Have you ever seen animals like crocodiles crawl their way up to the land and stay under the hot sun for a particular period of time? Well, when it comes to sun-bathing, not only humans, crocodiles enjoy it as well. However, reptiles like them do so solely because they need to. This behavioural response towards the surrounding temperature is essential as a mean of thermoregulation. Unlike crocodiles, we, humans, rely less on behavior and more on our physiological processes. Imagine that our body temperature soars every time we take a hot shower or drink a freshly brewed cup of coffee. Managing the state of internal environment is a principal challenge for the human body. The maintaining of internal environment of the body between limits is denoted as homeostasis, and body temperature is just one example of the many parameters which are controlled by homeostasis.
Figure 1 - An indication that homeostasis aims to provide a state of dynamic equilibrium.
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It is not an easy physiological process when it comes to maintaining homeostasis. As much as a unicellular organism needs to be able to take in oxygen and nutrients as well as to excrete waste products, multicellular organisms like humans also need to able to do those things. The mechanisms of homeostasis are complex enough to provide each cell with all that it needs. The integration of most of the systems in the human body leads to this particular purpose - homeostasis. A continuous bloodflow of nutrients must be adequately supplied. Vital organs such as the brain, kidney and heart need to have their activity monitored. The internal environment must always be in a relatively constant state, where the rate of exchange of cellular materials is done in such a manner that a dynamic equilibrium is considered.
There are many possible ways if we are to describe homeostasis. A good, simple one will probably be by using an analogy. Think of homeostasis as a scenario of someone walking up a descending escalator. When observing the person, he may seem to appear standing still if his speed when he walks up the escalator is the same as the speed of the escalator moving down. This is the case where an equilibrium is reached. Walking up faster than the escalator going down, the person will move up gradually. However, if he walks slower than the escalator, the result is going to be vice versa. In both cases, there is no equilibrium. It is only when the person's velocity of moving up is altered such that it is the exact opposite of the escalator moving down that equilibrium is restored. From this analogy, it is clear that homeostatic regulatory mechanism follows a particular pattern by which if a regulated variable increases, the system responds by making it decrease. In contrast, whenever the regulated variable decreases, the system reacts to make it increase. This manner of operation is referred to as the negative feedback mechanism.
Figure 2 - Negative feedback leads to a tight control situation whereby the corrective action taken by the controller forces the controlled variable toward the set point, thus leading the system to oscillate around equilibrium. (Source; http://controls.engin.umich.edu/wiki/index.php/Feedback_control)
Generally, in any feedback system, the level of a product feeds back to control the rate of its own production. A negative feedback mechanism works in a way that a change in levels always causes the opposite change, driving to a stabilizing effect. Both the nervous system and the endocrine system are both involved in monitoring the levels of variables. Small fluctuations above and below the set point will not usually result in a response. It is when the level rises significantly above or below the set point that it is altered by negative feedback accordingly. A homeostatic regulatory mechanism detects the regulated variables via the sensors. Sensors are basically cells which are sensitive to their corresponding variable. Certain blood vessels contain cells called chemoreceptors that are sensitive to concentrations of oxygen and carbon dioxide in the blood. Meanwhile, in the brain and other parts of the body, there are cells that are sensitive to temperature, and these cells are classified as thermoreceptors.
Flow of Information and The Concept of Homeostatic Set Point
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Sensors relay or transmit input/signals to the integrating center. The integrating center compares the regulated variable to the set point and orchestrates or coordinates the appropriate response. In response to the input it receives, the integrating center relays signals (now called outputs) to the targeted cells, tissues or organs that produce the final response. These cells, tissues or organs are entitled as effectors. The set point and normal ranges for homeostasis can change under various circumstances. One way in which the normal range of homeostasis may change is through acclimatization. This is when humans adjust to changes in the external environment. For instance, at high altitude, the partial pressure of oxygen at high altitude is lower than at sea level. Hemoglobin may not become fully saturated with oxygen as it passes through the lungs. As a result, body tissues may not have adequate supply of oxygen. Acclimatization will then occur when the body gradually ascends towards higher altitude, whereby extra erythrocytes are produced. Muscles produce more myoglobin and develop a denser capillary network.
As mentioned in the introductory paragraph, temperature is one of the variables of homeostasis. The process in which the internal temperature is regulated within tolerable range is named as thermoregulation. Where there is a change in temperature, there must be a change of heat provided. Basically, there are two sources of heat - internal and external environments. Organisms that are categorized under ectotherms get their heat source externally. These ectotherms include mostly amphibians, reptiles and invertebrates. Meanwhile, birds and mammals are mainly endothermic. Their internal metabolism provides the main source of heat. Many insects together with just a few nanovian reptiles as well as some fishes are endotherms. It is crucial to acknowledge the fact that endothermy and ectothermy are not mutually exclusive to one another. "A bird is, for instance, is mainly endothermic, but it may warm itself in the sun on a cold morning, much as an ectothermic lizard does." (Urry, Cain, Wasserman, Minorsky and Jackson, 2010).
