Burns, wounds and skin repair

The epidermis is avascular and is fed by diffusion from the dermis. An injury that stays in it does not bleed.

• Correct: Epidermis:
• Papillary layer of the dermis:
• Reticular layer of the dermis:

Histamine and other chemical messengers open gaps between endothelial cells, so water and proteins leak from plasma into the interstitial fluid and the tissue swells.

• They become less permeable:
• Correct: They become leakier, so fluid and proteins move into the tissue:
• They close completely:

The lipid sheets from lamellar granules seal the gaps between the keratin-filled cells of the stratum corneum, so water stays in.

• The dermal blood vessels:
• Correct: The lipid sheets between the dead cells of the stratum corneum:
• The fat of the hypodermis:

Across the integumentary, cardiovascular and urinary systems: a large burn removes the skin's barrier and makes capillaries leak body-wide. Water evaporates from the wound and plasma leaks into the tissues, so blood volume and blood pressure fall, heart rate rises to compensate, and urine output falls. That is why large burns need large volumes of fluid in the first day.

• Water lost by evaporation from the skin: up. The burned skin has lost its stratum corneum and lipid seal, so water evaporates freely from the wound surface.
• Volume of interstitial fluid: up. Heat damage and chemical messengers make capillaries leaky, so water, salts and proteins move from the plasma into the spaces between cells, and the tissue swells.
• Blood volume: down. Fluid leaves the vessels into the tissues and out through the burned surface faster than it is replaced.
• Blood pressure: down. With less blood volume, less blood returns to the heart and less is pumped into the arteries.
• Heart rate: up. The body responds to the falling blood pressure by speeding the heart.
• Urine output: down. Low blood pressure reduces blood flow to the kidneys, so they make less urine.

Front of the trunk = 18%; one whole upper limb = 9%. 18 + 9 = 27%.

• 18%: 18% is the front of the trunk alone; the arm adds another 9%.
• Correct: 27%: Correct. 18 + 9 = 27%.
• 36%: 36% is the whole trunk, front and back; only the front is burned.
• 45%: 45% would be the whole trunk plus one arm.

Only partial- and full-thickness burns count. The sunburn is superficial (red, painful, no blisters) and leaves the barrier intact, so it is left out. The blistered arm is partial-thickness: 9%.

• 27%: 27% adds the superficial sunburn, which is not counted.
• 18%: 18% is the sunburned back alone, which does not count.
• Correct: 9%: Correct. Only the arm's 9% counts.
• 36%: 36% is the whole trunk; neither the burns nor the rule support it.

A full-thickness burn destroys the whole dermis with its nerve endings, so its center is painless, white and leathery. The partial-thickness burn around it still has living nerve endings and capillaries, so it hurts and blisters. Painlessness here means the deepest damage.

• The painless center is a mild superficial burn: A superficial burn is red and painful, not white, leathery and numb.
• The whole area is one partial-thickness burn: Partial-thickness burns hurt and blister; a numb, leathery area has lost its nerve endings, which means full thickness.
• Correct: The painless center is full-thickness; the painful ring is partial-thickness: Correct. The numb center is the deepest part of the burn.
• The ring is full-thickness, and the numb center is a burn that has already healed: A red, blistered, painful ring has living nerve endings, which rules out full thickness; a burn does not heal from its center outward within hours.

Hair follicles and sweat gland ducts are lined by epithelium that reaches deep into the dermis, below a partial-thickness burn. Those cells divide and spread out across the wound bed, and the islands grow together into new epidermis. A full-thickness burn destroys them, so it cannot regrow epidermis from within.

• Fibroblasts of the reticular layer, which change into new keratinocytes: Fibroblasts make collagen and scar tissue; they do not become epithelial cells.
• Fat cells of the hypodermis: Fat cells lie below the dermis and do not form epidermis.
• Melanocytes that survive the burn: Melanocytes make pigment; they do not supply keratinocytes.
• Correct: Epithelial cells lining surviving hair follicles and sweat gland ducts: Correct. The epithelium of follicles and ducts regrows the surface.

A wound with separated edges and lost tissue heals by secondary union. It fills from the bottom with granulation tissue, its edges are pulled together by wound contraction, and it closes over weeks, leaving a wider scar.

• By primary union, with a thin line scar within days: Primary union needs edges held close together, as in a stitched cut; this wound is left open.
• By regeneration only, with no scar: A wound through the dermis with lost tissue fills partly with collagen, so it scars.
• Correct: By secondary union, with granulation tissue and contraction: Correct. Open wounds heal by secondary union.
• By primary union, once the scab falls off and the edges meet: Whether a wound heals by primary or secondary union depends on whether its edges are together, not on the scab.

Melanoma starts in melanocytes, which can break loose early, pass through the dermis into vessels and spread to lymph nodes and distant organs. Basal cell carcinoma rarely spreads, so it is common but rarely deadly.

• Correct: Melanoma cells can spread early to lymph nodes and distant organs: Correct. Early spread is what makes melanoma dangerous.
• Melanoma grows only on skin that cannot be seen: Melanoma can appear anywhere, including easily seen skin; that is why skin checks work.
• Melanoma cannot be removed by surgery: Thin melanomas removed early are usually cured.
• Melanoma always starts in an old mole: Most melanomas appear as new spots rather than in an existing mole.

Sustained pressure over a bony area, higher than the pressure inside the capillaries, flattens them. Blood stops flowing, the cells run out of oxygen and nutrients, and they begin to die by necrosis. Redness that does not blanch is the first sign of a pressure injury.

• Correct: Pressure squeezed his capillaries shut, starving cells of oxygen: Correct. Compressed capillaries starve the tissue.
• Heat from the mattress has burned the skin: A mattress is not hot enough to burn skin; the damage comes from pressure.
• Bacteria from the sheets have infected the intact skin of his heels: Infection can follow once skin breaks down, but the first damage is from lost blood flow.
• Too much sebum has plugged the follicles of the heel: The heel is thick skin with no hair and no sebaceous glands.