Fatty acids - long straight chains of carbon and hydrogen ending with an acid group at one end. Saturated fatty acids have no capacity to absorb more hydrogen atoms. Animal fats are mostly of this type and are considered less healthy as they end to accumulate in the linings of arteries.     Feedback - a system of control, where work being done is modified by the product. For example the blood pressure is maintained by the strength of the heart beat and the muscle tone of the arterioles. In the walls of the large arteries are receptors sensitive to the degree of stretch in the muscle wall. As the blood pressure increases, the wall are stretched, and the receptor sends signals via the brain to the sympathetic nervous system back to the heart and blood vessels, causing decreased pumping effort and more relaxed muscle tone in the arteries. In chemical reactions the accumulated product slows down the rate of production. For example if the oxygen level of the body falls, the rate of respiration.increases to restore the levels to normal. These control system are thus circular; what is produced returns to control the further production. They are examples of negative feedback, and are common in maintaining stability or homeostasis. Positive feedback is less common as it tends to be unstable. An example is the release by platelets of thrompotaxin. When the levels of thrombotaxin are high, they do not inhibit further production as occurs in a negative feedback system but actually stimulate more platelets to produce more thrombotaxin and so on until there is an explosive increase in the number of sticky platelets. This is useful in an emergency to s bleeding, but very dangerous when a clot forms inside a blood vessel.     Fibre - a long thin string-like structure constructed of smaller fibrils and even smaller microfibrils. Examples are collagen, elastic and keratin fibres. Collagen fibres are arranged parallel to each other in a tendon, to give it great resistance to tension (pulling).     Fibrinogen - a large soluble protein found in blood which is converted into fibrin during blood clotting.     Fibroblasts - cell of connective tissue which form both the intercellular matrix and fibres.     Fibronectin - a glycoprotein which is found in the extracellular matrix and is important for the attachments, and therefore the movement of cells.     Filamentous - long, thin, hair-like.     Fluorapatite- an apatite crystal in which fluoride has replaced hydroxyl ions.     Fluorosis - mottling of the teeth caused by an excess of fluoride in the drinking water. A fluorosis index recognises 4 stages of severity.     Foramina - the plural of foramen, which is a hole, for example Foramen Ovale.     Fossils - dead plant or animal remains whichhave become infused with minerals over many millions of years and are now hard and rock -like. The original shape of the animal or plant may be very well preserved.     Fractal dimension - a dimension which is some fraction in between a line(1) and a plane(2), or a plane and a solid (3). These fractal dimensions are useful in describing the quality of natural lines and surfaces, such as coastlines, trees, vascular branching and the patterns of trabecula bone     Freeway space - the space between the teeth when the jaw is in a rest position.