Hi Wei Jie,
We were taught the various organ systems and their functions during our TCM Diagnostics module. Some examples are the spleen governs the digestive system, the lung oversees immunity, and the heart is in charge of the mind.
The basis of TCM diagnosis is what we called syndromes classification (辨证型), which are sets of signs and symptoms grouped together.
A physician collects medical history, signs and symptoms through the four diagnostic methods, analyzes the information, and derives a syndrome. It then lays down the foundation for a treatment methodology and plan.
Four diagnostics (Inspection, Olfactory & Auscultation, Inquiry, Palpation) → Collect medical history, signs and symptoms (Sore throat, yellowish phlegm, fever) → Syndrome (Upper respiratory infection of the Wind-heat type 风热型) → Methodology (e.g. to expel Wind-heat 疏风散热)→ Treatment plan (e.g. herbal prescription such as Mulberry and Chrysanthemum Decoction 桑菊饮)