All living cells in the human body require oxygen to efficiently convert glucose into energy. For cells to receive oxygen, there needs to be adequate blood flow throughout the body so that oxygen-rich blood can deliver oxygen to the tissues and cellular respiration can occur. When there is impedance in the body's ability to perfuse blood to the tissues, shock may occur. Shock is the result of circulatory failure throughout the entire body, which results in cellular damage and can lead to multiple organ failure if not treated immediately.

Blood pressure is a major determinant of the perfusion of tissues. There needs to be pressure to push blood into the capillaries of each tissue. Blood pressure is determined by two factors: resistance to flow and cardiac output. Resistance to flow is determined by the length and width of the vessels, as well as the viscosity of blood. Cardiac output is determined by heart rate and stroke volume (amount of blood pumped out each beat). There are several factors that affect the body's blood pressure, and they are placed into 4 categories with several subcategories.

Hypovolemic Shock

Hypovolemic shock is a type of shock in which the body loses fluid volume. If the body loses 20% or more of its volume, it can be enough to affect its ability to perfuse the tissues. Hypovolemic shock can be hemorrhagic or non-hemorrhagic by nature. Non-hemorrhagic shock can be caused by severe dehydration from sweating, vomiting, or diarrhea without adequate replenishment of fluids. Hemorrhagic shock occurs when a person loses large amounts of blood—typically from a traumatic event. When the body loses a large amount of fluid, the blood pressure decreases, and the heart does not fill up completely before each beat, which decreases stroke volume. When stroke volume decreases, cardiac output also decreases, and the cells do not receive an adequate amount of oxygen from the blood.

Cardiogenic Shock

Cardiogenic shock occurs when the heart has been damaged to the point where it cannot pump enough blood to meet the body's demand for oxygen. The most common cause of cardiogenic shock is a heart attack or acute myocardial infarction. Following a heart attack, the muscles of the heart do not contract well or at all. This causes the heart to have weaker contractions, which in turn decreases stroke volume. The decrease in stroke volume decreases cardiac output and blood pressure. When the heart is not pumping at full capacity, the body cannot maintain a high enough blood pressure to perfuse blood to the tissues, resulting in shock.

Obstructive Shock

Like cardiogenic shock, obstructive shock affects the heart's ability to adequately pump blood, which results in a decrease in cardiac output and an overall decrease in blood pressure. For this reason, some classify obstructive shock as cardiogenic shock. However, the nature by which the heart is being affected is different in obstructive shock. Obstructive shock occurs when there is a physical obstruction in the heart or blood flow that causes a decrease in cardiac output. A common cause of obstructive shock is cardiac tamponade. Cardiac tamponade results when the pericardial sac surrounding the heart fills with blood following a traumatic accident or as a result of infection. The pressure from the blood surrounding the heart dampens the heart's ability to properly fill and contract, which decreases stroke volume, cardiac output, and blood pressure.

Distributive Shock

Distributive shock is a type of shock caused by excessive vasodilation that leads to a drop in the resistance of blood flow. As was mentioned earlier, the resistance of blood flow directly affects blood pressure. When the vessels dilate, there is less resistance, and blood moves too fast to unload oxygen. This results in a decrease in perfusion and distribution of blood to the tissues. There are several causes of distributive shock, the most common being septic shock. Septic shock is a result of bacteria in the bloodstream. The immune system response causes vasodilation, which results in a drop in blood pressure. Other forms of distributive shock are anaphylactic shock and neurogenic shock. Anaphylactic shock occurs when the body's immune system reacts to an allergen in the body, causing vasodilation. Neurogenic shock occurs when the central nervous system is damaged and cannot control the body's blood pressure, resulting in decreased perfusion.