Simplify the following text for a Grade 6 native English-speaking person.
Original: It constitutes 4 % of the total body weight of humans , i.e. approximately 2.6 kg ( 5.7 lbs . ) Bone marrow also prevents the backflow of lymph , working as a vital part of the lymphatic system .
Simplified: About 4 % of the weight of an adult human is bone marrow , that is about 2.6 kg .