Hair grows from the follicles; it’s the living part of the hair. The visible part, the hair shaft, is made up of dead, hard protein called keratin, so once the hair strand structure is damaged it won’t regenerate (heal back) by itself.
The hair growth phase, called anagen, lasts from two to six years. The longer the hair stays in the anagen phase, the faster and longer it will grow. While growing, the cells in one of the follicle’s parts (papilla) are dividing to produce new hair fibers. The same time follicle buries itself into the dermal layer of the skin to nourish the strand. About 85% of the hairs on one's head are in the anagen phase at any given time.
Hair falls out if the follicle has to rest to renew itself, and this process is called the catagen phase. This phase lasts about two weeks while the hair follicle shrinks due to disintegration cutting the hair strand off from its nourishing blood supply to make it go.
A healthy hair follicle needs to rest up to 4 months, and this process called the telogen phase.
The hair regrowth phase begins again once the hair follicle has got enough strength to produce new hair fibers. The lack of vitamins and vital minerals brought to hair follicles by blood circulation, poor blood circulation in scalp, mechanical or chemical scalp damage, all lead to hair thinning and slow hair regrowth.