Being mainly endotherms, human beings need to be able to regulate the internal body temperature without relying or depending on the external environment. The hypothalamus of the brain keeps an eye on the blood temperature and compares it with a set point, usually close to 370C. If the blood temperature is higher than the tolerated level, skin arterioles become wider, increasing bloodflow through skin. This blood transfers heat from the body core, raising the temperature of skin. Since the skin is the outermost organ, heat is lost from skin to the environment. The higher the temperature of the skin, the more heat is lost. Meanwhile, sweat glands secrete large amounts of sweat making the surface of the skin damp. Water that evaporates from the damp skin will bring with it the heat. Overall effect is that the body temperature lowers down, until it reaches the set point again. In contrast, when the body temperature decreases below the set point, skin arterioles become narrower so that less blood reaches the skin. Skeletal muscles do many small, rapid contractions to generate heat. This is called shivering. Sweat glands will stop secreting sweat and the skin remains dry.
Blood Glucose Concentration
The level of blood glucose in the blood is also one of the variables of homeostasis. Cells in the pancreas monitor the concentration and send hormone messages to targeted areas - the liver and muscle cells - when the level is low or high. In the case of a high blood glucose concentration, the Î²-cells in the pancreatic islets produce insulin. Insulin stimulates the liver and muscle cells to absorb glucose from the blood and convert it to glycogen. Granules of glycogen are stored in the cytoplasm of these cells. Other cells are stimulated to absorb glucose and use it in cell respiration instead of fat. These processes lower the blood glucose level. On the other hand, when the level of blood glucose declines way below the set point, the Î±-cells in the pancreatic islets produce glucagon. Glucagon stimulates the hepatocytes to break down glycogen down into glucose and release the glucose into the blood. This raises the blood glucose level.
Figure 3 - The control of blood glucose
(Source - http://www.get-discount-medical-supplies.com/images/blood-glucose-level.jpg)
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When the regulation of blood glucose level is not effective, the concentration can rise or fall beyond normal limits. This is referred to as diabetes mellitus. There are two forms of this condition. The table below provides a comparison between the two.
The beginning is usually during childhood.
Î±-cells produce insufficient insulin.
Insulin injections are used to control glucose levels.
Diet cannot by itself control the condition.
The onset is usually after childhood.
Target cells become insensitive to insulin.
Insulin injections are not usually needed.
Low carbohydrate diets usually control the condition.
Table 1 - The differences between Type 1 diabetes and Type 2 diabetes
(Source; Andrew Allot, 2007)
Comparison Between Endocrine and Nervous Systems
From the facts given above, it is shown that both endocrine system and nervous system have their roles in homeostasis. Even though both systems work together to achieve similar function, there are actually significance points of differences between the two. They both have distinct processes from each other. In the nervous system, nerves secrete chemicals called neurotransmitters. On the other hand, the chemicals secreted by endocrine system are hormones. While both regulate homeostasis, responses in nervous system are rapid and of short duration. In contrast, endocrine responses are slow but of long duration. On top of that, nerve impulses are transmitted via neurons whereas hormones are carried away in the bloodstream. One of the similarities between the two systems is that both involve regulated exocytosis. This ATP-dependent process happens during the secretion of neurotransmitters for the nervous system and hormones for the endocrine system.
The nervous system utilizes bioelectrical transmission. The depolarization of the nerve cell when an impulse is sent from the dendrites to the axons brings about an action potential on the membranes of neurons. The result is that a neuron is able to send information to the targeted cells in a very fast period. Theoretically, as long as there are neurons along the pathway, the signal will pass through without cease. This means that the process will carry on forever, given that synaptic cells are involved along the way. Meanwhile, the endocrine system utilizes endocrine glands which secrete hormones. Hormones are very specific type of proteins which are sent to targeted cells. Since the pathway is bloodstream, the process takes a longer period.
Figure 5 - shows the pathway of the hormones secreted by the endocrine gland cells, ending with the chemical messengers binding to the plasma membrane receptors on the target cells. However, if the hormone is steroid, they may pass through and bind to the receptor proteins in the cytoplasm, forming a hormone-receptor complex.
Figure 4 - shows the neurotransmitters being secreted from the pre-synaptic membrane to the post-synaptic membrane. Specific channel proteins on the post-synaptic neuron or the targeted cell must be there to allow chemicals to pass through.
Homeostasis is of utmost importance for the cells of the body to function really well. This explains the complexity of the systems that work out homeostasis. When the cells are able to meet their physiological needs, the organism will stay healthy. When homeostasis is not there to keep the internal environment stable, the organism is vulnerable to diseases, such as hypertension and diabetes. These diseases relate to the degrading mechanisms of negative feedback and positive feedback as one gets older. (http://www.123helpme.com)