question 14

What conditions might decrease cardiac output (caused by cardiac factors)?

Answer 14

severe coronary blood vessel blockage and consequent myocardial infarction, severe valvular heart disease, myocarditis, cardiac tamponade, cardiac metabolic derangements

Question 15

When the cardiac output falls so low that the tissues throughout the body begin to suffer nutritional deficiency, the condition is called?

Answer 15

cardiac shock (circulatory shock)

Question 16

What conditions can decrease cardiac output based on decreased venous return?

Answer 16

decreased blood volume, acute venous dilation, obstruction of the large veins, decreased tissue mass (especially skeletal muscle)

Question 17

What is the normal intrapleural pressure (chest cavity pressure)?

Answer 17

-4 mm/Hg

Question 18

Some of the factors that can alter the intrapleural pressure and thereby shift the cardiac output curve are the following:

Answer 18

cyclical changes of intrapleural pressure during respiration, breathing against a negative pressure (shift left), positive pressure breathing (shift right), opening the thoracic cage (shift right), cardiac tamponade (accumulation of a large quantity of fluid in the pericardial cavity around the heart with increase in external cardiac pressure and shift to right)

Question 19

Three principal factors that affect venous return to the heart from systemic circulation.

Answer 19

right atrial pressure (backward force on the veins to impede blood flow into the right atrium), degree of filling of systemic circulation (forces systemic flow towards the heart), resistance to blood flow between peripheral vessels and right atrium

Question 20

Both the arterial and the venous pressures come to equilibrium when all flow in the systemic circulation ceases at a pressure of 7 mm Hg, which, by definition, is the ____ ________ _______ _______ (Psf).

Answer 20

mean systemic filling pressure

Question 21

When right atrial pressure has fallen to about -2 mm Hg, the venous return will have reached a plateau. It remains at this plateau level even though the right atrial pressure falls to -20 mm Hg, -50 mm Hg, or even further. Why the plateau?

Answer 21

collapse of the veins entering the chest which prevents any additional flow of blood from the peripheral veins

Question 22

What is meant by the pressure gradient for venous return?

Answer 22

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

Question 23

How can we calculate venous return taking into account the resistance to venous return? Hint: Factor in mean systemic filling pressure (Psf), right atrial pressure (PRA), and resistance to venous return (RVR).

Answer 23

VR = (Psf - PRA) / RVR

In healthy adult, VR = 5 L/min, Psf = 7 mm/Hg, PRA = 0 mm/Hg, RVR = 1.4 mm/Hg.

Question 24

What occurs when right atrial pressure rises to equal the mean systemic filling pressure?

Answer 24

venous return becomes 0 because there is no pressure gradient to cause flow of blood

Question 25

After an increase in blood volume, cardiac output will increase initially but, after a period of time, return to normal. What are the compensatory effects that cause cardiac output to return to normal?

Answer 25

increased capillary pressure (fluid out); stress-relaxation (reducing mean systemic pressure); increase in the peripheral resistance (increasing resistance to venous return)