The digestion of food in the stomach is only the first step in delivering important nutrients to living cells, which need energy to operate and nutrients to build more cells. The main system of transporting nutrients to the body's cells is through the blood stream. But once a nutrient arrives at a needy cell, it is still inside a blood vessel while the cell lies outside. The lack of nutrients inside the cell, and between the cell and the blood vessel, creates a concentration gradient between the blood vessel and the cell. Due to the lower concentration in the cell, the nutrient diffuses through the blood vessel wall and into the cell.
Every organ in the body needs oxygen in order to survive, and the eyes are no different. However, the eyes lack a great number of blood vessels (which carry oxygen that diffuses into cells by the process above), and must have an extra supply of oxygen. The atmosphere provides the extra needed oxygen, which is consumed by the eye by the diffusion of oxygen through the cornea, the hard outer covering on the eye. In places with a very dirty atmosphere, oxygen concentration may be low, and the eye can dry out. You may have noticed this if you have ever been in a dust storm or had smoke from a fire blowing into your eyes.
From severe illness to a common headache, medicines are universally used to alleviate pain or cure sickness around the world. For medicines taken orally as pills, the medicine must somehow find its way into the bloodstream. Once in the stomach, if the pill capsule is a time release mechanism, the medicine must first diffuse out of the capsule. Once in the stomach, the medicine from the pill is absorbed into the lining of the stomach and then into the bloodstream, both processes of diffusion. For some medicines, the capsule cannot provide enough long term dosage, or the medicine does not last long enough because it is absorbed too quickly. Another possibility is the IV, but for most people, the IV is not quite a feasible day-to-day method of medicine intake. A solution to this problem, one that has seen increasing use over the past few years is the transdermal patch.
The transdermal patch consists of an adhesive layer that attaches it to the skin, and a reservoir that holds the medicine. The medicine must first diffuse out of the reservoir and onto the skin, and then through the skin and into the bloodstream. Since diffusion through the skin is a much slower process than diffusion through the stomach lining and into the bloodstream, and, since the patch reservoir is capable of holding a greater quantity of medicine than a pill capsule, the transdermal patch offers a method for increased dosage over a prolonged period of time. Click here for a pictorial representation of a transdermal patch.
