You're asking for the one thing that is NOT a function of human skin. Here are some of the key functions of skin:

* Protection: Skin is a barrier against bacteria, viruses, and other harmful substances.

* Regulation of temperature: Skin helps to regulate body temperature through sweating and blood flow.

* Sensation: Skin contains sensory receptors that allow us to feel touch, pressure, pain, heat, and cold.

* Excretion: Skin helps to excrete waste products through sweat glands.

* Vitamin D synthesis: Skin produces vitamin D when exposed to sunlight.

Let's consider some options that are NOT functions of human skin:

* Digestion

* Respiration

* Blood filtration

Let me know if you need more clarification!