The cardiovascular system and the lymphatic system form what is collectively called the circulatory system. Together, these systems transport oxygen, nutrients, cell wastes, hormones, and many other substances to and from all cells in the body.
The foods we eat—apples, pepperoni pizzas, leafy green salads—taste good to us, but cannot be used by the body as they are. The nutrition the cells of the body need to keeping growing and working must be in a simple form: amino acids, simple sugars, and fatty acids.
The endocrine system is the body's network of glands that produce more than fifty different known hormones or chemical messengers to maintain and regulate basic bodily functions. It is second only to the nervous system as the great controlling system of the body.
The integumentary system, formed by the skin, hair, nails, and associated glands, enwraps the body. It is the most visible organ system and one of the most complex.
The lymphatic system is often considered part of the cardiovascular system (see chapter 1). Excess fluid that leaks out of capillaries to bathe the body's cells is collected by the vessels of the lymphatic system and returned to the blood.
The muscular system is the body's network of tissues that controls movement both of the body and within it. Walking, running, jumping: all these actions propelling the body through space are possible only because of the contraction (shortening) and relaxation of muscles.
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