Homeostasis is the process through which an organism regulates its internal environment, keeping critical parameters within acceptable limits. Aging affects the ability to maintain and restore homeostasis because some of the mechanisms used by the organism are no longer as effective as in a young body. In many cases the inability to restore homeostasis can affect the body's activities and can result in reduced capabilities and disease. Typical parameters for which homeostasis has to be maintained or restored and which are affected by aging include the following:
- Body temperature
- Glucose levels
- Blood water balance
The mechanisms by which these parameters are kept within a desirable range include the action of hormones, the activities of cells and action on the part of the organism. If homeostatic regulation is not possible and the values of these parameters stay outside the required limits, the death of the organism may result.
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Aging Affects the Body's Response to Homeostatic Regulation
When a parameter is too high or too low, hormones trigger cell reactions that bring the value back to its normal level. For example, a too high temperature triggers counter measures in the skin, circulatory and respiratory systems. The hypothalamus gland sends hormones to these systems, signaling them to cool down the body. As the systems spring into action, body temperature tends to go down again. Homeostasis is restored.
Aging can affect the homeostatic response. The gland secreting the hormone may no longer be able to produce as much hormone as before. Even if the hormone is secreted in sufficient quantities, the target cells may no longer be as sensitive to the hormone. They may react less and the homeostatic response may be slower and weaker. The body is not able to restore homeostasis as quickly as when the organism was younger.
Homeostatic Imbalance Examples Demonstrate the Risks of Insufficient Regulation
If one or several of the important homeostatic parameters stay too high or too low for long, there is a risk of damage to cells and to the organism. If the body temperature remains too hot, the organism can suffer dehydration and impairment of brain function as the nerve cells stop operating properly. If the temperature is too low, bodily functions shut down, and if any part of the body freezes, the ice crystals damage cell membranes and tissue.
The levels of many substances are a key to cell activities. If glucose or water levels are too high or too low, cells can't function normally. Glucose is an important nutrient without which cells can't synthesize the proteins they need. A constant water level is needed for cell function and chemical signal diffusion. Homeostasis keeps these values close to their targets. If they remain too high or too low for long, the organism suffers damage.
Homeostasis and Aging Act in Opposite Directions
Homeostasis is the collection of mechanisms the body uses to keep its operating variables near their desired set points. Aging is a process that makes the mechanisms of homeostasis less effective. The tools used for homeostasis remain the same over the life of the organism, but with aging, there may be fewer tools and the tools don't work as well as before.
In homeostasis, cells produce chemical signals that target other cells and change their behavior. This happens in three ways:
- The targeted cells may take direct and individual action such as metabolizing more glucose.
- The cells may participate in a coordinated reaction in which an organ such as the heart beats faster.
- The cells may cause a feeling that makes the organism take action, such as drinking water in response to a feeling of thirst.
Aging hinders these actions. Many of the cells in an aging organism have lost some of their ability to perform their functions at peak efficiency due to mutations in their DNA, generalized damage or wear and tear. The cells may have fewer resources as a result of lost efficiency and may not be able to signal or receive signals as well as before.
Even when signaling works well and strong signals are received, cells are less able to take actions such as making the heart beat more quickly or having the organism look for water. While aging is not the same for all organisms or all humans, aging in general can reduce overall functionality, not only in restoring homeostasis.
Temperature Homeostasis Depends on Many Cell Functions
The homeostatic mechanism that keeps the temperature of organisms within limits has four branches. Its central command unit is the hypothalamus gland. It sends out chemical signals to nerve cells, skin cells, the circulatory system and the respiratory system. For temperatures that are too high, the four branches work as follows:
- The signals from the hypothalamus make the organism feel hot. In the case of humans, they remove clothing or find a cooler spot. This action is voluntary; the other three branches are involuntary, taking place automatically.
- The hypothalamus sends signals to skin cells. Receptors on the surface of sweat gland cells bind with the chemical signals and trigger activity within the sweat cells that eventually leads the cells to secrete sweat.
- Chemical signals are sent to the cells controlling the circulatory system and to the capillaries near the skin. The control cells are stimulated to send out a signal that speeds up the beating of the heart. Cells in the walls of capillaries expand and the capillaries dilate, bringing hot blood to the skin of the organism.
