Welcome to Heart Activity and Cardiac Output!

Have you ever wondered how your heart knows exactly how fast to beat when you're sprinting for a bus versus when you're chilling on the sofa? Unlike your arm or leg muscles, your heart doesn't wait for a "permission slip" from your brain to start beating—it has its own internal pacemaker. In this chapter, we explore the electrical "wiring" of the heart, how we can see that electricity on a screen (ECGs), and how your body calculates exactly how much blood you need to keep moving.

1. The Myogenic Heart: Beating to Its Own Drum

The human heart is myogenic. This means the signal for a heartbeat starts within the muscle itself, rather than from an external nerve impulse. If you took a heart out of the body (and kept it in the right nutrients), it would keep beating on its own!

The Electrical Relay Race

To ensure blood is pumped efficiently (atria first, then ventricles), a specific electrical pathway must be followed. Think of this as a relay race where the "baton" is a wave of electrical excitation.

Step 1: The SAN (Sinoatrial Node)
Located in the wall of the right atrium, the SAN is your natural pacemaker. It sends out a wave of electrical excitation that spreads across both atria, causing atrial systole (contraction).

Step 2: The Non-Conductive Barrier
The electricity cannot spread directly down into the ventricles because of a layer of non-conductive tissue. This is vital because it prevents the atria and ventricles from contracting at the exact same time.

Step 3: The AVN (Atrioventricular Node)
The electrical wave reaches the AVN. This node introduces a short delay. This delay is a "safety pause" that allows the atria to finish emptying their blood into the ventricles before the ventricles start to squeeze.

Step 4: The Bundle of His and Purkyne Fibres
The impulse is then conducted rapidly down the Bundle of His (specialised muscle fibres in the septum) to the apex (the bottom) of the heart. From there, it spreads up through the Purkyne fibres in the ventricle walls. This causes the ventricles to contract from the bottom up, efficiently pushing blood out into the arteries.

Quick Memory Tip: Use the phrase "S-A-B-P" (Stop And Beat Properly) to remember the order: SAN \(\rightarrow\) AVN \(\rightarrow\) Bundle of His \(\rightarrow\) Purkyne fibres.

Key Takeaway: The heart controls its own rhythm via the SAN, using a coordinated electrical pathway to ensure the chambers contract in the correct order.

2. Electrocardiograms (ECGs)

An ECG is a recording of the electrical activity of the heart over time. It doesn't show the muscle actually moving; it shows the electrical signals that tell the muscle to move.

The Standard ECG Trace

A normal ECG has three distinct "bumps" or waves:

  • The P wave: Represents atrial depolarisation (the electrical signal causing the atria to contract).
  • The QRS complex: The big spike! This represents ventricular depolarisation. It is much larger than the P wave because the ventricles have much thicker muscle walls.
  • The T wave: Represents ventricular repolarisation (the muscle cells recovering and "recharging" for the next beat).

Note: You might wonder where "atrial repolarisation" is. It actually happens during the QRS complex, but it's hidden because the ventricular signal is so much stronger!

Using ECGs for Diagnosis

Doctors look at the shape and timing of these waves to diagnose conditions like Cardiovascular Disease (CVD). Common abnormalities include:

  • Tachycardia: The heart rate is too fast (over \(100\) bpm at rest).
  • Bradycardia: The heart rate is too slow (under \(60\) bpm at rest).
  • Ectopic heartbeat: An "extra" beat that happens out of rhythm.
  • Fibrillation: A completely uncoordinated heartbeat where the muscle just quivers.

Key Takeaway: ECGs allow us to "see" the heart's electrical health. The P wave is the atria, the QRS is the ventricles, and the T wave is the recovery phase.

3. Cardiac Output

Cardiac output is the total volume of blood pumped by the heart (specifically the left ventricle) in one minute. It is vital for delivering oxygen to working muscles and removing carbon dioxide.

The Formula

You must be able to use this formula in your exam:

\(\text{cardiac output} = \text{stroke volume} \times \text{heart rate}\)

  • Heart Rate: The number of beats per minute (bpm).
  • Stroke Volume: The volume of blood pumped out of the left ventricle during one contraction (measured in \(cm^3\) or \(ml\)).

Example Calculation:
If an athlete has a heart rate of \(70\) bpm and a stroke volume of \(75\text{ }cm^3\):
\(\text{Cardiac Output} = 75 \times 70 = 5250\text{ }cm^3\text{ }min^{-1}\) (or \(5.25\text{ }litres\text{ }min^{-1}\)).

Did you know? During intense exercise, your cardiac output can increase significantly. Elite athletes often have a very high stroke volume, meaning their heart is so efficient it can pump more blood with fewer beats, which is why they often have a very low resting heart rate!

4. How the Brain Controls the Heart

While the heart is myogenic (it starts its own beat), the speed of that beat is controlled by the brain to meet the body's needs. This is a classic example of homeostasis.

The Cardiovascular Control Centre

In the medulla oblongata (the part of your brain at the top of the spinal cord), there is a cardiovascular control centre. It receives information about blood pressure and blood \(CO_2\) levels and sends signals to the SAN to either speed up or slow down the heart.

  • To speed up: The brain sends signals via the sympathetic nervous system. This is like the "accelerator" on a car.
  • To slow down: The brain sends signals via the parasympathetic nervous system. This is like the "brake."

Why does it change? During exercise, your muscles produce more \(CO_2\). This is detected by the body, and the medulla oblongata triggers an increase in heart rate and ventilation (breathing) to deliver more oxygen and flush out the \(CO_2\).

Key Takeaway: The heart starts the beat, but the medulla oblongata in the brain adjusts the speed via the sympathetic or parasympathetic nerves.

Quick Review: Avoid These Common Mistakes

  • Don't mix up the SAN and AVN: The SAN starts the wave; the AVN delays it.
  • Units matter: Always check if the question asks for cardiac output in \(cm^3\) or \(dm^3\) (litres). Remember \(1000\text{ }cm^3 = 1\text{ }dm^3\).
  • ECG Interpretation: Remember that the QRS complex is ventricular contraction. If the QRS complex is missing or weird, the problem is with the ventricles.

Final Thought: This chapter ties together everything about how the heart functions as a pump. Whether you're calculating cardiac output or identifying a P wave, always remember that the goal of the system is the same: keeping oxygenated blood moving to your cells!