In mammals, the exchange of nutrients and metabolic products occurs in the

Step 1: Analyze the question The question asks where nutrients and metabolic products are exchanged in mammals. This refers to the exchange between the blood and the body cells.

Step 2: Explain the concept Blood flows through capillaries, and the fluid that leaks out of them bathes the body cells. This is tissue fluid (also called interstitial fluid). Lymph is the tissue fluid that drains into the lymphatic vessels. Cells do not touch the blood directly. Instead:

• Nutrients and oxygen move from the blood into the tissue fluid/lymph, then into the cells.
• Metabolic wastes such as carbon dioxide and urea move from the cells into the tissue fluid/lymph, then into the blood.

So the medium through which the exchange takes place is the lymph (tissue fluid).

Step 3: Evaluate each option

• A. Lungs: These are for gaseous exchange (O₂ and CO₂) only, not general nutrient and metabolic product exchange.
• B. Oesophagus: This is just a tube that carries food to the stomach. No exchange happens here.
• C. Trachea: This is the windpipe, a passage for air with no exchange function.
• D. Lymph: This is the tissue fluid that serves as the go-between for the blood and the cells, so exchange of nutrients and metabolic products happens through it.

Step 4: Answer The correct option is **D.

Tip: Remember the chain Blood → Tissue fluid/Lymph → Cells. If a question mentions exchange between blood and cells, think "tissue fluid/lymph." If it mentions only gases, think "lungs" (or alveoli).