The hair shaft (remember, this is the visible part of the hair) comprises three layers; the innermost layer is called the medulla, the middle layer the cortex, and the outer layer the cuticle. The medulla, the inner layer of the hair shaft, is the honeycomb-like core of the hair structure. This nearly invisible layer is the softest and most fragile and serves as the pith or marrow of the hair. However, the medulla layer is not found in all hair types, and it is typically found only in coarser hair types. People with naturally blonde or fine hair do not generally have a medulla layer. The hair shaft's cortex is located between the outer hair cuticle and the inner medulla and is the thickest hair layer. The hair cuticle is the outermost part of the hair shaft. It is formed from cells overlapping layers, forming scales that strengthen and protect the hair shaft.
The size of our hair follicles determines the texture of our hair. Think of the follicle as a kind of hair extruder; the more significant the follicle, the coarser the hair will be that it extrudes. Hair texture is described as either fine, medium, or coarse. Fine hair has the smallest circumference and coarse hair, as the name implies, has the largest circumference, with medium hair falling between these two textures.
The size of our hair follicles and the number of active hair follicles in our scalp determine our hair's thickness or volume, which can either be thin, normal, or thick. Therefore, it is possible to have fine hair, but if you have lots of healthy follicles and minimize hair breakage, you can have a very thick, full head of hair. It is essential to realize that hair care products can not do anything (other than perhaps coat your hair artificially, yuck) to make your hair miraculously thick and full. However, the good news is that we can improve the health of our scalp and its army of follicles, and we can reduce breakage by using well-formulated products and treatments so that our hair will grow thicker over time.
The final takeaway is that when we think about our hair, it is best to consider the texture (the size and shape of each strand) and the volume separately. You can not change the texture of your hair, but you CAN increase the volume of your hair over time by taking care of your scalp and by using products, care routines, and treatments that minimize breakage.