Page 231.6 Homeostasis, Health, and DiseaseLEARNING OBJECTIVE1. Explain the general relationship of maintaining homeostasis to health and disease.In summary, homeostasis is a term that describes the many physiologic processes to maintain the health of the body. These characteristics are noted about homeostatic systems:- They are dynamic.- The control center is generally the nervous system or the endocrine system.- There are three components: receptor, control center, and effector.- They are typically regulated through negative feedback to maintain a normal value or set point.- It is when these systems fail that a homeostatic imbalance or disease results, ultimately threatening an individual's survival.Diabetes is an example of a homeostatic imbalance. Diabetes occurs when the homeostatic mechanisms for regulating blood glucose are not functioning normally, and blood glucose fluctuates out of the normal range, sometimes resulting in extremely high blood glucose readings. High blood glucose results in damage to anatomic structures throughout the body. Patients with diabetes must rely on other methods, such as diet restriction, exercise, and perhaps a medication to lower blood glucose.Sometimes a homeostatic imbalance results when critical changes from aging or disease cause a variable that is normally controlled by negative feedback, to be abnormally controlled by positive feedback. An example is when there is extensive damage to the heart, perhaps from a heart attack. This heart is less able to pump blood to the structures of the body including the heartitself. Consequently, the heart receives reduced amounts of nutrients and oxygen. The heart becomes progressively weaker, and even less able to pump blood to the body's structure. Ultimately, the heart becomes so weak that the heart stops beating.Treating patients generally involves determining a diagnosis, or the specific cause of the homeostatic imbalance. Once diagnosed, the patient is treated through the administration of medications or through other therapeutic avenues to facilitate the body in maintaining homeostasis.Health-care practitioners also need to understand how the drugs patients are taking may affect the normal homeostatic control mechanisms. For example, one type of medication for the treatment of depression is an SSRI, which stands for selective serotonin reuptake inhibitor. Paroxetine (Paxil), fluoxetine (Prozac), and sertraline (Zoloft) are examples of SSRIs. Serotonin is a type of neurotransmitter. Normally, a neurotransmitter is released from one nerve cell in response to a nerve impulse. The neurotransmitter accomplishes its communication task, and then is taken up again by the nerve cell for future use. Some depressed individuals may have lower levels of serotonin, so an SSRI blocks the reuptake of serotonin into the nerve cell.Therefore, serotonin stays outside the nerve cell for a longer period of time and its effects are prolonged, which may elevate the mood of the patient taking the SSRI.However, like all drugs, SSRIs come with some drawbacks. Some SSRI side effects include digestive system distress, such as nausea, upset stomach, diarrhea, or combinations of all three. As it turns out, serotonin is also used in the nerve cells of the digestive system. By tinkering withthe serotonin reuptake in the brain, the drug also affects serotonin reuptake in the digestive system. Essentially, the digestive system becomes a bit more excitable due to the intake of the SSRI drug, with the symptoms just described.Virtually all medications have some benefits and some side effects, many of which can be explained by examining the homeostatic control mechanisms with which they interact. Thus, an understanding of these mechanisms is a must for anatomists, physiologists, and health-care practitioners.WHAT DID YOU LEARN?What is an example of a disease process by which homeostasis is disrupted?Page 24CLINICAL VIEWMedical ImagingHealth-care professionals have taken advantage of sophisticated medical imaging techniques to extend their ability to visualize internal body structures noninvasively (e.g., without inserting an instrument into the body). Some of the most common techniques are radiography, sonography, computed tomography, digital subtraction angiography, dynamic spatial reconstruction, magneticresonance imaging, and positron emission tomography.RadiographyRadiography Radiography (rā-dē-og′ra-fē; radius = ray, grapho = to write) is the primary method of obtaining an image of a body part for diagnostic purposes. A beam of x-rays, a form of high-energy radiation, penetrates solid structures within the body. X-rays can pass through softtissues but they are absorbed by dense tissues, including bone, teeth, and tumors. Film images produced by x-rays passing through soft tissues leave the film lighter in the areas where x-rays are absorbed. Hollow organs can be visualized if they are filled with a radiopaque (rā-dē-ō-pāk; opacus = shady) substance that absorbs x-rays.Radiograph (x-ray) of the head and neck.The term x-ray also applies to the photograph (radiograph) made by this technique. Radiography is commonly used in dentistry, mammography, diagnosis of fractures, and chest examination. Disadvantages of x-rays are that they are difficult to interpret when organs overlap in the images,and they are unable to reveal slight differences in tissue density. In addition, the radiation of an x-ray is not without risk.SonographySonography The second most widely used imaging method is sonography (sŏ-nog′ră-fē; sonus =sound; grapho = to write), also known as ultrasound. A technician slowly moves a small, hand-held device across the body's surface. This device produces high-frequency ultrasound waves and then receives signals that are reflected from internal organs. The image produced is called a sonogram. Sonography is the method of choice in obstetrics, where a sonogram can examine the fetus and evaluate fetal age, position, and development. Sonography avoids the harmful effects of x-rays, and the equipment is inexpensive and portable. Until recently, its primary disadvantagewas that it did not produce a very sharp image, but technological advances are now markedly improving the images produced.Sonogram of a fetus.When radiography or sonography fail to produce the desired images, other more detailed but much more expensive imaging techniques are available.Computed Tomography (CT)Computed Tomography (CT) A computed