Chapter 20: Cardiac Output, Venous Return, and Their Regulation

What is the importance of the nervous system (nervous reflexes) in terms of regulating BP in response to PVR changes?

(example the book gave was excessively dilating vessels with dinitrophenol) Dilation --> DECREASE in PVR this decrease in PVR causes a reflexive increase in CARDIAC OUTPUT in order to maintain constant arterial BP 1. large vein constriction 2. increased HR 3. increased heart contractility all 3 mechanisms are mediated through baroreceptor reflex mechanisms, sensing a decrease in BP due to decreased PVR. this stimulates mechanisms (sympathetics) that will work to increase CO to keep BP from falling below normal. ---- example seen with exercise too (active tissue vessels dilate; this decreases PVR --> sympathetics stimulated to increase CO in order to deliver blood adequately (maintain BP) to active tissues/muscles

What is the normal intrapleural pressure (the external cardiac pressure)?

-4 mmHg

What two factors in combination can shift the cardiac output to the right + increased max level of CO? (shift to R and UP)

-increased intrapleural pressure --> shifts to R -increased pumping capacity (hypereffective heart) --> shifts max CO UP

What 4 factors directly affect a person's cardiac output?

1. Basic Level of Body Metabolism 2. Whether (or not) a person is exercising 3. Age 4. Size of body ----- cardiac output varies widely with the level of activity of the body!!!

What 2 circumstances may cause a low cardiac output?

1. Damage to heart (decreased pumping effectiveness) 2. Decreased venous return

What 2 factors make the heart a "better heart than normal" aka HYPEREFFECTIVE?

1. Hypertrophy of the heart muscle 2. Nervous stimulation

What can cause a hypoeffective heart (decreased maximum cardiac output)?

1. Increased arterial BP (HTN) 2. Inhibition of sympathetic innervation to the heart 3. coronary artery blockage (MI) 4. valvular heart disease 5. arrhythmias 6. congenital heart disease 7. myocarditis (inflammation of heart muscle) 8. cardiac hypoxia

What conditions/scenarios can shift the cardiac output curve?

1. cyclical changes in intrapleural pressure during respiration (normally increases +2mmHg during inspiration) 2. breathing positive pressure - shifts curve to RIGHT -breathing negative pressure shifts CO curve to LEFT 3. Opening the thoracic cage - increases the intrapleural pressure to 0mmHg 4. cardiac tamponade - shifts curve to the right

What 5 factors can result in decreased venous return, therefore decreasing cardiac output?

1. decreased blood volume 2. acute venous dilation 3. obstruction of the large veins 4. decreased tissue mass (esp in skeletal muscle) 5. decreased metabolic rate

What 3 principal factors affect venous return to the heart from systemic circulation?

1. right atrial pressure 2. systemic circulation filling 3. resistance to blood flow

For young, healthy women, what is the avg. resting cardiac output?

4.9 L/min

For young, healthy men, what is the avg. resting cardiac output?

5.6 L/min

What is the cardiac output (avg) for resting adults?

5L of blood/minute

Deficiency in what causes Beriberi?

B1 - thiamine diminished ability to use nutrients --> compensatory peripheral vasodilation --> decreased PVR --> long-term venous return & CO increase

What must be true in terms of cardiac output and venous return?

Cardiac Output must equal Venous Return blood pumped into aorta = blood returning into R atrium (except for a few heartbeats at a time when blood is temporarily stored in/removed from the heart and lungs)

What effect does the external pressure on the outside of the heart have on cardiac output (curves)?

