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Human Physiology and Homeostasis 16 Cells Are the Living Units of the Body 17 Body Organization: from cells to systems. 18 Body Organization: from cells to systems. 19 Body Organization: from cells to systems. 20 Body Organization: from cells to systems. 21 The “Internal Environment” 22 About 60 percent of the adult human body is fluid, mainly a water solution of ions and other substances. Although most of this fluid is inside the cells and is called intracellular fluid, about one third is in the spaces outside the cells and is called extracellular fluid. In the extracellular fluid are the ions and nutrients needed by the cells to maintain life. Thus, all cells live in essentially the same environment—the extracellular fluid. For this reason, the extracellular fluid is also called the internal environment of the body. 23 Cells are capable of living and performing their special functions as long as the proper concentrations of oxygen, glucose, different ions, amino acids, fatty substances, and other constituents are available in this internal environment. 24 Homeostasis—Maintenance of a Nearly Constant Internal Environment 25 Homeostasis Homeostasis refers to the dynamic mechanisms that detect and respond to deviations in physiological variables from their “set point” values by initiating effector responses that restore the variables to the optimal physiological range. 26 Essentially all organs and tissues of the body perform functions that help maintain these relatively constant conditions. For instance, the lungs provide oxygen to the extracellular fluid to replenish the oxygen used by the cells, the kidneys maintain constant ion concentrations, and the gastrointestinal system provides nutrients. 27 28 Homeostasis is a dynamic, not a static, process. • Physiological variables can change dramatically over a 24-hr. period, but the system is still in overall balance. • When homeostasis is maintained, we refer to physiology; when it is not, we refer to pathophysiology. 29 Blood glucose levels increase after eating. Levels return to their set point via homeostasis. This is an example of dynamic constancy. Levels change over short periods of time, but remain relatively constant over long periods of time. 30 Interpret the arrows in textbook’s flow charts as “leads to” or “causes.” (e.g., decreased room temperature causes increased heat loss from the body, which leads to a decrease in body temperature, etc.) 31 System Controls • Feedback loops or systems are a common mechanism to control physiological processes. • A positive feedback system enhances the production of the product. • A negative feedback system shuts the system off once the set point has been reached. 32 Most control systems of the body act by negative feedback “Active product” controls the sequence of chemical reactions by33inhibiting the sequence’s rate-limiting enzyme, “Enzyme A.” Type of Signals • Hormones are produced in and secreted from endocrine glands or in scattered cells that are distributed throughout another organ. Hormones travel through the blood to their target cells. • Neurotransmitters are chemical messengers that are released from the endings of neurons onto other neurons, muscle cells, or gland cells. 34 35 36 Each functional structure contributes its share to the maintenance of homeostatic conditions in the extracellular fluid, which is called the internal environment. As long as normal conditions are maintained in this internal environment, the cells of the body continue to live and function properly. Each cell benefits from homeostasis, and in turn, each cell contributes its share toward the maintenance of homeostasis. This reciprocal interplay provides continuous automaticity of the body until one or more functional systems lose their ability to contribute their share of function. When this happens, all the cells of the body suffer. Extreme dysfunction leads to death; moderate dysfunction leads to sickness. 37 Review of Cellular Biology 38 39 Organization of the Cell 40 The major parts of a cell are the nucleus and the cytoplasm. The nucleus is separated from the cytoplasm by a nuclear membrane, and the cytoplasm is separated from the surrounding 41 fluids by a cell membrane, also called the plasma membrane. The different substances that make up the cell are collectively called protoplasm. It is made of 5 substances: 1. Water constitutes 70 percent to 85 percent of most cells. 2. Ions/electrolytes provide inorganic chemicals for cellular reactions. Some of the most important ions in the cell are potassium, magnesium, phosphate, sulfate, bicarbonate, and small quantities of sodium, chloride, and calcium. 3. Proteins normally constitute 10 to 20 percent of the cell mass. They can be divided into two types: structural proteins and globular (functional) proteins, which are mainly enzymes. 42 4. Lipids constitute about 2 percent of the total cell mass. Among the most important lipids in the cells are phospholipids, cholesterol, triglycerides, and neutral fats. In adipocytes (fat cells), triglycerides account for as much as 95 percent of the cell mass and represent the body’s main energy storehouse. 5. Carbohydrates play a major role in nutrition of the cell. Most human cells do not store large amounts of carbohydrates, which usually average about 1 percent of the total cell mass but may be as high as 3 percent in muscle cells and 6 percent in liver cells. The small amount of carbohydrates in the cells is usually stored in the form of glycogen, an insoluble polymer of glucose. 43