Homeostasis 14.09.2024 PDF

# Homeostasis ## Ammar Boudaka ## Associate Professor of Physiology *** ## Objectives - Explain the meaning of the term internal environment and homeostasis and appreciate the importance of constancy of the statics (milieu interior). - State that homeostasis involves maintaining the internal e...

# Homeostasis ## Ammar Boudaka ## Associate Professor of Physiology *** ## Objectives - Explain the meaning of the term internal environment and homeostasis and appreciate the importance of constancy of the statics (milieu interior). - State that homeostasis involves maintaining the internal environment at a constant level (within narrow limits). - Explain that homeostasis involves monitoring levels of variables and correcting changes. - Understand the components of a feedback system. - Contrast the operation of negative and positive feedback mechanisms in control systems with examples. *** ## Introduction - **Anatomy** is the study of body structures and the relationships among them. - **Physiology** is the study of body functions including the study of homeostasis (keeping the organs systems of the body in balance). - **Pathophysiology** is how physiological processes are altered in disease or injury. Structure and function are closely related (structure mirrors function), e.g. the thin air sacs of the lungs permit movement of gases from the lungs to the blood. ## Levels of Structural Organization - **Chemical:** atoms combined to form molecules. - **Cellular:** cells are made of molecules. - **Tissue:** consists of similar types of cells. - **Organ:** made up of different types of tissues. - **Organ System:** consists of different organs that work closely together. - **Organism:** made up of the organ systems. ## Homeostasis - Homeostasis is the maintenance of a steady state in the body's internal environment despite changes in the external environment. It is a condition of equilibrium (balance) in the body's internal environment. - Maintenance of a relatively stable internal environment. - The term 'homeostasis' is derived from two Greek words: Homeo which means 'unchanging' and Stasis which means 'standing' it means staying the same. - It is a dynamic condition meant to keep body functions in the narrow range compatible with maintaining life. - **What should stay the same?** The internal environment. - **What is the internal environment?** The conditions inside an organism (mainly the extracellular fluid). **Cellular function depends on the regulation of composition of internal environment** ## Homeostasis and Body Fluids **Claude Bernard wrote:** The proper functioning of the cells depends on the precise regulation of the composition of their surrounding fluid. He called the surrounding fluid the internal environment. **This is the underlying principle of homeostasis.** - An important aspect of homeostasis is maintaining the **volume and composition** of body fluids. - **Body fluids** are defined as dilute, watery solutions containing dissolved chemicals inside or outside the cell. - **Intracellular Fluid (ICF)** is the fluid within the cells. - **Extracellular Fluid (ECF)** is the fluid outside the cells. ECF also exists in: - **Brain and spinal cord:** Cerebrospinal fluid (CSF) - **The lymphatic vessels:** Lymph - **The joints:** Synovial fluid - **The eyes:** Aqueous humor and vitreous body - The proper functioning of body cells depends on the regulation of the composition of the **interstitial fluid**. - The composition of interstitial fluid changes as substances move between plasma and the interstitial fluid. - Movement back and forth across capillary walls provides nutrients (glucose, oxygen, ions) to tissue cells and removes waste (carbon dioxide). **Body cells work best if they have the correct:** >- Volume of extracellular fluid. >- Blood volume. >- Blood pressure. >- Concentration of oxygen and carbon dioxide. >- Concentration of nutrients and waste products. >- Concentration of electrolytes. >- pH of the internal environment. >- Etc... **Our bodies have mechanisms to keep the cells in a constant internal environment.** ## Examples of Constancy of the Internal Environment - **Body core temperature:** 37°C - **Blood pressure:** 120/80 mmHg - **Arterial Blood:** PaO2 100 mmHg; PaCO2 40 mmHg - **Blood Sugar (glucose):** 100 mg/dL (5 µmol/L) - **Electrolytes:** Na+ = 140 mmol/L. K+ = 4 mmol/L - **pH:** blood pH= 7.4. stomach pH = 2-4, small intestinal pH = 8 and urine pH = 6 **All body cells and systems contribute towards this constancy.** ## All organs of the body systems help in the maintenance of homeostasis - Nervous System - Endocrine System - Cardiovascular System - Respiratory System - Gastrointestinal System - Excretory System - Musculoskeletal System - Integumentary System - Reproductive System ## Control of Homeostasis - Control of homeostasis is continually being disturbed by: - Physical insults from external environment such as intense heat or lack of oxygen. - Changes in the internal environment such as a drop in blood glucose due to lack of food. - Psychological stress such as demands of work or school. - In most cases, the disruption of homeostasis is mild and temporary, and the responses of body cells quickly restore balance in the internal environment. - In some cases, the disruption of homeostasis is intense and often