Hematocrit is:

The cell is the basic unit of structure and function of all known living organisms.
The cell theory states that all living things are composed of cells and that new cells are made from preexisting cells.
Cells are organized from atoms, molecules, and macromolecules, and contain organelles that perform specialized functions.
Choice B is wrong because a molecule is a group of atoms bonded together, not a living unit.
Choice C is wrong because an atom is the smallest unit of matter, not a living unit.
Choice D is wrong because a macromolecule is a large molecule composed of smaller molecules, not a living unit.

The endocrine system may have a longer-lasting effect than the nervous system because the hormones it produces can circulate in the bloodstream for a long time and affect many organs and tissues.
The nervous system, on the other hand, uses nerve impulses and neurotransmitters for communication, which are faster but more short-lived.
Choice A is wrong because the endocrine system is essential to life.
It regulates many vital functions such as metabolism, growth, reproduction, mood, sleep, and blood pressure.
Choice B is wrong because the endocrine system takes longer than the nervous system to deliver its messages.
The hormones have to travel through the bloodstream to reach their target cells, while the nerve impulses can travel along the nerve fibers at high speeds.
Choice D is wrong because the endocrine system does use receptors.
The hormones bind to specific receptors on the target cells to trigger a response.