notes by dr Claudio Italiano
The heart is a hollow muscle-membranous organ that determines, with its rhythmic contractions, the movement of the blood stream. It is located in that space between the two lungs (mediastinum) and rests on the convexity of the diaphragm. It has the shape of a flattened cone whose apex rests at the bottom left and forward. And indeed the impact of the tip of the heart is appreciated in the fifth intercostal space on the left. Its base looks up and back. It weighs approximately 300 grams. A fibrous sac, the pericardium, envelops it. This sack is formed of a thickened connective wall and inside it there are two thin serous sheets bathed by a small quantity of a liquid, which increases physiologically in the slow agonies and moreover particularly in pathological conditions (pericarditis).
Of the two sheets of the pericardium, the one covering the heart is called the visceral pericardium, while the outer one, adherent to the fibrous sac, is called the parietal pericardium. The myocardium forms the muscular wall of the heart. The heart muscle fibers have a structure that offers some characters of the smooth muscle fibrocellula and others of the striated fiber. The fibers of the myocardium are joined together, constituting a characteristic muscular syncyte.
The myocardium is, therefore, a special muscle tissue due to the fact that its contraction is characterized by the automatism and by the tireless and regular rhythm of the heart. The inner tunic of the heart, called the endocardium, is covered by an endothelium resting on a connective layer endowed with elastic fibers. The heart of Man shows 4 cavities: 2 lower ones called ventricles and 2 upper ones called atriums.
A sagittal septum separates the atrium and the right ventricle from the two homonymous left cavities, thus subdividing the heart into a right half and a left half. Since the atrium and the right ventricle are full of venous blood, the right half is called the venous heart, while the left half, in which arterial blood flows, is called the arterial heart. To limit each atrium from the underlying ventricle an atrioventricular cavity is arranged that surrounds a large orifice, which connects the atrial cavity with the ventricular cavity. These orifices are equipped with characteristic funnel-shaped valves at the apex face towards the ventricle, valves consisting of flaps subtended by tendons that attach themselves to the muscles of the ventricles. The valves serve to prevent the reflux of blood, from the ventricles to the atria. The right atrioventricular orifice is held by the tricuspid valve, with three flaps or cusps. The left atrioventricular port has the bicuspid valve, formed of only two valvular flaps and therefore called mitral valve, because it resembles a miter. Two large arteries originate from the ventricles whose orifice is provided with three valves, which have the shape of a swallow's nest and are called semilunar valves. It should be remembered that the right ventricle emerges from the pulmonary artery, which carries the venous blood to the lung, while from the left one arises the aorta artery, which pours oxygenated blood into the current of the great circulation. The atria have very thin walls and the veins open in them: the right atrium receives the superior or descending vena cava and the inferior or ascending vena cava and the sinus of the coronary veins collecting the waste blood from the heart. In the vault of the left atrium, the orifices of the four pulmonary veins open.
Blood circulation in Vertebrates can be double or simple, complete or incomplete. It is called double when the blood passes twice for the heart, covering two circuits, one that starts from the left ventricle and returns to the right atrium after having covered the whole body and takes the name of great circulation, the other that starts from the right ventricle and back to the left atrium after passing through the capillary networks of the lung: the latter is the pulmonary circulation or small circulation. In the great circulation, the blood, which comes out of the left ventricle and goes towards the periphery, is called arterial, and has a bright red color for the overall color of the red blood cells that are enriched with oxygen; the blood returning from the periphery to the right atrium takes on a bluish tinge for the carbon dioxide which has dissolved in the blood; and it is called venous blood. In the small circulation, the blood of the pulmonary artery has a venous tint and becomes an arterial color in the pulmonary veins that terminate in the left atrium. Circulation is called simple when blood passes only once through the heart. Moreover, the circulation is complete if the arterial blood does not mix with the venous blood; incomplete if such mixing takes place. In the man the circulation is double and complete. Small circulation vessels. - The pulmonary artery originating from the right ventricle, heads up and to the left and after a short course divides into a right trunk and a left trunk. These two branches of division (pulmonary arteries) are brought to the respective lungs and penetrating, through the ilo, they are divided into numerous branches until they are reduced in a thick capillary network that tightens around the pulmonary alveoli. From this capillary network the pulmonary veins begin that in each lung are collected in correspondence of the hilum in two pulmonary veins. The four pulmonary veins, two on the right and two on the left, flow into the left atrium.
