Bleeding out is the cause of approximately 40% of trauma-related deaths worldwide. Penetrating trauma is particularly dangerous as the victim faces a rapid drop in blood pressure from heavy bleeding, eventually leading to organ failure and death if unattended. The body, however, undergoes various physiological changes to preserve blood pressure and volume.
This essay will consider the impact of a stab wound, specifically homeostatic mechanisms that will come into play to maintain blood pressure in compensation for the heavy blood loss. These mechanisms include the autonomic and renal system. Further consideration will also be given to factors that may influence the patient’s condition and prognosis, such as pregnancy or infection
Physiological changes to Blood Pressure
It is important to consider the role blood plays within the human body to understand what physiological changes occur as a result of heavy blood loss. About 7-8% of a person’s total body weight is blood. It is vital for the functioning of the human body as it transports oxygen from the lungs to cells around the body, where it is needed for metabolic activities. Also, with 3700-10,500 white blood cells per microlitre of blood, blood is well adapted to fighting infections. Blood also provides cells with hormones and nutrients, which play a role in feedback chain mechanisms as well as removing waste products for excretion.
Therefore, a multitude of problems would arise for a patient who is experiencing blood loss from a stab wound. It becomes particularly worrisome if more than 40% of their total blood volume is lost because at this point, blood pressure drops significantly as less force is exerted onto vessel walls. Cellular aerobic respiration shuts down and anaerobic metabolism increases, which leads to the production of lactic acid and eventually metabolic acidosis. Consequently, the patient can go into haemorrhage shock, the oxygen demands of tissues cannot be met, which may result in organ failure. Therefore, the process of homeostasis is vital to maintain this blood pressure. Two crucial systems involved in homeostasis of the blood pressure after a stab wound are described below.
The Autonomic System
The body’s autonomic system induces a series of mechanisms to counter the drop in blood pressure. This involves baroreceptors. Baroreceptors are pressure sensitive sensory receptors which are stimulated by stretching of vessel walls. As illustrated in figure 1, they relay information derived from blood pressure to the cardiovascular (CV) centre (located in the medulla oblongata), allowing for an increase or decrease of sympathetic stimulation from a feedback chain (Fig 1) to regulate blood pressure within the body. Due to decreased blood pressure levels from the haemorrhage, baroreceptors within the carotid sinuses and aortic arch stretch less after each pulse and consequently send nerve impulses at slower rates to the CV centre through glossopharyngeal (IX) nerves.
In response, the CV centre inhibits parasympathetic stimulation of the heart via vagus nerves and increases sympathetic stimulation via cardiac accelerator nerves (Gordan, 2015). Sympathetic stimulation induces homeostatic mechanisms. It leads to the secretion of the hormone/ neurotransmitter noradrenaline (also known as norepinephrine) by the adrenal medulla. Noradrenaline plays a key role in the body’s “fight or flight” response, and it increases cardiac output by increasing the rate and force of heart contraction by acting on cardiac myocytes. Furthermore, noradrenaline acts on smooth muscle cells of blood vessels (Fig 1) which causes them to constrict (vasoconstriction). Vasoconstriction increases systemic vascular resistance, consequently increasing blood pressure so that more oxygen can reach essential tissues of vital organs.
Chemoreceptors further help in maintaining blood pressure. They are sensory receptors that monitor the composition of oxygen, carbon dioxide and hydrogen ions in the blood. Chemoreceptors can detect conditions such as hypoxia, reduced oxygen levels in blood and acidosis, excess acid in the blood due to lactate build up from the haemorrhage. In response to these conditions, chemoreceptors send impulses to the CV centre to increase sympathetic stimulation, reinforcing the baroreceptor reflex by increasing vasoconstriction of blood vessels.
The renal system
As the patient is losing blood, their renal system would attempt to counter these losses by conserving blood volume and therefore, blood pressure. When the volume of blood is low, juxtaglomerular cells in kidneys secrete renin into circulation. Plasma renin carries out the conversion of angiotensinogen released by the liver to angiotensin-1. In sequence, renin and angiotensin converting enzyme (ACE) act on their substrates to produce the active hormone angiotensin II, which is a potent vasoactive peptide that causes blood vessels to constrict, resulting in increased systemic vascular resistance ((Samuel, Francis & Anthony, 2018). In addition, angiotensin II causes the adrenal cortex to release aldosterone, which increases reabsorption of sodium ions and water back into the blood (Gordan, 2015). Consequently, blood volume and pressure are conserved.
The patient may experience hypovolemic shock: a state of low extracellular fluid levels which means that vital organs cannot receive enough blood.
During this state, baroreceptors detect changes in arterial blood volume and relay this information to the vagus nerve. This initiates Anti-Diuretic Hormone (ADH) being produced by the hypothalamus and released by the posterior pituitary gland to help reduce the severity of the shock. It functions by binding onto V-receptors of kidney principal cells and activating protein kinase A, a phosphorylating enzyme. Intracellular aquaporin-2 storage vesicles (Fig 2) are phosphorylated as a result of a phosphorylation cascade, which promotes their movement. This process causes the body to produce an increased concentration of urine and more water is reabsorbed back into the blood. This helps to maintain blood pressure.
Further Considerations
Whilst blood levels would be of high concern for a patient, other factors such as infection and pregnancy cannot be ignored when dealing with this case.
The nature of stabbing can cause further problems for the patient. If the weapon already contained blood before the stabbing, then the patient is at risk of HIV. There is also a threat of infection if the wound is exposed to pathogens. Whilst a stab wound to the spine rarely causes infection, studies showed that penetrating trauma to the abdomen had a 15% infection rate (Petersen and Waterman, 2011). Furthermore, the gut contains 100 times more bacteria than cells in the human body, therefore the risk of infection rises substantially if the gut is punctured.
If the patient is female, there is a possibility of pregnancy. In this scenario her abdominal organs are pushed upward by the growing uterus therefore she is more susceptible to bowel or splenic injuries resulting from penetrating trauma to the upper abdomen. As pregnancy evolves, uterus size increases and goes beyond the pelvic cavity. This exposes the uterus to potential trauma and uterine lesions, which can cause foetal death. However, if the injury is lower, there are still high risks to the developing foetus as vasoconstriction occurs to preserve the mother’s blood pressure. In doing so, blood flow is diverted away from the uterus, and directed to the mother’s vital organs, placing the foetus in danger of hypoxia.
Conclusion
The stab wound resulting in haemorrhage initiates a cascade of physiological changes to the patient’s blood pressure. Blood pressure plummets and in response, homeostatic and autonomic systems elicit changes in order to maintain the blood levels. Factors such as infection and pregnancy need to be considered as it can heavily impact the severity and nature of the injury.