Final answer:
The reddish-brown organ in the upper part of the abdominal cavity is the liver, which is essential for metabolism and regulation. The right kidney is located near the middle lobe of the liver, slightly lower due to the liver's placement.
Step-by-step explanation:
The reddish-brown organ located in the upper part of the abdominal cavity is the liver. The liver is the largest gland in the body and is considered one of the most important organs for metabolism and regulation. It is found in the right upper quadrant of the abdominal cavity, protected by the ribs and lying just below the diaphragm to the right of the stomach. Near the middle lobe of the liver, the right kidney can be found.
Due to the liver's size and position, the right kidney is slightly lower than the left, and this arrangement also affects the location of other organs such as the adrenal glands and the pancreas, with the latter situated behind the stomach and extending towards the left with its head nestled in the curvature of the duodenum.