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Plant and animals vary tremendously in their water requirements. Intake increases with size and, interestingly enough, in a mathematical way. Also of interest is the time required for a given member of a species to imbibe a quantity of water equal to its own weight. A mouse take 5 days; a cow, 2 weeks; a camel, 3 months; a tortoise, 1 year; a human being 1 month;and a cactus, 29 years.

Regardless of the amount of water taken in, however, the body normally balances the gain with an equivalent loss. Conversely, the body balances a loss with an equivalent gain. Either way, the central idea is balance. Humans possess phenomenal balance, in a 24 hour period a person's weight may vary less than 250 g.

The respiratory tract includes the nose, pharynx (throat), larynx (voice box), trachea (windpipe), bronchus, and lung in this order. Lining the tract is a special ciliated epithelium. The nasal cavities are fashioned into tortuous passageways, and the trachea and primary bronchi are composed of cartilaginous rings strung together by connective tissue and smooth muscle. We also find smooth muscle throughout the small bronchi and bronchioles.

The terminal bronchioles enter alveolar ducts, and these in turn enter alveolar sacs, each of which resembles a bunch of hollow grapes. Each "grape" of the sac, continuing with the analogy, is called an alveolus, and because the alveolar wall is only one cell thick, gases easily diffuse across it between the air within and the blood in the surrounding capillaries. The terminal bronchioles, the alveolar sac, and the associated vessels together make up what is called a pulmonary unit; millions of these units compose the substance of the lungs.

The skeletal muscle make up the bulk of the human body. They are the muscle masses that  attach to bones for the purpose of movement and posture. Muscles vary greatly in size and shape, ranging from the tiny stapedius of the middle ear to the bulky gluteus maximus of the buttock. The typical muscle has a main part (called the body) and two extremities ( called the body), both of which are attached to bone or cartilage. When such an attachment terminates as a strong white cord, we call it a tendon.

By convention, the origin is the extremity attached to the stationary bone (during contraction), and the insertion is  the extremity at the movable end. In not a few instances, however, the origin and insertion are interchangeable, depending on the particular movement. The muscle whose contraction actually produces the desired movement is called  the agonist, whereas the muscle that concomitantly relaxes ( to permit the movement) is referred to as the antagonist. Only rarely is a single muscle responsible fo a given movement, and for the sake of smoothness the agonist is aided by neighboring muscle called synergist. For instance, in extending the leg four large muscles are called into play.