The cardiovascular system consists of three elements:
(1) blood - a fluid that circulates through the body and brings substances to the cells and drives others away;
(2) blood vessels - conduits through which blood circulates;
(3) the heart - a muscle pump that distributes blood flow in the vessels.
The cardiovascular system can distribute substances throughout the body more quickly than it can spread, as the molecules in the blood move inside the circulating liquid like water particles in a river. In the bloodstream the molecules move faster because they do not proceed at random, forward-backward or zig-zag as in the diffusion, but in a precise and orderly manner.
Blood circulation is so crucial to our existence that if the blood flow stops at some point, we will lose consciousness within a few seconds and expire after a few minutes. Obviously the heart has to perform its function continuously and correctly, every minute and every day of our lives.
Heart
The heart is contained in the center of the rib cage, located anteriorly and slightly shifted to the left. Its shape resembles roughly that of a cone, whose base faces upwards (to the right), while the tip faces downwards, to the left.
The myocardium, that is the heart muscle, allows the heart to contract, drawing blood from the periphery and pumping it back into the circulation.
Internally, the heart is covered by a serous membrane, called endocardium. Externally, on the other hand, the heart is contained in a membranous sac called pericardium, which constitutes the space within which the heart is free to contract, without necessarily having to give rise to frictions with the surrounding structures. The pericardium cells secrete a liquid that has the task of lubricating the surfaces to avoid such frictions.
The cavity of the heart is divided into four areas: two atrial areas (right atrium and left atrium) and two ventricular areas (right ventricle and left ventricle).
The two right cavities (atrium and ventricle) are communicating with each other thanks to the right atrioventricular orifice, which is cyclically closed by the tricuspid valve. The two left cavities are in communication via the left atrio-ventricular orifice, closed cyclically by the bicuspid or mitral valve.
The right cavities are completely separated from the left cavities; this separation takes place by two septa: the interatrial one (which separates the two atriums) and the interventricular one (which separates the two ventricles).
The functioning of the tricuspid valve (formed by three connective flaps) and that of the mitral valve (formed by two connective flaps) allow the blood to flow along only one direction, starting from the atria, up to the ventricles, and not vice versa.
The right ventricle originates from the pulmonary artery, and is separated from it through the pulmonary valve (consisting of three connective flaps). The left ventricle is separated from the aorta through the aortic valve, which presents a morphology that is completely superimposable to the pulmonary valve.
These two valves allow blood to flow from the ventricle to the blood vessel (pulmonary artery and aorta), without this changing direction.
The right atrium receives blood from the periphery via two veins: the superior vena cava and the inferior vena cava. This blood, called venous, is poor in oxygen and reaches the heart muscle precisely for re-oxygenation. On the contrary, the left atrium receives arterial blood (rich in oxygen) from the four pulmonary veins, so that the same blood can be poured into the circulation and perform its functions: re-oxygenate and nourish the various tissues.
The heart, like skeletal muscles, contracts in response to an electrical stimulus: for skeletal muscles this stimulus comes from the brain through the various nerves; for the heart, on the other hand, the impulse is formed autonomously, in a structure called the sino-atrial node, from which the electrical impulse reaches the atrioventricular node.
The bundle of His originates from the atrioventricular node, leading the impulse downwards; the His bundle is divided into two branches, the right and the left, which descend respectively on the right and left sides of the interventricular septum. These bundles are progressively branching, reaching, with their ramifications, the entire ventricular myocardium, where the electrical impulse produces the contraction of the heart muscle.
Small circulation
The small circulation begins where the large ends: the venous blood from the right atrium descends into the right ventricle, and here, through the pulmonary artery, brings blood to each of the two lungs. Inside the lung the two branches of the pulmonary artery divide into smaller and smaller arterioles, which become pulmonary capillaries at the end of their path. The pulmonary capillaries flow through the pulmonary alveoli, where the blood, poor in O 2 and rich in CO 2, is re-oxygenated.
It is interesting to note how in the pulmonary circulation the veins carry arterial blood and venous blood arteries, contrary to what happens in the systemic circulation.
The large circle starts at the aorta and ends at the capillaries
The aorta, through successive branches, gives rise to all the minor arteries that reach the various organs and tissues. These branches become progressively smaller and smaller, until they become capillaries deputed to the exchange of substances between blood and tissues. Through these exchanges, nutrients and oxygen are added to the cells.
ELEMENTS OF CARDIOVASCULAR PHYSIOLOGY
The heart has four fundamental properties:
1) the ability to contract;
2) the ability to self-stimulate at certain heart rates;
3) the ability of the myocardial fibers to transmit to the neighboring ones the received electrical stimulus, also making use of preferential conduction routes;
4) the excitability, that is the ability of the heart to respond to the electrical stimulus given to it.
The cardiac cycle is the time between the end of a cardiac contraction and the beginning of the next. In the cardiac cycle we can distinguish two periods: diastole (the period of relaxation of myocardial musculature and filling of the heart) and systole (period of contraction, ie the expulsion of blood in the systemic circulation by means of the aorta).
From the atrial sinus node the electrical impulse reaches the atrio-ventricular node, where it undergoes a slight slowing down and where it spreads, following the two branches of the His bundle (and their terminal branches), to the whole ventricular myocardium, causing its contraction .
Most (about 70%) of the blood that reaches the heart during diastole passes directly from the atria to the ventricles, while the remaining quantity is pumped from the atria to the ventricles by the contraction of the atria themselves, at the end of the diastole. This latter quantity of blood is not particularly important in conditions of rest; it becomes indispensable during the effort when the increase in heart rate shortens the diastole (ie the period of filling the heart) making the time available for filling the ventricles shorter. During atrial fibrillation (ie the condition in which the heart beats in a completely irregular manner) there is a functional limitation of cardiac performance, which is particularly manifested during exertion.
The time that elapses between the closure of the atrioventricular valves and the opening of the semi-lunar valves is called isometric contraction time, because, even if the ventricles become tense, the muscle fibers do not shorten.
At the end of the systole, the ventricular musculature is released: the endoventricular pressure falls to levels much lower than those present in the aorta and pulmonary artery, causing the closure of the semilunar valves and, subsequently, the opening of the atrioventricular ones (because the endoventricular pressure has become less than intra-atrial pressure).
The period between the closure of the semi-lunar valves and the opening of the atrioventricular valves is called the isovolumetric relaxation period, as the muscle tension collapses, but the volume of the ventricular cavities remains unchanged. When the atrioventricular valves open, the blood flows again from the atria to the ventricles and the described cycle begins again.
The movement of the heart valves is passive: they open and passively close as a consequence of the pressure regimes existing in the chambers separated by the valves themselves. The function of these valves is therefore to allow the flow of blood in a single direction, the anterograde one, preventing the blood from turning back.
Edited by: Lorenzo Boscariol
