Homeostasis is the control of internal conditions, be it temperature, specific blood conditions or other variables within living organisms. The term ‘Homeostasis’ was first defined by the French physiologist, Claude Bernard, in 1865.
Homeostasis in a general sense refers to stability, balance or equilibrium. It is the body’s attempt to maintain a constant internal environment. Maintaining a stable internal environment requires constant monitoring and adjustments as conditions change. This adjusting of physiological systems within the body is called homeostatic regulation.
Homeostatic regulation involves three parts or mechanisms: 1) the receptor, 2) the control center and 3) the effector.
The receptor receives information that something in the environment is changing. The control center or integration center receives and processes information from the receptor. And lastly, the effector responds to the commands of the control center by either opposing or enhancing the stimulus. This is an ongoing process that continually works to restore and maintain homeostasis. For example, in regulating body temperature there are temperature receptors in the skin, which communicate information to the brain, which is the control center, and the effector is our blood vessels and sweat glands in our skin.
Because the internal and external environment of the body are constantly changing and adjustments must be made continuously to stay at or near the set point, homeostasis can be thought of as a synthetic equilibrium.
Since homeostasis is an attempt to maintain the internal conditions of an environment by limiting fluctuations, it must involve a series of negative feedback loops.
When a change of variable occurs, there are two main types of feedback to which the system reacts:
- Negative feedback: a reaction in which the system responds in such a way as to reverse the direction of change. Since this tends to keep things constant, it allows the maintenance of homeostasis. For instance, when the concentration of carbon dioxide in the human body increases, the lungs are signaled to increase their activity and expel more carbon dioxide. Thermoregulation is another example of negative feedback. When body temperature rises, receptors in the skin and the hypothalamus sense a change, triggering a command from the brain. This command, in turn, effects the correct response, in this case a decrease in body temperature.
- Positive feedback: a response is to amplify the change in the variable. This has a destabilizing effect, so does not result in homeostasis. Positive feedback is less common in naturally occurring systems than negative feedback, but it has its applications. For example, in nerves, a threshold electric potential triggers the generation of a much larger action potential. Blood clotting in which the platelets process mechanisms to transform blood liquid to solidify is an example of positive feedback loop. Another example is the secretion of oxytocin which provides a pathway for the uterus to contract, leading to child birth.
- Harmful Positive Feedback
- Although Positive Feedback is needed within Homeostasis it also can be harmful at times. When you have a high fever it causes a metabolic change that can push the fever higher and higher. In rare occurrences the body temperature reaches 113 degrees Fahrenheit / 45 degrees Celsius and the cellular proteins stop working and the metabolism stops, resulting in death.
PATHWAYS THAT ALTER HOMEOSTASIS
- A variety of homeostatic mechanisms maintain the internal environment within tolerable limits. Either homeostasis is maintained through a series of control mechanisms, or the body suffers various illnesses or disease. When the cells in the body begin to malfunction, the homeostatic balance becomes disrupted. Eventually this leads to disease or cell malfunction. Disease and cellular malfunction can be caused in two basic ways: either, deficiency (cells not getting all they need) or toxicity (cells being poisoned by things they do not need). When homeostasis is interrupted in your cells, there are pathways to correct or worsen the problem. In addition to the internal control mechanisms, there are external influences based primarily on lifestyle choices and environmental exposures that influence our body’s ability to maintain cellular health.
- Nutrition: If your diet is lacking in a specific vitamin or mineral your cells will function poorly, possibly resulting in a disease condition. For example, a menstruating woman with inadequate dietary intake of iron will become anemic. Lack of hemoglobin, a molecule that requires iron, will result in reduced oxygen-carrying capacity. In mild cases symptoms may be vague (e.g. fatigue), but if the anemia (British English: anaemia) is severe the body will try to compensate by increasing cardiac output, leading to palpitations and sweatiness, and possibly to heart failure.
- Toxins: Any substance that interferes with cellular function, causing cellular malfunction. This is done through a variety of ways; chemical, plant, insecticides, and/or bites. A commonly seen example of this is drug overdoses. When a person takes too much of a drug their vital signs begin to waver; either increasing or decreasing, these vital signs can cause problems including coma, brain damage and even death.
- Psychological: Your physical health and mental health are inseparable. Our thoughts and emotions cause chemical changes to take place either for better as with meditation, or worse as with stress.
- Physical: Physical maintenance is essential for our cells and bodies. Adequate rest, sunlight, and exercise are examples of physical mechanisms for influencing homeostasis. Lack of sleep is related to a number of ailments such as irregular cardiac rhythms, fatigue, anxiety and headaches.
- Genetic/Reproductive: Inheriting strengths and weaknesses is part of our genetic makeup. Genes are sometimes turned off or on due to external factors which we can have some control over, but at other times little can be done to correct or improve genetic diseases. Beginning at the cellular level a variety of diseases come from mutated genes. For example, cancer can be genetically inherited or can be caused due to a mutation from an external source such as radiation or genes altered in a fetus when the mother uses drugs.
- Medical: Because of genetic differences some bodies need help in gaining or maintaining homeostasis. Through modern medicine our bodies can be given different aids, from anti-bodies to help fight infections, or chemotherapy to kill harmful cancer cells. Traditional and alternative medical practices have many benefits, but like any medical practice the potential for harmful effects is present. Whether by nosocomial infections, or wrong dosage of medication, homeostasis can be altered by that which is trying to fix it. Trial and error with medications can cause potential harmful reactions and possibly death if not caught soon enough.
The factors listed above all have their effects at the cellular level, whether harmful or beneficial. Inadequate beneficial pathways (deficiency) will almost always result in a harmful waver in homeostasis. Too much toxicity also causes homeostatic imbalance, resulting in cellular malfunction. By removing negative health influences, and providing adequate positive health influences, your body is better able to self-regulate and self-repair, thus maintaining homeostasis.
HOMEOSTASIS THROUGHOUT THE BODY
Each body system contributes to the homeostasis of other systems and of the entire organism. No system of the body works in isolation, and the well-being of the person depends upon the well-being of all the interacting body systems. A disruption within one system generally has consequences for several additional body systems. Here are some brief explanations of how various body systems contribute to the maintenance of homeostasis:
Since the nervous system does not store nutrients, it must receive a continuous supply from blood. Any interruption to the flow of blood may bring brain damage or death. The nervous system maintains homeostasis by controlling and regulating the other parts of the body. A deviation from a normal set point acts as a stimulus to a receptor, which sends nerve impulses to a regulating center in the brain. The brain directs an effector to act in such a way that an adaptive response takes place. If, for example, the deviation was a lowering of body temperature, the effector acts to increase body temperature. The adaptive response returns the body to a state of normalcy and the receptor, the regulating center, and the effector temporarily cease their activities. Since the effector is regulated by the very conditions it produced, this process is called control by negative feedback. This manner of regulating normalcy results in a fluctuation between two extreme levels. Not until body temperature drops below normal do receptors stimulate the regulating center and effectors act to raise body temperature. Regulating centers are located in the central nervous system, consisting of the brain and spinal cord. The hypothalamus is a portion of the brain particularly concerned with homeostasis; it influences the action of the medulla oblongata, a lower part of the brain, the autonomic nervous system, and the pituitary gland.
The nervous system has two major portions: the central nervous system and the peripheral nervous system. The peripheral nervous system consists of the cranial and spinal nerves. The autonomic nervous system is a part of peripheral nervous system and contains motor neurons that control internal organs. It operates at the subconscious level and has two divisions, the sympathetic and parasympathetic systems. In general, the sympathetic system brings about those results we associate with emergency situations, often called fight or flight reactions, and the parasympathetic system produces those effects necessary to our everyday existence.
The endocrine system consists of glands which secrete hormones into the bloodstream. Each hormone has an effect on one or more target tissues. In this way the endocrine system regulates the metabolism and development of most body cells and body systems. To be more specific, the Endocrine system has sex hormones that can activate sebaceous glands, development of mammary glands, alter dermal blood flow and release lipids from adipocytes. MSH can stimulate melanocytes on our skin. Our bone growth is regulated by several hormones, and the endocrine system helps with the mobilization of calcitonin and calcium. In the muscular system, hormones adjust muscle metabolism, energy production, and growth. In the nervous system, hormones affect neural metabolism, regulate fluid/electrolyte balance and help with reproductive hormones that influence CNS development and behaviors. In the Cardiovascular system, we need hormones that regulate the production of RBC’s (red blood cells), which elevate and lower blood pressure. Hormones also have anti-inflammatory effects and stimulate the lymphatic system. In summary, the endocrine system has a regulatory effect on basically every other body system.
