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LEC: CONTROL OF INTERNAL ENVIRONMENT BY DR FARIHA RIZWAN Homeostasis: homeo = same; stasis = standing Homeostasis =constancy of the internal fluid environment. Definition: Maintenance of internal environment Homeostasis Maintenance of nearly constant conditions in the internal environment during unstressed conditions Hemostasis Stoppage of bleeding when blood vessel is injured The term homeostasis is defined as the maintenance of a constant internal environment during unstressed conditions. A similar term, steady state , is often used to denote a steady and unchanging level of some physiological variable (e.g., heart rate). The term steady state is also defined as a constant internal environment, but this does not necessarily mean that the internal environment is at rest and normal. When the body is in a steady state, a balance has been achieved between the demands placed on the body and the body's response to those demands So the term homeostasis is generally reserved for describing normal resting conditions and the term steady state is often applied to exercise where in the physiological variable in question (i.e., body temperature) is unchanging but may not equal the “homeostatic” resting value CONTROL SYSTEMS OF THE BODY The body has literally hundreds of different control systems, and the overall goal of most is to regulate some physiological variable at or near a constant value The most intricate of these control systems reside inside the cell itself. These cellular control systems regulate cell activities such as protein breakdown and synthesis, energy production, and maintenance of the appropriate amounts of stored nutrients Almost all organ systems of the body work to help maintain homeostasis For example, the lungs (pulmonary system) and heart (circulatory system) work together to replenish oxygen and to remove carbon dioxide from the extracellular fluid. The fact that the cardiopulmonary system is usually able to maintain normal levels of oxygen and carbon dioxide even during periods of strenuous exercise is not an accident but the end result of a good control system. Although much is known about how specific control systems of the body operate, the details of how many control systems work to maintain homeostasis remain a mystery. This remains an active area of research in exercise physiology NATURE OF CONTROL SYSTEMS To develop a better understanding of how the body maintains a stable internal environment, let's begin with the analogy of a simple, nonbiological control system such as a thermostat-regulated heating and cooling system in a home. Suppose the thermostat is set at 20° C. Any change in room temperature away from the 20° C “set point” results in the appropriate response by either the furnace or the air conditioner to return the room temperature to 20° C. If the room temperature rises above the set point, the thermostat signals the air conditioner to start, which returns the room temperature to 20° C. In contrast, a decrease in temperature below the set point results in the thermostat signaling the heating system to begin operation In both cases the response by the heating and cooling system was to correct the condition, low or high temperature, that initially turned it on. COMPONENTS OF CONTROL SYSTEMS Similar to the example of a mechanical control system, a biological control system is a series of interconnected components that maintain a chemical or physical parameter of the body near a constant value . Biological control systems are composed of three elements: (1) a sensor (or receptor); (2) a control center (i.e., center to integrate response) (3)effectors (i.e., organs that produce the desired effect) The signal to begin the operation of a control system is the stimulus that represents a change in the internal environment (i.e., too much or too little of a regulated variable). The stimulus excites a sensor that is a receptor in the body capable of detecting change in the variable in question. The excited sensor then sends a message to the control center. The control center integrates the strength of the incoming signal from the sensor and sends an appropriate message to the effectors to bring about the appropriate response to correct the disturbance (i.e., desired effect). The return of the internal environment to normal results in a decrease in the original stimulus that triggered the control system into action. This type of feedback loop is termed negative feedback and is the primary method responsible for maintaining homeostasis in the body NEGATIVE FEEDBACK Most control systems of the body operate via negative feedback An example of negative feedback can be seen in the respiratory system's regulation of the CO 2 concentration in extracellular fluid. In this case, an increase in extracellular CO 2 above normal levels triggers a receptor, which sends information to the respiratory control center (integrating center) to increase breathing. The effectors in this example are the respiratory muscles. This increase in breathing will reduce extracellular CO 2 concentrations back to normal, thus reestablishing homeostasis. The reason that this type of feedback is termed negative is that the response of the control system