Lecture 13: Cardiac Output And Blood Flow In Muscle Tissues Flashcards
Define 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 of the tissues of the body.
Define Cardiac Index
- Cardiac output per square meter of body surface
- To calculate cardiac index:
- Normal human being: 70 kg
- Body surface area = 1.7 square meters
- Cardiac output = 5 L 5
- Cardiac index = 5 L/1.7 square meters = 3 L/min/m2
What occurs to cardiac index over time?
Reduces with age…declining muscle mass often also observed.
What is the Flick Principle of Blood Flow?
Cardiac Output = (O2 consumption) / (Pulmonary Vein Oxygen - pulmonary artery oxygen)
- Measure oxygen consumption for the whole body.
- Measure pulmonary vein [O2] in systemic arterial blood.
- Measure pulmonary artery [O2] in systemic mixed venous blood.
- See Slide 8 and 10
What is the determining factor that controls how much blood the heart pumps out?
- The heart is a “demand” pump.
- The heart pumps out whatever blood comes back into it from the venous system.
- It is the amount of blood returning to the heart that determines how much blood the heart pumps out.
Describe the two pumps of the cardiac output system connected in Series
- The left and right ventricles are the two pumps.
- The two circuits are the pulmonary and systemic circuits.
- Because the two circuits are connected in series:
- Flow must be equal in the two circuits.
- Cardiac output and rate of the two circuits are equal.
- All pressures are higher in the systemic circuit.
- Chemical composition of pulmonary venous blood is similar to that of systemic arterial blood.
- Chemical composition of venous blood entering the right atrium is the same as the composition of pulmonary arterial blood.
What are factors that directly affect cardiac output
- Basic level of body metabolism
- Whether the person is exercising
- Age
- Size of the body
Normal Values:
- Young, healthy men: 5.6 L/min
- Women: 4.9 L/min
- Resting adult: 5.0 L/min
Describe control of the cardiac output by venous return
- Frank-Starling Law: Heart automatically pumps whatever amount of blood that flows into the right atrium.**
- Stretching of the heart causes the heart to pump faster.
- Stretched right atrium initiates Bainbridge reflex (See next slide).
- Under most normal non-stressful conditions, the cardiac output is controlled almost entirely by peripheral factors that determine venous return.
- Ohm’s law: Any time the long-term level of total peripheral resistance changes, the cardiac output changes quantitatively in exactly the opposite direction.
- Refer to Figure 20-4.
- Therefore, peripheral factors are more important controllers of cardiac output.
What is the Bain bridge reflex?
- Both the Bainbridge reflex and the baroreceptor reflex control heart rate:
- The Bainbridge reflex (atrial stretch reflex) responds to changes in blood volume as detected by stretch receptors in the right atrium.
- Contrast with baroreceptors which respond to changes in arterial pressure.
- Not significant in humans: However, does occur after birth, when a large volume of the uteroplacental blood returns to the mother’s circulation and results in tachycardia.
- See Slide 17
What are factors that can cause a hypereffective heart
- Nervous stimulation
- Hypertrophy of heart
- Exercise via the nervous system:
- Intense increase in metabolism in active skeletal muscles causes the muscle arterioles to relax.
- This allows more blood into these arterioles.
- Brain sends motor signals to the muscles and to the ANS centers of the brain to excite circulatory activity
- This causes a large vein constriction
- This leads to increased heart rate, and increased contractility of the heart.
What are factors that cause a hypoeffective heart?
- Increased arterial pressure (i.e., hypertension)
- Inhibition of nervous excitation of the heart
- Pathological factors causing abnormal heart rhythm/rate
- Coronary artery blockage
- Valvular heart disease
- Congenital heart disease
- Cardiac hypoxia
Describe the ventricular function curve
- Graph of ventricular function curves under conditions of a normal, hypoeffective, and hypereffective heart.
- Demonstrates the Frank-Starling mechanism:
- As ventricles fill in response to higher atrial pressures, each ventricular volume and strength of cardiac muscle contraction also increases and results in an increase in cardiac output.
- See Slide 22
What are factors that cause decreased cardiac output?
- Cardiac Factors:
- Severe blood vessel blockage → myocardial infarction
- Severe valvular disease
- Myocarditis
- Cardiac tamponade
- Cardiac metabolic derangements
- See Slide 25
- Non-cardiac Factors:
- Decreased blood volume
- Acute venous dilation
- Obstruction of large veins
- Decreased tissue mass (esp., muscle mass)
- Decreased metabolic rate of tissues
- See Slide 27
Describe how right atrial pressure affects the venous return to the heart from systemic circulation
- Venous return → 0 when right atrial pressure → +7 mm Hg
• = mean systemic filling pressure
• If right atrial pressure → -2 mm Hg, venous return reaches a plateau.
• Caused by collapse of veins entering chest
Describe how the Degree of Filling of Systemic Circulation affects venous return to the heart from systemic circulation
When heart pumping stops:
• All blood flow ceases
• Pressures everywhere in the body become equal
• = mean circulatory filling pressure
• = 0 when blood volume = 4 L
• = 7 mm Hg when blood volume = 5 L
• Almost equal to mean systemic filling pressure