The integumentary system (the skin) is involved in protecting the body from invading microbes (mainly by forming a thick impenetrable layer), regulating body temperature through sweating and vasodilation/vasoconstriction, or shivering and piloerection (goose bumps), and regulating ion balances in the blood. Stimulation of mast cells also produce changes in blood flow and capillary permeability which can effect the blood flow in the body and how it is regulated. It also helps synthesize vitamin D which interacts with calcium and phosphorus absorption needed for bone growth, maintenance, and repair. Hair on the skin guards entrance into the nasal cavity or other orifices, preventing invaders from getting further into our bodies. Our skin also helps maintain balance by excretion of water and other solutes (i.e.) the keratinized epidermis limits fluid loss through skin. It also provides mechanical protection against environmental hazards. We need to remember that our skin is integumentary; it is our first line of defense.
As the structural framework for the human body, the skeletal system consists mainly of the 206 or so bones of the skeletal system but also includes cartilages, ligaments, and other connective tissues that stabilize and interconnect them. Bones work in conjunction with the muscular system to aid in posture and locomotion. Many bones of the skeleton function as levers, which change the magnitude and direction of forces generated by skeletal muscle. Protection is a pivotal role occupied by the skeletal system, as many vital organs are encased within the skeletal cavities (eg. cranial and spinal), and bones form much of the structural basis for other body cavities (ex: thoracic and pelvic cavities). The skeletal system also serves as an important mineral reserve. For example, if blood levels of calcium or magnesium are low and the minerals are not available in the diet, they will be taken from the bones. Also, the skeletal system provides calcium needed for all muscular contraction. Finally, red blood cells, lymphocytes and other cells relating to the immune response are produced and stored in the bone marrow.
The muscular system is one of the most versatile systems in the body. The muscular system contains the heart, which constantly pumps blood through the body. The muscular system is also responsible for involuntary (e.g. goosebumps, digestion, breathing) and voluntary (e.g. walking, picking up objects) actions. Muscles also help protect organs in the body’s cavities. The muscles in your body contract, which increases your body heat when you’re cold. The act of shivering occurs when internal temperature drops. Muscles around vital organs contract, breaking down ATP, and thereby expending heat, which is then distributed to the rest of the body.
The cardiovascular system, in addition to needing to maintain itself within certain levels, plays a role in maintenance of other body systems by transporting hormones (heart secretes Atrial Natriuretic Peptide and Brain Natriuretic Peptide, or ANP and BNP, respectively) and nutrients (oxygen, EPO to bones,etc.), taking away waste products, and providing all living body cells with a fresh supply of oxygen and removing carbon dioxide. Homeostasis is disturbed if the cardiovascular or lymphatic systems are not functioning correctly. Our skin, bones, muscles, lungs, digestive tract, and nervous, endocrine, lymphatic, urinary and reproductive systems use the cardiovascular system as its “road” or “highway” as far as distribution of things such as nutrients, oxygen, waste products, hormones, drugs, etc. There are many risk factors for an unhealthy cardiovascular system. Some diseases associated are typically labeled “uncontrollable” or “controllable.” The main uncontrollable risk factors are age, gender, and a family history of heart disease, especially at an early age.
The cardiovascular system also contains sensors to monitor blood pressure, called baroreceptors, that work by detecting how stretched a blood vessel is. This information is relayed to the Medulla Oblongata in the brain where action is taken to raise or lower blood pressure via the autonomic nervous system.
The lymphatic system has three principal roles. First is the maintenance of blood and tissue volume. Excess fluid that leaves the capillaries when under pressure would build up and cause edema. Secondly, the lymphatic system absorbs fatty acids and triglycerides from fat digestion so that these components of digestion do not enter directly into the blood stream. Third, the lymphatic system is involved in defending the body against invading microbes, and the immune response. This system assists in maintenance, such as bone and muscle repair after injuries. Another defense is maintaining the acidic pH of urine to fight infections in the urinary system. The tonsils are our bodies “helpers” to defend us against infections and toxins absorbed from the digestive tract. The tonsils also protect against infections entering into our lungs.
The respiratory system works in conjunction with the cardiovascular system to provide oxygen to cells within every body system for cellular metabolism. The respiratory system also removes carbon dioxide. Since CO2 is mainly transported in the plasma as bicarbonate ions, which act as a chemical buffer, the respiratory system also helps maintain proper blood pH levels, a fact that is very important for homeostasis. As a result of hyperventilation, CO2 is decreased in blood levels. This causes the pH of body fluids to increase. If acid levels rise above 7.45, the result is respiratory alkalosis. On the other hand, too much CO2 causes pH to fall below 7.35 which results in respiratory acidosis. The respiratory system also helps the lymphatic system by trapping pathogens and protecting deeper tissues within. Note that when you have increased thoracic space it can provide abdominal pressure through the contraction of respiratory muscles. This can assist in defecation. Remember the lungs are the gateway for our breath of life.
