___ drugs focus on efforts to change the concentration of neurotransmitters in the brain. For example, selective serotonin reuptake inhibitors concentrate serotonin in synapses between neurons.

The amount of oxygen that is actually used by the body during the rest or a hand workout is determined by the heart rate, the stroke volume, and the amount of a. blood circulating in the vascular system b. oxygen removed from the vascular system c. oxygen provided to the vascular system d. blood that individual has in the body e. hemoglobin in the red blood cells