Normal intrapleural pressure (external cardiac pressure) = -4 mmHg when there is a rise in intrapleural pressure, the right atrial pressure increases in order to maintain normal cardiac output levels

What effect does anemia have on cardiac output?

anemia causes an increase in cardiac output bc of 2 factors: 1. decreased viscosity of blood --> decreases PVR 2. decreased oxygen delivery to tissues --> local vasodilation to compensate --> decreased PVR

What type of pathological conditions cause a high cardiac output?

any pathologic condition that decreases PVR ----- 1. Beriberi 2. AV fistula/AV sunt 3. Hyperthyroidism 4. Anemia

What is the result of an AV fistula?

blood from an artery flows directly into a vein this decreases total PVR causes increase in venous return and cardiac output

What is the cardiac index?

cardiac output increases in proportion to surface area of the body: the cardiac index is a way of expressing cardiac output it is the cardiac output per sq. meter of body surface area can calculate: 3L/min/m^2 of body surface area

What is the relationship between cardiac output and PVR?

cardiac output inversely related to PVR when PVR increases, cardiac output will decrease when PVR decreases, cardiac output will increase

What is considered to be the primary controller of cardiac output?

cardiac output is controlled by venous return: NOT the heart controlling CO it is the various factors of the peripheral circulation, affecting the flow of blood into the heart from the veins (venous return) that mainly controls the cardiac output

What can lead to cardiac shock? What can lead to circulatory shock?

cardiac shock - damage to heart (low cardiac output) circulatory shock - more general term; anything that causes decreased cardiac output

What can cardiac shock result from?

cardiac shock can result from severely low cardiac output (due to damage to the heart) tissues of the body suffer nutritional deficiency

How does cardiac tamponade affect the cardiac output curve differently that an increase in intrapleural pressure?

cardiac tamponade shifts the upper parts of the curve farther right than the lower curve bc the external "tamponade" pressure rises to higher values as the chambers of the heart full to increased volumes (during high CO)

Why does decreased tissue mass lead to decreased cardiac output?

decreased tissue mass means decrease in total oxygen consumption --> decreases blood flow needs (esp of skeletal muscle) --> decreased CO

What is the best example of low cardiac output caused by acute venous dilation?

fainting (usually due to loss of sympathetic activity) causes blood to pool in the vessels, reducing venous return --> LOW CO

What is the most common noncardiac peripheral factor that leads to LOW cardiac output?

hemorrhage - decreased blood volume

Why does hyperthyroidism result in an overall increase in cardiac output?

hyperthyroidism increases tissue metabolism this increases tissue oxygen consumption increases vasodilator substances --> decrease PVR results in increased venous return & cardiac output

What can cardiac function curves (cardiac output curves) tell you about a heart?

if it is functioning normally, hypereffective, or hypoeffective in a normal heart, the cardiac output increases in response to right atrial pressure until CO reaches 2.5x normal, or about 13L/min so, if the cardiac function curve shows that the cardiac output plateau/maximum limit is 25 L/min, then the heart is said to be "hypereffective" if the CO maximum (point in which CO plateaus in response to increased R atrial pressure) is 5L/min, that heart is said to be "hypoeffective"

What effect does systemic filling have on venous return?

increased systemic filling forces systemic blood towards the heart

What is the effect of right atrial pressure on venous return?

right atrial pressure exerts a backward force on veins --> impedes venous return

What is the Bainbridge reflex?

stretch of the right atrium initiates a nervous reflex --> signals are sent to the vasomotor center of the brain via sympathetics and vagus nerves to increase heart rate

What feature of the sinus node allows it to alter the heart's rhythm?

stretch of the sinus (SA) node in the wall of the R atrium has a direct effect on the rhythmicity of the node --> increases heart rate (as much as 10-15%)

What happens when the intrapleural pressure increases?

the cardiac output curve shifts to the RIGHT

What is venous return?

the quantity of blood flowing from veins into R atrium each minute

What is cardiac output?

the quantity of blood pumped into the aorta each minute

Most tissue blood flow increases mainly in proportion to what?

tissue blood flow increases in proportion to that tissue's metabolism (oxygen consumption)

What is the Frank-Starling law of the heart?

when increased quantities of blood flow into the heart, the increased blood STRETCHES the walls of the heart chambers this STRETCH (on the atria esp) the ventricles will contract with increased force (within limits), *emptying the extra blood that has entered the heart/circulation* excess blood to the heart is *automatically* pumped, without delay, into the aorta --> flows again into circulation