While taking part in the rat dissection lab, a student cuts open the rat on its ventral side. Asides from the intestines, the first internal organ he notices is a large, four-lobed, dark red organ resting on top of the lower portion of the thoracic cavity. This organ is most likely the rat's:

Liver.

Explanation:

The liver is a dark reddish organ that is situated in the right hypochondriac or upper right hand region of the abdomen, just below the diaphragm and anterior to the stomach in the epigastric region, the right kidney and the small intestines.

It is the second largest organ of the body, second only to the skin. The liver is divided into four lobes which include;

1) The right lobe

2) The left lobe

3) The quadrate lobe and

4) The caudate lobe

The blood supply to the liver include;

1) The hepatic artery

2) The hepatic portal vein which supplies the liver with nutrients-filled blood from the small intestine. It is the main blood supply to the liver.

The venous drainage is through the hepatic veins.

The nerve supply is through the hepatic plexus made up of sympathetic and parasympathetic nerves.

The liver plays a major role in the processing and metabolism of food and ingested substances and also detoxification.