Cardiac output

What is cardiac output?

- Volume of blood pumped per ventricle minute.

- Heart rate x stroke volume

How is heart rate initiated?

- SAN reaches threshold potential resulting in action potential which spreads through the heart.

- This induces heart to contract.

- Parasympathetic (↓) and sympathetic (↑) fibres control the rate of this.

How is heart rate increased?

- Increased secretion of plasma epinephrine/adrenaline.

- Increased activity of the sympathetic nerves.

What are the two types of load the heart can be under?

- Preload

- After load

What is preload?

Volume of blood in ventricles at the end of diastole.

What is afterload?

Resistance left ventricle must overcome to pump blood.

How is stroke volume affected?

- Changes in venous return

- Changes in sympathetic stimulation

- Changes in afterload which occurs in disease.

What is End Diastolic volume (EDV)?

The amount of blood in the ventricles before contraction.

How does increasing End diastolic Volume increase stroke volume?

- Increased EDV stretches the cardiac muscle which optimises the overlap between muscle cells which enhances the force of contraction

- This results in a greater volume of blood being ejected causing stroke volume to increase.

How does sympathetic stimulation affect stroke volume?

- Noreadrenaline binding to B1 receptors increases strength of contraction.

- This is independent of EDV so more complete ejection of blood.

How does elevated blood pressure increase the load on the heart?

- When blood pressure is high, the heart must work harder to pump blood into the arteries (Increased afterload)

- This increased effort can lead to hypertrophy (thickening) of the heart muscle as it tries to compensate for the higher resistance.

- Hypertrophy can have negative effects on the heart.

Why is hypertrophy of the heart dangerous?

- Hypertrophied heart muscle needs more oxygen, risking ischemia and heart attacks.

- Thickened muscle can become stiff, leading to diastolic dysfunction and heart failure.

- Over time, the heart's ability to compensate diminishes, leading to heart failure.

How does a stenotic exit valve increase the load on the heart?

- If the exit valve (aortic or pulmonary) is narrowed (stenosis), the heart must generate more pressure to push blood through the smaller opening.

- This increases the workload on the heart which can lead to heart failure in older or weakened hearts.