Vena cava - The system of the vena cava

The vena cava system is divided into superior vena cava and inferior vena cava; each of these two routes conveys to the heart the oxygen-poor and carbon dioxide-rich blood coming from the upper half (suprradiaphragmatic) and lower half (subdiaphragmatic) respectively of the body.

Indeed:

through the superior vena cava blood flows from the head, neck, chest and upper limbs to the heart.
through the inferior vena cava, on the other hand, the blood that has supplied the pelvis, the lower limbs and the abdomen flows to the heart.

In other words, within the framework of the general blood circulation, the vena cava system is responsible for collecting and making it flow towards the heart, precisely in the right atrium, all the blood poor in oxygen and rich in carbon dioxide coming from the tissues and collected by the other components from the venous tree. This blood will then be introduced from the right ventricle into the pulmonary circulation to re-oxygenate it and, subsequently, collected and pumped into the systemic circulation from the left side of the heart.

Structural and anatomical characteristics of the veins

The veins differ from the arteries for a smaller thickness and a greater delicacy of the walls. Despite this, the venous walls have a greater distension capacity and, in general, a lower risk of injury than arterial vessels. This is possible because the blood flowing inside them has a lower pressure. They tend to be larger in diameter than the arteries, consequently the quantity of blood flowing through them is higher.
From the point of view of the structure, the muscular and elastic components in the veins are lower than in the arteries. Collagen, on the other hand, is a highly present component.
The veins are also made up of the typical three cassocks, the intimate, the medium and the adventitia.

The intimate is the innermost lining of the vessel. Made up of endothelial cells, the blood flows inside it and, precisely because of the presence of the endothelium, it is possible that material is absorbed by the fluid that flows inside.
The media is supplied with muscle fibers and elastic fibers. The prevalence of one type of fiber over the other depends on the role played by the vessel in a specific area of the human body.
Finally, the adventitia is the external wrapping of the vessels and is formed by connective tissue. It represents the point of contact between the vessel and the external environment. It can contain muscle and elastic fibers.

It is difficult to classify and accurately describe the histology of the cassocks since the structure of the veins varies according to the local function. In other words, the capillaries, the medium caliber vessels and the larger blood ducts have a different histological composition, to level of the three cassocks, according to the districts in which they are located and the functions they perform. In some veins, the smooth muscle component may be almost completely missing, as in the veins of the skin, spleen or retina. In other venous vessels, instead, the smooth muscle fibers are very present, as for example in the uterus of a pregnant woman or in the internal jugular.

The system of the vena cava, superior and inferior, presents a peculiarity: in the terminal part, in contact with the right atrium of the heart, the adventitious tunic is made up of layers of myocardium.

Superior vena cava

The role played by the superior vena cava is a collector for all the other veins and capillaries that have supplied the tissues and organs of the supradiaphragmatic parts. In fact, the two anonymous or brachiocephalic veins, right and left, unite in it; anonymous veins which in turn derive from the union of subclavian and internal jugular. Imagine, therefore, the superior vena cava as the collection terminal of a series of increasingly larger venous vessels. The point of confluence of the two anonymous veins. , at the level of the right rib, it identifies the point of origin of the superior vena cava.
It is 7-8 cm long, has a diameter of about 22 mm and reaches the right atrium of the heart at the third right rib.
Along its path, the superior vena cava is partly extrapericardial (in the initial tract), and partly intrapericardial (in the final tract). Intra- and extrapericardial refers to the anatomical characteristic of being or not covered by the pericardium. The pericardium is a thin membrane that surrounds the heart and has a protective function.

In its extrapericardial portion, the superior vena cava is in contact with:

the thymus, anteriorly.
the right phrenic nerve, laterally.
the right lung, the bronchial lymph nodes and the right vagus nerve, posteriorly.
the ascending aorta, in the medial point.

