The correct answer is (B) Vena cava. In frogs (Rana tigrina), the circulatory system is closed and double-natured, though the heart is only three-chambered.

Deoxygenated blood from various parts of the frog's body is collected by a system of veins. The main vessels that bring this blood back to the heart are called the Vena cava (specifically, two anterior vena cavae and one posterior vena cava). These vena cavae empty the deoxygenated blood into a special triangular chamber called the Sinus Venosus, which is attached to the dorsal side of the right atrium. From the Sinus Venosus, the blood enters the right atrium.

In contrast, the Pulmonary vein carries oxygenated blood from the lungs back to the left atrium, and the Aorta (through the conus arteriosus) distributes blood from the ventricle to the rest of the body.

The frog possesses a well-developed circulatory system consisting of a heart, blood vessels, and blood. Understanding the flow of blood is crucial for NEET biology.

1. Structure of the Frog Heart

The frog heart is a muscular organ located in the upper part of the body cavity. It has:

• Three Chambers: Two atria (Right and Left) and one muscular Ventricle.
• Sinus Venosus: A triangular structure that receives deoxygenated blood from the three vena cavae.
• Conus Arteriosus: A sac-like structure on the ventral side of the heart through which blood leaves the ventricle.

2. The Path of Blood Flow

The circulation in a frog follows this general sequence:

• Deoxygenated Path: Body tissues → Vena cava → Sinus venosus → Right atrium.
• Oxygenated Path: Lungs/Skin → Pulmonary vein → Left atrium.
• Mixing: Both atria contract, sending blood into the single ventricle, where some mixing of oxygenated and deoxygenated blood occurs.
• Distribution: The ventricle contracts, pumping blood out through the Conus arteriosus into the aortic arches.

3. The Portal Systems

Frogs have two unique venous connections called portal systems:

• Hepatic Portal System: Connects the digestive tract (liver) and the intestine before the blood reaches the heart.
• Renal Portal System: Connects the lower parts of the body to the kidneys. This is a distinguishing feature compared to mammals, where the renal portal system is absent.

4. Composition of Frog Blood

Frog blood consists of plasma and cells (RBCs, WBCs, and Platelets). Notably, Frog RBCs are nucleated and contain hemoglobin, unlike the enucleated RBCs found in humans.

5. Key Comparison Points for NEET