The organs of the respiratory system include the nose, pharynx, larynx, trachea, bronchi and lungs. Together these organs permit the movement of air into the tiny, thin walled sacs of the lungs called alveoli. It is in the alveoli that oxygen from the air is exchanged for the waste product carbon dioxide, which is carried to lungs by the blood so that it can be eliminated from the body.
Without a regular supply of energy and nutrients from the digestive system, all body systems would soon suffer. The digestive system absorbs organic substances, vitamins, ions, and water that are needed all over the body. In the skin, the digestive tract provides lipids for storage in the subcutaneous layer. Note that food undergoes three types of processes in the body: digestion, absorption, and elimination. If one of these is not working, you will have problems that will be extremely noticeable. Mechanics of digestion can include chemical digestion, movements, ingestion absorption, and elimination. In order to maintain a healthy and efficient digestive system, we have to remember the components involved. If these are disturbed, digestive health may be compromised.
Toxic nitrogenous wastes accumulate as proteins and nucleic acids are broken down and used for other purposes. The urinary system rids the body of these wastes. The urinary system is also directly involved in maintaining proper blood volume (and indirectly blood pressure) and ion concentration within the blood. One other contribution is that the kidneys produce a hormone (erythropoietin) that stimulates red blood cell production. The kidneys also play an important role in maintaining the correct water content of the body and the correct salt composition of extracellular fluid. External changes that lead to excess fluid loss trigger feedback mechanisms that act to inhibit fluid loss.
The Reproductive System is unique in that it does little to contribute to the homeostasis of the organism. Rather than being tied to the maintenance of the organism, the reproductive system relates to the maintenance of the species. Having said that, the sex hormones do have an effect on other body systems, and an imbalance can lead to various disorders (e.g. a woman whose ovaries are removed early in life is at much higher risk of osteoporosis).
Excretory System is responsible for removing wastes, excess water and salt in the urine. Regulates the volume and pH of the internal environment. The human excretory system maintains homeostasis by removing metabolic waste such as water, salt and metabolite concentrations in the blood. The kidneys, which are the primary excretory organs, are major organs of homeostasis because they excrete nitrogenous wastes, and regulate water-salt balance and acid base balance. This section will examine the kidney in details.
EXAMPLES OF HOMEOSTASIS
Control of Temperature
The nude body is capable of maintaining a normal body temperature somewhere between 37° C and 38° C even if the external temperature varies between 16° C and 54° C. The metabolic activity of cells is the furnace of the body because cellular reactions give off heat as a by-product. When the body is at rest, body heat is generated primarily by the liver, heart, brain, and endocrine glands but when the muscles are active they generate many times the heat produced by these organs.
Therefore, increased muscle activity by rubbing the hands or stamping the feet is used as a short-term measure to raise body temperature. On a long-term basis, the hormone thyroxin produced by the thyroid gland stimulates cells to a higher metabolic rate. Therefore, we would expect persons living in a cold climate to have a higher metabolic rate than those who live in a moderate climate.
The regulatory center for body temperature, located in the hypothalamus, is sensitive to temperature changes in arterial blood flowing through it. Depending on the body temperature, the regulatory center brings about the adaptive responses listed in table 5, and body temperature then increases or decreases.
Regulation of the size of superficial arterial blood vessels and the activity of sweat glands is an important means by which body heat can be either conserved or dissipated. We can liken these activities to either closing or opening the windows of a house. The autonomic nervous system controls these reactions; the sympathetic system brings about the effects that conserve heat, and the parasympathetic acts to release heat.
The body cools when blood vessels lying in the skin are dilated and the warm blood passing through them loses heat to the environment by radiation. Sweating also cools the body because as perspiration evaporates, the body loses heat. Evaporation is more efficient on dry days than on humid days; humidity, then, does affect our ability to cool off.
If body temperature falls too low, shivering in addition to vasoconstriction will occur. Shivering requires that nerve impulses be sent to the skeletal muscles.
Humans contribute to the regulation of body heat by wearing appropriate clothing. In cold climates, humans wear clothing that traps an insulating layer of warm air next to the body to compensate for a lack of body hair. The formation of “goose bumps” is an ineffective attempt to raise the now absent hairs of the body to achieve a layer of trapped air naturally. In warm climates, clothing is worn to protect the body against the burning rays of the sun, but such clothing should be loose so that heat may still be lost by radiation.
Control of Blood Pressure
Blood pressure, which is the pressure of blood against blood vessel walls, is created by a beating of the heart. The mean arterial blood pressure averages about 100 mm Hg, although it increases progressively from birth to old age due to decreasing elasticity and narrowing of the arteries.
