Vitamin A is a fat-soluble vitamin which does not occur in nature by itself but which is formed from yellow substances (carotene) occurring in plants, by the walls of the intestines and in the liver and kidneys. The liver is the main storage depot for Vitamin A; thus it is found in such high concentration in fish-liver oils. It has several known functions in the human body: It is essential for maintaining the mucous membranes of the mouth, nose, throat, lungs, ears and other body organs in healthy condition; it is necessary for healthy skin, bones and teeth and normal growth; it plays a part in reproduction and lactation; and it is indispensable for maintaining good vision, especially in dim light. It combines with protein to form a pigment in the retina of the eye called visual purple. Light—whether artificial light or sunlight—reaching the eyes breaks down part of this pigment, setting up nerve impulses which inform the brain of what is seen. The formation and breakdown of visual purple is a continuous cycle whose efficiency is dependent on an adequate supply of Vitamin A.

The earliest sign of vitamin A deficiency is usually night blindness. (A person who, when driving a car at night finds it difficult to see the road clearly after being "blinded" by oncoming lights, is suffering from a degree of night blindness.) Mild deficiencies result in roughness and dryness of the skin, especially the elbows, knees, and buttocks. The hair becomes dry and the fingernails may become ridged. A degree of night blindness may appear. More sever deficiencies lead to inflamed eyelids, eyestrain, frequently occurring sties, the drying up of the various mucous membranes of the throat, ears, and lungs and disturbances in digestion. Very severe deficiencies may lead to growth retardation in children and eye diseases severe enough to result in blindness. Deficiencies are caused by factors other than inadequate. Those whose diets are low in fat frequently are deficient in Vitamin A.

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Wikipedia.org: Vitamin A Deficiency
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