Homeostasis Lecture Notes PDF

Metabolism Module Session 1 Darya S. Abdulateef MBChB, PhD Physiology  Intended learning outcomes Define homeostasis and explain its importance in health and disease. Describe the main features of control systems in the body. Define negative and positi...

Metabolism Module Session 1 Darya S. Abdulateef MBChB, PhD Physiology  Intended learning outcomes Define homeostasis and explain its importance in health and disease. Describe the main features of control systems in the body. Define negative and positive feedbacks and their role in homeostasis. Describe examples of biological rhythm. Darya S. Abdulateef 2  Homeostasis - Homeostasis, is a state of maintaining “a similar condition” or Maintains Internal Stability - To cope with external variability in the challenging environment Importance of homeostasis in health and disease - The body monitors its internal state - takes action to correct disruptions that threaten its normal function. - If the body fail to maintain homeostasis of the critical variables  normal function is disrupted  a disease state, or pathological condition may result 3  List of variables that are under homeostatic control  Environmental factors that affect cells: osmolarity, temperature, and pH  Materials for cell needs: nutrients, water, sodium, calcium, other inorganic ions, oxygen  Internal secretions: hormones and other chemicals that our cells use to communicate with one another. Internal secretions having general and continuous effects Darya S. Abdulateef 4  How diseases produce? Diseases fall into two general groups :  either arises from internal failure of some normal physiological process.  or from some outside source. - In both internally and externally caused diseases, when homeostasis is disturbed, the body attempts to compensate Darya S. Abdulateef 5  What Is the Body’s Internal Environment? - Extracellular fluid (ECF): is the watery internal environment that surrounds the cells, a “sea within” the body. - It serves as the transition between an organism’s external environment and the intracellular fluid (ICF) inside cells (buffer zone between cells and the outside world). - Thus, several physiological processes have evolved to keep its composition relatively stable. Darya S. Abdulateef 6 Fluid content of the body Darya S. Abdulateef 7 Darya S. Abdulateef 8 Q. Calculate total body water (TBW) for a 70 kg man TBW = 60% of body weight TBW = 60% X 70 = 42 L of water e.g.; (CSF, ocular fluid and joint fluid)  Differences Between Extracellular and Intra- cellular Fluids The body compartments are in a dynamic steady state but are not at equilibrium. Ion concentrations are very different in the extracellular fluid compartment (ECF) and the intracellular fluid compartment (ICF). * Special mechanisms for transporting ions through the cell membranes maintain the ion concentration differences between the extracellular and intracellular fluids Major cations and anions of the intracellular and extracellular fluids Darya S. Abdulateef 10 The extracellular fluid The intracellular fluid (internal environment) (ICF)  Contains large amounts of sodium, chloride, and  Differs significantly from the bicarbonate ions plus nutrients for ECF; for example, it contains the cells, such as oxygen, glucose, large amounts of potassium, fatty acids, and amino acids. magnesium, and phosphate ions instead of the sodium and chloride ions found in  It also contains carbon dioxide the ECF. that is being transported from the cells to the lungs to be excreted,  plus other cellular waste products that are being transported to the kidneys for excretion. Darya S. Abdulateef 11 Normal Ranges and Physical Characteristics of Important Extracellular Fluid Constituents Darya S. Abdulateef 12  Homeostatic Control Systems Functionally interconnected network of body systems that operate to maintain a given factor in the internal environment. Example: Temperature, fluid & nutrient levels, chemical composition This is done from the cellular to body systems level in order to achieve steady state. This control system must be able to: 1. Detect deviations from “normal” or desired steady state level 2. Integrate this information with other relevant information 3. Make appropriate adjustments to return this factor to ‘normal’ or desired steady state level Darya S. Abdulateef 13  ‘Homeostatic Components’ All control systems have 3 components: (1) an input signal (sensed by receptor) (2)a controller, or integrating center, that integrates incoming information and initiates an appropriate response (3)an output signal that creates a response (by effector). Darya S. Abdulateef 14  Components of Homeostatic control mechanisms All have at least three interdependent components for the variable being regulated:  Sensor (receptor),  Integrating center (control center)  Effector 1-A sensor or receptor: Detects changes in the internal or external environment; for example; chemoreceptor and thermoreceptor. An example is baroreceptor, which detect stretch of the arterial wall during high blood pressure. Darya S. Abdulateef 15 2-The integrating center or control center It receives information from the sensors and initiates the response to maintain homeostasis. Two important control centers in the brain are the hypothalamus and the medulla oblongata in the brain stem. The hypothalamus is involved in the control of the endocrine system and the medulla are involved in the control of ventilation and the cardiovascular system Darya S. Abdulateef 16 3-An effector is any organ or tissue that receives information from the integrating center and acts to bring about the changes needed to maintain homeostasis. One example is the kidney, which retains water if blood pressure is too low. Darya S. Abdulateef 17  “Homeostatic” Mechanisms of the Major Functional Systems There are two basic patterns of control mechanisms: ① local control ② long-distance reflex control Darya S. Abdulateef 18  Local Control It is Restricted to a Tissue  In local control, a relatively isolated change occurs in a tissue.  