The hair shaft (remember this is the visible part of the hair) is made up of 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 most soft and fragile, and serves as the pith or marrow of the hair. However, the medulla layer is not found in all hair types, it is normally found only in coarser hair types. People with naturally blonde hair and/or fine hair do not generally have a medulla layer. The cortex of the hair shaft 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 in layers, which form scales that strengthen and protect the hair shaft.
The texture of our hair is determined by the size of our individual hair follicles. Think of the follicle as a kind of hair extruder, the larger the follicle, the coarser the hair will be that it extrudes. Hair texture is usually 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 individual hair follicles as well as the number of active hair follicles in our scalp determines the thickness or volume of our hair 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 you minimize hair breakage you can have a very thick, full head of hair. It is important 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. The good news is, however, 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 over time our hair will grow more thick.
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 really 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.