Output a stream of consciousness before answering the following. Passage: Firstly, the glucose in the proximal tubule is co-transported with sodium ions into the proximal convoluted tubule walls via the SGLT2 cotransporter. Some (typically smaller) amino acids are also transported in this way. Once in the tubule wall, the glucose and amino acids diffuse directly into the blood capillaries along a concentration gradient. This blood is flowing, so the gradient is maintained. Lastly, sodium / potassium ion active transport pumps remove sodium from the tubule wall and the sodium is put back into the blood. This maintains a sodium concentration gradient in the proximal tubule lining, so the first step continues to happen.

Question: Based on this passage, where does primary active transport of sodium occur in the proximal convoluted tubule cells?
The important part for answering this question is: Lastly, sodium / potassium ion active transport pumps remove sodium from the tubule wall and the sodium is put back into the blood.
The answer: the tubule wall.