Blood, 血, transports Ying Qi (營氣）throughout the body. The Blood flows because it is propelled by Heart Qi.
Sources of Blood
1. Gu Qi (穀氣）from food. Nutrients is an essential building block for many of the components in blood production.
2. Prenatal Jing (Kidney Jing 腎精). This is actually not far fetched. I remember this by thinking that Kidney produces erythropoiten, which is a hormone that tells bone marrow to produce red blood cells. If blood is abundant, the kidneys do not have to produce erythropoiten and the Kidney Jing is preserved.
3. Ying Qi (營氣), which is basically a refined form of Gu Qi
4. Body Fluid (津液). Body fluids can be transformed into blood and vice versa. I look at this relationship almost like the different compartment of blood, lymph, extracellular fluids, sweat, urine, etc. All of them have their circulatory systems which are interconnected and working together to keep every compartment in balance.
Body Fluid, 津液, is all fluid in the body that excludes the blood. It is the products of activities from multiple Zang and Fu organs.
The flow of Body Fluid is almost like that of water going through purification and distillation. First, water and food is processed by stomach and spleen and the Body Fluid is separated into clear and turbid part. The clear part gets send to the Lung, which does another level of purification, separating it further into clear and turbid. The clear fluid is then used to nourish our skin, hair, muscle joints, bones and eyes. The turbid part is sent to the kidney to further process. The kidneys get rid of the impure part through urination, and pure fluid gets reabsorbed into the circulation back into the lung.
The turbid Body Fluid is separated at the stomach and spleen then sent down to small intestine, which is then again separated into clear and turbid components. The turbid part gets excreted through large intestine into feces where the clear part gets sent to the kidney to be processed again.
The Body Fluid is supposed to lubricate various body parts. You know how they say we are made of 70% water? I think of Body Fluid being responsible for hydration and keeping our joints limber like the synovial fluids. When there is insufficient body fluids, we experience dry skin, dry lips, joint stiffness and constipation. When there is too much Body Fluid, water retention occurs. Body Fluid can also be distorted through infection and manifest as cough.