What Is the Sinus Venosus?

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  • Written By: Mary McMahon
  • Edited By: Shereen Skola
  • Last Modified Date: 15 September 2019
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The sinus venosus is an embryonic structure that later becomes part of the heart. Errors in the formation of this structure and subsequent development into heart tissue can cause a type of Atrial Septal Defect (ASD) known as sinus venosus for the structure involved. In patients with this type of congenital abnormality, blood passes freely between the upper chambers of the heart, called the right and left atria. This allows deoxygenated venous blood to mix with freshly oxygenated arterial blood, which can create a problem.

In the process of embryonic development, a single fertilized egg grows and divides numerous times into a series of stem cells that will gradually develop into increasingly advanced structures. These migrate through the body of the embryo to take up their positions so they can start growing into bones, organs, and other tissues in the body. This process is extremely complex; in humans, it takes a full 40 weeks for organism to fully develop.

As one of the structures that will become part of the heart, the sinus venosus migrates to the chest area and starts developing. Over time, it will shift slightly, and cells will become more advanced. It is fed throughout the process by a series of blood vessels to deliver nutrients, with corresponding vessels to remove cellular wastes. Errors can occur because of environmental pressures, random quirks of cell division, or genetic diseases, sometimes causing a sinus venosus defect.


Such defects are located close to the area that once was the sinus venosus. They may need to be treated with surgery to close the hole and improve the patient’s heart function, depending on their precise size. Some are challenging to diagnose, which may leave a patient living with a sinus venosus defect for an extended period of time before the problem is identified, usually doing a routine cardiac evaluation. Diagnosis can be complicated because of the position of the defect, which makes it hard to spot in medical imaging studies of the heart.

Surgical treatment tends to be most successful when it is performed before the patient reaches 20 years of age. This gives the heart time to recover and grow after the surgery. It is possible to correct defects after this point if they have just been identified, but the patient can face an increased risk of side effects and a shorter life expectancy. An untreated sinus venosus defect may lead to a shorter life expectancy overall.


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