Luke, When we are stressed, we release excitatory neurotransmitters. Your body responds to these hormones by increasing your heart rate, blood pressure, and respiration. More blood is pumped into your muscles, sending more oxygen to your muscles, brain, heart and lungs. Blood flow may increase 300 percent to 400 percent, preparing your lungs, muscles, and brain for added demands. The amount of sugar of glucose increases in your blood, which accelerates your metabolism so you can take immediate action in an emergency. Your blood thickens as platelets prepare to stop bleeding quickly. As the blood thickens, oxygen increases in red cells and promotes better function of the white cells that prevent infections. The spleen discharges red and white blood cells, allowing the blood to transport more oxygen.