Functions of the Spleen
The Spleen is the root of all post-natal Qi. Combined with the Stomach it takes all things consumed and converts them into energy. If any lack of qi is found in the body, the Spleen is the first place to look for an imbalance. It is internally-externally related to the Stomach. Its element is earth.
Governs Transformation and Transportation
The spleen is responsible for the intake, processing, and shipping out of food and fluid. From the food and fluid, the spleen derives key components of making qi and blood and is thus the primary organ looked at when these two essential substances are not balanced. The spleen extracts the qi of the food (gu qi), and sends it up to the heart and lungs. In the heart, it combines with the pre-natal qi to form blood. In the lungs, it combines with the qi of the air (kong qi) and the original qi (yuan qi) to form the True or Upright Qi (zhen qi). It is this final type of qi that will be sent through the blood and channels of the rest of the body. When functioning properly: qi is strong, digestion goes smoothly, body is kept moist. When malfunctioning: qi is weak (lassitude), appetite is poor, digestion is sluggish, stools may be loose, phlegm and damp may accumulate.
Controls the Blood
As just said, the spleen is the key to the manufacturing of blood and is thus in control of its quantity in the body. Additionally, the spleen qi holds the blood in the vessels. Thus if the spleen qi is weak there may be hemorrhaging or spilling over of blood.
Controls the Muscles and Four Limbs
This ties into the transformation and transportation function of the spleen to spread nutrients to the muscles and tissues. If the spleen is weak, the limbs and muscles aren't nourished and they become weak and tired.
Opens into the Mouth
Mastication is necessary to the function of the spleen, thus it is functionally related. Also, if the spleen is weak, the sense of taste may dull.
Manifests in the Lips
Red and vibrant lips are the domain of the spleen. When the spleen is deficient the lips may turn pale from lack of nourishment.
Controls Upright Qi
The upright qi is that which holds the body, particularly the organs right and in place. It is like the gravity fighting force of the body, that without the organs would all by lying at the bottom of your abdomen. Thus if the qi is weak, prolapses may occur.
The spleen is directly related to our capacity for thinking. How well we manage things that require concentration is dependent on the strength of the spleen. So if you wanna ace that test, make sure your spleen qi is strong. Some translation use different words, some indicate worry as much as thought. Possibly it is meant that the spleen controls how much we overthink and analyze and when strong the spleen is weak it can't keep the mind from running through things over and over.