The intrapericardial portion, on the other hand, enters into a relationship with:

the right auricle, that is a small indented appendix that acts as an extension of the atrium, below.
the pulmonary artery (right branch) and the right superior pulmonary vein, posteriorly.

The superior vena cava is devoid of valves and has no muscle cells, except for a lining of striated muscle fibers dependent on the musculature of the "atrium. This last characteristic is typical of the veins that open into the heart: in general, in fact, they present, in the tract that connects to the heart - and precisely at the level of the adventitious tunic - a layer of myocardium, that is the typical musculature of the heart, with a circular or spiral course.

Tributaries of the superior vena cava

The main tributary venous vessel of the superior vena cava is the azygos vein. The azygos vein collects the blood coming from the intercostal veins and therefore also acts as a collector of smaller vessels. More precisely, the azygos vein is part of a larger venous system, composed of two other vessels called hemiazygos and hemiazygos. The azygos vein directly conveys the blood coming from the right intercostal veins. The other two veins instead convey the blood coming from the left intercostal vessels and only then pour it into the azygos vein. The azygos vein system is located in the mediastinum posterior, i.e. in that area of the chest between the vertebrae and the pericardium.
The superior vena cava can also receive blood flow from the right superior thyroid vein, pericardial veins, and internal mammary veins.

The superior vena cava and lymph

The system of the superior vena cava also flows all the lymph coming from the upper areas of the body. The point of contact occurs, more precisely, at the level of the large venous vessels located at the base of the neck, between the right subclavian vein and the right internal jugular vein. The lymph that flows into the venous system has already passed through the tissues: it is therefore rich in waste substances, collected by the tissues themselves, but also in chylomicrons absorbed in the intestine.

Inferior vena cava

The inferior vena cava collects blood from the subdiaphragmatic part of the human body. In other words, through this path, the oxygen-poor blood, which has supplied the lower parts of the body, returns to the heart.

It forms at the level of the 4th or 5th lumbar vertebrae, at the confluence of the two common iliac veins. In its path through the abdominal cavity, the inferior vena cava first passes on the posterior aspect of the liver, then crosses the diaphragm by means of an orifice and, finally, enters the thorax; here, it bends slightly forward and to the left to reach the right atrium of the heart.
It is 21-22 cm long: 19 cm along the abdominal portion, the remaining 2-3 cm are located in the thoracic area. The diameter of the inferior vena cava is variable: at the origin, ie at the confluence of the two iliac veins, it is 20 mm; at the level of the renal veins, which flow into the inferior vena cava, it is 30 mm; in the terminal part it reaches 33 mm. mm and, finally, at the level of the diaphragmatic orifice, it measures 27 mm. Due to these characteristics, the inferior vena cava is the largest vein in the human body.

The abdominal portion of the inferior vena cava comes into contact with:

the abdominal aorta, to his left.
the vertebral bodies, the psoas muscle and the sympathetic trunk, in the posterior part.
the ureter, the spermatic vein and the medial margin of the right kidney, laterally.
the parietal peritoneum (i.e. the membrane that lines the inner surface of the abdominal and pelvic cavity walls) the duodenum (in the lower portion), the head of the pancreas and the liver in the anterior part.

The thoracic portion, on the other hand, comes into contact with:

the pericardium, in the inferoposterior area
the base of the right lung.

Unlike the superior vena cava, the inferior vena cava has a valve, called the Eustachian valve. Its structure is also different: the wall, in the abdominal tract, is thick and made up of two muscle layers. Similarly to what happens for the superior vena cava, in the thoracic portion, the only musculature present is that of atrial origin, of the myocardial type.

Tributaries of the inferior vena cava

The main tributaries of the inferior vena cava are of two types: parietal and visceral. The lower lumbar and phrenic veins belong to the group of parietal veins. The renal, middle and hepatic suprarenal veins belong to the group of visceral veins. In women and men, ovarian veins and spermatic veins, both of the visceral type, respectively flow into the inferior vena cava.

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