Lecture 12 Cardiac Output and Blood Flow in Muscle Tissues Flashcards Preview

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Definitions of Cardiac Output

Quantity of blood pumped into the aorta each minute by the heart*
Quantity of blood that flows through the circulation
Sum of all the blood flows to all the tissues in the body


Definition of Cardiac Index

Cardiac output per square meter of body surface


Calculation for Cardiac Index

Normal Human Being:
70 kg
Body surface area = 1.7 square meters
Cardiac Output = 5 L
Cardiac index = 5 L/1.7 square meters = 3L/min/m2


Fick Principle of Blood Flow

Used to calculate blood from through an organ


Equation used to calculate blood flow through an organ:

Cardiac Output = (O2 consumption) / ([O2]pul vein - [O2]pul art)
pul vein = systemic arterial
pul artery = systemic venous


Patient has a resting O2 consumption of 250 mL/min
Systemic arterial O2 content of 0.20 mL O2/mL of blood
Systemic mixed venous O2 content of 0.15 mL O2/mL of blood
Heart rate of 72 beats/min
What is the cardiac output?
What is the stroke volume?

Cardiac Output = 250/(0.20 - 0.15) = 5000 mL/min or 5 L/min

Stroke Volume = (5000 mL/min) / (72 beats/min) = 70 mL/beat


What is the determining factor that controls how much blood the heart pumps out?

It is the amount of blood returning to the heart that determines how much blood the heart pumps out


The cardiovascular system consists of two pumps and two circuits connected in series:
What are the two pumps?
What are the two circuits?

Pumps = left and right ventricles
Circuits = pulmonary and systemic


Because the two circuits are connected in series:

Flow must equal the two circuits
Cardiac output & rate of the two circuits are equal
All pressures are higher in the systemic circuit
Chem. composition of pulmonary venous blood is similar to that of systemic arterial blood.
Chem. composition of venous blood entering the right atrium is the same as the composition of pulmonary arterial blood.


Factors that directly affect cardiac output:

Basic level of body metabolism
Whether the person is exercising
Size of the body


Normal Cardiac Output values

Young, healthy men: 5.6 L/min
Women: 4.9 L/min
Resting Adult: 5.0 L/min


Control of cardiac output by venous return:

Frank-Starling Law
Stretching of heart causes heart to pump faster.
Stretched r. atrium initiates Bainbridge reflex.
Undermost normal non-stressful conditions, the CO is controlled almost entirely by peripheral factors that determine venous return.
Ohm's Law


Frank-Starling Law (cardiac output)

Heart automatically pumps whatever amount of blood that flows into the right atrium... Therefore, peripheral factors are more important controllers of cardiac output


Ohm's Law (cardiac output)

Any time the long-term level of total peripheral resistance changes the cardiac output changes quantitatively in exactly the opposite direction


Bainbridge Reflex (atrial stretch reflex)

Responds to changes in blood volume as detected by stretch receptors in the right atrium
Not significant in humans, but does occur after birth


Factors that cause a hypereffective heart

Nervous stimulation
Hypertrophy of heart
Exercise via the nervous system


Describe how exercise via the nervous system causes a hypereffective heart

Intense inc. in metab. in active skeletal muscles causes muscle arterioles to relax --> Allows more blood into these arterioles --> Brain sends motor signals to the muscles & to the ANS centers of the brain to excite circulatory activity --> Causes large vein constriction --> Leads to increased HR, & increased contractility of the heart


Factors that cause a hypoeffective heart

Increased arterial pressure (i.e. hypertension)
Inhibition of nervous excitation of the hear
Pathological factors causing abnormal heart rhythm/rate
Coronary artery blockage
Valvular heart disease
Cardiac hypoxia


Review Ventricular Function Curve

Figure 20-5 or slide 22
Cardiac output curves for the normal heart, hypoeffective heart, and hypereffective heart


Cardiac Factors Causing Decreased Cardiac Output

Severe blood vessel blockage --> myocardial infarction
Severe valvular disease
Cardiac tamponade
Cardiac metabolic arrangements


Non-Cardiac Factors Causing Decreased Cardiac Output

Decreased blood volume
Acute venous dilation
Obstruction of large veins
Decreased tissue mass (esp. muscle mass)
Decreased metabolic rate of tissues


Factors that affect venous return to the heart from the systemic circulation:

Right atrial pressure
Degree of filling of systemic circulation
Resistance to blood flow


Right atrial pressure:

Venous return --> 0 when r. atrial pressure --> +7 mmHg
= Mean systemic filling pressure
If r. atrial pressure --> -2 mmHg, venous return reaches plateau (caused by collapse of veins entering chest)


Degree of filling of systemic circulation:

When heart pumping stops:
all blood flow ceases
pressure everywhere in the body becomes equal
= mean circulatory filling pressure
= 0 when BV = 4L
= 7 mmHG when BV = 5L
Almost equal to mean systemic filling pressure


The grater the difference between the mean systemic filling pressure and the right atrial pressure, the _______ the venous return



The difference between the mean systemic filling pressure and the right atrial pressure = ?

Pressure gradient for venous return


About 2/3 of the resistance to venous return is determined by:

Venous resistance (Because of vein distensibility, there is little rise in venous pressure)


About 1/3 of the resistance to venous return is determined by:

Arteriolar and small artery resistance (Accumulation of blood overcomes much of the resistance)


Venous return equation

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


Factors that affect venous return

Right atrial pressure
Mean systemic filling pressure
Blood flow resistance between peripheral vessels and right atrium