4. Which organ system in humans is most directly involved in the transport of oxygen?
excretory
nervous
circulatory
O digestive

C. Circulatory System

Explanation:

The circulatory system (cardiovascular system or the vascular system) is an organ system that permits blood to circulate and transport nutrients (such as amino acids and electrolytes), oxygen, carbon dioxide, hormones, and blood cells to and from the cells in the body to provide nourishment and help in fighting diseases, stabilize temperature and pH, and maintain homeostasis.

answer: the blood-brain barrier is a layer of specialized membranes and cell processes designed to keep undesirable substances from passing from the bloodstream into the precious tissues of the brain. while the blood-brain barrier does keep most bad things out of the central nervous system, it is not impermeable.

explanation:

not sure if this is what you'r looking for eubacteria and archaebacteria