Injury from physical agents
How physical energy damages tissue: force scrapes, tears and cuts, heat coagulates proteins, cold freezes cells and injures vessels, electric current heats the tissues it crosses, and ionising radiation breaks DNA in dividing cells.
By the end, you can
- Classify mechanical wounds (abrasion, contusion, laceration, incised and stab wounds) by how the force acts on tissue.
- Explain how the depth of a burn determines its appearance, its pain and how it heals.
- Explain how freezing and non-freezing cold injure cells and small vessels, and how the core temperature fails in hypothermia and heatstroke.
- Relate the path and resistance of an electric current to the injuries it causes, including deep muscle damage.
- Explain why ionising radiation harms dividing cells first, and distinguish its early and late effects.
- Recognise the classic wound, bruise, frostbite and electrical marks, the temperature bands and tests examiners ask about, and explain how wound care, cooling, rewarming and radiation countermeasures act on the mechanism.
What it is
Physical agents injure tissue by delivering energy to it: mechanical force (trauma), heat, cold, electricity, ionising and ultraviolet radiation, and sudden changes in pressure. The damage depends on the kind of energy, how much is delivered, how fast and over what area, and how well each tissue resists or carries it away.
4 types to tell apart
- Mechanical
- Blunt force compresses, stretches and shears tissue; sharp force cuts it. Abrasions, contusions, lacerations, incised and stab wounds, fractures and crush injury.
- Thermal
- Heat coagulates proteins (burns, and heatstroke when the whole body overheats); cold freezes tissue (frostbite), injures it without freezing (trench foot) or slows the whole body (hypothermia).
- Electrical
- Current crosses the body along tissues of low resistance, disturbing the heart and nerves and heating the tissues on its path.
- Radiation
- Ionising radiation (X-rays, gamma rays, particles) damages DNA directly and through free radicals; ultraviolet light damages the DNA of skin cells.
US spelling: fibres → fibers.
Normal function
How healthy tissue resists and sheds energy
Most physical energy meets the skin first. The resists friction and, when dry, has a high electrical resistance. Collagen and elastic fibres of the give tensile strength and stretch, and the cushions blows.
Heat is carried away by blood flowing through the and by sweat from the ; in the cold these vessels constrict to hold heat in the core. The hypothalamus keeps the core temperature close to 37 °C by adjusting both. Melanin in the epidermis absorbs ultraviolet light.
Tissues renew themselves from dividing cells, such as those of the , the bone marrow and the gut crypts. Their ability to divide lets them repair injury, but also makes them the first casualties of ionising radiation.
Underlying change · Step 1 of 5
Force: scraping, tearing, crushing and cutting
The full lesson is part of Student Pro
It takes you through all 5 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .
Underlying change · Step 2 of 5
Heat: proteins coagulate, layer by layer
The full lesson is part of Student Pro
It takes you through all 5 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .
Disrupted function · Step 3 of 5
Cold: ice, damaged vessels and a slowing body
The full lesson is part of Student Pro
It takes you through all 5 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .
Disrupted function · Step 4 of 5
Electricity: current follows the easiest path
The full lesson is part of Student Pro
It takes you through all 5 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .
Consequences · Step 5 of 5
Radiation: broken DNA in dividing cells
The full lesson is part of Student Pro
It takes you through all 5 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .
Symptoms and signs
Symptoms, signs and findings
The full lesson is part of Student Pro
It takes you through all 5 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .
Exam detail
Exam detail
The full lesson is part of Student Pro
It takes you through all 5 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .
Treatment
How treatment works
The full lesson is part of Student Pro
It takes you through all 5 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .
Recap
Recap
The full lesson is part of Student Pro
It takes you through all 5 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .
Self-check
Check your understanding
The full lesson is part of Student Pro
It takes you through all 5 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .