a Journey of Life

The most common source of energy in animals including humans is the carbohydrate glucose. Using oxygen via a specific biochemical pathway known as `glycolysis', glucose is oxidized to provide energy yielding the products carbon dioxide and water. Glucose levels are replenished by means of the diet. Food is absorbed from the gut into the bloodstream, and can be stored in the liver, muscle, and fat. Glucose is stored mainly in the liver and muscles as glycogen. Since glucose is the preferred source of energy in the body, the fate of absorbed glucose depends on the energy requirements of the body. If the cells need to produce energy, they need a steady supply of glucose, so there are mechanisms within the body to mobilize its stores of glucose. The detection of an inadequate blood glucose level is by glucose receptors predominantly within the brain and pancreas. The brain processes the information and sets into motion the feeling of hunger within the person such that they may acquire and ingest food. The pancreas releases a protein hormone called glucogen which causes the stores of glycogen to be converted to free glucose in the blood. These two mechanisms among others provide the cells with their required glucose.

If, conversely, the blood glucose is greater than the need of the cells, then following detection by receptors, the pancreas releases another hormone called insulin. This favors glucose being stored as glucogen rather than being free in the blood. Also, the brain gives the feeling of being satiated, so no more food is ingested.

Therefore, it can be seen that the blood glucose level is regulated within a fairly tight range to preserve its balance. If this is disturbed, the effects can be dangerous and problematic for the individual. For example, if not enough glucose reaches the brain, the cells cannot survive. In the short term, the person feels dizzy and has a feeling of hunger. If this lack of glucose continues for a longer period of time, alternative sources of energy are utilized. If inadequate, it leads to brain cell death and eventually to bodily death.

Alternatively, if there is too much glucose in the blood, this leads to other problems. The medical condition defined by raised blood glucose is Diabetes Mellitus, and can be related to heart and blood vessel disease, loss of vision, loss of sensation, gangrene and kidney failure, if uncontrolled. It has been shown to be a problem with insulin either not being released or not having its desired effect.

In the human body also animal, there is a system to regulate and control temperature. In mammals --warm-blooded animals-- the optimum temperature is approximately 37.5 degrees C (98.6 degrees F). This means that although the external temperature may vary greatly from place to place and time to time, the body has mechanisms to compensate for the variations.

On Earth, the temperature has been measured below -60 degrees C in Arctic conditions and above 60 degrees C in desert conditions. The greatest variation in internal body temperature consistent with cellular life is around +- 4 degrees C, so if body temperature were to rise and fall with external temperature, the cells would soon die.

In the brain, specifically the hypothalamus, there is a temperature monitoring device somewhat like the thermostat that I described earlier. This receives input from temperature receptors (thermometers) scattered throughout the body, both on the skin and in internal organs.

If the hypothalamic `temperature centre' detects that the body temperature is too high i.e. greater than 37.5 degrees C, then cooling processes are activated. These include diverting blood nearer to the body surface which allows the excess of heat to be lost to the environment. In addition, sweat glands are activated which produce perspiration which evaporates and cools the skin. Also, the body's metabolic rate (rate at which food is being burned) is decreased so less heat is produced.

If the `temperature centre' detects that the body temperature is too low, the systems of heat conservation are set into motion. These include diverting blood away from the surface to prevent further heat loss, effecting the burning of food stores, the mechanisms of shivering and teeth-chattering to generate heat by movement and the concurrent production of heat-generating hormones. There are many problems associated with either extreme of body temperature. If it is too high, it leads to a state of dehydration, heat exhaustion, heat stroke etc. Conversely, if body temperature is too low, the resulting hypothermia is associated with cell death and heart problems. It is obvious that the issue of water control balance is also closely linked to temperature control so this is a good example of the integration of balancing systems within the human body.

Most individuals maintain body weight within a very small range of +- 1-2% over most of their lives. Body weight and energy balance are regulated by genetic and environmental influences. These control food intake, appetite, diet selection, absorption in the gut, energy expenditure and fat storage/breakdown. Integration of various signaling processes results in constant weight in normal circumstances. Any disturbance of this finely tuned energy balance results in a net increase or decrease of weight producing obesity (weight gain) or cachexia (weight loss).