The human venous system has progressively adapted to the functional demands imposed by evolution.
A sort of "compromise" has therefore gradually been reached, which now, allows us to schematically describe three main venous sectors:
that of the head, that of the deep viscera, which represents a true venous blood reservoir, and, finally, that of the lower limbs.
Examination of the temporal and occipital bones of our distant ancestors provides decisive elements concerning the progressive "specialization" of the venous system of the head. These bones
are riddled with small orifices, the emissary foramina, fossil remnants of the development of emissary veins. By crossing the skull, these
vessels are responsible for exchanges, especially heat exchange, between brain tissue and the extracranial environment. The number of these foramina
increased during the course of evolution. From Australopithecus to Homo sapiens, these vessels have multiplied, in parallel with the adaptation of man
to the upright posture and the development of the brain. This diversification of the cephalic venous system was justified functionally by the absolute
necessity to ensure thermoregulation of an increasingly larger brain, unable to support a temperature rise of more than 5°C.
The problem was very different in the lower limbs. As the predominant tissue in the limbs is muscle,
the main function of these veins has become the transport of muscle toxins. Very complex interconnections
between the five main components of venous hemodynamics, muscles, fascia, venous walls, valves, and hydrostatic
pressure, therefore gradually developed. This extremely complex equation is further complicated by the effects
of other parameters, such as the respiratory and cardiac pumps, tissue pressure, or residual blood pressure.
The return of venous blood therefore corresponds to the movement of multiple parameters. Mathematical
equations have attempted to describe this process, but it is generally unpredictable and appears to
obey the laws of more chaotic systems.
In practice, the important point to consider is the extreme variability of this venous system composed
of duplicated or triplicated. sinuous, plexiform veins.
From a hemodynamic point of view, the situation is even
more complex in view of the fact that it is intimately related to pressure variations, which are also due to multiple origins:
the subject's position, which determines the hydrostatic pressure;
respiration (breath-holding effort, coughing, etc);
compressions, strictures, and obstruction of veins:
deep veins often pass through fibrous channels, which adhere to their walls. Under normal conditions, the venous lumen remains open,
but certain situations of flexion or extension can induce compression in the calcaneal channel (plantar veins), superior orifice
of the interosseous membrane (anterior tibial veins), ring of the soleus muscle (posterior tibial veins), arcade of flexor hallucis
muscle (peroneal veins), as well as other forms of compression, such us Cockett’s syndrome, edges of chairs, and crossing of legs:
the diameter of venous lumens:
due to its viscoelastic properties, the venous wall can distend during prolonged standing or in response to neurochemical or hormonal stimuli.
The valvulomuscular pump
certainly represents the most effective way of decreasing hydrostatic pressure. It can be broken down
into three motor units: the plantar pump, the calf pump, and the femoral pump. The first is composed of very large medial and lateral plantar veins, which converge posteriorly towards deep tibial veins and, to a lesser degree, anteriorly towards anterior tibial veins via pedal veins.
The leg pump is essentially composed of the gastrocnemius
and soleus veins. This second pump confers the role of venous crossroads to the popliteal vein, a potential site of venous
hypertension and circulatory obstruction. Finally, the femoral pump consists of the femoral, quadriceps, hamstring, and
gluteal veins, subsequently drained by the thoracoabdominal respiratory pump.
At rest, the vein essentially opposes a pressure rise by means
of its parietal tone. During effort, the capacity of the valvulomuscular pumps to control venous pressure depends on:
the force of calf muscle contraction,
the amplitude of contraction-stretching movements,
the number of contractions,
the absence of circulatory impediments.
All these concepts confirm the intimate relations between venous anatomy and
venous hemodynamics. Assessment of these relationships, described throughout
the three volumes of this Atlas, constitutes an essential step in the determination
of the pathogenic nature of any abnormality, especially blood reflux.