prolonged -> disease (poisoning or severe infections) or death. - Control of homeostasis requires a communication system. - Fortunately, the body has many regulating systems that usually bring the internal environment back into balance. - Most often, the nervous system and endocrine system, working together or independently, provide the needed corrective measures. ## Homeostasis is maintained by feedback mechanisms (systems) - A feedback system (loop) is a cycle of events in which the state of the body is continually monitored, evaluated and changed. - The output of a system "feeds back" to either reverses or strengthen the action taken by the system. - A feedback system includes three basic components: - **Receptor** - **Control (integrating) center** - **Effector** - **Variable (controlled condition)** is a value that changes (e.g. body temperature, blood glucose). - **The set point** is the normal range of the variable (determined by the control center) that will be optimum for the system to operate ( e.g. body temperature around 37°C, blood glucose around 70-100 mg/dL). ## Components of a Feedback System - **Receptor:** - A receptor (sensor) is a body structure that monitors changes in a controlled condition (such as body temperature) and sends input to the control center - Specialized nerve endings in the skin act as temperature receptors (they cause a nerve to fire in response to temperature changes). - **Control (integrating) center:** - The control center sets the range of values to be maintained (usually this is done by the brain). - The control center evaluates input received from receptors, evaluates the signals and compared them to the set point. Then it generates output command (output involves nerve impulses, hormones, or other chemical agents). - **Example:** Brain acts as a control center receiving nerve impulses from skin temperature receptors. - **Effector:** - The effector receives output from the control center and produces a response or effect that changes the controlled condition (variable) to bring the system back to the set point. - Nearly every organ or tissue can serve as an effector. - **Example:** If body temperature drops -> brain sends an impulse to the skeletal muscles to contract -> shivering occurs to generate heat. ## Interactions among the components of a homeostatic control system - **Stimulus:** Produces change in variable. - **Receptor (sensor)**: Change detected by receptor. - **Input:** Information sent along afferent pathway to the control center. - **Control Center:** Output: information sent along efferent pathway to the effector. - **Effector:** Response of effector feeds back to influence magnitude of stimulus and returns variable to homeostasis ## Types of Feedback Systems - **Negative feedback system (most control systems)** - The response reverses the change in the controlled condition. - Tends to stabilize a system, correcting the deviation of a variable from the set point - **Example:** Regulation of BP and regulation of body temperature - **Positive feedback system:** - The response enhances (strengthens or enforces) the changes in the body's controlled condition. - Tends to disturb the system. - **Example:** Normal child birth and blood clotting. ## Homeostatic regulation of blood pressure - **Negative Feedback System** - **Example 1:** - **Stimulus:** Increase in BP. - **Receptors:** Baroreceptors. - **Control Center:** Brain. - **Effector:** Heart and blood vessels. ## Homeostatic regulation of breathing in response to changes in O2 and CO2 - **Negative Feedback System** - **Example 2:** - **Stimulus:** Increase in arterial CO2 or decrease in arterial O2. - **Receptors:** Chemoreceptors. - **Control Center:** Brain stem. - **Effector:** Respiratory muscles. ## Control of Body Temperature - **Negative Feedback System** - **Example 3:** - Humans maintain a constant body temperature around 37°C ## Control of Labor Contraction During Child Birth - **Positive Feedback System** - **Example 1:** - **Stimulus:** Stretch of the uterus. - **Receptors:** Stretch receptors (cervix). - **Control center:** Brain. - **Effector:** Uterine muscles. ## Control of labor contraction during child birth - **Positive feedback:** causes variables to continue in the same direction as the initial change. ## Blood Clotting - **Positive Feedback System** - **Example 2:** - **Break or tear occurs in blood vessel wall.** - **Platelets** adhere to site and release chemicals. - **Released chemicals** attract more platelets. ## Homeostatic Imbalance - It occurs when the fine control of the variable falls outside the normal range. - If the control system cannot maintain homeostasis, imbalance occurs. - If the imbalance is **mild or moderate**, disorder or disease occurs. - **Disorder:** Abnormality of function. - **Disease (illness):** Characterized by symptoms and signs - **Symptoms:** headache, nausea - **Signs:** fever, high blood pressure, paralysis. - If the imbalance is **severe**, death may result. ## Failure of any component of a control system results in a disturbance of homeostasis - **Organism in Homeostasis** - **External Change** - **Internal Change:** Internal Change results in loss of homeostasis. - **Organism attempts to compensate** - **Compensation fails:** Illness or disease - **Compensation succeeds:** Wellness ## Thank you!