Cardiovascular System - Control of Cardiac Output

Question 1

Define cardiac output.

Answer 1

Cardiac output is the volume of blood pumped by the left ventricle in one minute
Calculated by heart rate x stroke volume
Cardiac output altered by varying either one
Left ventricular output = right ventricular output
Ejection fraction measures cardiac efficiency/ contractility

Question 2

What is the sympathetic effect on cardiac output?

Answer 2

Speeds up heart rate - positive chronotropic effect
β1 receptor opens L calcium ion channels
Increases slope of pre potential, more rapid SA firing
Speeds up AV conduction
R symps to SA node, L to AV node.

Question 3

What is the parasympathetic effect on cardiac output?

Answer 3

Slows down heart rate - negative chronotropic effect
M2 receptor opens K+ channels slowing opening of calcium ion channels
Hyperpolarises cell decreasing pre potential slope
Slows AV conduction

Question 4

How is cardiac output altered?

Answer 4

Cardiac output can be influenced by altering stroke volume
These changes may depend on the intrinsic properties of cardiac muscle, on autonomic stimulation and on hormones
Heterometric regulation - changes in cardiac output dependent on initial length of cardiac muscle fibre
Homeometric regulation - changes in cardiac output independent of such changes

Question 5

What is the Fick principle?

Answer 5

The Fick principle is based on the principle of conservation of mass, which states that the amount of a substance taken up or released by an organ is equal to the amount entering or leaving the circulation.
Oxygen consumption (VO2) is determined by measuring the difference in oxygen content between arterial and venous blood and the blood flow to the organ
Cardiac output can then be calculated using the equation: CO = VO2 / (Arterial O2 content - Venous O2 content)

Question 6

What is the dye dilution technique?

Answer 6

This method involves injecting a known quantity of an indicator dye into a central vein.
The concentration of the dye is measured in a peripheral artery over time using a spectrophotometer or colorimeter.
By analyzing the dilution curve, cardiac output can be calculated based on the principle of indicator dilution

Question 7

What is the thermodilution technique?

Answer 7

injecting a cold saline solution into a central vein, typically the right atrium or pulmonary artery.
A thermistor-tipped catheter is placed in a large artery to measure temperature changes caused by the injected solution
By analyzing the temperature changes over time, the cardiac output can be calculated using the principle of heat exchange

Question 8

What is the Frank-Starling relationship?

Answer 8

Describes the relationship between the preload (the initial stretching of the cardiac muscle fibers before contraction) and the stroke volume (the volume of blood pumped out by the heart with each beat)
This relationship is fundamental to the regulation of cardiac output and ensures that the heart pumps an appropriate amount of blood to meet the body’s demands
Stroke volume and preload are proportional, if preload decreases, stroke volume decreases.