Flashcards in Chapter 20 - Guyton Deck (25):
Define cardiac output.
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)
Define venous return.
Venous return is the quantity of blood flowing from the veins into the right atrium each minute.
Factors that directly affect cardiac output:
basic level of body metabolism, exercise status, age, size of the body
What is the Frank-Starling law of the heart?
when increased quantities of blood flow into the heart, the heart chambers stretch and the cardiac muscle contracts with more force
Why will the stretching of the heart that occurs with increased blood flow into the heart also result in an increased heart rate?
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
What primarily controls cardiac output?
peripheral factors that determine venous return, heart will only become limiting factor when the returning blood is more than the heart can pump
As total peripheral resistance decreases, cardiac output will ________.
increase, and vice versa
About how much can cardiac output increase?
Factors that can cause hypereffective heart.
nervous stimulation, hypertrophy of the heart muscle
What two nervous system events occur to increase heart pumping?
sympathetic stimulation and parasympathetic inhibition (increase heart rate and contractility)
Factors that cause a hypoeffective heart.
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
During exercise, why does the nervous system function to increase arterial pressure?
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.
Name some conditions that can increase cardiac output (decrease peripheral resistance rather than excessive excitation of the heart).
Beriberi disease, arteriorvenous fistula (shunt), hyperthyroidism, anemia
What conditions might decrease cardiac output (caused by cardiac factors)?
severe coronary blood vessel blockage and consequent myocardial infarction, severe valvular heart disease, myocarditis, cardiac tamponade, cardiac metabolic derangements
When the cardiac output falls so low that the tissues throughout the body begin to suffer nutritional deficiency, the condition is called?
cardiac shock (circulatory shock)
What conditions can decrease cardiac output based on decreased venous return?
decreased blood volume, acute venous dilation, obstruction of the large veins, decreased tissue mass (especially skeletal muscle)
What is the normal intrapleural pressure (chest cavity pressure)?
Some of the factors that can alter the intrapleural pressure and thereby shift the cardiac output curve are the following:
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)
Three principal factors that affect venous return to the heart from systemic circulation.
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
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).
mean systemic filling pressure
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?
collapse of the veins entering the chest which prevents any additional flow of blood from the peripheral veins
What is meant by the pressure gradient for venous return?
The greater the difference between the mean systemic filling pressure and the right atrial pressure, the greater becomes the venous return.
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).
VR = (Psf - PRA) / RVR
In healthy adult, VR = 5 L/min, Psf = 7 mm/Hg, PRA = 0 mm/Hg, RVR = 1.4 mm/Hg.
What occurs when right atrial pressure rises to equal the mean systemic filling pressure?
venous return becomes 0 because there is no pressure gradient to cause flow of blood