What happens to sinus venosus in mammals?

The sinus venosus is a large quadrangular cavity which precedes the atrium on the venous side of the chordate heart. In humans, it exists distinctly only in the embryonic heart, where it is found between the two venae cavae. In the adult, it is incorporated into the wall of the right atrium to form a smooth part called the sinus venarum, also known as the venarum sinus, which is separated from the rest of the atrium by a ridge of fibres called the crista terminalis. The sinus venosus also forms the SA node and the coronary sinus.

In the embryo, the thin walls of the sinus venosus are connected below with the right ventricle, and medially with the left atrium, but are free in the rest of their extent. It receives blood from the vitelline vein, umbilical vein and common cardinal vein.

It originally starts as a paired structure but shifts towards associating only with the right atrium as the embryonic heart develops. The left portion shrinks in size and eventually forms the coronary sinus and oblique vein of the left atrium, whereas the right part becomes incorporated into the right atrium to form the sinus venarum.

In simple terms, an atrial septal defect (ASD) is a deficiency of the atrial septum. Atrial septal defects account for about 10-15% of all congenital cardiac anomalies and are the most common congenital cardiac lesion presenting in adults.1 Sinus venosus atrial septal defects account for only 10% of atrial septal defects. The remaining atrial septal defects are ostium secundum type (70%), ostium primum type (20%), and unroofed coronary sinus, or coronary sinus septal defects, (<1%). Most children with sinus venosus atrial septal defects are asymptomatic but may develop symptoms as they age.

Excellent surgical results with a mortality rate near 0% can be expected. This is particularly true in patients who undergo repair when younger than 15 years. An atrial septal defect was the first lesion repaired using cardiopulmonary bypass in 1954 by John Gibbon, MD, at the Mayo Clinic.

Pathophysiology

The more common sinus venosus type defect (often referred to as the "usual type") occurs in the upper atrial septum and is contiguous with the superior vena cava (SVC). The lesion is rostral and posterior to the fossa ovalis (where secundum type defects occur) and is separate from it. It is almost always associated with anomalous pulmonary venous drainage of the right upper pulmonary vein into the SVC.

Less commonly, the defect may occur at the junction of the right atrium and inferior vena cava (IVC) and be associated with anomalous connection of the right lower pulmonary vein to the IVC. Rarely, sinus venosus defects occur posterior to the fossa ovalis without bordering the SVC or IVC. The predominant hemodynamic consequence is a left-to-right shunt through the defect.

Mammals and birds are both homeothermic vertebrates and they have converged upon similar circulation patterns, presumably because the high metabolic rate demanded a very efficient system of blood flow. Some differences remain however. For example, in birds, the sinus venosus is reduced to a small anatomically discrete area. In mammals, the sinus venosus is reduced to a patch of Perkinje fibers in the wall of the right atrium known simply as the pacemaker, or the sinoatrial (SA) node. In both birds and mammals, the double system of aortic arches inherited from the fishes (one arch passing through the gills on each side of the pharynx) has been reduced to a single arch. In birds, the right arch is retained and the left is lost. In mammals, the left side is retained and the right side is lost. This is why the aortic arch in humans exits the heart cranially (toward where the gills used to be) then arches sharply (180_) to the left as it descends toward the visceral cavity.

In both, the reptilian conus arteriosus is replaced by a pulmonary trunk and a single aortic trunk. Blood flow is much simplified compared to reptiles:

Blood from systemic circulation (low O2) _ right atrium _ right ventricle _ pulmonary trunk to the pulmonary arteries _ lungs _ pulmonary veins (high O2) _ left atrium _ left ventricle _ aorta _ systemic circulation.

Diving birds and mammals do not have a cardiac shunt to bypass pulmonary circuit because they are descendent from terrestrial ancestors. Instead, diving birds and mammals slow their heart rate (bradycardia), convert their skeletal muscles to use anaerobic respiration, and blood flow is directed toward critical areas (brain) and away from the lungs, digestive system, and skeletal muscles.

Sinus Venosus

Sinus Venarum