Internal Environment & Homeostasis (Najran University) PDF

‫ال‬ ‫مملكة الع برية السعودبة‬ ‫التع‬ ‫وزازة ليم‬...

‫ال‬ ‫مملكة الع برية السعودبة‬ ‫التع‬ ‫وزازة ليم‬ ‫جامعة نجران‬ ‫كلية الطب‬ (1) Internal Environment & Homeostasis 1444 - 2023 ‫العام‬ By Dr. Najla Atef Assistant Prof of Physiology MBBCH, MSc Physiology, MD Physiology, DHPE Email: [email protected]  Objectives of the lecture By the end of the lecture, the student should be able to: Recognize the concept of the internal environment. Understand the meaning of homeostasis. Define set point. Clarify the basic components of a control system. Outline mechanisms of homeostasis. Discuss some examples of homeostasis.  Internal Environment It is the environment that immediately surrounds the cells. Because all body cells are surrounded by extracellular fluid (ECF), therefore, ECF is the internal environment of our body.  Homeostasis It means the ability of the body to maintain a stable or constant internal environment. i.e. keeping its physical and chemical composition constant. Importance : Cells can live, survive and function only, when chemical composition (like pH and osmolarity) and physical state (like temperature) of internal environment are suitable for them.  Homeostatic / Control mechanisms The body has hundreds of homeostatic mechanisms including those which are involved for example in the regulation of: Blood pressure (BP), Body pH, Blood sugar, Water, electrolyte Acid-base balance.  Set Point / Reference value / Normal range Normal value of each controlled condition is called set point. Examples : Normal value of pH of ECF is 7.4 Normal value of arterial BP is 120 / 80 mmHg Stimulus: Means a change in the environment i.e. any change in normal value of a controlled condition or factor.  Basic components of a control system (1) Sensory Receptors : Detect change in a controlled condition ( = stimulus) and send this information (= input) to a control center. Example: Baroreceptors (2) Control Center : Analyzes information received from sensory receptors and if needed, it sends its order (output) to an effector. (3) Effector : Produces necessary action (= response) that brings the value of a controlled condition back to normal.  Coordinating and integrating systems Monitoring and regulation of different body functions are coordinated through : (1) Nervous system (2) Endocrine system  Systems Involved in Maintaining Homeostasis Almost all systems of the body are involved in maintaining homeostasis : (1) Circulatory system: carries O2 and nutrients to the body cells and removes waste products. (2) GIT: provides absorbed nutrients, water & electrolyte to the internal environment (3) Respiratory system: provides O2 and removes CO2 from the internal environment. (4) Renal system: maintains pH and excretes waste products from the internal environment. (5) Endocrine system: provides hormones needed for the metabolism of nutrients.  Mode of action of homeostatic control system Mostly depends on feedback mechanisms There are two types of feedback mechanisms : (1) Negative Feedback (NFB) (2) Positive Feedback (PFB)  ‫ال‬ ‫مملكة الع برية السعودبة‬ ‫التع‬ ‫وزازة ليم‬ ‫جامعة نجران‬ ‫كلية الطب‬ (2) Feedback Mechanisms 1444 - 2023 ‫العام‬ By Dr. Najla Atef Assistant Prof of Physiology MBBCH, MSc Physiology, MD Physiology, DHPE Email: [email protected]  Objectives of the lecture: By the end of the lecture the student should: Discuss feedback mechanisms. Differentiate between positive and negative feedback. Explain examples of feedback mechanisms. Refer to Vicious cycle. Describe role of feedback mechanism in the control of hormonal secretion.  What is Feedback? Feedback is the process in which part of the output of a system is returned to its input in order to regulate its further output.  Negative Feed Back Control System If the activity of a particular system is increased or decreased, NFB system initiates a series of changes that return the activity back to normal. This type of FB system is inhibitory, beneficial & most commonly used mechanism in the body.  Examples of Negative Feedback Mechanisms Regulation of body temperature. Regulation of blood pressure. Regulation of blood glucose level. Regulation of body water content. Regulation of calcium.  Regulation of Blood Pressure (BP) If a stimulus causes BP to rise; baroreceptors in blood vessels are activated and send impulses (= Input) to the brain (= control center). Brain then sends signals (= Output) to the heart and blood vessels (= Effectors) which produce a response (decreased heart rate & vasodilation) that decreases BP back to normal ( = restoration of homeostasis).  Example: Glucose homeostasis Glucose intake occurs during digestion of food that is needed for energy expenditure to perform routine physical Pancreas activities. The pancreas is the key organ that regulates the glucose levels in body by secreting two hormones, insulin and Liver glucagon. The liver also helps to store the excess glucose in form of glycogen to be utilized later.  