Shock is best defined as a state of reduced end-organ oxygenation caused by an imbalance between tissue oxygen delivery and demand resulting in an oxygen debt.
Oxygen delivery is determined by cardiac output and oxygen content of the blood. Oxygen content is a composite of haemoglobin and arterial oxygen saturation, and the cardiac output is a product of stroke volume and heart rate. Stroke volume is affected by preload (filling), left ventricular contractility (pump function), and afterload as measured by the systemic vascular resistance.
Differences in the preload, afterload, and contractility generally differentiate the aetiologies of shock. These are broadly classified into hypovolaemic, cardiogenic, obstructive, and distributive types. 
Whatever the aetiology, shock is characterised by release of cytokines and other inflammatory mediators that cause a systemic inflammatory response syndrome mediated by tissue hypoxia. This causes alterations in flow at the level of the microcirculation   that can usually be reversed by intravascular volume resuscitation and inotropic support. 
Shock passes through an early reversible stage of compensated shock where the body's homeostatic mechanisms compensate for decreased perfusion by increasing the rate and force of contraction of the heart, initially maintaining arterial blood pressure. The circulation is centralised, due to peripheral vasoconstriction, so that blood flow to non-vital organs is reduced. Respiratory rate increases to compensate for metabolic acidosis and urine output falls as a result of release of antidiuretic hormone from the posterior pituitary. These adaptive responses may be inapparent to the casual observer. Unrecognised, this evolves to overt shock manifested by decreased BP and altered mental status. This occurs in the setting of an effective loss ≥30% of plasma volume and/or cardiac index of <2.2 L/minute/m^2. Untreated at this stage, it can cause irreversible cell death and organ damage.
The lack of tissue oxygenation leads to accumulation of products of anaerobic metabolism such as lactate. Significant tissue hypoxia leads to a systemic pro-inflammatory state with excess cytokine release; when prolonged ( >6 to 12 hours) this causes irreversible cellular damage. This is clinically manifest as multi-organ dysfunction and/or failure with increased mortality.