55 Trauma and Healing – Effects on Heart Rate

When trauma has occurred and tissue repair is required, the activated cells at the site of damage require more energy.  In order to accommodate the elevated enzymatic reactions required to undergo mitosis, clean up debris and eliminate infectious agents, the cells take up more fuel molecules and building blocks from blood plasma.  At the same time, inflammation at the injured site will cause local vasodilation to occur in blood vessels that supply the damaged site.  While vasodilation and increased capillary permeability assist in delivering WBCs, blood flow velocity in the zone slows down which assists with increased nutrient delivery and waste removal.  However, there is an optimal delivery rate of oxygen and nutrients (i.e. not too slow and not too fast) in order for the receiving cells to uptake enough of these resources and keep up with enzymatic reactions.  Therefore, maintaining adequate blood flow and blood pressure is important.  Additionally, knowing that blood circulation is a closed circuit within the human body, with ~5L of blood circulating at all times, it may make sense that if some blood vessels need to dilate, others blood vessels (servicing less active cells) will need to constrict in order to maintain appropriate levels of blood flow and blood pressure.  Not only will the ability to adjust vasoconstriction and vasodilation assist with maintaining adequate levels of blood flow and blood pressure, but also will the ability to adjust heart rate and heart contractility (i.e. force of contraction, stroke volume).  This will likely give one insight in answering the next question.

Why would you see an increased heart rate during trauma?

There’s quite a few reasons actually.

Overall, biological stressors (such a trauma, extreme changes in temperature, illness, etc.) require the body to expend more energy in order to ensure the survival of all tissues and organs and the body.  As such, the sympathetic nervous system (SNS) is typically stimulated which can immediately assist in the increased delivery of fuel molecules, building blocks and oxygen to cells of the body.  Specifically, activation of SNS, can assist in the maintenance of homeostasis of blood pressure, blood glucose levels, blood oxygen levels and blood pH levels.

1. Increasing Fuel Availability:  In response to low blood glucose, SNS can stimulate hepatocytes (liver cells) to undergo lipolysis, glycogenolysis, and gluconeogenesis, which increases fuel molecule (e.g. glucose and lipids) concentrations in the blood stream.

2. Increasing Blood Flow:  The SNS can increase delivery rates of nutrients and oxygen by speeding up blood flow. To do this, SNS increases heart rate, increases heart contractility and stroke volume, increases vasoconstriction, and increases breathing rates.

3. Increasing Blood Oxygenation Rates:  The increased breathing rates help to increase blood oxygenation rate within the pulmonary capillaries, which is necessary as cardiac output (pumped blood volume per minute) and blood velocity increase with higher heart rates and stroke volumes.

4. Increasing CO2 Removal Rates and maintaining Blood pH:  Increased breathing rates also assist with blood pH maintenance through the removal (exhaling) of carbon dioxide (which reduces carbonic acid levels in the blood).  As such, SNS maintains blood carbon dioxide and blood oxygen levels as well as blood pH levels.  SNS simultaneously helps to control blood flow to the kidneys (through vasoconstriction/vasodilation), so that excess H⁺ ions from acidic waste produces can be removed from the blood and eliminated in the form of urine which also helps to maintain blood pH7.4.  Blood pH homeostasis ensures that cellular enzymes are able to work in optimal conditions.

5. Maintaining Blood Pressure:  The SNS can adjust for a drop in blood pressure which may occur if the injury has been severe. For example, a drop in blood pressure can occur during trauma, if

   • there has been a loss of blood volume (due to hemorrhaging) and/or
   • there is significant amount of vasodilation in the injured site (due to inflammation).
     • To ensure that blood pressure is maintained in both of these situations, SNS will increase both heart rate and heart contractility as well as induce vasoconstriction in regions of the body that are not injured and less active.  Additionally, SNS will induce venoconstriction if required.

*Side note:

Besides, injuries, there are other biological stressors that induce can SNS stimulation of increased heart rate, heart contractility (stroke volume), vasoconstriction, and/or breathing rates in order to supply and maintain increased demands for sufficient blood oxygen, blood glucose, and blood pH maintenance.

For example, exercising, a biological stressor that is good for you, increases the demands by skeletal muscles cells for oxygen and glucose as well as for increased waste removal (e.g., lactate and creatinine). Therefore, when exercising you may notice your SNS working for you.  Your heart rate and breathing rate increase during exercise as a result of SNS stimulation, in order to support muscle cell enzymatic activity.

Other energetically demanding situations such as pregnancy, exposure to extreme temperatures inducing sweating or shivering, recovering from illness can also lead to elevated heart rates in order to supply and maintain increased demands for sufficient blood oxygen, blood glucose, and blood pH maintenance.