The cardiovascular system

Through the blood flow, the cardiovascular system transports nutriments, oxygen, carbon dioxide, hormones, and blood cells to and from the body’s organs. Its essential components are the heart and the blood vessels (arteries, veins, and capillaries). This system consists of two circulatory systems, the pulmonary circulation and the systemic circulation.

The pulmonary or small circulation ensures re-oxygenation of the blood through the lungs.

The systemic or large circulation provides the oxygenated blood for the muscles and organs of the rest of the body (Papillo & Shapiro, 1990).

1.4.2.1. Heart

The heart, located just behind and slightly left of the breastbone, is a muscular organ composed of four chambers: right atrium, right ventricle, left atrium and left ventricle. The heart is self-excitable, which means that electrical impulses generated by specialized cells within the heart—pacemaker cells—initiate the mechanical contraction of the heart muscle.

The electric activation of the sinoatrial node in the right atrium and of the atrioventricular node in the right ventricle provide the electrical trigger for the contraction of the cardiac muscle. There are two different pumps, the right one and the left one. The right pump produces the energy needed to send the deoxygenated blood into the pulmonary circulation, while the left pump allows the blood to enter the systemic circulation and reach different organs and muscles of the body. More precisely, the right atrium receives blood poor in oxygen from the superior vena cava and inferior vena cava. Once the atrium is filled with blood, the tricuspid valve opens, and the blood fills the right ventricle. The right ventricle contracts and the blood is ejected through the pulmonary valve into the lungs for oxygenation (through the pulmonary artery). During the same contraction, the blood rich in oxygen, goes back into the heart through the pulmonary veins, first into the left atrium and then into the left ventricle, with the opening of the mitral valve (Figure 2). With the ventricular contraction, the blood is ejected into the aorta through the aortic valve to the all body. During each cardiac contraction, blood is simultaneously ejected into the pulmonary artery and the aorta.

Figure 2. The pathway of blood flow through the heart (Figure retrieved from https://www.quora.com/How-does-blood-flow-through-the-heart).

1.4.2.2. The cardiac cycle

The cardiac cycle begins with contraction of the atria (the beginning of one heartbeat) and ends with ventricular relaxation (the beginning of the next heartbeat). It describes the cycle of electrical and mechanical events that occur when the heart beats (Figure 3). It is composed of two main periods: the systole and the diastole. Both the atria and ventricles undergo systole and diastole. The cardiac cycle starts with the depolarization of the sinoatrial node during the diastole, which is followed by the atrial contraction. As the atrial muscles contract from the upper part of the atria toward the atrioventricular septum, pressure rises within the atria and blood is pumped into the ventricles through the open atrioventricular—

tricuspid and mitral—valves. Once the blood is ejected into the ventricles, the atrioventricular valves are closed, which produces the familiar sound of the first beat of the heart. Then follows the depolarization of the ventricles and the ventricular systole. When the pressure within the ventricles is greater than the pressures in the pulmonary trunk and the aorta, the semilunar—

pulmonary and aortic— valves open and the blood is ejected from the heart. Once the blood is ejected, the semilunar valves close, which corresponds to the second beat which is sharper

than the first one. At the end of the systole follows repolarization of the ventricles with ventricular relaxation, or diastole. During this period, the ventricular muscles relax and the pressure on the remaining blood within the ventricles begins to fall. When the pressure in the ventricles drops below the pressure in the atria, blood flows from the atria into the ventricles, pushing the tricuspid and mitral valves to open. As the pressure in the ventricles drops even further, blood flows from the major veins into the relaxed atria and then into the ventricles.

Both the atria and the ventricles are in diastole, the atrioventricular valves are open, and the semilunar valves remain close. One cardiac cycle is complete. Its total duration is on average 800 ms, the diastolic phase lasts on average 600 ms, whereas the systolic phase lasts on average 200 ms (Papillo & Shapiro, 1990).

Figure 3. Mechanical and electrical events of the cardiac cycle (Figure retrieved from https://www.printablediagram.com/wiggers-diagrams/).

1.4.2.3. Sympathetic and Parasympathetic Nervous Systems

Among the factors influencing the cardiovascular system, we can find the two branches of the autonomic nervous system: the sympathetic and parasympathetic nervous systems which influence cardiac activity in two opposite ways. Activation of the sympathetic nervous system prepares the organism for activity. This system is responsible for homeostasis—

maintenance of internal stability—of the body. It is also associated with the activity of two main neurotransmitters, named norepinephrine and acetylcholine, which stimulate β-adrenergic and α-β-adrenergic receptors. An increase of the sympathetic activation may lead to either vasoconstriction or vasodilation depending on the type of the adrenergic receptor. The sympathetic nervous system is usually identified as responsible of the increase of cardiac activity—increase in myocardial contractility and in heart rate—due to an increased β-adrenergic stimulation. It also influences the cardiovascular system by affecting the contraction of the blood vessels by increased α-adrenergic impact. Activation of the parasympathetic system leads to a general slowdown of the organism’s functions. It is necessary for digestion, energy storage, and reproduction. It is mainly associated with the neurotransmitter acetylcholine, which stimulates muscarinic receptors that are responsible for the inhibition of sympathetic effects decreasing myocardial contractility and heart rate (Cacioppo, Tassinary, & Bernston, 2000; Papillo & Shapiro, 1990).