Integumentary System: Hypodermis, Subcutaneous Injections, Burns, and Aging

Hypodermis: structure, function, and clinical relevance

• The hypodermis is not considered part of the skin. The skin consists of the epidermis and dermis, but it is still part of the integumentary system and plays an important role.
• It is the deepest part of the integumentary system and is also referred to as the subcutaneous layer.
• The cutaneous layer is formed by the epidermis and dermis together; the hypodermis sits below them.
• Composition: high quantity of collagen and elastic fibers, a lot of body fat (adipose tissue), and fibroblasts forming connective tissue that helps anchor the skin to the body.
• Adipocytes are the most numerous cell type in the hypodermis. The exact number of adipocytes varies with an individual’s body fat level.
• The hypodermis connects the skin to underlying tissues via connective tissue (primarily dense regular connective tissue).
• It contains larger blood vessels that supply the dermis with blood.
• Functions:
  • Energy reserve storage via adipose tissue.
  • Insulation and warmth maintenance.
  • Anchoring skin to underlying tissues and houses blood vessels that feed the dermis.
• Subcutaneous injections: medications injected into the subcutaneous layer are absorbed more slowly than intravascular or intramuscular routes.
  • Insulin is a classic example of a drug that is formulated for subcutaneous injection to achieve a gradual absorption and avoid rapid hypoglycemia.
  • If insulin were injected directly into the bloodstream, it could cause a rapid drop in blood glucose that could be dangerous or fatal.
• Injection sites: green-shaded areas on diagrams typically indicate regions with more fat—common subcutaneous injection sites.
• Quick takeaway: the hypodermis stores fat, houses vessels and nerves, helps anchor the skin, and serves as an energy reserve and insulator.

Burns: overview and key concepts

• Burns can be caused by extreme heat, flames, electricity, radiation, and chemical exposure.
• Sunburns are typically first-degree or second-degree burns and rarely third-degree; burns are categorized by degree of severity.
• The rule of nines is used to estimate the percentage of body surface area involved in a burn. It is not a perfect partition but uses factors of nine to approximate surface area.
• Commonly used to estimate burn severity and necessary medical attention:
  • First-degree burns: damage limited to the epidermis (partial thickness).
  • Second-degree burns: damage to the epidermis and dermis (partial thickness).
  • Third-degree burns: involve the hypodermis and potentially deeper tissues (full thickness).
• Degrees and thickness:
  • First degree: epidermis damaged only (partial thickness).
  • Second degree: dermis involved (epidermis + dermis; partial thickness).
  • Third degree: hypodermis and beyond involved (full thickness).
  • Fourth or fifth degree: rarely used; most references describe up to third degree.
• Partial thickness vs full thickness:
  • Partial thickness burns: first or second degree; do not go through full dermal thickness.
  • Full thickness burns: third degree; extend through the full thickness of the skin.
• Appearance indicators:
  • Blisters: common in second-degree burns.
  • Third-degree burns: blisters may be absent or poorly formed; tissue may appear oozing with a charred, white, brown, or black appearance.
  • Leathery appearance: often indicates hypodermal involvement.
  • Electrical burns may show charred patches and can involve deeper tissues.
• Rule of nines (proximate percentages):
  • Head (front + back) = $9 ext{\%}$
  • Torso (front) = $18 ext{\%}$; Torso (back) = $18 ext{\%}$; Torso total = $36 ext{\%}$
  • Each arm: $9 ext{\%}$ (front 4.5\% + back 4.5\% per arm)
  • Each leg: $18 ext{\%}$ (front 9\% + back 9\% per leg)
  • Genitals: $1 ext{\%}$
  • Sum check: $9 ext{\%} + 36 ext{\%} + 18 ext{\%} + 36 ext{\%} + 1 ext{\%} = 100\%$
• High-risk areas requiring more urgent attention (even if surface area is small):
  • Hands and feet (impact on function and mobility)
  • Face (airway risk and cosmetic/functional concerns)
  • Buttocks (sitting and weight-bearing areas)
  • Areas over joints (immobilization needed; healing risk due to movement)
  • Genital area (central to function and high risk for infection and dehydration)
• Depth-specific triage implications:
  • Burns over joints or large areas, or with airway involvement or signs of inhalation injury, require urgent care.
  • Chemical and electrical burns are often more complex and benign surface appearance can be deceptive.