A nearby cell or group of cells senses the change and responds, usually by releasing a chemical.  The response is restricted to the region where the change took place. Darya S. Abdulateef 19  An example of Local Control Dilation of regional blood vessels in response to tissue hypoxia A single muscle that is highly active uses up O2,  reducing O2 levels nearby (tissue hypoxia)  Nearby blood vessels sense this chemical change and dilate to bring in more blood and oxygen to that particular muscle * The other blood vessels throughout the body does not affected Tissue hypoxia cause dilation of the blood vessel supply that tissue Darya S. Abdulateef 20 Reflex Control Uses Long-Distance Signaling  Changes that are widespread throughout the body, or systemic in nature, require more complex control systems to maintain homeostasis.  Reflex control that uses the nervous system, endocrine system, or both  Output signals may be chemical signals, electrical signals, or a combination of both Darya S. Abdulateef 21 An example of Long-Distance Signaling Low blood pressure cause narrowing of most blood vessels throughout the body, How? Low blood pressure is sensed by receptors throughout the body.  The nervous system sends signals to the heart, kidneys and blood vessels  Cardiac output & blood volume increased and blood vessels constricted  to increase blood pressure back to normal Darya S. Abdulateef 22 Negative Feedback  A stimulus elicits a response that result in an effect opposite to the (-‐ve FB) initiating stimulus.  It brings a system back to its level of normal functioning.  Most of the system & function of the body controlled through the –ve FB mechanism.  e.g Adjustments of blood Darya S. Abdulateef pressure, 23 metabolism, and body temperature  Negative Feedback mechanisms (always leads to stability) In endocrine system, Negative feedback (-‐ve FB) prevent over-‐activity of hormone systems The controlled variable is often the degree of activity of the target tissue. Therefore, only when the activity of the target tissue rises to an appropriate level FB signals to the endocrine gland become powerful enough to slow further hormone secretion. e.G of –ve FB Control of Thyroid hormone Darya S. Abdulateef (T3 & T4) secretion 24  Positive feedback (most of the time leads to instability; but there are few exceptions)  Positive feedback enhances or accelerates output created by an activated stimulus (the response enhance the stimulus further) There are few example of positive feedback in the body, in some instances the body uses positive feedback to its advantage or physiologic usefulness: e.g  Platelet aggregation and accumulation in response to injury is an example of positive feedback. Darya S. Abdulateef 25  Positive Feedback in Blood clotting  When a blood vessel is ruptured and a clot begins to form,  multiple enzymes called clotting factors are activated within the clot itself. Some of these factors activate other factors of the immediately adjacent blood,  thus causing more blood clotting.  This process continues until the hole in the vessel is plugged and bleeding no longer occurs. Darya S. Abdulateef 26 Darya S. Abdulateef 27  Positive feedback (vicious circle) Positive feedback does not lead to stability but to instability and, in some cases, can cause death Example: Excessive bleeding (Shock) decrease BP Decreased BP affects cardiac perfusion Cardiac contractility Cardiac output decrease More decrease in BP and More If the blood loss is less, the body could deterioration in heart function control it by negative FB mechanism, as we saw in previous slides of low blood pressure control, Darya S. Abdulateef 28 Death  Biological Rhythms Result from Changes in a Set point  Rather than the set point being a fixed steady value, it can vary over time, giving rise to biological rhythms.  The menstrual cycle is an obvious example of a biological rhythm and women’s body temperature varies during the cycle.  A sudden increase in body temperature can be used as a marker of ovulation  For example the levels of the hormone cortisol in the blood varies during the day from a peak at about 7.00 am to a very low level at about 7 pm. 29 Darya S. Abdulateef  Disruptions of Homeostasis The body maintains a range of each factor in order to prevent illness, disease and death: - Ideal Value - Optimal range- where body functions still efficient - Range of tolerance- can still function, but not optimal - Minimum or Maximum set point for steady state (Moving pass max or min may result in illness, disease, death) Darya S. Abdulateef 30