is negative (opposite) to the stimulus. POSITIVE FEEDBACK Although negative feedback is the primary type of feedback used to maintain homeostasis in the body, positive feedback control loops also exist. Positive feedback control mechanisms act to increase the original stimulus. This type of feedback is termed positive because the response is in the same direction as the stimulus. A classic example of a positive feedback mechanism is child birth. Extracellular Fluid 60 % of human body is fluid. (Total fluid =42 L,ICF=28 L) One third of it is present in the spaces outside the cell and called extracellular fluid (ECF). (Blood + interstitial fluid+trans cellular fluid) = (3 + 11 = 14L) ECF is in constant motion in exchange with blood and body cells. Contains ions and nutrients needed to maintain cell life and receive cell wastes. Provide internal environment (milieu interior) for body cells. The body cells are capable of living, growing and performing special functions as long as internal environment remain constant. The internal environment in the body is the extracellular fluid in which the cells live. It’s the fluid outside the cell and it constantly moves throughout the body. It includes the blood which circulates in vascular system and fluid present b/w the cells called interstitial fluid. ECF contains nutrients ,ions and all other substances necessary for the survival of the cells. Differences between intracellular and extracellular fluids Extracellular fluid 14 L Large amount of Sodium, chloride and bicabonate ions. Oxygen and carbon dioxide. Nutrients: Glucose, fatty acids and amino acids. Intracellular fluid 28 L Potassium, magnesium and phosphate ECF Ions & nutrients form constant internal environment / milieu (Bernard 19th cent.) Sodium, chloride, bicarbonate, oxygen, glucose, fatty acid, amino acid, carbon-dioxide (cell lungs), wastes (cell kidneys). ICF Potassium ion. Magnesium ion. Phosphate ion. Why ECF is called the internal environment? ECF has ions & nutrients needed by the cells to maintain cell life. ECF = internal environment / milieu interieur Cell growth & functions depend on proper concentration of components of internal environment (oxygen, glucose, different ions, amino acids, fatty substances etc). Examples of homeostatic control Regulation of body temperature (37.5 degree C) Regulation of blood glucose Stress proteins assists in regulation of cellular homeostasis The pH of the extracellular fluid has to be maintained at the critical value of 7.4 The respiratory system and the kidney help in the regulation of pH. The supply of nutrients must be adequate Nutrients must be digested absorbed into the blood and supplied to the cells.(digestive system) THERMOREGULATION REGULATION OF BLOOD GLUCOSE Homeostasis is also a function of the endocrine system. The body contains eight major endocrine glands, which synthesize and secrete blood borne chemical substances called hormones. Hormones are transported via the circulatory system throughout the body as an aid to regulate circulatory and metabolic functions . An example of the endocrine system’s role in the maintenance of homeostasis is the control of blood glucose levels. For example, the hormone insulin regulates cellular uptake and the metabolism of glucose and is therefore important in the regulation of the blood glucose concentration. After a large carbohydrate meal ,the blood glucose level increases above normal The rise in blood glucose signals the pancreas to release insulin, which then lowers blood glucose by increasing cellular uptake. Failure of the blood glucose control system results in disease Diabetes REGULATION OF BLOOD GLUCOSE TO MAINTAIN HOMEOSTATSIS STRESS PROTEINS ASSIST IN REGULATION OF CELLULAR HOMEOSTASIS A disturbance in cellular homeostasis occurs when a cell is faced with a “stress” that surpasses its ability to defend against this particular type of disturbance. ” The cellular stress response is a biological control system in cells that battles homeostatic disturbances by manufacturing proteins designed to defend against stress A brief overview of the cellular stress response control system and how it protects cells against homeostatic disturbances follows. STRESS PROTEINS ASSIST IN REGULATION OF CELLULAR HOMEOSTASIS At the cellular level, proteins are important in maintaining homeostasis. For example, proteins play critical roles in normal cell function by serving as intracellular transporters or as enzymes that catalyze chemical reactions. Damage to cellular proteins by stress (e.g., high temperature) can result in a disturbance in homeostasis. To combat this type of disruption in homeostasis, cells respond by rapidly manufacturing protective proteins called stress proteins After synthesis, these stress proteins go to work to protect the cell by repairing damaged proteins and restoring homeostasis. The above mentioned figure provides an overview of how this control system regulates protein homeostasis in cells. The process starts with a stressor that results in protein damage. Stresses associated with exercise that are known to produce cellular protein damage include high temperatures, reduced cellular oxygen, low pH, and the production of free radicals. Damaged proteins become signals for the cell