What Does the Term Metabolism Mean

Each and every second of every day our cells are engaged in the operations that help keep them alive and well. At the same time the efforts of each cell also contribute to the proper functioning of our body as a whole. To do so each cell must perform an incredible number of chemical reactions every second. The term metabolism refers to those chemical reactions collectively.

The term metabolism is somewhat general. For instance, total body metabolism refers to all the energy released from all the chemical reactions and associated processes in our body. Said differently, total body metabolism is the total of all reactions taking place in each cell added together. However, if we wanted to describe just those chemical reactions within a specific tissue, such as muscle or bone, we would say "muscle metabolism" or "bone metabolism." We can be even more focused and use the term metabolism to describe only those reactions associated with a single nutrient or nutrient class. For example, if we were discussing the

How Our Body Works 27 Table 2.3 Primary Functions of the Major Tissue and Organs in Our Body

Bone Provides structure and the basis of movement of limbs and our entire body. Also serves as a mineral storage. Primarily composed of minerals and protein and smaller amount of cells, nerves and blood vessels.

Skeletal muscle We have three kinds of muscle (skeletal, cardiac (heart) and smooth), which is largely water and protein and to a lesser degree carbohydrate and fat. Contraction of muscle results in movement of some type. Skeletal muscle is connected to bone and provides movement of our limbs and body.

Heart and blood Our heart is mostly muscle (cardiac). Contraction of cardiac muscle establishes the blood pressure in our heart, which drives blood through our blood vessels. We have about 100,000 miles of blood vessels and our blood is, for the most part, a delivery medium!

Smooth muscle Smooth muscle lines tubes in our body such as airways, blood vessels, digestive tract, reproductive tract, etc.) Smooth muscle is responsible for regulating the flow of content (gases, fluids, semi-solids) through those tubes.

Lungs Serves as the site of oxygen and carbon dioxide exchange between our body and the air around us.

Liver Perhaps the "hub" of nutrition. Our liver is involved in maintain blood glucose, regulating blood lipid levels, processing amino acids, making plasma proteins (e.g., clotting factors, transport proteins), and bile and metabolizing and storing many vitamins, minerals, and other nutrients.

Kidneys Regulate the composition of our body fluid. They do this by filtering and regulating the composition of our blood, which in turn regulates the composition of the fluid in-between our cells and inside of our cells.

Adrenal glands Our adrenals are steroid hormone producing factories. They produce cortisol (stress hormone), aldosterone, a lot of DHEA and lesser amount of androstenedione, testosterone, and estrogens.

Thyroid gland Produces the hormones thyroid hormone and calcitonin.

Thyroid hormone is one of the most influential hormones in regulating our energy expenditure.

Brain and spinal Our brain is an information processing center and the spinal cord cord is the conduit for signals to leave (or be carried to) our brain to the rest of our body. Our brain initiates and regulates muscle activity, processes sensory information and controls body temperature and appetite.

Skin Site of heat removal and protective coating. Some vitamin D is produced in our skin.

Pancreas Produces the hormones insulin and glucagon and digestive enzymes.

Pituitary gland Produces a slew of hormones including thyroid stimulating hormone (TSH) and adrenocorticotrophic hormone (ACTH).

chemical reactions that involve only proteins or carbohydrates, we would be discussing protein or carbohydrate metabolism, respectively.

In general, chemical reactions and/or pathways will release energy. Ultimately, this extra energy will be converted to heat. Since body temperature remains fairly constant, the heat produced in metabolism must be removed from our body. Therefore, our total body metabolism can be estimated by measuring how much heat is lost from our body. Researchers can do this in specialized laboratories as discussed in a later chapter.

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