A mass of  tissue is seen in the lower left corner of the right atrium close to the atrio-ventricular septum called the atrio-ventricular node (AVN).

The correct answer is option 'C' - AVN (Atrioventricular Node).

Explanation:

The mass of tissue seen in the left corner of the right atrium close to the atrioventricular septum is the AVN (Atrioventricular Node). Here's a detailed explanation:

Atrioventricular Node (AVN):

- The AVN is a specialized mass of conducting tissue located at the bottom of the interatrial septum, close to the tricuspid valve in the right atrium.
- It serves as the electrical gateway between the atria and ventricles in the heart.
- The AVN receives electrical impulses from the SA node (Sinoatrial Node) and delays their transmission to allow the atria to contract and fill the ventricles with blood before ventricular contraction.
- The delay in conduction at the AVN is important as it ensures proper coordination and synchronization of atrial and ventricular contractions.

Function of AVN:

1. Electrical Gateway:
- The AVN acts as the only electrical connection between the atria and ventricles, allowing the electrical signals generated by the SA node in the atria to reach the ventricles.
- It receives the electrical impulses from the atria and conducts them to the ventricles.

2. Electrical Delay:
- The AVN introduces a slight delay in the conduction of electrical signals from the atria to the ventricles.
- This delay is crucial as it allows the atria to contract and pump blood into the ventricles before ventricular contraction.
- The delay also ensures that the ventricles are filled with blood completely before they contract, optimizing cardiac output.

3. Protection against Rapid Ventricular Contraction:
- The AVN acts as a protective mechanism against rapid ventricular contractions.
- If the electrical impulses from the atria were to be conducted directly to the ventricles without any delay, it could result in excessively fast ventricular contractions, leading to inefficient pumping and reduced cardiac output.
- The AVN slows down the conduction of impulses, preventing rapid ventricular contractions.

In summary, the mass of tissue seen in the left corner of the right atrium close to the atrioventricular septum is the AVN (Atrioventricular Node). It acts as an electrical gateway, introducing a delay in conduction and protecting against rapid ventricular contractions.