Pathophysiology is the study of the functional changes associated with disease or trauma. Recognizing the signs and symptoms of these functional changes can help you determine when a patient is seriously ill or injured. These changes include respiratory compromise, level of consciousness, severe bleeding, shock, pain, and skin characteristics.

Respiratory Compromise

One of the most common indications of illness or injury is respiratory compromise (breathing difficulty or respiratory arrest). Respiratory compromise can result from a blocked airway, inadequate oxygen, and/or inadequate depth or rate of ventilations.

Level of Consciousness

Level of consciousness (LOC) is a strong determiner of the severity of the patient’s injury, illness, or other condition. LOC can be measured using the AVPU scale or (especially following a head injury) the Glasgow Coma Scale.

A decreasing LOC may manifest itself as an inability to focus or answer questions, a lack of coherence, a glassy stare, a drifting off to sleep, or complete unresponsiveness.

Severe Bleeding

Severe bleeding, whether internal or external, can contribute to seriousness of any injury. The lowering of blood volume contributes to shock (see the topic that follows) and leads to the failure of the circulatory system, affecting all other body systems by this failure.


Shock is the failure of the circulatory system to delivery oxygen-rich blood (perfusion) to all parts of the body. Shock is present to some degree in every illness or injury, but, when the condition is minor, the body adapts quickly to the physical stresses involved as the patient recovers. When the condition is serious, however, the body may fail to meet the demands for oxygen by vital organs, contributing to the severity of the condition and leading to cellular dysfunction, tissue damage, and even death.


Pain is an unpleasant to intolerable feeling associated with illness or trauma. It is the most common reason for consulting a doctor in the United States. In a first aid setting, pain can be an indicator of an injured body part or a serious medical condition such as a heart attack.

Skin Characteristics

The color, temperature, moisture, and appearance of the skin can tell you a great deal about the health and well-being of the patient. Healthy skin is smooth, supple, and without blemish. It is pink in nonpigmented areas (e.g., mucous membranes, nail beds, etc.), neither too cold nor too warm, and neither dry nor clammy. When skin characteristics differ from this, an illness or injury may be indicated.