The Heart

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As it is World Heart Day on the 28th September, we thought we'd show this very important organ some love!

Despite being no bigger than a large, clenched fist, the heart beats around 100,000 times every day. By doing this, it distributes an amazing 2,000 gallons of blood around the body daily. It is an organ, but unlike the other organs in the body, the heart is actually made entirely of muscle.

Anatomy

The veins carry deoxygenated blood from around the body into the heart to be pumped into the lungs to be breathed out. The arteries carry oxygenated blood from the lungs into the heart to be pumped around the body.

Blood from the veins enters into the right atrium, the first of four chambers in the heart, via the inferior and superior vena cava. There are also four valves that separate the four chambers, ensuring that the blood always flows in the correct direction. The blood next flows downwards through the tricuspid valve that separates the right atrium from the right ventricle. The blood makes its exit from the right ventricle and into the lungs via the pulmonary artery once it flows through the pulmonary valve.

When breathing in, the blood is oxygenated in the lungs, and travels from them via the pulmonary veins into the left atrium of the heart. It then moves through the mitral valve downwards into the left ventricle. From here it enters the aorta arteries by flowing through the aortic valve.

The ventricles both contract together at once, whilst the atria are relaxed. When the atria contract, the ventricles are relaxed. This allows for a constant supply of blood to enter and exit the heart. Major problems can occur if this status quo is interrupted.

Heartbeat

The heart has an electrical system that controls the previously described pumping mechanism via electrical impulses.

In the right atrium, there is a collection of specialised cells called the sinoatrial node (SA node), whose job is to set the rhythm and rate of the heartbeat (colloquially it is called the natural pacemaker). The impulse starts here, and spreads through the atrium's walls, reaching into the left atrium too, which causes the muscles in both the right and left atrium to contract.

This electrical signal travels downwards towards the ventricles, but is slowed down by the atrioventricular node (AV node), which are more of these specialised cells found between the right atrium and right ventricle. Slowing down the signal gives the atria enough time to contract fully before the impulse causes the muscles of the ventricles to contract.

Finally, the AV node redirects the signal through the centre of the heart and along a pathway of fibers called His-Purkinje Network (HPS). These fibres spread out the impulse along the walls of muscle surrounding both ventricles, causing them to contract.

Once the ventricles are empty, the cycle starts again back in the atria, as the SA node releases another impulse.

Transportation

The heart pumps blood around the body via a huge network of veins, arteries and capillaries, which provide the essential route for transportation. The blood contains nutrients and oxygen that the cells in the body need to carry out their tasks, alongside waste material such as urine and carbon dioxide.

The biggest vessels are the arteries and veins, which transports the blood in large quantities over long distances very quickly. From these, thousands and thousands of thinner tubes branch off, and get smaller and smaller in diameter, the smallest being capillaries. They are so narrow that the red blood cells struggle to fit through them. These capillaries are where the oxygen and carbon dioxide is exchanged between the blood and cells.

Heart related AnatomyStuff resources

Anatomy for Children: The Heart Poster
achheaAnatomy of the Heart Chart / Poster - Laminated
hc1000Budget Heart Model (2 part)
xc-307aBudget Jumbo Heart Model (4 times life size, 3 part)
xc-307Understanding Heart Disease Chart / Poster - Laminated
hd1000

Further reading / sources

Heart anatomy
https://www.texasheart.org/heart-health/heart-information-center/topics/heart-anatomy/
Heartbeat
https://my.clevelandclinic.org/health/articles/17064-heart-beat

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