- Similar signals are sent to the respiratory system control cells. These cells react to send signals to speed up breathing. This reaction is especially important for animals that use panting as a means of cooling down.
For temperatures that are too cold, similar signals have opposite effects such as making the organism look for a warm space or shrinking the capillaries near the skin. In each case many systems have to interact in a coordinated fashion to restore temperature homeostasis.
Aging Can Reduce Temperature Homeostasis Capability
Aging cells don't carry out cell functions as efficiently as younger cells. In the case of temperature homeostasis, temperatures in aging organisms may stay too high or too low longer than for young organisms. This can lead to further cell damage or further inefficiencies in the production of hormones and other chemicals.
Poor temperature homeostasis due to aging may be due to a lack of hormone production at the hypothalamus. Hormones are proteins produced by ribosomes attached to the endoplasmic reticulum (ER) of cells. The ER processes, stores and exports the hormones in special vesicles via the Golgi apparatus. The vesicles fuse with the outside cell membranes and leave their contents outside the cell as endocrine secreted hormones. These different steps are less efficient in aged cells leading to less hormone secreted.
At the other end of the signaling chain, hormone receptors on the outer membrane of cells may be fewer and some may be damaged. The hormones then produce less effect than in younger cells. Fewer cells change their behavior and those that react to the hormones may change their behavior only slightly. As a result of all these influences, aging can reduce the effectiveness of temperature homeostasis.
Glucose Homeostasis is Critical for Cell Functions
Cells constantly consume glucose and oxygen to produce energy for cell functions. Glucose is distributed to every cell in the body through the circulatory system and its level in the blood has to be kept constant. Both low levels of glucose or hypoglycemia and high levels or hyperglycemia can lead to death.
The level of glucose in the blood is controlled by the pancreas through the hormone insulin. In glucose homeostasis, insulin is secreted by cells in the pancreas and distributed through blood vessels. When glucose is too high, insulin levels in the blood increase as well and insulin receptors on the outside of cells are triggered by the insulin. The triggering releases chemicals inside the cell that increase metabolism and consume glucose. The level of glucose in the blood goes back down.
If the glucose level is too low, the organism experiences a feeling of hunger. The organism eats and the food is digested and broken down into components including glucose in the digestive tract. The glucose is absorbed by blood vessels around the digestive tract and the glucose level in the blood is restored.
When Glucose Homeostasis is Reduced by Aging, Diabetes can Result
Glucose homeostasis is affected by the same aging factors as for temperature. Cells in the pancreas produce less insulin and the cell receptors don't work as well. But there are additional ways in which aging can affect the blood's levels of glucose. The risk of high glucose levels causing diabetes increases in older people.
There are two types of diabetes. Type I is caused by a lack of insulin, either due to a destruction of the insulin-producing cells of the pancreas or the cells producing less insulin. Type II diabetes is caused by the receptors on the target cells becoming desensitized because of continuous exposure to high levels of insulin. This effect is often due to obesity or the long term consumption of food with high levels of easily-digested glucose. All these factors are more severe and more common in old age.
Aging Can Affect the Blood Water Balance
Maintaining the right amount of water in the blood is important for cell chemical reactions. If blood contains too much water, water will enter the cells and dilute cell solutions. If there is too little water, cells lose water and chemical diffusion is affected. Blood water homeostasis is controlled by the hypothalamus via two channels as follows:
- If there is too much water in the blood, the hypothalamus sends a signal to the pituitary gland to secrete an antidiuretic hormone called ADH. ADH targets cells in the kidneys that allow more water into the urine.
- If there is too litle water in the blood, the hypothalamus creates a sensation of thirst in the organism. The organism drinks water , which is absorbed through the digestive system into the blood.
Aging does not affect the control path in which a low water level leads to thirst but aging kidneys lose mass and are no longer as responsive to signals as younger organs. As a result, cells may allow water to pass into the urine even when the hypothalamus has not given the corresponding signal or water may be retained even when the blood water level is too high. Overall, blood water homeostasis is no longer as accurate as in younger organisms.
In general, aging affects the maintenance and restoration of homeostasis negatively. The performance of aging cells often deteriorates and they are less sensitive to cell signaling. Even when the cells carry out their functions, the aged organism is often less able to take the actions required. However, the actual effects of aging for individual cases may vary widely. Aging can have these negative effects but not all aging cells and aging organisms display the same deterioration in functionality.