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《金匮要略》has this famous line on preventive medicine:

夫治未病者，见肝之病，知肝传脾，当先实脾。四季脾王不受邪，即勿补之。

In prevention of disease, when you see a “liver” disease, you must know that “liver” diseases pass on to the “spleen”, hence it is imperative to first tonify the spleen. In the case where the spleen is abundant, then such tonification is not necessary.

Note here that liver and spleen refer not to the anatomical liver and spleen, but to the functional systems as described by chinese medicine and labelled with an organ name.

Liver diseases can be of two types: the deficient (虚) and the replete (实). In the case of the deficient liver, 虚则补之, hence we tonify the liver. How? Liver deficiencies are in the blood (肝阴血虚) and that would mean tonifying blood and yin, the purview of many well-known prescriptions.

That said, we meet an answer in the next line of the text. It says:

补用酸，助用焦苦，益用甘味之药调之。

Tonify with sour [flavor], assist with burnt bitter, benefit using sweet flavored herbs to regulate it all.

So we have 芍药、五味子、山萸肉、酸枣仁、乌梅 as sour flavors; 炒枳壳 as a burnt bitter; 淮山药、地黄、淮小麦 as sour-sweets. Also worth considering are 黄芪 and 党参 as sweets.

How the sour flavor is able to tonify “liver yin and blood” (肝阴血):

Presently, I can’t think of a good answer to how sour-tasting herbs can help tonify liver blood and liver yin. Many sour-tasting herbs have organic acid constituents as well as tannins. 酸能收能涩 means that the sour taste in herbs often attribute to them astringent properties.

Take tannins as an example. They can be applied to external wounds, much like how tea tree oil is to close a wound, form a protective later and as an antiseptic. Their astringent properties see them used in the treatment of inflammatory digestive disorders, anywhere from the stomach (gastritis) to the intestines (IBS).

But still no definite answer.

I think the way to approach this is not how astringents interact with mucosa. My approach will be first to state the defining characteristic of weak “liver” function. I’ll describe it as a feeling of strain on the body, a strain which causes weakness by is due to excess - perhaps an “over-intense” central nervous system, or perhaps an “over-tight” cardiovascular system.

The core idea here is that you’re tonifying the liver and not reducing it. There are drugs that “reduce” by “clearing the qi channels” e.g. 青皮 and 川楝子 as definitive examples. There are those that “tonify and move the blood”, and hence act on the cardiovascular system, e.g. 当归，川芎. Then, there are others like 五味子 and 山萸肉 that are considered astringents, but really are tonics.

How to tonify the spleen:

Tonifying the spleen very often refers to tonifying spleen qi, but also involves drying off spleen-dampness and regulating qi. More on this another time.