Vena cava - The vena cava system

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

Indeed:

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

In other words, in the context of general blood circulation, the vena cava system takes care of collecting and flowing towards the heart, precisely in the right atrium, all the oxygen-poor blood, rich in carbon dioxide from the tissues and collected by the other components from the venous tree. This blood will then be injected by 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 features of the veins

The veins differ from the arteries due to a lower thickness and a greater delicacy of the walls. Despite this, venous walls have a greater relaxation capacity and, in general, a lower risk of injury compared to arterial vessels. This is possible because within them the blood that flows there has lower pressure. Then, compared to the arteries, they tend to have a larger diameter, consequently the amount of blood flowing there is greater.

From the point of view of the structure, in the veins the muscular and elastic components are lower than in the arteries. Collagen, on the other hand, is a highly present component.

Even the veins are made up of the typical three cassocks, the intimate, the medium and the adventitia .

The intimate is the innermost covering of the vase. Made up of endothelial cells, blood flows through it and, precisely because of the presence of the endothelium, it is possible that material is absorbed by the fluid that flows inside.
The average 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. May 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, vessels of medium caliber and the largest blood ducts have a different histological composition, at the level of the three coats, 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 absent, such as in the veins of the skin, spleen or retina. In other venous vessels, however, smooth muscle fibers are very present, such as in the uterus of a pregnant woman or in the internal jugular.

The vena cava system, upper and lower, has a peculiarity: in the terminal part, in contact with the right atrium of the heart, the adventitious frock is made up of layers of myocardium.

Upper vena cava

The role played by the superior vena cava is a collector for all the other veins and capillaries that have sprayed the tissues and the organs of the supradiaphragmatic parts. In it, in fact, the two anonymous or brachiocephalic veins are joined, right and left; anonymous veins that 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 venous vessels gradually getting larger. The point of confluence of the two anonymous veins, at the level of the right rib, 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). By intra- and extrapericardial is meant the anatomical feature 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, instead, enters into relationship with:

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

The superior vena cava is devoid of valves and does not have muscle cells, except for a covering of striated muscle fibers dependent on the muscles 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 habit - a layer of myocardium, that is the typical musculature of the heart, to circular or spiral course.

Tributaries of the superior vena cava

The main venous affluent vessel of the superior vena cava is the azygos vein . The azygos vein collects blood 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, consisting of two other vessels called emiazygos and accessory hemiazygos. The azygos vein directs the blood from the right intercostal veins directly. The other two veins convey, instead, the blood coming from the left intercostal vessels and only later pour it into the azygos vein. The system of the azygos vein is found in the posterior mediastinum, that is in that area of the thorax comprised between vertebrae and pericardium.

The superior vena cava can also receive the blood flow coming from the right upper thyroid vein, from the pericardial veins and from the internal mammary veins.

The superior vena cava and the lymph

All the sap from the upper parts of the body also flows into the superior vena cava system. The point of contact occurs, more precisely, at the level of the large venous vessels placed at the base of the neck, between the right subclavian vein and the right internal jugular vein. The sap 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 of chylomicrons absorbed in the intestine.

Lower vena cava

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

It forms at the level of the IV or V lumbar vertebra, at the confluence of the two common iliac veins. In its path of crossing the abdominal cavity, the inferior vena cava passes first on the posterior aspect of the liver, then crosses the diaphragm by means of an orifice and, finally, penetrates into the thorax; here, it makes a slight curve forward and to the left to reach the right atrium of the heart.

It is 21-22 cm long: 19 cm run through the abdominal portion, the remaining 2-3 cm are in the thoracic area. The diameter of the inferior vena cava is variable: at the origin, that is 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, is 30 mm; in the terminal part it reaches 33 mm and, finally, at the level of the diaphragm orifice, it measures 27 mm. Because of these characteristics, the inferior vena cava is the largest vein of 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 (ie the membrane that covers the inner surface of the walls of the abdominal cavity and the pelvic cavity) the duodenum (in the lower portion), the head of the pancreas and the liver in the anterior part.

Instead, the thoracic portion makes 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 . Even its structure is different: the wall, in the abdominal tract, is thick and consists of two muscular layers. Similarly to what happens for the superior vena cava, in the thoracic portion, the only musculature present is that of atrial origin, of 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 middle and hepatic renal, suprarenal veins belong to the group of visceral veins. In women and men, the ovarian veins and spermatic veins, both of the visceral type, flow into the inferior vena cava respectively.