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The hypodermis, also called the subcutaneous layer, lies directly below the dermis. Though not technically part of the integumentary system, it connects the skin to underlying fascia of bones and muscles. It consists of loose, areolar connective tissue and adipose tissue, functioning as an energy reserve and insulator for the body.

Adipose tissue in the hypodermis stores fat in cells called adipocytes, accounting for approximately 80% of body fat. This stored fat serves as an energy reserve, insulates the body to prevent heat loss, and cushions underlying structures from trauma. Fat distribution varies with hormones and genetic factors.

The hypodermis contains well-vascularized connective tissue with arteries that nourish the skin. These arteries form two networks: the cutaneous plexus along the border of the hypodermis and reticular layer, and the papillary plexus, which supplies smaller arteries to the papillary dermis.

The hypodermis contains pressure-sensing receptors called Pacinian corpuscles, similar to those found in the reticular dermis. These receptors enable the skin to detect pressure and contribute to the sensory functions of the skin, allowing the body to respond to external stimuli.

Fat distribution in the hypodermis depends on hormones including testosterone, estrogen, insulin, glucagon, and leptin, as well as genetic factors. Males typically accumulate fat in the neck, arms, lower back, and abdomen, while females tend to accumulate fat in the breasts, hips, thighs, and buttocks.

Body mass index (BMI) is derived from a mathematical formula comparing body weight to height and is often used to measure fat. However, its accuracy as a health indicator is unreliable, particularly in exceptionally physically fit individuals who may have high muscle mass.

Changes in diet and exercise are the best ways to control body fat accumulation in the hypodermis. Reducing high-calorie food consumption and increasing physical activity help prevent excess adipose tissue buildup, which can increase the risk of heart disease and diabetes.