Friday, December 10, 2010


                                   Homeostasis, or the regulation of an organism’s internal environment is necessary to maintain conditions suitable for life.  The internal equilibrium of the body is the ultimate gauge of its proper function.  Homeostasis involves the maintenance of a consistent range in the concentration of certain molecules in the body. Disruption of homeostasis in the body systems can make an organism susceptible to disease and possibly lead to death. Body temperature and hormone levels are examples of mechanisms that are regulated by the body to maintain homeostasis. Homeostasis encompasses many body processes. We probably think of maintaining a constant body temperature, but homeostasis also includes water balance, which is influenced by the amount of water in the external environment and whether it is fresh or salt water. Water balance is regulated through such things as thirst and urination. Other factors which are under regulation include internal salt concentration, pH (Despite the fact that cellular respiration creates CO2 which dissolves in our blood to make carbonic acid, the pH of our blood is buffered at 7.4 or we would die.), nutrients and various chemicals (regulated by factors like blood sugar level, feelings of hunger, or cravings for certain foods). Homeostasis is controlled by feedback loops (positive and negative), most of which are negative feedback loops. An example of a positive feedback loop (this particular one is not involved in homeostasis) is the process of giving birth to a baby. Labor contractions push the baby against the cervix causing the cervix to dilate. This, in turn, triggers the production of oxytocin, a hormone which triggers stronger contractions. A negative feedback loop works in the opposite direction from what it is trying to accomplish. An example of a negative feedback loop involved in homeostasis is maintenance of body temperature. As a person’s body gets too hot, he begins to sweat in an attempt to lower the temperature. If someone’s body is too cool, he will begin to shiver in an attempt to increase the temperature.

The systems of the body cooperate in maintaining homeostasis, that is, the relative constancy of the internal environment despite external environmental changes. The circulatory system is critical to the internal environment in that tissue fluid is nourished and purified by the movement of small molecules across capillary walls. The digestive system contributes nutrients to the blood, while the excretory system removes wastes. The respiratory system takes in oxygen and excretes carbon dioxide. Oxygen is used during cellular respiration and carbon dioxide is a waste product of cellular respiration. The nervous and endocrine systems exert the ultimate control over homeostasis because they coordinate the functions of the body's systems. Main examples of homeostasis in mammals are as follows:
  • The regulation of the amounts of water and minerals in the body. This is known as osmoregulation. This happens primarily in the kidneys.
  • The removal of metabolic waste. This is known as excretion. This is done by the excretory organs such as the kidneys and lungs.
  • The regulation of body temperature. This is mainly done by the skin.
  • The regulation of blood glucose level. This is mainly done by the liver and the insulin and glucagon secreted by the pancreas in the body.
These hormones associated with the regulation of blood glucose are considered antagonistic because their actions have opposite effects; an increase in glucose concentration following glucagon secretion is counteracted by an insulin secretion.

The overall effect of a disruption in the body’s internal environment is disease. Disease is a change that disrupts homeostasis in the body. Heart disease is the leading cause of death in the United States and is a major cause of disability. Almost 700,000 people die of heart disease in the United States annually. That is about 29% of all U.S. deaths. Heart disease and strokes are common cardiovascular diseases. They are the third and first top cause of death for both genders. The most common heart disease in the United States is coronary heart disease, which often appears as a heart attack. Cancer is the second leading cause of death in the United States. Cancer refers to any one of a large number of diseases characterized by the development of abnormal cells that divide uncontrollably and have the ability to infiltrate and destroy normal body tissue. Cancer can spread throughout your body. Cancer doesn't discriminate when it comes to race, sex or age anyone can get cancer. The American Cancer Society estimates that half the men and one-third of the women in the United States will develop cancer in their lifetimes. The American Cancer Society estimates that about 1.4 million new cases of cancer are expected in 2007, and about 560,000 people will die of the disease.

