CRS Human Physiology Topic 1: Homeostasis PDF

CRS HUMAN PHYSIOLOGY TOPIC 1: HOMEOSTASIS OBJECTIVES 1. Describe homeostasis. 2. Explain the importance of homeostasis. 3. Explain the conditions maintained in the human body to achieve homeostasis. 4. Discuss how negative...

CRS HUMAN PHYSIOLOGY TOPIC 1: HOMEOSTASIS OBJECTIVES 1. Describe homeostasis. 2. Explain the importance of homeostasis. 3. Explain the conditions maintained in the human body to achieve homeostasis. 4. Discuss how negative and positive feedback mechanisms operate to maintain homeostasis. 5. Enumerate the three components of a feedback mechanism. 6. Compare negative and positive feedback mechanisms through examples. 7. Explain the role of the following in homeostasis: 7.1 Body fluids a. Intracellular b. extracellular 7.2 Composition of body fluids 7.3 Cell membrane 7.4 Cell transport mechanisms a. Diffusion b. Osmosis c. Facilitated diffusion d. Active transport e. Bulk transport 7.5 Resting membrane potential 7.6 Action potential 8. Relate the roles of the organ systems in maintaining the conditions for homeostasis 9. Explain the importance of monitoring vital signs in relation to homeostasis. CONCEPTUAL MAP Figure 1. Conceptual map Homeostasis Many textbooks emphasize the role of feedback mechanisms, particularly negative feedback, in maintaining homeostasis. The conceptual map depicts what homeostasis is all about. The map illustrates the bigger picture of these seven (7) conditions also called variables that are being maintained within narrow limits to achieve a homeostatic state. There are many more details no longer included in the conceptual map to simplify and make clearer the bigger picture. If any of these conditions (variables) are not met, homeostatic imbalance results which is the basis for pathology and disease. So, homeostasis is important because it keeps the body healthy and free from disease. The seven conditions maintained in homeostasis considered as variables: 1. Body fluid volumes – The total body water TBW is 60%. 40% is intracellular fluid, 20% is extracellular fluid The cell membrane separates the intracellular from the extracellular fluid and plays an important role 2. Blood pressure BP is dependent on body fluid volume as well as body fluid composition. If body fluid volume is low, BP drops 3. Body fluid composition – Includes the ions potassium, sodium, chloride, calcium; small molecules, and water These ions and water move in and out of the cell through transport mechanisms that occur on the cell membrane such as diffusion, osmosis, or active transport 4. Body fluid pH – pH between 7.37 and 7.43 provides an optimal environment for cellular enzyme activity and membrane integrity (Zaoutis & Chiang, 2007). 5. Blood gases carbon dioxide and oxygen 6. Blood glucose 7. Body temperature Cell membrane Cell membranes are bilayers of two main components: phospholipids and proteins. Study the composition of the cell membrane presented in a fluid mosaic model. Notice that the phospholipid bilayer is arranged such that the charged/polarized hydrophilic heads (circles) sandwich the hydrophobic tails (threads between the circles). This explains why the cell membrane is selectively or semi-permeable (some substances can pass while others cannot). Remember: 1. Lipid-soluble substances such as gases--oxygen and carbon dioxide--and steroid hormones cross cell membranes easily because they can dissolve in the hydrophobic lipid bilayer. 2. Water-soluble substances such as sodium, chloride, glucose, and water cannot dissolve in the lipid of the membrane, but may cross through water-filled channels, or pores, or may be transported by carriers (Costanzo, 2006). 3. There are several proteins in the membrane that control the activities of the cell. These proteins are in the form of receptors, channels, enzymes, pumps. 4. The glycocalyx is the sugar-coating at the exterior of the cell that allows cells to recognize each other. This prevents our own cells from being attacked by our immune system such as the antibodies. Failure of the glycocalyx may lead to autoimmune disorders. Figure 2. Cell membrane. (Guyton, 2011). Figure 3. Cell membrane. (BC Open Textbooks, nd). The cell membrane is critical in maintaining the normal body fluid volumes and composition which include water and ions (also called electrolytes: Na for sodium, K for potassium, and Ca for calcium). Disrupting the integrity of the cell membrane can affect its function in maintaining body fluid volumes and composition. Maintaining the normal ion composition of body fluids is important in establishing the resting membrane potential RMP which is necessary for action potentials to develop. These action potentials occurring at cell membranes represent how neurons and muscles communicate. All functions of the nervous system are carried through transmission of information and signals by action potentials. This explains how important the cell membrane is in sustaining homeostasis. Feedback mechanism Organ systems function to contribute to homeostasis. How do organ systems do this? Through feedback mechanisms. It is the negative feedback mechanism that operates most of the time to achieve homeostasis. Please review the systems in charge of contributing to conditions 4, 5, 6, and 7 in the conceptual map. The nervous system is almost always involved together with other systems in maintaining homeostasis. To appreciate this better, let us understand how a feedback mechanism operates. Let us define the following terms to understand the feedback mechanism: 1. Variable – any of the conditions that are controlled in narrow limits/ranges to maintain homeostasis 2. Stimulus – a change in the variable (increased or decreased) 3. Receptor – the sensor that monitors the environment and detects the stimulus; once detected, the receptor relays the information to the control/integrating center. 4. Control center or integrating center – a structure in the brain or spinal cord (central nervous system) that interprets or processes the meaning of the stimulus (input) and determines the response (output) 5. Afferent - refers to sensory functions; the transmission of action potentials in afferent fibers is from the receptor that detects the stimulus (which could be a change in the variable) towards the control/integrating center. The integrating center is usually a structure in the central nervous system (CNS). 6. Efferent - refers to motor functions; the transmission of action potentials in efferent fibers is from the control/integrating center to the effector (which performs the response).  Figure 4. Elements of a homeostatic control system. (Marieb & Hoehn, 2007). Figure 5. Mechanism of negative feedback (Seeley, Stephens & Tate 2003). The output in a negative feedback mechanism is the opposite of the stimulus. Examples:  If the stimulus is an increase in body temperature, the output is a decrease in body temperature.  If the stimulus is an increase in blood sugar, the output is a decrease in blood sugar.  If the stimulus is an increase in blood pressure, the output is a decrease in blood pressure. The output in a positive feedback mechanism is the exaggeration of the stimulus. Examples:   If the stimulus is that the baby's head is pushing through the cervix during childbirth, the output is the release of oxytocin from the posterior pituitary gland causing the uterus to contract and making the head push against the cervix more. This example is a beneficial positive feedback. Most of the feedback mechanisms that sustain homeostasis are of the negative type. A positive feedback mechanis,m may not always maintain homeostasis. Example:  If the stimulus is decreased blood flow to the cardiac muscle, this results to a decreased pumping of the heart that causes the blood pressure to decrease. The decrease in blood pressure results to a further decrease in blood flow to the cardiac muscle causing the cardiac muscle to become weaker as a pump. This is an example of a harmful positive feedback Role of vital signs (VS) What can help in monitoring to check if homeostasis is sustained? The usual vital signs like blood pressure (BP), respiratory rate (RR), heart/cardiac rate (HR), and body temperature (BT) are useful to detect any serious homeostatic imbalance that necessitates immediate attention and treatment. Resting membrane potential Responsible for establishing the resting membrane potential (-70 to -90 mv inside the cell) 1. Leaky potassium channels (proteins on the cell membrane) that allow potassium to exit from inside to outside the cell through diffusion. The concentration of potassium is greater inside than outside the cell. Thus, potassium moves from inside to outside. The exit of potassium which are positively charged ions called cations contribute to the negative potential inside cell. There are also sodium channels that allow Na to enter but these are very few in number compared to the leaky potassium channels. 2. Negatively charged proteins inside the cell 3. Na-K pump – with ATP for energy, 3 Na ions are allowed to exit while 2 K ions enter the cell. Both are cations (positively charged ions). This Na-K pump maintains the resting membrane potential. Since there are more cations leaving than entering the cell, this contributes to maintaining the RMP. The Na-K pump is also referred to as the Na-K ATPase in the cell membrane. Figure 6. Na-K pump and K "leak" channels. (Guyton & Hall, 2011). Action potentials Movement of ions across the cell membrane occurs through transport mechanisms such as diffusion or active transport. This movement of ions is responsible for developing the action potential. How does an action potential look like? How does it take place? An action potential can only occur if the resting membrane potential is established. The parts of the action potential include: 1. Depolarization – occurs due to entry of sodium (Na) ions through sodium channels 2. Repolarization – occurs due to exit of potassium (K) ions through potassium channels 3. Hyperpolarization – occurs due to an excess of potassium (K) ions exiting through potassium channels  Figure 7. Components