to produce stress proteins. After synthesis, these stress proteins work to repair damaged proteins and restore homeostasis Digestive system-GASTROINTESTINAL TRACT: Origin of Nutrients in ECF: Absorption of dissolved nutrients (carbohydrates, fatty acids, amino acids) from ingested food ECF. Skin and temperature regulation Sensation of cold Shivering, We look for warmth Excretory system: Removal of Metabolic End Products Removal of CO2 by the lungs: Simultaneous processes: O2 from lungs blood CO2 from blood lung alveoli atmosphere (expiration). Note: Most abundant end product of metabolism is CO2 Removal of other end products of cellular metabolism (besides CO2) Kidneys: Urea Uric acid Excess of ions Excess of water Filtration of plasma followed by reabsorption of useful substances (glucose, amino acids, water, ions) Un-useful substances (urea) are poorly reabsorbed renal tubules urine Nervous system Fit to survive under varying conditions Hunger Hunger center in hypothalamus we seek food. Maintains homeostasis by generating new beings to take the place of those that are dying. REPRODUCTION: ROLE IN HOMEOSTASIS The autonomic nervous system regulates all the vegetative functions of the body essential for homeostasis. Water and electrolyte balance should be maintained . Otherwise it may lead to dehydration or water toxicity . Kidneys ,skin , salivary glands and GIT take care of this. NATURE OF CONTROL SYSTEMS A feedback system or feedback loop is a cycle of events in which the status of a body condition is monitored, evaluated, changed, re-monitored, reevaluated, and so on. Each monitored variable, such as body temperature, blood pressure, or blood glucose level, is termed a controlled condition. Any disruption that changes a controlled condition is called a stimulus. A feedback system includes three basic components—a receptor, a control center, and an effector. Normal feedback system Negative feedback system A negative feedback system reverses a change in a controlled condition. When thyroxine secretion is increased ,it inhibits the secretion of TSH from pituitary so that secretion of thyroxine from thyroid decreses and vice versa. Another example is maintenance of water balance in the body Positive feedback system A positive feedback system tends to strengthen or reinforce a change in one of the body’s controlled conditions. Examples of positive feedback system • 1) BLOOD CLOTTING • 2) CHILDBIRTH Local disease affects one part or a limited region of the body. Systemic disease affects either the entire body or several parts of it. A person with a disease may experience symptoms, Subjective changes in body functions that are not apparent to an observer, e.g. headache, nausea, and anxiety. Objective changes that a clinician can observe and measure are called signs. Signs of disease can be either anatomical, such as swelling or a rash, or physiological, such as fever, high blood pressure, or paralysis. CONCLUSION: All body structures are so organized by nature that they help to maintain the automaticity & continuity of life. Gain of a Control System The precision with which a control system maintains homeostasis is called the gain of the system. Gain can be thought of as the “capability” of the control system. This means that a control system with a large gain is more capable of correcting a disturbance in homeostasis than a control system with a low gain. As you might predict, the most important control systems of the body have large gains. For example, control systems that regulate body temperature, breathing (i.e., pulmonary system), and delivery of blood (i.e., cardiovascular system) all have large gains. The fact that these systems have large gains is not surprising, given that these control systems all deal with life-and-death EXERCISE: A TEST OF HOMEOSTATIC CONTROL Muscular exercise can be considered a dramatic test of the body's homeostatic control systems Because exercise has the potential to disrupt many homeostatic variables. • For example, during heavy exercise, skeletal muscle produces large amounts of lactic acid, which causes an increase in intracellular and extracellular acidity . This increase in acidity represents a serious challenge to the body's acid-base control system . Additionally, heavy exercise results in large increases in muscle O 2 requirements, and large amounts of CO 2 are produced. These changes must be countered by increases in breathing (pulmonary ventilation) and blood flow to increase O 2 delivery to the exercising muscle and remove metabolically produced CO 2 . Further, during heavy exercise the working muscles produce large amounts of heat that must be removed to prevent overheating. The body's control systems must respond rapidly to prevent drastic alterations in the internal environment. SUMMARY Exercise represents a challenge to the body's control systems to maintain homeostasis. In general, the body's many control systems are capable of maintaining a steady state during most types of sub maximal exercise in a cool environment. However, intense exercise or prolonged work in a hostile environment (i.e., high temperature/humidity) may exceed the ability of a control system to maintain a steady state, and severe disturbances in homeostasis may occur. THANK YOU