The heart: All you need to know

Our heart beats 100,000 times a day, pushing 5,000 gallons of blood, every 24 hours, through our body. It supplies our tissues with oxygen- and nutrient-rich blood, and carries waste away. Here, we explain how this magnificent organ does this critical function.

The heart is a muscular organ roughly the size of a closed fist. It sits in the middle, slightly to the center-left.

It pumps blood around the body, as the heart contracts. This takes deoxygenated blood into the lungs where it fills with oxygen and discharges carbon dioxide, a metabolism waste product.

Combined heart, blood , and blood vessels are called circulatory system. An average human has about 5 liters of blood (8 pints) which is constantly pumped throughout the body.

In this article we will describe the heart structure, how it pumps blood throughout the body and how it regulates the electrical system.

Basic anatomy of the heart

The heart consists of four chambers:

  • Atria: the two upper chambers (they receive blood).
  • Ventricles: the two lower chambers (they discharge blood).

The left atria and left ventricle are separated by a muscle wall called the septum from the right atria and right ventricle.

The wall of the heart consists of three layers of tissue:

  • Epicardium — protective layer mostly made of connective tissue.
  • Myocardium — the muscles of the heart.
  • Endocardium — lines the inside of the heart and protects the valves and chambers.

In a thin protective coating called the pericardium, these layers are covered.

How the heart works

Depending upon many factors the heart contracts at different rates. It may beat about 60 times a minute at rest, but it can rise to 100 beats per minute or more. Heart rate can be influenced by exercise, emotions, fever, disease and some medication.

The left and right side of the heart work in unison. The right side of the heart receives deoxygenated blood and sends it to the lungs; blood from the lungs is received from the left side of the heart and pumped to the rest of the body.

The atria and ventricles in effect contract and relax, producing a rhythmic heartbeat:

Right side

  • The right atrium receives deoxygenated blood from the body through veins called the superior and inferior vena cava (the largest veins in the body).
  • The right atrium contracts and blood passes to the right ventricle.
  • Once the right ventricle is full, it contracts and pumps the blood through to the lungs via the pulmonary artery, where it picks up oxygen and offloads carbon dioxide.

Left side

  • Newly oxygenated blood returns to the left atrium via the pulmonary vein.
  • The left atrium contracts, pushing blood into the left ventricle.
  • Once the left ventricle is full, it contracts and pushes the blood back out to the body via the aorta.

Each heartbeat can be split into two parts:

Diastole: the atria and ventricles relax and fill with blood.

Systole: the atria contract (atrial systole) and push blood into the ventricles; then the ventricles contract (ventricular systole) and pump blood out of the heart as the atria begin to relax.

As blood is sent to the lungs through the pulmonary artery, it passes through tiny capillaries on the alveoli (air sacs) surface of the lung. Oxygen travels into the capillaries, and carbon dioxide travels into the air sacs from the capillaries, where it is breathed out into the atmosphere.

The heart ‘s muscles must receive oxygenated blood, too. They are fed to the surface of the heart by the coronary arteries.

Where blood passes near the body’s surface, such as at the wrist or neck, your pulse may be felt; this is the rush of blood, as it is pumped through the heart through the body.

The valves

A diagram of the heart’s valves.
Image credit: OpenStax College, Anatomy & Physiology, 2013
A diagram of the heart’s valves.
Image credit: OpenStax College, Anatomy & Physiology, 2013

The heart has four valves that help ensure that blood only flows in one direction:

Aortic valve: between the left ventricle and the aorta.

Mitral valve: between the left atrium and the left ventricle.

Pulmonary valve: between the right ventricle and the pulmonary artery.

Tricuspid valve: between the right atrium and right ventricle.

The sound of a human heartbeat is familiar to the majority of people. It’s often referred to as a “lub-DUB” sound. The “lub” sound is produced by the closing of the tricuspid and mitral valves, and the “DUB” sound is caused by the pulmonary and aortic valve closure.

The heart’s electrical system

The heart muscles have to be perfectly coordinated to pump blood throughout the body — squeezing the blood in the right direction, at the right time , at the right pressure. The heart ‘s activity is coordinated by electrical impulses.

The electrical signal starts at the Sino-Atrial (or sinus, SA) node — the pacemaker of the heart, above the right atrium. This signal causes the atria to contract, pushing blood down into the ventricles.

The electrical impulse travels down the right atrium to an area of cells called the atrioventricular (AV) node. These cells act as a gate; they slow the signal down so that the atria and ventricles do not contract at the same time — there needs to be a slight delay.

From here the signal is carried inside the ventricle walls through special fibers called Purkinje fibres; they transfer the impulse to the muscle of the heart, causing the ventricles to contract.

Blood vessels

There are three types of blood vessels:

Arteries: carry oxygenated blood into the rest of the body from the heart. Arteries are strong and stretchy, helping push blood through the circulatory system. Its elastic walls help to maintain consistent blood pressure. Branch the arteries into smaller arterioles.

Veins: These bring deoxygenated blood back to the heart and increase in size as they reach the heart. Veins have thinner walls than arteries.

Capillaries: The smallest arteries are connected to the smallest veins. They have very thin walls which allow compounds to be exchanged with surrounding tissues such as carbon dioxide , water, oxygen, waste, and nutrients.

In a nutshell

The heart is an important and strong organ although we rarely consider it. It pumps oxygen and nutrients around our body constantly, never pausing. Powered by muscles and perfectly synchronized by electrical signals, it is one of the finest engineering feats of nature.

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