The genus Aristolochia
Aristolochia is the scientific name for the genus Umanosuzukusa.  The name, Umanosuzukusa came from the fact that the shape of its fruit resembles the bell for a horse.  It is thought that this genus contains more than 500 species, which are widely distributed among tropical, sub-tropical and temperate regions in the world, and their flower shapes are varied.  Many of them have no petals, and the pipe-shaped or trumpet-shaped floral tube is a modified calyx.  In the depths of the floral tube there is a column which is the unified stamens and pistil.
This unique flower is called a trap-flower and has a skillful pollination strategy to avoid decreasing its genetic diversity by self-pollination.  It releases a smell which is foul for humans but favorable for flies to attract it into the column in the floral tube.  When it is in the female stage, hairs inside of the tubal part face inwards to trap flies.  After that the column part shrinks, the flower comes into the male stage to release pollen, and the flies are smeared with pollen. Then the hairs in the neck part shrink to release the flies.  The flies enter other flowers in the female stage to complete pollination.  It is a very skillful strategy to prioritise cross pollination.
The relatives of the genus Aristolochia contain aristolochic acid I and its relatives aristolochic_acid-I
  aristolochic acid I
which are nitro compounds and are extremely rare as natural compounds.  Aristolochic acids are chemical defence materials for these plants.  These show nephrotoxicity and possibly carcinogenicity and mutagenicity.  In fact, there are cases where an herbal medicine containing a plant belonging to the genus Aristolochia has caused serious nephropathy.  In China an herbal medicine using Kidach-umanosuzukusa (Aristolochia manshuriensis Kom.) is called Kanmokutsu and it is sometimes abbreviated to Mokutsu which refers to the stems of Akebia in Japan.  Careful attention is required not to confuse them.
On the other hand the relatives of Atrophaneura, such as the butterflies, Chinese Windmill (Atrophaneura alcinous) and Birdwing (the genus Ornithoptera and Trogonoptera), use the relatives in the genus Aristolochia as foodplants.  The relatives of Atrophaneura accumulate aristolochic acids in their body during its larval period and keep it for their life span to obtain chemical defence.

Mokutsu, prescribed in Japanese pharmacopeia, is climbing stems of Chocolate Vine (Akebia quinata (Houtt.) Decaisne) and Mitsuba-akebi (Akebia trifoliata (Thunb.) Koidzumi) and does not contain aristolochic acid.