The autonomic nervous system (ANS) regulates internal biological process without thought such as breathing and blood pressure. ANS is broken into two divisions; sympathetic and parasympathetic. The sympathetic division operates in response to stimuli by stimulating a process. The parasympathetic division usually operates with inhibiting a process.
In stress responses the sympathetic division of the ANS will begin to operate. The brain will send out a panic alarm along the vertebral ganglia, which are nerves connect directly to the spinal cord. This signal will cause body systems to either stimulate or relax depending on the immediate need. Three major functions which will be affected are the heart rate, breathing and blood pressure.
The heart rate will increase to increase the blood flow to the brain and muscles. The increased flow allows muscles to work harder due to the increase of oxygen and nutrients supplied by the blood. This will allow the person to run faster to the bus stop and experience greater flexibility and agility. Due in part to the increase in blood flow, it is important to note the increase of adrenaline, norepinephrine and epinephrine also contributes to muscular durability.
The increased heart rate will necessarily increase blood pressure, which is the measure of pressure of blood flow through the body. Pressure is a key ingredient because it is in reality the measure of how hard your heart is pumping. The blood ratio in the body is fairly constant and an increased heart rate alone would only speed the flow of blood in the body. However, by increasing the arterial pressure the body is forcing the flow of blood at higher pressures as well. Think of it like a person punching a bag. If they speed up the amount of times they punch they can do more damage, but if they speed up the amount and punch harder they are doing more damage.
It is also necessary for the breathing to increase to handle the influx of required oxygen by the blood. The shortness of breathing is due to the sympathetic nerves telling the diaphragm to increase blood/oxygen exchange.
When the person gets to the bus stop, the immediate stress is no longer present and the parasympathetic division will kick in and start to relax the body to a normal operating state. It may not fully return, but will to some extent.