Image credit: Patrick J. Lynch, CC BY 2.0
What is the heart?
The heart is a conical shaped cavitary organ that is found between the lungs, with the apex facing downward, forward and to the left. The heart is approximately the size of a closed fist.
Its main function is to send blood in the circulatory system, acting as a pump. The flow rate or the amount of blood pumped per minute is on average 5 liters.
The heart is made up of muscle tissue, myocardium, a special type of striated muscle that can't be found anywere else in the body. On the outside it is wrapped in a hard bag of fibrous tissue, called pericardium. The pericardium is made up of two sheets separated by a fine layer of liquid designed to remove the friction that occurs through the contraction and the distension of the heart.
How does it work?
Heart muscle cells can be divided into two categories: regular "working" muscle cells that form the majority muscle mass, and more specific muscle cells that form the heart conduction system and which have a very important feature: spontaneous capacity to emit and drive electrical impulses that will later materialize in the contraction of the cardiac muscle.
These cells, which have the spontaneous ability to emit electrical impulses, are medicaly called "pacemaker" cells, meaning cells capable of imparting a certain rhythm / frequency to the heart.
The heart's conduction system consists of the following structures:
- The sino-atrial node that physiologically emits impulses at a frequency of 60-100 beats per minute (this range also represents the normal number of heart beats per minute)
- Atrioventricular node
- The His Beam that divides into two main branches, left and right
- The Purkinje network
Image credit: Patrick J. Lynch, CC BY 2.0
Image credit: Rawpixel Ltd, CC BY 2.0
The heart and the circulatory system
The heart consists of four small rooms or cavities: two atria and two ventricles. These cavities, through the functions they perform, divide the heart into two halves, right heart (right atrium and right ventricle) and left heart (left atrium and left ventricle). Physiologically the right heart does not communicate directly with the left one except during intrauterine life.
Communication between the heart's cavities is done through structures called valves. These are thin, fibrous sheets that impress the blood with a unique sense of flow (prevents it from returning). Thus, the right atrium communicates with the right ventricle through the tricuspid valve, and the left atrium communicates with the left ventricle through the mitral valve.
The vessels that leave the heart are called arteries, and those that return to the heart, veins. The arteries that start from the heart are represented by the aorta, that starts from the left ventricle and the pulmonary artery that starts from the right ventricle. At the emerging zone of these arteries, there are also aortic and pulmonary valves, which have the same role of imparting a unique sense of flow to the blood. The veins returning to the left heart are called pulmonary veins, they are four and open in the left atrium. The veins returning to the right heart are called venae cavae, they are two, one upper and one inferior, and open in the right atrium.
Through the large circulation, arteries deliver oxygenated blood and nutrients and through the veins, deoxygenated blood, with carbon dioxide and substances that need to be removed from the circulation. This rule only applies for the blood that is pumped from the left heart (left atrium -> left ventricle -> aorta artery -> organs). As for the small circulation (right atrium -> right ventricle -> pulmonary artery -> lungs), the situation is the opposite: the deoxygenated blood is delivered through the arteries, which is oxygenated in the lungs, and through the pulmonary veins the oxygenated blood will be driven in the left atrium.