Role in Embryology and Fetal Circulation
During cardiac development between weeks 2 and 5, endocardial tubes, the myoepicardial mantle, the formation of the atria and ventricles, and the primitive vascular system develop. The vascular apparatus includes the venous sinus, which empties into the atrium.
In placental fetal circulation, blood from the inferior vena cava is directed by the valve through the foramen ovale into the left atrium. Blood from the superior vena cava and the coronary sinus (sinus coronarius) enters the right atrium and is directed into the right ventricle.
From the right ventricle, blood enters the pulmonary trunk. Because the lungs are non-functional in utero, most of this blood bypasses them via the ductus arteriosus into the aortic arch.
Phylogeny of the Venous Sinus
In the vertebrate venous system, blood reaches the venous sinus via two main pathways:
Somatic drainage:
- Blood from the posterior body drains into the posterior cardinal veins;
- Blood from the anterior body drains into the anterior cardinal veins;
- On each side, the anterior and posterior cardinal veins merge to form the ducts of Cuvier (common cardinal veins);
- The right and left ducts of Cuvier carry blood into the venous sinus.
Visceral drainage:
- Blood from the gut travels via the subintestinal vein;
- This forms the hepatic portal system;
- From the liver, blood travels via the hepatic veins into the venous sinus.
The vessels opening into the venous sinus and carrying deoxygenated blood include: the right duct of Cuvier, the left duct of Cuvier, and the hepatic veins.
In reptiles, the venous sinus undergoes reduction. Deoxygenated blood from the posterior body collects into the inferior vena cava and enters the right atrium; blood from the anterior body drains into two superior venae cavae and subsequently into the right atrium.
Physiological Significance: The Stannius Experiment
The role of the venous sinus in cardiac automatism is demonstrated by the first Stannius ligature.
- Site of application: The sulcus between the venous sinus and the atria.
- Type: An isolating ligature, tied securely.
- Objective: To separate the sinuatrial node (primary pacemaker) from the rest of the heart.
- Effect: Isolation of the venous sinus from the atria and ventricles.
- Result: The atria and ventricles stop beating; the mechanogram registers a flat line. The venous sinus continues to contract at its intrinsic rhythm.
- Physiological meaning: This experiment proves that the venous sinus acts as the primary pacemaker. The arrest of the downstream chambers occurs because subordinate pacemaker centers have lower lability, require time to "warm up," or are normally overdrive-suppressed by high-frequency impulses from the sinus.
Clinical Significance and Accessory Vessels
Morphological features of the coronary circulation include a network of accessory vessels structurally similar to capillaries. They maintain myocardial perfusion when major coronary arteries become occluded.
Thebesian veins (venae cordis minimae):
- Connect venules and small veins directly with the heart chambers;
- They channel 20–50% of deoxygenated coronary venous blood directly into the cavity of the right ventricle and other chambers;
- They prevent venous stasis even when outflow through the coronary sinus is completely obstructed, such as during elevated right atrial pressure secondary to tricuspid stenosis.
Viessenius vessels (arteriololuminal vessels): Connect arterioles directly with the heart chambers.
Do Not Confuse: Scleral Venous Sinus
In anatomy, a similarly named structure exists—the scleral venous sinus (sinus venosus sclerae), commonly known as the canal of Schlemm.
It belongs to the aqueous humor drainage pathway of the eye. Aqueous humor drains from the anterior chamber through the trabecular meshwork (iridocorneal angle) into the scleral venous sinus, and subsequently into the aqueous veins.
The aqueous outflow pathway sequence is described as: spaces of Fontana → canal of Schlemm (scleral venous sinus) → episcleral venous plexus.