EMT Exam: Circulatory Conditions Involving Pump, Pipes, and Fluid

By Arthur Hsieh

For the EMT exam, you will need to be aware of conditions that affect circulation. Think about how water moves through your house or apartment. There has to be a pump that pushes water from your well or your town’s water supply into the dwelling.

Once there, pipes channel that water to a variety of appliances, from faucets and showers to toilets and sprinklers. There has to be a certain pressure within the system in order to make all the appliances function properly. Lower the water pressure and you have showers that dribble and toilets that clog.

What might cause pressure to fall? It could be the pump at the well or station. Perhaps a break occurred in one of the pipes. Maybe the well ran dry. In any case, unless something changes to compensate for the loss, your water pressure falls and eventually water stops flowing. Not good!

Your body essentially does the same thing with circulation, or perfusion. The heart has to pump at a certain rate and with enough force so that blood is pushed into the vasculature. What’s nice about your system is that, unlike a house’s plumbing, it automatically tries to compensate for changes in pressure.

Specialized cells called baroreceptors are found in the first part of the aorta as it leaves the left ventricle. Baroreceptors sense subtle changes in falling pressure and send that information to the brain. The brain interprets these signals and sends commands to the heart to increase both heart rate (chronotropy) and contraction of the muscle (inotropy) resulting in more pressure generated out of the heart and into the circulatory system.

Meanwhile, the rest of the body reacts as well. As pressure falls, smooth muscles throughout various parts of the body contract, causing parts of the vasculature to constrict and decrease blood flow from areas of the body that don’t need very much circulation during a crisis.

Areas such as the skin and gastrointestinal tract are not crucial to the body during low-pressure conditions. The brain, heart, lungs, and kidneys must have blood flow maintained at virtually all costs.

The body also senses, through chemoreceptors in the brainstem, the potential loss of oxygen and worsening ability to remove carbon dioxide. As a result, signals to the diaphragm and intercostal muscles trigger faster breathing.

Put it all together: cool, pale skin; tachycardia; tachypnea. Sound familiar? It should; these are the early signs of shock. When the body can maintain blood pressure and perfusion to the vital organs, it’s in compensated shock; when it begins to fail and pressure falls, it’s entering decompensating shock. Let it persist long enough, and enough cells will die from lack of perfusion to cause organ failure and probable death.

You may wonder whether the body can compensate for falling pressure by increasing the amount of blood it has. In fact, it does do this by stimulating red blood cell production in the bone marrow. However, it takes days to increase cell production, which is why it’s not helpful in the sudden crisis of shock.