Arterial blood pressure will rise whenever blood volume increases or whenever there is a decrease in the cross-sectional area of the arteries. Sympathetic neurons under the control of regulatory centers (called cardiac and vasomotor centers) located in the medulla oblongata of the brain, can increase the heartbeat and constrict the arteries. A faster heartbeat temporarily increases the amount of blood within the arteries, and constriction of blood vessels, usually those of the skin and intestines, reduces their cross-sectional area.
The vasomotor center can be activated by impulses received from pressoreceptors located in the aorta and carotid arteries. When pressoreceptors are stimulated by a decrease in blood volume, as when we stand up suddenly after lying down, nerve impulses are sent to the vasomotor center and then blood pressure rises. The vasomotor center can also be effective when blood volume suddenly decreases, as when hemorrhaging occurs. At these times, it causes the blood reservoirs of the body (i.e., the veins, spleen, and liver) to contract and send more blood into the arteries.
The kidneys are also involved in monitoring blood pressure because of the role they play in regulating blood volume. When blood pressure decreases, the kidneys release renin, an enzyme that leads to the formation of angiotensin II, a powerful vasoconstrictor that also stimulates the adrenal cortex to release aldosterone. Under the influence of aldosterone, the kidneys retain sodium. As sodium is reabsorbed, water follows passively and both blood volume and blood pressure rise. In the presence of high blood pressure, the heart releases atrial natriuretic hormone, which has the opposite effect on the kidneys. This illustrates that homeostasis is often regulated by the contrary actions of hormones.
Control of pH
The normal pH of arterial blood is 7.4. A person is considered to have acidosis when the pH is below this value and to have alkalosis when it rises above pH 7.4. The lower limit at which a person can live is about pH 7.0 and the upper limit is about pH 7.8. To prevent a change in the pH of the body, all body fluids including cytoplasm are buffered. A buffer is a chemical or a combination of chemicals that can absorb either hydrogen ions (H+) or hydroxide ions and therefore maintain a relatively constant hydrogen ion concentration.
Proteins are effective chemical buffers both within cells and within blood. Hemoglobin is the most active protein buffer within blood, and it absorbs excess hydrogen ions when it is not carrying oxygen.
There are two other types of chemical buffers in body fluids–the carbonate and phosphate buffer systems. The phosphate system (NaH2PO4 and Na2HPO4) effectively buffers urine and cytoplasm. The carbonate system (a mixture of carbonic acid, H2CO3, and sodium bicarbonate, NaHCO3) is present in both tissue fluid and blood. The concentration of buffering substances is regulated by the lungs and/or kidneys. For example, when carbon dioxide is exhaled by the lungs or the bicarbonate ion is excreted by the kidneys, the concentration of the buffering substances can return to their most effective levels.
If the hydrogen ion concentration of the blood remains high, the respiratory center of the medulla oblongata is stimulated and the breathing rate increases. As carbon dioxide is excreted the pH shifts toward normal. This respiratory regulation of the acid-base balance is a physiological type of buffer system that is an important adjunct to the chemical systems discussed.
The kidneys are a powerful mechanism by which the pH may be regulated. The kidneys may form either an acid or alkaline urine, bringing the hydrogen ion concentration back toward normal. When the kidneys form an acid urine, they excrete H+, and when the kidneys form an alkaline urine, they excrete the bicarbonate ion. However, the full effect of the kidneys is not realized for ten to twenty hours.
Notice that the pH of the body is regulated in three ways. Chemical buffers both within cells and within body fluids react immediately to regulate the hydrogen ion and hydroxide ion concentrations. The pulmonary system requires a few minutes to bring about its effects while the kidneys take from ten to twenty hours. The kidneys, however, are the most powerful of the three.
Control of Glucose Concentration
A number of different hormones are active in keeping the blood glucose level at about 0.1%. The most important of these is insulin. Immediately after eating, increased glucose concentration stimulates the pancreas to release insulin. Insulin also promotes the uptake of glucose by cells including the liver. Insulin stimulates the conversion of glucose to glycogen in the liver. Between eating, when insulin is not being produced, the liver converts glycogen to glucose and therefore the blood glucose level remains constant. This conversion of glycogen is stimulated by both glucagon and, in times of emergency, also adrenalin.
If the supply of glycogen should run out and the blood glucose level remains low, both thyroxin and glucocorticoids stimulate gluconeogenesis, or the conversion of amino acids and glycerol to glucose by the liver.