The main artery of the great circulation is the aorta. This voluminous trunk originates from the left ventricle with the name of ascending aorta, goes up again in the pericardium, then leaves it and goes from right to left and from the front backwards it forms a wide curve with a lower concavity, called arch of the aorta. The arch embraces the large left bronchus and then descends into the thorax and abdomen respectively with the names of thoracic aorta and abdominal aorta. At the level of the fourth lumbar vertebra is divided into the two common iliac arteries. At this point of bifurcation stands out from the aorta, the middle sacral artery thus called because it rests at the front of the sacrum; this is the caudal artery of animals with tail. At its beginning, when it is still in the pericardium, the two coronary arteries, the right and the left, are detached from the aorta. A spasm of these arteries (angina pectoris) or the occlusion of a branch of them (myocardial infarction) can lead to sudden death.
They bring to the heart, with the arterial blood, the quantity of oxygen essential for the good function of the heart muscle. From the convexity of the aortic arch, the brachiocephalic trunk that opens into the right carotid and the right subclavian to the right and then the left carotid and the left subclavian stand out from the arch. The carotids bring blood to the head, while the succlavias provide with their branches to nourish the walls of the chest and partly the neck and continue to the root of the upper limbs with the name of axillary arteries, and then humeral. These, below the elbow, give rise to the radial, ulnar or cubital arteries, which supply the forearm.
The thoracic aorta detaches the intercostal arteries, which run in the intercostal spaces, and also the branches destined to the thoracic viscera (pericardial, bronchial, esophageal arteries, etc.). After crossing the diaphragm the aorta descends into the abdomen and takes the name of abdominal aorta and from visceral branches to the stomach (gastric artery), the liver (hepatic artery), the spleen (splenic artery), the intestine (mesenteric arteries), kidney (renal arteries), adrenal capsules (adrenal arteries) and the walls of the abdomen. The two common iliac arteries, branches of division of the aorta, after a short course, are divided into the internal iliac which provides for the circle of the organs of the pelvis and in the external iliac that continues in the inguinal region with the femoral artery, which thick branches irrigate the entire lower limb and also provide for the foot's circulation.
The veins of the great circulation converge to form the system of the two hollow veins, the upper which collects blood from the head, from the thorax and from the upper limbs, the inferior which collects blood from the abdominal walls and from the lower limbs. The veins of the heart bring back the blood, which circulated in the cardiac walls in the right atrium through the outlet of the coronary sinus. All the veins of the brain and all those of the skull, face and neck are part of the jugular vein system. The largest are the two internal jugular veins, one on each side. The jugular veins joining the subclavian veins, which collect the venous blood from the upper limb, from the upper part of the thorax and from the lower part of the neck, make up the two anonymous veins.
The anonymous left vein and left anonymity vein join together to form the superior vena cava, which opens in the right atrium of the heart after receiving the outlet of the azigos vein that congeals the veins of the thorax. The inferior vena cava reaches from the bottom the right atrium and is designed to collect blood from the kidneys (renal veins), the organs of the pelvis and the lower limbs (iliac veins) and the liver (veins above hepatic).
A particular venous system is that of the portal vein: a voluminous trunk that collects blood from the stomach (left gastric vein), from the intestine (superior and inferior mesenteric veins), from the spleen (splenic vein) and from the pancreas and conveys it to the liver. In this large glandular organ the portal vein is capillarized and from this capillary network, which is in close contact with the cells of the liver, the so-called superhepatic veins originate, leading to the inferior vena cava. It should therefore be remembered that all the venous blood coming from the intestinal tract of the intestine does not go directly into the general venous circulation but reaches the liver through the portal vein.
Cardiology