 The term zàng () refers to the five entities considered to be yin in nature–Heart, Liver, Spleen, Lung, Kidney–, while fǔ () refers to the six yang organs–Small Intestine, Large Intestine, Gallbladder, Urinary Bladder, Stomach and Sānjiaō.The zàng's essential functions consist in production and storage of qì and xuě; they are said to regulate digestion, breathing, water metabolism, the musculoskeletal system, the skin, the sense organs, aging, emotional processes, and mental activity, among other structures and processes.The fǔ organs' main purpose is merely to transmit and digest () substances such as waste and food.