Mammalian venous system and its developmental anomalies

Blood drainage from the anterior and posterior parts of the body

The anatomy of the mammalian venous bed has a number of fundamental differences established during evolution. Venous blood from the cranial (anterior) part of the trunk is collected via jugular and subclavian veins. Uniting, these major trunks form superior venae cavae, which empty directly into the right atrium. A crucial feature of normal human and most mammalian anatomy is that during ontogenesis, the left superior vena cava degenerates. Thus, the right vena cava remains the primary drainage pathway for the upper body.

The collection of venous blood from the caudal (posterior) half of the body occurs into the large inferior vena cava. This vessel receives azygos and hemiazygos veins, which are rudimentary remnants of the posterior cardinal veins of ancestral forms. The formation of the azygos venous system is a specific trait exclusive to the class Mammalia.

Another critically important characteristic is the absolute absence (reduction) of the renal portal system, which radically distinguishes mammals from lower vertebrate classes.

Afferent and efferent vessels of the heart

Cardiac hemodynamics in mammals are ensured by a strict separation of blood flows, as well as a specific topography of afferent (bringing blood to the chambers) and efferent (carrying blood away) vessels.

A special place is occupied by the pulmonary circulation: after oxygenation in the lung tissues, arterial blood flows through the pulmonary veins into the left atrium.

Afferent pathways (flowing into the heart):

• Right superior vena cava: brings venous blood to the right atrium. This is the main drainage pathway from the upper body.
• Left superior vena cava: normally regresses in humans. When persistent, it empties into the right atrium.
• Inferior vena cava: collects venous blood from the posterior (lower) part of the body into the right atrium.
• Pulmonary veins: deliver arterial blood from the lungs, completing the pulmonary circuit in the left atrium.

Efferent pathways (draining from the heart):

• Left aortic arch: originates from the left ventricle, carrying arterial blood into the systemic circulation.
• Pulmonary trunk: originates from the right ventricle, transporting venous blood to the lungs for gas exchange.

Developmental anomalies of the venous bed in humans

Many congenital vascular anomalies in humans are viewed through the prism of phylogenesis as recapitulations—that is, the appearance in the embryo of structures characteristic of distant evolutionary ancestors. However, not every pathology is a return to the past.

1. Persistence of the left superior vena cava. If this vessel does not regress during embryogenesis, a person is born with two superior venae cavae. From the standpoint of evolutionary biology, this is a classic recapitulation of the circulatory system structure of ancestral mammalian forms.
2. Persistence of the posterior cardinal veins. Normally, during development, they should transform, leaving only remnants—the azygos veins. Their complete persistence in an adult is interpreted as a profound recapitulation referring to the circulatory system anatomy of reptiles.
3. Atresia (absence) of the inferior vena cava. This pathology represents the absence or obliteration of the major vessel. Unlike previous examples, this anomaly is not a recapitulation. Its nature is purely an ontogenetic disorder (a failure during organ formation in a specific embryo) unrelated to phylogenetic memory.

What else to study on this topic

• Mechanisms of reduction of the renal portal system in mammals
• The process of formation and transformation of the superior venae cavae in embryogenesis
• Clinical significance of the persistence of the left superior vena cava
• Formation of the azygos system from the posterior cardinal veins
• Evolutionary transformations of the circulatory system from reptiles to mammals

Other topics in the section «Phylogeny of the vertebrate circulatory system»

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