of an action potential. (Koeppen & Stanton, 2008). Figure 8. Threshold and subthreshold stimuli. (Koeppen & Stanton, 2008). Threshold stimulus means that the intensity or strength of the stimulus that reached the cell membrane is strong enough to cause an action potential. If the intensity of the stimulus does not reach threshold, no action potentials can develop. So, it’s either a threshold intensity is reached to cause an action potential or not at all. This explains simply the All or None Law. Remember, only a stimulus of threshold intensity will cause an action potential. What is the role of action potentials? Action potentials are how transmission or conduction of signals and information takes place. The direction of transmission in afferent/sensory fibers is from receptor to the control/integrating center. The direction of transmission in efferent/motor fibers is from the control/integrating center to the effector (muscle). Expect a more detailed discussion of action potentials in another topic in physiology. To facilitate further understanding of concepts, you must study these videos. Learning from these videos will enable you to answer the review questions in the Test Yourself exercises. You are encouraged to read further in any physiology book listed in the references. https://www.youtube.com/watch?v=tdBWJsUKFTQ The Cell- organelles (11:56 min) Lisa Johnson-DiMarco https://www.youtube.com/watch?v=rcacx09VODc Resting Membrane Potential (8:51 min) Lisa Johnson-DiMarco https://www.youtube.com/watch?v=VwoSoj6kPcc Neuron Membrane Physiology2 (9:01 min) Lisa Johnson-DiMarco https://www.youtube.com/watch?v=_obgJ2zc3ZU Homeostasis & Feedback Mechanisms (3:46 min) Ron Manalastas https://www.youtube.com/watch?v=BGeSDI03aaw&t=107s Active, Passive, and Bulk Cell Transport (4:34 min) RicochetScience (4:34 min) https://www.youtube.com/watch?v=154t3n1vB7M Establishing Resting Membrane Potential (1:49 min) Michael Leonard Test Yourself Part I* https://www.youtube.com/watch?v=tdBWJsUKFTQ The Cell Organelles (11:56 min) Lisa Johnson-DiMarco 1. Osmosis is a form of A. Passive transport B. Active transport 2. Which is transported in osmosis? A. Ions B. Protein C. Water 3. Serve as receptors, channels, enzymes, and pumps in the cell membrane A. Water B. Protein C. glycocalyx 4. What happens to the cell in a hypotonic solution? A. It shrinks. B. It swells. C. Nothing. 5. What happens to the cell in a hypertonic solution? A. It shrinks. B. It swells. C. Nothing. 6. Nerve and muscle cells depend on this potential prior to transmission of information A. Action potential B. Resting membrane potential Test Yourself Part II* https://www.youtube.com/watch?v=rcacx09VODc Resting Membrane Potential (8:51 min) Lisa Johnson-DiMarco 7. Charged particles (positive or negative) are able to pass the cell membrane easily. A. True B. False 8. Lipid-soluble substances are able to pass the cell membrane easily. A. True B. False 9. Ions with negative charges A. Anions B. Cations 10. Which ion is greater intracellularly in a resting state? A. Potassium B. Sodium 11. Which condition/s contribute to the negative potential inside the cell? A. Presence of negatively charged proteins inside the cell B. Leaky potassium channels allowing potassium to leave the cell C. Sodium-potassium pump causing 3 sodium ions to enter the cell and 2 potassium ions to exit the cell D. A and B E. A, B, and C Test Yourself Part III* https://www.youtube.com/watch?v=VwoSoj6kPcc Neuron Membrane Physiology2 (9:01 min) Lisa Johnson-DiMarco 12. In depolarization, ___ channels open A. Sodium B. Potassium 13. In repolarization, ___ channels open A. Sodium B. Potassium 14. During depolarization the membrane potential becomes A. positive B. negative 15. During repolarization, the membrane potential becomes A. positive B. negative 16. Describe the membrane potential during depolarization. It becomes A. less negative to positive. B. positive to negative. 17. What is the direction of movement of potassium ions when potassium channels open? These ___ the cell. A. exit B. enter 18. Which best describes hyperpolarization? A. The membrane potential becomes more negative due to an excess of potassium leaving the cell B. The membrane potential becomes more negative due to an excess of sodium leaving the cell Test Yourself Part IV* https://www.youtube.com/watch?v=_obgJ2zc3ZU Homeostasis & Feedback Mechanisms (3:46 min) Ron Manalastas 19. Which best describes the receptor in a feedback mechanism? A. Sends information from the control/integrating center to the effector B. Is responsible for the change in stimulus C. Detects the change in stimulus 20. Which best describes the stimulus in a feedback mechanism? A. Interprets the meaning of the stimulus B. Represents a change in the variable C. Detects the change in stimulus 21. Which best describes the effector in a feedback mechanism? A. Detects the change in stimulus B. Executes the output as the response C. Interprets the meaning of the stimulus and decides the output 22. Which best describes the control/integrating center in a feedback mechanism? A. Executes the output as the response B. Interprets the meaning of the stimulus and decides the output C. Detects the change in stimulus 23. The most common feedback mechanism in maintaining homeostasis is ___. A. Negative B. positive 24. Input in a feedback mechanism refers to the A. transmission along sensory fibers from the receptor to the control center. B. information from the receptors. C. A and B 25. Output and input in a feedback mechanism are A. signals or information transmitted through action potentials. B. from the control/integrating center. C. from the receptor. Test Yourself Part V* https://www.youtube.com/watch?v=BGeSDI03aaw&t=107s Active, Passive, and Bulk Cell Transport (4:34 min) RicochetScience. 