The Definition And Overview Of Homeostasis

An introduction to homeostasis:

Homeostasis is the ability to maintain relatively constant internal conditions despite fluctuating changes in the external environment (1). Having mechanisms in place in order to control internal conditions allows us to adapt to our surroundings and enables us to go wherever we want. The external environment mainly refers to the extracellular fluid which bathes our cells, including blood plasma, and the changes in our external environment refers to changes in our surroundings, for example, noise levels and temperature. “The literal translation of homeostasis is ‘unchanging’ however, it is really a dynamic state of equilibrium in which internal conditions vary within limits” (2). Homeostasis is essential for the survival of organisms and it ensures that all of our bodily processes are functioning properly. For example, the smooth flow of blood throughout our circulatory system needs to be monitored for sufficient delivery of oxygen and nutrients to cells and tissues and for the removal of waste products such as carbon dioxide (2). Temperature and pH also need to be balanced so that enzymes can function at their optimum levels. Other examples of features of the internal environment that are regulated include blood pressure, respiration rate, cardiac output, stroke volume, and blood glucose concentration (1). There is no fixed value for these measurements, instead, we have a range of values that are a result of variation within organisms of the same species. Values may vary within a range due to lifestyle factors, genetic history, and medication, etc.

In order for homeostatic balance to be maintained, communication between different cells in the body is essential (2). This is achieved by the nervous and endocrine systems. The endocrine system uses blood-borne hormones to carry information and the nervous system uses nerve impulses to transmit information (2). For example, nerve cells in the skin detect a change in temperature and use signals to send information along the afferent pathway to the brain which then processes the information and compares it to the setpoint (3). The body is able to regulate homeostasis by both intrinsic and extrinsic mechanisms. Intrinsic refers to mechanisms within the body and extrinsic refers to behavioural mechanisms such as wearing a jumper in cold weather in order to keep warm.

Overview of homeostatic control mechanisms:

Homeostatic control mechanisms involve three main components: the receptor, control center, and effector. A stimulus (change away from the setpoint) is detected by a receptor which then passes information along the afferent pathway to the control center. The control center processes the information it receives and decides on the appropriate course of action to take. It then sends this along the efferent pathway to the effector. The effector carries out the response determined by the control center. It may amplify the effect of the stimulus, reverse the direction of the stimulus, or shut it off (2). Negative feedback is a common homeostatic control mechanism in the human body (4). When there is a deviation away from the set point, it works to reverse the direction of the stimulus in order to bring it back within the normal range. An example of this is the maintenance of blood glucose concentration, which I will discuss in more detail later on. On the other hand, positive feedback mechanisms are much less frequent and they work to reinforce a stimulus so that the change is further amplified. For example, positive feedback is involved in blood clotting. When there is a tear in the blood vessel wall, positive feedback is initiated and platelets in the blood adhere to the site of injury and release chemicals that attract more platelets, forming a temporary plug (2). The positive feedback cycle ends when the plug is formed. Positive feedback mechanisms are often referred to as cascades. This is because they have an amplifying effect which can be very rapid (1).

Contribution of cell functioning to the maintenance of homeostasis:

Cells play an important role in homeostatic balance, for example, they are involved in the regulation of blood glucose concentration. This is achieved by the alpha and beta cells in the Islets of Langerhans which are tissues found in the pancreas. In response to fluctuating changes of glucose in the bloodstream, the alpha and beta cells release the hormones glucagon and insulin, respectively (11). At times of high concentration of glucose present in the blood, beta cells secrete insulin via exocytosis and the insulin then binds to complementary receptors on the liver and muscle cells which act as effectors (5-6). This activates glycogenesis allowing the excess glucose to be converted into glycogen to be stored in the liver and muscle cells for later use. This brings the levels back within the normal range. On the other hand, when low levels of glucose are detected, beta cells stop secreting insulin, and alpha cells release glucagon by exocytosis (15). The hormone binds to receptors on liver cells and this allows for the conversion of stored glycogen back into glucose in order to bring the levels back within the appropriate range. Gluconeogenesis is activated in instances when glycogen stores are not sufficient to meet the needs of the body. This is the production of glucose from non-carbohydrate sources such as amino acids (14).

This negative feedback loop highlights the importance of cell functioning to the maintenance of homeostasis in the body. Cell theory states that cells are the basic structural and functional unit of life and that the life of any organism depends on the life of its individual cells (53). As a result, disruption in the functioning of one cell for example, due to a mutation in a gene coding for proteins can in turn, have drastic effects on the individual as a whole.

Homeostatic imbalance:

Stressors originating from both the external and internal environment such as oxygen levels, noise, temperature, aging, and pregnancy may lead to homeostatic imbalance (13). The body is able to deal with these stressors within limits using the mechanisms previously mentioned. As the body begins to age, it becomes less efficient in dealing with stressors and begins to decline in its ability to regulate homeostasis (13). Most diseases occur as a result of failure in homeostatic mechanisms (13).

Cystic Fibrosis:

“Cystic fibrosis (CF) is a severely life-shortening genetic disease resulting from abnormalities in the cystic fibrosis transmembrane conductance regulator (CFTR), a chloride channel found in cells lining the lungs, intestines, pancreatic ducts, sweat glands, and reproductive organs (8).” CF is an autosomal recessive disorder which means that the mutated gene needs to be inherited from both parents for it to be present (10). The role of the chloride channel is to pump out chloride ions into thick extracellular secretions as this results in a high concentration of chloride ions in the secretions which allows water to move in by osmosis, causing the secretions to become thinner (10). Mutations in the CTFR gene can either reduce the number of channels in the membrane, impair the function of the chloride channels present in the membrane, or have an effect on both number and function (12). As a result, the movement of chloride ions is affected and water is unable to move in by osmosis resulting in thick secretions outside the cell. In the lungs, cilia waft mucus containing pathogens up and out of the airways, but in people with Cystic Fibrosis, the mucus is thick and the cilia are unable to waft the mucus out. This leads to blockage in the airways.

Conclusion:

The maintenance of constant internal conditions in the face of varying changes to the external environment is vital for the smooth functioning of all biological processes as each process has optimal conditions at which it functions best. The body has both intrinsic and extrinsic mechanisms in place in order to deal with internal and external stressors within limits, but some stressors such as aging may lead to a decline in its ability to maintain balance. Cells are the basic unit of life and their impairment may lead to effects on the whole organism, such as disease. An example of this is demonstrated by mutations in the CFTR protein which result in Cystic Fibrosis.