What is the heart?

The heart is a muscular organ which pumps blood throughout the body and carries oxygen received from the lungs to the cells of tissues and organs to feed them and eliminates carbon dioxide from the bloodstream to the lungs.

It is divided into four chambers: upper left and right atria; and lower left and right ventricles. The chambers are divided from each other by walls called septa. At birth, the heart of an infant weighs about 20-21 grams. In adults, it reaches 250-300 grams.


How does the heart work?

The heart is located in the chest, behind the breastbone and it is positioned slightly to the left and placed on the diaphragm.It has a conical shape and is composed of the following layers:

  • Pericardium– a membrane that produces serous fluid, which surrounds and protects the heart from any friction between it and its surrounding organs
  • Myocardium–  the "muscle" of the heart
  • Endocardium– the layer which constitutes of the inner walls of the heart
  • Epicardium– the visceral layer of the pericardium


The heart is formed by involuntary striated muscle tissue, but unlike the other muscles of the human body, the heart is capable of contracting without nervous stimulation and this enables the heart to be transplanted. The myocardium, in particular, is composed of muscle fibers and cells called cardiac myocytes. These cells are capable of triggering nerve impulses to contractions, that is, heartbeats that allows blood to be circulated throughout the body. The “generator” of this stimulus is the sinoatrial node, located between the superior vena cava and right atrium, which transmits the impulse to contract around the heart through the muscle fibers. Located in the heart are also autonomous nerve fibers that make up the cardiac plexus, while the vagus nerve and sympathetic fibers connect the organ with the sympathetic, parasympathetic and sensory nerve stimulations, ensuring the coordination of the heart with brain stimuli. 

The heart consists of four valves, which act to control unidirectional blood flow between the atria and ventricles (the tricuspid valve and the mitral valve, called the atrioventricular valves) and between the heart and blood vessels (pulmonary valve and aortic valve, called semilunar valves). The blood vessels that communicate with the heart ensure the exchange of oxygen and carbon dioxide to and from tissues and organs. These include: the veins (inferior vena cava and superior vena cava), which carry blood full of carbon dioxide from organs and tissues to the heart; the pulmonary artery and the pulmonary vein, which ensure blood flow between the heart and lungs; and the aorta, which is the main artery of the human body, that reports and distributes oxygenated blood from the left ventricle of the heart to every organ.

Parts of the heart include:

  • Pulmonary valve
  • Tricuspid valve
  • Aortic valve
  • Mitral valve
  • Right ventricle
  • Left ventricle
  • Right atrium
  • Left atrium


What function does the heart serve?

The heart acts as a pump and its main function is to carry oxygen-rich blood to the rest of the body ( cells, tissues and organs) in order to feed them, and receive carbon dioxide enriched blood to be sent to the lungs, where the oxygen exchange takes place. Oxygenated blood (arterial blood) travels towards organs and tissues in the body through the aorta, the artery that carries nourishment to the whole body and further splits into smaller and smaller branches and capillaries. Carbon dioxide enriched blood returns to the heart through veins, which transform it to oxygen.

During physical activity, the heart is able to pump up to 20-30 liters of blood per minute, in order to ensure the increased requirement for blood from the muscles. At rest, the heart provides approximately 5 liters of oxygenated blood per minute.Considered optimal pressure is between 130 (systolic or high) and 80 (diastolic or lower) mmHg.

The heart provides this continuous loop through coordinated movements. In fact, the cardiac cycle consists of two phases that are repeated on average 70-80 times per minute at rest: a relaxation phase (the diastole) and a contraction phase (the systole). The frequency of the cardiac cycle is based on the different pressure that the blood exerts on the cardiac structures, causing closure to prevent retrograde flow. During the first stage (the diastole phase), the valves between the atria and ventricles are open, and the whole heart is relaxed. At this stage, the semilunar valves are closed, preventing blood from entering the vascular beds. During the second stage (the systole phase), the atria and ventricles contract in a coordinated manner: first the atria (atrial systole) and then the ventricles (systole). During ventricular contraction, blood pressure pushes up the atrio-ventricular valves, causing them to close, not allowing blood into the atria, and the blood is pumped into the effluent veins, namely those through which the blood comes out from the heart. In this case, the heart needs to be “fed”. This role falls to the coronary arteries, which are responsible for supplying oxygen-rich blood to the heart muscle.

When blood flow through the myocardium is diminished, due to a narrowing of the coronary arteries and other associated factors, the heart and its tissues suffer from lack of oxygen. In these cases, myocardial ischemia (ischemic heart disease or myocardial ischemia) may lead to myocardial infarction (or ACS). Other diseases that can affect the heart are caused by inflammation, valvular injury and  nerve conduction problems such as arrhythmia.