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In anatomy, a vein is a blood vessel that carries blood from the capillaries to the heart. Generally, the veins are characterized because they contain deoxygenated blood (which is reoxygenated as it passes through the lungs), and because they carry carbon dioxide and metabolic waste from the tissues, in the direction of the organs in charge of its elimination (the lungs, the kidneys or liver). However, there are veins that contain blood rich in oxygen: this is the case of the pulmonary veins (two left and two right), which carry oxygenated blood from the lungs to the cavities on the left side of the heart, so that it can be pumped to the heart. rest of the body through the aorta artery, and the umbilical veins.

The human body has more veins than arteries and their exact location is much more variable from person to person than that of the arteries. The structure of the veins is very different from that of the arteries: the cavity of the veins (the "lumen") is generally larger and more irregular in shape than those of the corresponding arteries, and the veins are devoid of elastic sheets.

Veins are high-capacity vessels, containing about 70% of the total blood volume.

Histology of veins

Like arteries, veins are made up of three layers:

• Internal, intimate or endothelial; the limits between this layer and the following are often misdefined.
• Media or muscle; little developed in the veins, and with some elastic tissue. Made mainly of connective tissue, with some smooth muscle fibers arranged concentrically.
• External or adventiciawhich forms most of the venous wall. Formed by conjunctive lax tissue that contains collage fibers and makes muscle cells arranged longitudinally.

However, some veins with a propulsive function have relatively important musculature both in the media (in a concentric arrangement) and in the adventitia (in a longitudinal arrangement). These types of veins are called "muscular veins."

Veins have a thinner wall than arteries, due to the lesser thickness of the muscular layer, but they have a larger diameter than arteries because their wall is more distensible, with more capacity to accumulate blood. Inside the veins there are structures called semilunar valves, which prevent the return of blood and favor its movement towards the heart.

Although the veins of the extremities have intrinsic vasomotor activity, the return of blood to the heart depends on extrinsic forces, provided by the contraction of the skeletal muscles that surround them, and on the presence of valves, which They ensure movement in only one direction.

Division of the venous systems

Three venous systems can be considered: the pulmonary system, the general (or systemic) system, and the portal system.

• General system veins: By the veins of the systemic or general circulation the poor blood circulates in oxygen from the capillaries or blood microcirculation of the tissues to the right side of the heart. The veins of the systemic circulation also have valves, called semilunar valves that prevent the return of blood to capillaries.
• Pulmonary system: Through the veins of the lung circulation circulates the oxygenated blood of the lungs to the left part of the heart.
• Porta system: The veins of the porta systems circulate blood from a capillary system to another capillary system. There are two porta systems in the human body:
  • Hepatic porta system: The veins originated in the capillaries of the digestive tract (from the stomach to the rectum) that transport the products of digestion, are transformed back into capillaries in the liver's hepatic sinuseides, to form new veins that lead to systemic circulation.
  • Hypophysical carrier system: The upper hypophysary artery from the internal carotid, is branched into a first network of capillaries located in the average eminence. These capillaries form the hypophysial veins that descend through the hypophysial stem and originate a second network of capillaries in the adenohipophysis that drain into the inner jugular vein.

Names of the main veins

Normally, each vein is associated with an artery, often with the same name (although sometimes there are differences: for example, carotid arteries are associated with jugular veins). The names of the main veins are:

• Come jugular.
• Come subclavian.
• Coronary veins.
• Vena cava superior (VCS) and lower (VCI).
• Pulmonary veins.
• Kidney vein.
• Come femoral.
• Vena safena mayor y menor.

Venous pressure

Venous pressure is a general term that defines the mean pressure of blood within the venous compartment. A more specific term is central venous pressure, which defines the pressure of blood in the inferior vena cava at the entrance to the right atrium of the heart. This pressure is important, because it defines the filling pressure of the right ventricle, and therefore determines the ejection stroke volume, according to the Frank-Starling mechanism.

The stroke volume of ejection is the volume of blood that the heart pumps in each beat, essential to ensure the correct supply of blood to all tissues of the body. The Frank-Starling mechanism states that an increase in venous return (the amount of blood that reaches the right atrium through the vena cava) causes an increase in ventricular preload (simplified, the filling volume of the left ventricle), and this generates an increase in the stroke volume of ejection.

Veins and arteries in the transport of substances

Arteries and veins present several differential characteristics in terms of the transport of substances. Arteries carry oxygen and nutrients to the tissues. At the level of the capillaries, these substances pass by diffusion from the blood to the tissue cells down a concentration gradient, to supply the raw materials necessary for cellular metabolism. Conversely, the waste products of cellular metabolism (CO₂ and other metabolites) leave the cells and enter the capillaries down a concentration gradient. Specifically, deoxygenated hemoglobin has high affinity by CO₂, forming carbaminohemoglobin. So that the arterial blood, rich in oxygen and nutrients, when passing through the capillaries exchanges its content with the cellular content, and the cellular waste products pass into the veins and are distributed to the different organs in charge of their elimination from the organism:

• CO₂ is eliminated in the form of gas in the lungs, and as bicarbonate (HCO)₃^-through the kidneys;
• a large part of the ions and metabolic products are eliminated through the kidneys: sodium, potassium, magnesium, calcium, ammonia, urea, etc;
• Some waste products are eliminated by the liver, through bile: for example bilirubin, a product of the degradation of hemoglobin.

From a gasometric point of view (content of dissolved gases), what differentiates arterial blood from venous blood is the partial pressure of oxygen, or pO₂ (which varies from 95 mmHg on average in the arteries to 40 mmHg in the veins), since the pCO2 is very similar (40 in the arteries, and 46 in the veins). However, only the fraction of a gas dissolved in a liquid contributes to the value of its partial pressure, and both O₂ (in the direction of the tissues) and CO₂ (generated in the tissues) are transported in different ways in the blood. While oxygen is transported in two ways (98% bound to hemoglobin and only 2% dissolved), CO₂ is transported either bound to hemoglobin (30%), either in the form of bicarbonate (70%), well dissolved (10%). So the partial pressure values only reflect a part of the composition of the blood. In erythrocytes, bicarbonate is transformed into water and CO₂, in a reaction catalyzed by carbonic anhydrase. This CO₂ diffuses into the pulmonary alveoli and is exhaled, as is the case with dissolved CO₂ in the blood and bound CO₂ to hemoglobin. Therefore, exhaled air has a pCO₂ of 27 mmHg, while atmospheric air only has a pCO₂ of 0.3 mmHg. That is, like all living organisms (with some exceptions), we expel CO₂ into the environment, which was generated in the mitochondria as a result of cellular metabolism.

The products resulting from cellular metabolism, CO₂ and other waste products, must be removed because they are toxic. The elimination of these compounds is essential for the balance of the organism, and if they are not eliminated properly they can cause problems: thus, an accumulation of CO₂ (because there is hypoventilation, for example) can produce acidosis.

Diseases of the veins

• Varices.
• Flebitis.
• Deep vein thrombosis.
• Kidney vein thrombosis.