Cardiac Output, Stroke Volume and Heart Rate

Question 1

What is cardiac output? Equation?

Answer 1

Measure of heart’s efficiency in pumping blood (5L / min in resting adult)

CO = HR x SV

(stroke volume is amountof blood ejected by ventricle in 1 contraction, so end diastolic volume - end systolic volume (EDV - ESV)

Stroke volume increases and EDV increases.

Question 2

Special thing about cardiac muscle cells

Answer 2

Can depolarise and contract rhythmically without need for nerve input (myogenic rhythmicity or autorhythmicity)

Question 3

How does the heart initiate heartbeat

Answer 3

Sinoatrial Node (SA Node):
- Natural pacemaker of the heart.
- Located in the right atrium near the superior vena cava.
- Fires electrical impulses spontaneously (about 60–100 times/min at rest).

Atrioventricular Node (AV Node):
- Located between the atria and ventricles.
- Delays the impulse slightly (about 0.1 seconds) to allow the atria to finish contracting before ventricles contract.

Bundle of His:
- Pathway that carries impulses from the AV node into the ventricles.

Bundle Branches:
- Split the impulse into the right and left ventricles.

Purkinje Fibers:
- Fine network of fibers spreading through the ventricles, triggering coordinated ventricular contraction.

Question 4

Nervous System Breakdown

Answer 4

Question 5

Answer 5

Question 6

How does sympathetic activity involve adrenaline and noradrenaline to increase heartrate

Answer 6

Question 7

Chronotropic effect (how fast heart beats)

Answer 7

Question 8

What are the major determinants of venous return to the heart? How do changes in blood volume affect venous return and central venous pressure (CVP)?

Answer 8

Venous return is driven by pressure gradients between the thorax, abdomen, and right atrium. Key factors include:

• Thoracic pressure (Pthorax): Decreases during inspiration, pulling blood toward the heart.
• Right atrial pressure (Pright atrium): Lower pressure promotes venous return; higher pressure opposes it.
• Abdominal pressure (Pabdomen): Increased pressure pushes blood toward the thorax.
• Skeletal muscle pump: Muscle contractions compress veins, and venous valves prevent backflow, ensuring one-way blood flow to the heart.

Increased blood volume (e.g., renal failure, fluid retention via RAAS) raises venous return and CVP.

Decreased blood volume (e.g., haemorrhage, dehydration) lowers venous return and CVP.

Question 9

How to increase contractility

Answer 9