Disease-producing agents such as bacteria, protozoans, fungi, viruses and other parasites are called pathogens. The main sources of pathogens are soil, contaminated water, and infected people or animals. Any disease caused by the presence of pathogens in the body is called an infectious disease. One-half of all human diseases are infectious. Not all diseases are caused by pathogens. Some diseases can be inherited, such as sickle cell anemia or be due to body aging (wear and tear) like osteoarthritis. Pathogens can be transmitted in four main ways. The first way is through direct contact, for example  STD’s and influenza are easily spread through contact. The second way is through food or drink contamination which can result in poisoning, Salmonella, Botulism and E-coli are some common examples of food contamination. Some disease-causing germs travel through the air in particles considerably smaller than droplets. These tiny particles remain suspended in the air for extended periods of time and can be carried by air currents. If you breathe in an airborne virus, bacterium or other germ, you may become infected. Tuberculosis and SARS are two infectious diseases usually spread through the air, in both particle and droplet forms. The final way that someone is susceptible to disease is through intermediate organisms (vectors), for example, malaria which is spread by mosquitoes. 

Diseases can be classified into two categories, endemic and epidemic. Endemic diseases are diseases that are constantly present in a population like “the common cold”. Epidemic disease occurs when many people in a given area are afflicted with the same disease in a short period of time. A very prominent epidemic occurred when people became infected with polio in the 1950’s. There are more common diseases that affect us in today’s world to which polio has fallen into the shadows of, and that is the common flu virus. Seasonal (or common) flu is a respiratory illness that can be transmitted person to person. Most people have some immunity, and a vaccine is available. Avian (or bird) flu (AI) is caused by influenza viruses that occur naturally among wild birds. Low pathogenic AI is common in birds and causes few problems. H5N1 is highly pathogenic, deadly to domestic fowl, and can be transmitted from birds to humans. There is no human immunity and no vaccine is available. At the time of this writing H5N1 does not seem to spread easily from person to person. Pandemic flu is virulent human flu that causes a global outbreak, or pandemic, of serious illness. Because there is little natural immunity to that “pandemic” strain of flu virus, the disease can spread easily from person to person. The influenza pandemic of 1918-1919 killed more people than the Great War, known today as World War I, at somewhere between 20 and 40 million people. It has been cited as the most devastating epidemic in recorded world history. More people died of influenza in a single year than in four-years of the Black Death, or Bubonic Plague, from 1347 to 1351. Known as "Spanish Flu" or "La Grippe" the influenza of 1918-1919 was a global disaster. 
Carriers are people who harbor a disease without showing any signs, yet they can pass the disease on to others. Until 1876 it was quite difficult to determine the cause of a disease, in that year Robert Koch provided definitive proof of the germ theory by isolating the cause of anthrax and showing it to be a bacterium. From this came the development of Koch's Postulates, a set of rules for the assignment of a microbe as the cause of a disease.
1. The specific organism should be shown to be present in all cases of animals suffering from a specific disease, but should not be found in healthy animals.
2. The specific microorganism should be isolated from the diseased animal and grown in pure culture on artificial laboratory media.
3. This freshly isolated microorganism, when inoculated into a healthy non-immune laboratory animal, should cause the same disease seen in the original animal.
4. The microorganism should be re-isolated in pure culture from the experimental infection.
This is his most famous contribution to science and it is a testament to the utility of these postulates that they are stilled used today to discover the cause of new emerging diseases. Koch went on to apply these principles in the study of many other diseases including tuberculosis, cholera and sleeping sickness. It should be pointed out that Koch’s postulates cannot be applied to all diseases. Also, it is not always possible to obtain a disease-causing microbe in pure culture. Koch developed the tools for obtaining pure cultures to attack the problem of disease. Advances in science often come from innovations in the available technology. Robert Koch was an important microbiologist because his pioneering work in the isolation and characterization of bacterial diseases helped to identify the causes of many of the maladies plaguing humanity. Further work by other scientists then began the long road to conquering them.