From the esophagus down, the 6-9 meter long digestive tube is made up of four layers: the inner (A)__________ of epithelial cells, a sub mucosa, smooth muscle, and serosa. Nutrients from the tube are absorbed across the layers into the bloodstream.
(B)__________ are circular muscles that control the flow of food through the digestive tract.
To find the answer, let's break down the question:
(A) The question is asking for the inner layer of the digestive tube from the esophagus downward. According to the information provided, the inner layer is made up of epithelial cells.
(B) The question is asking for circular muscles that control the flow of food through the digestive tract. These circular muscles are called sphincters.
So, to answer the first part of the question, the inner layer of the digestive tube is composed of epithelial cells. For the second part, the circular muscles that control the flow of food are called sphincters.
To get this answer, I analyzed the provided information about the layers of the digestive tube and my knowledge of the human digestive system.