Blood
Similar to the common concept of Blood, it is the carrier and substance of nourishment in the body. Given its iron content it has always been considered in Chinese Medicine as rich, moistening and a tonic like qualities.
The relationship between Qi and Blood may be equated to the way the oxygen is carried in blood by red blood cells. They are inter-connected and inter-dependent. Treatment for a disharmony in one substance often involves the other.
Qi
This concept was coined thousands of years ago in describing the body's life force. As we now know, the human body is made up of atoms that have electrons spinning around them, as do all things. We also know our bodies work due to electric impulses from our brain that are sent all over our bodies. This constant movement of energy, micro and macro, is what we call Qi.
According to Chinese Medicine Qi is our energy levels, it moves the blood in our vessels, keeps the heart beating regularly, makes the mind think more sharply and much more.
Yin
This is both a classification and debatably a substance itself in the body. Yin is feminine, dark, moist and nurturing, it is night time, the moon, rest, relaxation & hibernation. Yin is replenished at night when we are resting, as our body temperature and respiratory rate drops while we sleep - Yin takes over to repair and nurture the body.
Most commonly referred to with its counterpart - yang, the two should balance each other. Most commonly when illness occurs there is an imbalance of yin and yang ing the body.
Yang
As above but Yang is the masculine, strong & moving, it is day, light and the sun. Yang is responsible for our feeling of alertness during the day, it keeps us warm and keeps the bodily functions active. Yang is replenished and is in its element during the daylight hours.