Homeostasis

Homeostasis is the balance, continuation and overall equilibrium of a system based on a balance of a specific set of values considered normal in maintaining the specific system. This can refer to an environment, planet, or body. All living things must maintain a level of homeostasis to survive. The term homeostasis is often considered in science and in medicine as a means of maintaining the equilibrium of the body. In medicine, a review of a person’s homeostasis is paramount in determining the cause of an illness or the treatment of a disease. Physiological homeostasis can be defined as the maintenance of a variable around an ideal normal value with a living system. This value fluctuates around a set point to establish a normal rage of values and maintain equilibrium.

How does homeostasis work in the human body?

Homeostatic regulating factors include the following three elements: the receptor, the control center and effector. The receptor receives information that the
homeostasis (balance of normal values in the body) has been disrupted, where the control center reviews this information and sends a message to the effector
in order to instigate some form of change in the body. For example, in regulating blood pressure, there are baro-receptors in the large vessels of the body, which detect a raise in blood pressure, which is then received as a message in the control center (brain), which then messages various effectors in the body, such as the heart to stop beating so forcefully and therefore reduce the blood pressure.

Here is a link to a video that demonstrates homeostasis:

httpv://www.youtube.com/watch?v=DFyt7FJn-UM&feature=related

Mechanisms of Homeostasis in the Body

There are two main mechanisms that maintain homeostasis in the body and these are: positive and negative feedback mechanisms.

Negative Feedback Mechanism

A negative feedback system is the most common type of regulation used by the body. In this system, the body acts to remove or hinder and deviation from the set ‘ideal’ state. The body does this with three main components. A receptor that acknowledges that something has deviated from the ‘ideal,’ a control center, which establishes the set point in which a variable is maintained, and an effector, which is capable of changing the variable.

An example of a Negative Feedback Mechanism in the human body would be:

A patient loses blood and consequently the blood pressure is decreased. As the body’s normal blood pressure deviates lower than the ‘ideal’ the body’s receptors (primarily located in large blood vessels around the heart and neck) pick up this deviation. The receptors then tell the control center, which identifies the deviation, and tells the effector (in this case, primarily the heart – to start contracting stronger and faster, and the kidneys to start retaining salt and therefore fluid). This will then increase blood pressure.

Another example of a Negative Feedback System in the human body includes the Renin Angiotensin
Aldosterone Pathway (RAA).

Positive Feedback Mechanism

Positive responses are not common in healthy people. Positive mechanisms basically mean that when a deviation from a normal value occurs, the response of the system is to make hat deviation even greater. Therefore, a cycle is created, that leads away from homeostasis.

An example of a positive feedback mechanism would include the following:

During child birth the mother’s body produces the hormone oxytocin to increase the pressure and regularity of uterine contractions. The more the baby puts more pressure on the vaginal wall (as a result of the contractions), the body releases more oxytocin, and the contractions get stronger again. This cycle continues until the baby is born. However, if the baby cannot be born (such as the case in cephalic pelvis disproportionation –too large a head for the pelvis) the cycle will continue until both mother and baby eventually die.
Homeostasis in the Blood

Homeostasis in blood is called haemostasis. Haemostasis, like homeostasis refers to maintaining systems within the body within normal, pre-determined ranges, and in the case of haemostasis, it specifically refers to maintaining a stable flow and level of blood within the circulatory system. Mechanisms of haemostasis include: platelets, blood proteins, and vasculature. Haemostasis is a sign of a negative feedback mechanism, where the body recognises that it is bleeding, and consequently causes the vasculature to constrict, develop platelet plugs and cause the blood to clot in order to cease haemorrhaging.

Homeostasis in the Lungs

CO2 increases in the blood are detected by carotid chemoreceptors and cause the lungs to breathe deeper and faster. Lack of CO2 causes the lungs to breathe slower. This is a negative feedback mechanism, in which the body makes physiological changes to improve the CO2 and O2 ratios in the blood. The body also identifies the pH of blood and works around a physiological normal value of 7.35 to 7.45. If a person becomes acidotic (<7.35) he or she will increase respiratory effort to remove the acidic CO2; if a person becomes laccolitic (>7.45) then the respiratory effort is slowed to allow more CO2 into the circulatory system.

Homeostasis for Blood Pressure

Blood pressure homeostasis is managed by the negative feedback in the RAA Pathway when the blood pressure is identified in the baro-receptors in the carotid arteries and determined as too low. When the blood pressure is too high, the reflex bradycardia response is initiated and causes the heart to beat slower and therefore reduce the blood pressure, while conserving the energy in the heart.

Homeostasis for the Kidneys

Homeostasis for the kidneys is also managed by the negative feedback mechanisms involved in the RAA Pathway.

Homeostasis of the Blood Glucose Levels

If a patient’s blood glucose levels are below a normal value (such as <4 mmols) then the liver converts glycogen stores (found in the liver) into glucose through a negative feedback process called glycogenolysis. If a patient has abnormally high blood glucose levels (such as >18 mmols) then the patient’s pancreas will produce more insulin to remove the blood glucose.