Subcutaneous Tissue: The Innermost Layer of Skin

Diagram of human skin

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Subcutaneous tissue, which is also known as the hypodermis, is the innermost layer of skin. It's made up of fat and connective tissues that house larger blood vessels and nerves. Subcutaneous tissue acts as an insulator and regulates body temperature. The thickness of this layer varies throughout the body and from person to person.

Subcutaneous Tissue Composition

The skin is composed of three layers: the epidermis, the dermis, and subcutaneous tissue. There are several structures and specialized cells that exist within subcutaneous tissue, including:

  • Collagen and elastin fibers
  • Fat cells
  • Blood vessels
  • Sebaceous glands
  • Nerve endings
  • Hair follicle roots

Subcutaneous tissue is largely composed of adipose tissue (fat tissue) that is made up of adipocytes, or fat cells. The amount of adipose tissue varies throughout the body. It's thickest in the buttocks, the palms of the hands, and the soles of the feet. The size of adipocytes is determined by an individual's nutritional habits. Generally speaking, a person that maintains a healthy diet and exercise habits has smaller adipocytes and is less likely to be overweight.

Functions of Subcutaneous Tissue

The adipose tissue acts as an energy reserve. Once the body uses up energy that's acquired from carbohydrate consumption, it turns to adipose tissue as a fuel source, which can lead to weight loss. Adipocytes can swell or shrink depending on whether fat is being stored or used.

Additionally, this fat acts as armor that protects muscles, bones, organs, and more delicate tissues. Think of subcutaneous tissue as the protective gear that athletes like football and ice hockey players wear. It's like the body's natural padding. It cushions the body and protects its insides whenever a person takes a hit or a fall. Falling on the ground would hurt a lot more if subcutaneous tissue didn't exist.

It also regulates body temperature by making sure that the internal temperature isn't too hot or too cold. Subcutaneous tissue essentially insulates the body, which allows a person to go outside on a cold day without freezing to death.

Subcutaneous tissue starts to thin out as a person ages. This weakened layer of insulation makes the body more prone to hypothermia because less tissue makes it harder to stay warm. The loss of subcutaneous tissue also causes the body to sweat less, which, in turn, makes it harder to stay cool. It can also affect the body's reaction to certain medications that are absorbed by the subcutaneous tissue.

Anatomically speaking, the location and thickness of subcutaneous tissue differ by gender. For example, men tend to accumulate more subcutaneous tissue around the abdomen and the shoulders, while women tend to accumulate it around the thighs, the hips, and the buttocks.

Subcutaneous Injection

Some medications need to be injected intravenously, like an IV drip. Others are injected directly into the skin. Subcutaneous injections are used to administer medications like insulin and morphine. A common type of subcutaneous injection is an epinephrine auto-injector, or an EpiPen, which treats anaphylaxis. Since subcutaneous tissue contains blood vessels, medications can be immediately absorbed, but its high-fat content also allows medications to be absorbed gradually over time.

The entire body contains subcutaneous tissue; some parts more than others. Parts of the body that have greater concentrations of subcutaneous tissue make them ideal injection sites. These injection sites include:

  • The outer part of the upper arm
  • The middle part of the abdomen
  • The front of the thigh
  • The upper back
  • The upper part of the buttocks
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