Physiology of the organ of vision

The physiology of the organ of vision examines the functional processes occurring in the eye. This section contains information about the interaction of water and oxygen for the normal functioning of the cornea and tear film, the role of glucose (a nutrient for cells) and salts in eye metabolism.

Water balance

The normal water content in a healthy cornea is 78%. The cornea contains biophysical systems that maintain this level constant. The equilibrium ratio of water/ions (dissolved salts) between the tear fluid and the cornea is called osmolarity or tonicity. With an isotonic tear film, the cornea maintains normal thickness, since the amount of water entering and leaving the cornea is the same.

From a physiological point of view, the process of maintaining balance — exchange of water and salt molecules between the cornea, tear and intraocular fluids. The optimal equilibrium ratio is achieved when the tear film contains 0.9% salts (tonicity). This is why most contact lens solutions have the same tonicity: the lenses do not change this equilibrium ratio when they are in the eye. When the concentration of salts in the tear film exceeds 0.9%, a hypertensive state develops.

In this case, water flows from the cornea into the tear fluid and dehydration or wrinkling of the cornea occurs. The epithelium acts primarily as a semi-permeable membrane and does not affect the passive diffusion of water.

When the salt concentration in the tear film is below 0.9%, a hypotonic state occurs and water enters the cornea, which swells.

The main function of the endothelial layer — maintain cornea cleanliness. Ion “pump” controls the flow of water and ions from and into the cornea so that the water content remains at 78%. Glucose is the main source of energy for this pump.

Glucose is found in our food. It is mainly delivered to the endothelium by the intraocular fluid, but first it is necessary for glucose to be metabolized or broken down into energy units (ATP - adenosine triphosphate). Glucose metabolism occurs in one of two biochemical pathways: anaerobic or aerobic. In anaerobic glycolysis (which means “without oxygen”), one molecule of glucose produces two units of energy (ATP).

This amount of energy is not enough for the “pump” to work effectively, and therefore water accumulates in the cornea, causing it to swell (edema). Aerobic glycolysis (metabolizing glucose “with oxygen”) is much more efficient. During aerobic metabolism, one molecule of glucose produces 36 units of energy (ATP). This is a sufficient amount of energy to maintain the endothelial “pump” the necessary balance of water in the cornea.

The tear film is the main supplier of nutrients — oxygen, glucose, salts and minerals — into the cornea. Oxygen from the air is contained in the tear film in a dissolved state. Without contact lenses, up to 21% of the total oxygen in the air can enter your open eyes. With eyes closed and without lenses (during sleep), the amount of oxygen decreases to 7%, and oxygen mainly comes by diffusion from the blood vessels of the conjunctiva. Contact lenses make it much more difficult for oxygen to enter the eye. This is not significant when the eye is open. However, when your eyes are closed, for example while sleeping with contact lenses for long-term wear, vital processes in the cornea may decrease.

Cellular metabolism of glucose begins to proceed along the less efficient anaerobic pathway. The ion “pump”, which is responsible for the water content in the cornea, begins to work with voltage. As a result, corneal edema may develop. When selecting lenses, doctors usually take into account and fully trust the oxygen consumption indicator. They often prefer lenses that maximize oxygen supply during sleep. In general, the oxygen permeability of lenses is higher the more water they contain and the thinner they are. Hard lenses allow only 1-3% of the total oxygen available to the cornea to pass through. In fact, these lenses are impermeable to oxygen and the indicated 1-3% comes with tear fluid penetrating under the lenses and from the blood vessels of the limbus.

Ion “pump”

Ion “pump” The endothelial layer of the cornea plays a major role in maintaining the water balance in the eye. Metabolized glucose serves as fuel for the endothelial “pump.”

Glucose can be metabolized anaerobically (without oxygen) or aerobic (with oxygen) methods. For the metabolism of glucose in the amount determined by the work of the endothelial “pump”, oxygen is required. If glucose metabolism is slower than required to produce the amount of energy required by the pump, the water balance in the cornea is disrupted and edema develops.

Contact lenses reduce the supply of oxygen to the eye. Because oxygen is so important for normal vision, doctors usually prefer contact lenses that are as permeable to oxygen as possible. The oxygen consumption of contact lens material can be determined in several ways. The most commonly used tests are dK (oxygen permeability) and dK/L (oxygen permeability).