Heart

The **heart** is a hard-working, fist-sized muscle found in people and animals. It pumps out blood through tube-like structures called blood vessels. The heart and these vessels together make up the body-wide circulatory system. Blood carries in oxygen-rich air and nutrient-packed food to the body and takes away waste-filled substances like carbon dioxide to the lungs. In people, the heart is about the size of a closed-hand and sits in between the lungs, in the chest-centered cavity.

In humans, the heart has four parts called chambers. These are the upper-left and upper-right atria, and the lower-left and lower-right ventricles. People often talk about the right-side atrium and ventricle together as the right heart, and the left-side ones as the left heart.

In a well-working heart, blood goes through the heart in one direction. Special flaps called valves keep out blood from going backward. The heart sits inside a fluid-filled protective bag called the pericardium. This bag holds in a small amount of fluid to help the heart move smoothly.

The heart wall has three layered parts: the epicardium (outer layer), the myocardium (middle layer), and the endocardium (inner layer).

## Structure

The human heart has four chambers or closed spaces. Some animals have only two or three chambers.

In humans, the four chambers are two _atria_ and two _ventricles_. Atria is talking about two chambers; atrium is talking about one chamber. There is a right atrium and right ventricle. These get blood that comes to the heart. They pump this blood to the lungs. In the lungs blood picks up oxygen and drops carbon dioxide. Blood from the lungs goes to the left atrium and ventricle. The left atrium and ventricle send the blood out to the body. The left ventricle works six times harder than the right ventricle because it carries oxygenated blood.

Blood is carried in blood vessels. These are arteries and veins. Blood going to the heart is carried in veins. Blood going away from the heart is carried in arteries. The main artery going out of the right ventricle is the _pulmonary artery_. (Pulmonary means about lungs.) The main artery going out of the left ventricle is the _aorta_.

The veins going into the right atrium are the _superior vena cava_ and _inferior vena cava_. These bring blood from the body to the right heart. The veins going into the left atrium are the _pulmonary veins_. These bring blood from the lungs to the left heart.

When the blood goes from the atria to the ventricles it goes through heart _valves_. When blood goes out of the ventricles it goes through valves. The valves make sure that blood only goes one way in or out.

The four valves of the heart are:
- Atria to ventricle valves
    - _Tricuspid valve_ – blood goes from right atrium to right ventricle
    - _Mitral valve_ – blood goes from left atrium to left ventricle
- Ventricles to arteries
    - _Pulmonic valve_ – blood goes out of the right ventricle to the lungs (through the _pulmonic artery_)
    - _Aortic valve_ – blood goes out of the left ventricle to the body (through the _aorta_)

The heart has three layers. The outer covering is the pericardium. This is a tough sack that surrounds the heart. The middle layer is the myocardium. This is the heart muscle. The inner layer is the endocardium. This is the thin smooth lining of the chambers of the heart.

## Cardiac cycle

A _heart beat_ is when the heart muscle _contracts_. This means the heart pushes in and this makes the chambers smaller. This pushes blood out of the heart and into the blood vessels. After the heart contracts and pushes in, the muscle _relaxes_ or stops pushing in. The chambers get bigger and blood coming back to the heart fills them.

When the heart muscle contracts (pushes in) it is called _systole_. When the heart muscle relaxes (stops pushing in), this is called _diastole_. Both atria do systole together. Both ventricles do systole together. But the atria do systole _before_ the ventricles. Even though the _atrial systole_ comes before _ventricular systole_, all four chambers do diastole at the same time. This is called _cardiac diastole_

The order is: atrial systole → ventricular systole → cardiac diastole. When this happens one time, it is called a _cardiac cycle_.

## Heart's pacemaker

Systole (when the heart squeezes) happens because the muscle cells of the heart gets smaller in size. When they get smaller we also say they _contract_. Electricity going through the heart makes the cells contract. The electricity starts in the _sino-atrial node_ (acronym SA Node) The SA Node is a group of cells called pacemaker cells in the right atria. These cells start an _electrical impulse_. This electrical impulse sets the rate and timing at which all cardiac muscle cells contract. This motion is called 'atrial systole'. Once electrical impulse goes through the _atrio-ventricular node_ (AV Node). The AV Node makes the impulse slow down. Slowing down makes the electrical impulses get to the ventricles later. That is what makes the ventricular systole occur _after_ atrial systole, and lets all the blood leave the atria before ventricle contracts (_meaning squeeze_).

After the electrical impulse goes through the AV Node, the electrical impulse will go through the _conduction system_ of the ventricle. Conduction means heat or electricity traveling through something. This brings the electrical impulse to the ventricles. The first part of the conduction system is the _bundle of His_. _His_ is named for the doctor (Wilhelm His, Jr) who discovered it. Bundle means strings or wires grouped together in parallel. Once the bundle (_meaning a group of strings or wires going in parallel directions_) goes through the ventricle muscle, it divides into two _bundle branches_, the left bundle branch and the right bundle branch. The left bundle branch travels to the left ventricle and the right bundle branch travels to the right ventricle. At the end of the bundle branches, the electrical impulse goes into the ventricular muscle through the Purkinje Fibers. This is what makes ventricle contraction take place and makes ventricular systole.

The order is:
Sino-Atrial Node → Atria (systole) → Atrio-Ventricular Node → Bundle of His → Bundle branches → Purkinje Fibers → Ventricles (systole)

## ECG

ECG is an acronym for **E**lectro**C**ardio**G**ram. It is also written EKG for **E**lectro**K**ardio**G**ram in German. The ECG shows what the electricity in the heart is doing. An ECG is done by putting electrodes on a person's skin. The electrodes see the electricity going through the heart. This is written on paper by a machine. This writing on the paper is the ECG.

Doctors learn about the person's heart by looking at the ECG. The ECG shows some diseases of the heart like heart attacks or problems with the _rhythm_ of the heart (how the electricity goes through the heart's conduction system.)

The ECG shows atrial systole. This is called a **P-wave**. Then ventricular systole happens. This is called the **QRS** or QRS-complex. It is called a complex because there are three different waves in it. The Q-wave, R-wave, and S-wave. Then the ECG shows ventricular diastole. This is called the **T-wave**. Atrial diastole happens then too. But it is not seen separate from ventricular diastole.

The **PR-Interval** is the space between atrial systole (P) and ventricular systole (QRS). The **QT-Interval** is from when the QRS starts to when the T ends. The **ST-segment** is the space between the QRS and T.

## Other websites

- Symptoms of heart disease
- What is the heart? - NIH
- The gross physiology of the cardiovascular system (2nd Ed., 2012) – Robert M. Anderson, M.D. (CC-BY-NC)
- Atlas of human cardiac anatomy
- healthy heart  " Blog