Heart Beat Under The Microscope – Lesson One – Cells and Electrical Charge

Heart Beat Under The Microscope - Lesson One - Cells and Electrical ChargeHeart Beat Under The Microscope – Lesson One – Cells and Electrical Charge

This is the first post in a series on how the heart beats, but on a cellular level. Unfortunately to explain the hearts ‘beat’ fully we have to cover some principles that a seem quite out of place. Stick with me though and you will have a FANTASTIC understanding of how the heart beats!

Cells have an Electrical CHARGE!

The first thing to know is that cells in the human body have an electrical charge.  By an electrical charge I am referring to that physics class at school where you learned that opposites attract! So something with a positive charge and something with a negative charge will be attracted (pulled) towards one another. Things of the same charge (both negative or both positive) will be repelled from one another!
These forces have a HUGE affect on how the CELLS in our bodies work.

How Can a Cell have an Electrical Charge?


There are many molecules/atoms (tiny building blocks of matter) in the body. If these atoms have a charge then they are called an ION (Ion just means a variation of an atom or molecule that carries a net positive or negative charge!). If you want to go into more detail than this then feel free to google IONS but essentially if there are more Electrons than Protons in an atom it will have a NEGATIVE charge and if there are more Protons than Electrons it will have a positive charge. 

Types of Ion

Heart Beat Under The Microscope - Lesson One - Cells and Electrical Charge
So there are many types of IONS in the human body and human cells. Below is a list of the most plentiful and whether they are a positive ION or a negative ION.
Each cell TYPE contains a varying number and type of IONS. It is the combination of these different types of IONs within a cell that dictates its overall charge.

For example if you had a cell with 20 x Sodium Ions (+’ve) and 10 x Potassium Ions (+’ve) then the cell will be positively charged. If you have a cell made up entirely of Chloride Ions then the cell would be Negatively charged. If a cell contains a mixture of positive and negative IONS the charge of the cell will depend on whether the charge of the Negative Ions or the Positive Ions is more dominant.

Got it? Awesome!

Quite interestingly and just to confuse things, ions can often move freely in and out of cells.

So What Decides the Number and Type of Ions in Different Cells?

Well the most obvious one is when and how it was created but there are other forces in place that affect the distribution of ions.

There are essentially three ‘laws’ that affect the movement of Ions in and out of a cell.

This is really cool stuff…

Permeability of the Membrane

Cell Membrane Permeability

Lets start with a simple one.

The Membrane of the cell is like a clever skin, keeping everything in that it wants and everything out that it doesn’t. It does this through special ‘door ways’ in the cell membrane that will allow certain ions to pass but not other types of Ion. A Calcium Ion Channel would allow Calcium Ions to pass through it.
Like a Doorman outside a nightclub the cell membrane controls which particles can pass through. If your name isn’t on the list then you are not coming through.

Unsurprisingly these ‘gateways’ in the cell membrane are called Ion channels and these channels have a huge influence on Ion concentrations.


Diffusion is something you are familiar with, even if you don’t realise it. What happens if you urinate in a swimming pool? Does a cloud of yellow water follow you around for the rest of the day? No. The particles in the water diffuse. They spread out so they are equally distributed around the pool. They become so diffused that you can no longer see your shameful act. This principle is at work within cells and their extracellular fluid (the fluid that they float around in). If there is a large concentration of Ions on the outside of a cell they will try and diffuse to the area with a low concentration (into the cell) and vice versa. Anyway the pictures explains this better than I do. This process is obviously dependent on the Membrane Permeability that I just discussed.

Diffusion Heart Cell Membrane Potential

Electromagnetic Forces

We touched on this earlier but basically the Ions that are stuck within the cell attract, stick or pull some of those Ions to them that would otherwise be wishing to leave the cell because of diffusion or attraction from else where. This is just another force that is affecting the distribution and concentration of Ions inside and outside of the cell. Think of giant magnets pulling Ions into the cell that would otherwise be looking to leave. See image below…

The Attraction of Ions Heart Cell Membrane Potential

So we have a few different forces all trying to influence the movement of Ions the only solution is that a compromise has to be reached.  The compromise between these forces decides how many Ions of different types are inside and outside the cell.

Lets look at a fictional example cell…

Heart Cell Membrane Potential

See how the cell has an electrical charge because of the 3 ‘laws’? Perfect!

Forces of Ions Heart Cell Membrane Potential

In actual fact you are familiar with forces working along side one another during every day life! It happens everywhere…

Think of a plane staying in the air – gravity wants the plane to come down, propulsion wants it to move forward and the force of the air passing the wings pushes the plane upwards. None of the ‘forces’ really get totally what they want. It is the compromise of the forces keeps the plane in a state of linear motion through the air.

P.S. Thats the best plane I have ever drawn!

SO…. there we have an explanation of HOW a cell can have and sustain an electrical charge! This is going to be key in understanding how the heart beats!

BEFORE I let you escape though, I just want to familiarise you with two terms that will be important in my next post, Membrane Potential and Action Potential.

Membrane Potential and Action Potential

The difference between the electrical charge inside the cell and the electrical charge outside the cell (in the extracellular fluid) is what is known as the Membrane POTENTIAL. This is because the cell has a potential to release energy based on this imbalance!

Think of an inflated ballon, this has a ‘membrane potential’. A potential energy caused by the difference in number of atoms held inside the balloon and that on the outside. This potential energy is not released until there is a change in circumstance (i.e. the membrane permeability is changed by a child and a large sharp pin!) at which point the energy is released! BANG!

Cells also hold a Potential Energy. When this potential energy (Membrane Potential) changes very rapidly and then returns to how it was originally… this is known as ACTION POTENTIAL.

Action Potentials are responsible for the heart beat and will be explained fully in the next post.

This is a tough topic (tough to explain!) so if you have any questions then pop them in the comments and I will try and clear them up!

If you have read and understood this, fantastic go and have a cup of tea!

Thank You for Reading

Cardiac Technician

Image courtesy of sscreations / FreeDigitalPhotos.net

Comments 1

  1. Hi, very good first lesson on the physiology of the heart function, helped me alot! But I can’t find the second lesson anywhere… is it just me that is blind or is there somewhere else where I can find it?

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