An important aspect of our body’s stress response is the increase in cardiovascular output. The reason for this is not far to see. From the point of view of evolution, in the face of a physical stressor, oxygen and energy need to be diverted to muscles for flight or fight. The heart is the transport system pump; the delivery routes are the blood vessels. Using blood as the transporting medium, the heart propels oxygen, nutrients, wastes, and other substances to and past the body cells. Naturally, this (cardiovascular) output needs to be increased when faced with a stressor.
In simple terms, the heart works like an ordinary water pump—well, actually two pumps laid out side by side. The blood vessels are the ‘pipes’ that carry blood throughout the body. To give a very brief description, the right side of the heart works to pump impure blood to the lungs where it is purified and returned back to the left side of the heart. From there, it is pumped to the rest of the body through the arteries. When the body has depleted the oxygen in the blood, it is returned to the heart via the veins. For a more detailed description of the action of the heart, please refer to Appendix 3.
The question that comes to mind is how does the heart go about automatically pumping blood hour after hour, day after day? The answer lies in a special group of cells that have the ability to generate electrical activity on their own. These cells separate charged particles. Then they spontaneously leak certain charged particles into the cells. This produces electrical impulses in the pacemaker cells, which are spread over the heart, causing it to contract. These cells do this more than once per second to produce a normal rate of 72 beats per minute.
Who sets the rate at which the heart beats? The natural pacemaker of the heart is called the sinoatrial node (SA node). The heart also contains specialized fibres that conduct the electrical impulse from the pacemaker (SA node) to the rest of the heart.
Here is an interesting aside—the heart weighs only around 300 gm. An average heart pumps 70 ml per heartbeat. Assuming average heartbeats, the heart pumps 5 1 per minute. Keeping the calculations going, it pumps 7,200 1 per day or 18 million 1 by the time someone is 70 years old. That’s not bad for a 300 gm pump!
To get additional output from a pump, you need to speed it up. In the case of the heart, it needs to beat faster and harder. In response to stress, the brain turns down the parasympathetic nervous system and turns up the sympathetic nervous system. The sympathetic nervous system triggers the heart’s pacemaker to speed up resulting in faster heartbeats. The net result is an increase in the blood flow (both faster and with more force) from the heart. As can be expected, this process is far more complex than what is described here but the details are beyond the scope of this book. Typically, when we experience maximum stress, the blood flow is five times that of the normal flow.
Our body has another trick to increase the force with which the blood flows. Imagine for a moment, that you have a pipe through which water is flowing. If you would like to increase the force of the water but cannot increase the supply, you partly cover the mouth of the pipe with your finger. The narrowing of the opening causes the water to come out with force. This is the same trick that the body employs. Tiny circular muscles surround all the major arteries of the body.
The sympathetic nervous system triggers those muscles to tighten (causing the arteries to narrow) and the blood pressure goes up. As a result of the constriction of the major arteries, blood is now being delivered with greater speed to the exercising muscles. At the same time, there is a dramatic decrease in the blood supply to the non-essential parts of the body including the blood supplied to the stomach, kidneys and the skin. This can explain the blanching or paleness observed in people under intense stress.
In a typical physical stressor (the scenario of the wounded animal running for life) there is loss of fluid—blood from the wound and sweat. Naturally, there is not enough time to drink the fluids required. If the blood volume goes down the oxygen and energy cannot be delivered to the muscles howsoever fast the heart may be beating. The body’s stress response has another trick up its proverbial sleeve. It directs the kidneys to stop urine formation and reabsorb some of the water into the circulatory system. The messenger in this case is the hormone vasopressin. Needless to add, there are a whole lot of other hormones involved in regulating the water balance in the blood.