26. Semi-permeable membrane A. means that the membrane is selective in allowing substances to pass; some may, some may not B. is due to the composition of the membrane being a phospholipid bilayer C. A and B 27. All forms of cellular transport occur in the A. cytoplasm. B. cell membrane. C. nucleus. 28. Diffusion A. is a form of passive transport. B. does not require energy. C. occurs if there is a difference in concentration gradient. D. A, B, and C 29. Facilitated diffusion A. allows charged particles such as ions to pass the membrane. B. needs energy. C. needs channels or carrier proteins. D. A and C 30. Osmosis A. is the diffusion of water. B. needs energy. C. needs enzymes. D. A, B, and C 31. Active transport A. needs energy in the form of ATP. B. needs protein such as a pump. C. refers to the movement of substances from a higher to a lower concentration gradient. D. A and B E. A, B, and C  32. Describe/s the Na-K pump in the cell membrane A. Allows 3 Na ions to exit while 2 K ions enter B. Needs energy in the form of ATP C. Important to maintain the resting membrane potential (RMP) D. A, B, and C 33. Bulk transport A. includes phagocytosis, pinocytosis, endocytosis, and exocytosis. B. needs energy. C. refers to the movement of bigger substances. D. A, B, and C Test Yourself Part VI* https://www.youtube.com/watch?v=154t3n1vB7M Establishing Resting Membrane Potential (1:49 min) Michael Leonard 34. Describes the resting membrane potential (RMP) A. The potential inside the cell is negative while outside the cell is positive B. Required for action potentials to develop C. A and B 35. The resting membrane potential (RMP) is maintained by A. Large, negatively-charged proteins inside the cell B. The Na-K pump C. Leaky potassium channels D. A, B, and C 36. In a resting membrane potential A. Na is greater extracellularly B. K is greater intracellularly C. A and B 37. Negative 70 mv or -70 mv refers to the A. Action potential B. Depolarization C. Resting membrane potential D. B and C *Please log-in at Moodle to enter your answers within the period allowed. Enter your answers for numbers 1-37 following the sequence above and know your scores. LEARNING SCENARIO Three hours after eating a meal, a 60-year-old female had several episodes of vomiting/diarrhea and became very weak with a blood pressure (BP) of 100/70. Intravenous fluids were immediately given in the emergency room and after a few hours, she felt better with a blood pressure (BP) of 120/80. Why did she become weak after suffering from severe vomiting/diarrhea? What made her feel better? What is the importance of monitoring blood pressure and other vital signs? References: Guyton, A. & Hall, J. (2011). Textbook of Medical Physiology. (12th ed., p. 46). Saunders, imprint of Elsevier. Guyton, A. & Hall, J. (2011). Textbook of Medical Physiology. (12th ed., p. 59). Saunders, imprint of Elsevier. Johnson-DiMarco, L. (2014. August 31). The cell organelles[Video file]. Retrieved from https://www.youtube.com/watch?v=tdBWJsUKFTQ Johnson-DiMarco, L. (2014. November 15). Resting Membrane Potential[Video file]. Retrieved from https://www.youtube.com/watch?v=rcacx09VODc Johnson-DiMarco, L. (2014. November 15). Neuron Membrane Physiology2[Video file]. Retrieved from https://www.youtube.com/watch?v=VwoSoj6kPcc Koeppen, B. & Stanton, B. (2008). Berne and Levy Physiology (6th ed., p. 69). Mosby, imprint of Elsevier. Leonard, M. (2015, July 8). Establishing Resting Membrane Potential[Video file]. Retrieved from https://www.youtube.com/watch?v=154t3n1vB7M. Manalastqas, R. (2015, August 16). Homeostasis & Feedback Mechanisms[Video file]. Retrieved from https://www.youtube.com/watch?v=_obgJ2zc3ZU Marieb, E., Hoehn, K. (2007). Homeostatic control mechanisms. Human Anatomy and Physiology (7th ed.). San Francisco, CA: Pearson Benjamin Cummings. RicochetScience. (2016, April 15). Active, Passive, and Bulk Cell Transport[Video file]. Retrieved from https://www.youtube.com/watch?v=BGeSDI03aaw&t=107s Seeley, R., Stephens, T. & Tate, P. (2003). Chapter 1: The human organism. Anatomy and Physiology (6th ed., p. 10). Boston, MA: McGraw Hill. Zaoutis, L. & Chiang, V. (2007). Acidosis. Comprehensive Pediatric Hospital Medicine (1st ed. P. 125). Retrieved at https://www.sciencedirect.com/topics/medicine-and-dentistry/blood-ph