Positive Feed Back Control System (PFB) If activity of a particular system is increased or decreased , PFB system further reinforces or enhances that activity. Therefore, this type of feed back system is stimulatory, enhances the change (that is initiated) and may create a vicious cycle. Some beneficial examples : Generation of nerve impulse, LH surge, clotting of blood & labor (child birth) It is rare and less commonly used mechanism in the body.  Positive Feedback : It also occurs in control systems, however, in most cases it is dangerous to the body. In the few cases where positive feedbackfeedbackk occurs, it is soon overcome by counterbalancing negative feed-back mechanism.  Any positive feedback is intrinsically unstable; the reason why such instability does not usually get out of hand is that other feedback mechanisms are also operating to keep it under control. For instance, while positive feedback can occur at one particular level of the hormone, once the concentration has reached another critical level, it can trigger a negative feedback mechanism which will then oppose the further rise in the hormone concentration.  Vicious cycle: Positive feedback system may create a vicious cycle which once started, causes the condition to become more and more worse and may lead to death. Example: When a person has suddenly bled 2 Liters of blood, a vicious cycle (of progressive weakening of the heart) is set, which finally leads to death.  Feedback mechanisms in the control of hormonal secretion The basic function of a hormone is to regulate the activity of its target cells in a specific manner. To maintain this function it is essential that the endocrine gland receives constant, rapid information about the state of the systems being regulated  Endocrine System The endocrine system is composed of glands that produces chemical messengers called hormones. Hormones are produced in one part of the body and travel to target organs through the bloodstream.  Endocrine system Glands of the endocrine system include: Pituitary gland Thyroid Gland Parathyroid glands Thymus Adrenal glands Pancreas Ovary and Testis  Endocrine System The brain continuously sends signals to the endocrine glands to secrete and release hormones and the glands, in turn, send feedback to the nervous system. The hypothalamus in the brain is the master switch that sends signals to the anterior pituitary gland which can release up to eight hormones into the bloodstream.  The hormone travels to its target organ and usually results in the release of another hormone into the bloodstream. The hypothalamus then detects the rising hormone levels from the target organ and decreases the release of hormones from the pituitary which results in a decrease in hormone release from the target organ.  Negative feed-back: Each gland tends to over-secrete its hormone, but once the normal physiological effect of the hormone has been achieved, information is transferred back to the producing gland to check further secretion. Also, if the gland under-secretes, the physiological effects of the hormone decreases, and the feedback decreases, thus, stimulates the glands to begin secreting the hormone again  Negative feed-back: It relates the rate of production of the hormone to the blood concentration of that chemical substance which it controls or to a chemical product of the metabolic process that it regulates, so that the response to the hormone opposes the stimulus for its release. An example is the relationship between the hormone insulin and the principal variable which it controls i.e. the blood glucose level: Rise in blood glucose concentration increase the rate of insulin production act on its target cells to restore the blood glucose back to normal levels  A typical feedback loop consists of: Gland A liberates hormone A which stimulates (+) gland B. This releases hormone B which regulates the liberation of hormone A. The response of the receptors at gland A can be moved up or down. The purpose of this arrangement is to control the plasma level of hormone B.  Usually, an endocrine gland continually receives signals from a variety of sources and the actual rate of hormone synthesis and secretion from the gland is determined by the integration of these different signals.  Levels of feed back mechanisms in the endocrine system (1) Long feedback loop: Here peripheral gland, hormone or substances (arising from tissue metabolism) exert negative feedback control on both hypothalamus and anterior pituitary. (2) Short feed back loop: Here anterior pituitary hormones inhibit secretion of hypothalamic-releasing hormones. (3) Ultrashort feedback loop: Here hypothalamic hormones inhibit their own secretion. Example : GHRH inhibits its own release