The cornea is the transparent outer layer of the human eye. Because it must be transparent to light, it does not normally contai
n blood vessels. Therefore, it must receive its nutrients via diffusion. Oxygen from the surrounding air diffuses to the cornea through the surface tears whereas other nutrients diffuse to the cornea from the inner parts of the eye, such as the vitreous humor and lens. During operation, the cornea produces waste in the form of CO2 gas that must be expelled to keep the eye healthy and functioning. This is accomplished by the simultaneous diffusion of CO2 from the cornea to the surrounding atmosphere, which generally features a low CO2 concentration.
It is therefore critical that modern contact lens materials allow sufficient diffusion rates of oxygen and carbon dioxide. Without oxygen, the cornea will warp, loose transparency, and become susceptible to scarring. The body may also react by growing additional blood vessels into the eye, which can damage the cornea.
If an increased steady-state flow rate of O2 (oxygen molecules per second) to the cornea is desired, which of the following contact lens / ambient condition modifications is not likely to be useful?
Note: the flow rate is equal to product of the diffusion flux and an area of interest through which diffusion occurs.
(a) Increase the contact lens thickness
(b) Increase the diffusivity of oxygen gas by increasing the contact lens porosity
(c) Increase the ambient temperature
(d) Increase the ambient partial pressure of oxygen gas
(e) All of the suggestions (a-d) are useful for increasing the flow rate of oxygen
the correct answer is b, since increasing the porosity of the contact lens allows it to work as a good method of gas exchange, simulating the lung membranes that present fenestrated capillaries that promote gas exchange. For this to happen, the permeability must always be high and therefore this is directly related to the porosity of the element with which the contact lens is made.
During the summer months, the water table tends to fall, due in part to plants taking up water from the soil surface before it can reach the water table. The water table level is also influenced by human extraction of groundwater using wells; groundwater is pumped out for drinking water and to irrigate farmland.
DNA structure. DNA is made up of molecules called nucleotides. Each nucleotide contains a phosphate group, a sugar group and a nitrogen base. The four types of nitrogen bases are adenine (A), thymine (T), guanine (G) and cytosine (C).