Burn treatment and management by type

• General principles for all burns:
  • Keep the patient warm and hydrated; major burns risk hypothermia and dehydration.
  • Prevent infection: wounds tend to be highly susceptible due to epidermal barrier loss; cover and keep moist to prevent dehydration and infection.
  • Antibiotics may be used prophylactically in major burns due to high risk of infection.
  • Oxygen support may be required if airway or lung involvement is suspected or if inhalation injury is present.
  • IV fluids are often necessary for major burns to maintain hydration and perfusion.
• Chemical burns:
  • Do not add water to chemical burns.
  • Wear gloves and gently brush off dry chemical; avoid spreading the chemical to other areas.
  • Do not rinse with water when the chemical is in a dry, reactive form that could become more harmful upon dissolution (e.g., powdered lye).
  • After removal, keep the wound clean and covered; moisture retention is critical but avoid contamination.
• General wound care:
  • Most burns require keeping the wound clean, covered, and moist to prevent infection and promote healing.
  • The epidermis provides a moisture barrier; its loss accelerates dehydration and increases infection risk.
• Major burn interventions:
  • Skin grafts may be required if there is extensive tissue loss and no remaining dermal tissue to close the wound.
  • Physical therapy may be needed if joints, muscles, or bones are involved.
  • In some cases, antibiotics (preventive or therapeutic) are used due to infection risk.
  • Supplemental oxygen and respiratory support if airway injury is suspected.
• Healing and reconstruction considerations:
  • The extent and depth of burns influence the likelihood of needing grafts and rehabilitation.
  • Immobilization may be required to prevent wound reopening over joints.

Effects of aging on the skin

• The epidermis generally becomes thinner with age, reducing repair capacity and slowing wound healing.
• Dermal collagen and elastin decline, leading to reduced skin firmness and elasticity and resulting in wrinkles or crepey skin.
• Cumulative UV exposure accelerates aging and increases cancer risk.
• Hair and nail growth may slow, and hair/nail may become thinner or more brittle.
• Glandular secretions decrease with age (sweat and oil glands), reducing the skin’s ability to stay moisturized and regulate temperature.
• Overall, aging reduces moisturizing, cooling ability, and protective capabilities of the skin, increasing vulnerability to injury and dehydration.

Quick recap: the hypodermis and its place in the integumentary system

• Is the hypodermis technically part of the skin? No, it is not part of the skin proper, but it is part of the integumentary system.
• Primary functions of the hypodermis:
  • Fat storage to insulate and provide energy reserves.
  • Hosting blood vessels that supply the dermis.
• Major components:
  • Adipose tissue (majority).
  • Connective tissue, blood vessels, and nerves.
• Key takeaway: the hypodermis stores fat, promotes warmth, and serves as a pathway for vasculature to reach the dermis; it also helps anchor the skin to deeper tissues.

Recap: Connections to broader concepts and clinical relevance

• The integumentary system includes the skin (epidermis + dermis) and the hypodermis; the latter is not skin but is essential for function and health of the skin.
• The dermal and epidermal layers provide barrier protection, immune defense, and regulate temperature; the hypodermis supports these functions through insulation, energy storage, and vascular supply.
• Understanding burn depth, surface area, and high-risk areas informs triage, treatment plans, and likely need for grafts or rehabilitation.
• Practical implications for clinical care include careful chemical burn management, avoidance of improper irrigation in chemical burns, and attention to airway, breathing, and circulation in severe burn scenarios.

Takeaway checklist

• Hypodermis: not skin, but part of the integumentary system; primary components and functions.
• Adipose content varies with body fat; major energy reserve and insulation.
• Dense regular connective tissue helps anchor skin to underlying tissues; large vessels run through the hypodermis.
• Subcutaneous injections target the hypodermis; insulin example for gradual absorption.
• Burns: degrees, partial vs full thickness, and the rule of nines for surface area estimation.
• High-risk burn areas require aggressive evaluation and management.
• Chemical burn management: never add water in most cases; remove chemical carefully; maintain moisture and cleanliness.
• Major burns: IV fluids, antibiotics, oxygen, possible grafts, and rehabilitation.
• Aging effects on skin: thinning epidermis, reduced collagen/elastin, wrinkles, slower healing, reduced gland activity.
• End-of-section check: confirm hypodermis is part of integumentary system but not the skin proper.