CoverModule 1.0. Homeostasis, Membranes, Electrophysiology and ANS1.1. Homeostasis1.1.1. Homeostasis Defined1.1.2. Homeostatic Control Systems1.1.3. Feedback Response Loop1.2. Cell Transport; Water & Solutes1.2.1. Fluid Compartments1.2.2. Osmosis1.2.3. Diffusion of Solutes1.2.4. Active Transport1.2.5. Bulk Transport1.3. Electrophysiology1.3.1. Ions and Cell Membranes1.3.2. Membrane Potentials1.3.3. Graded Potential1.3.4. Action Potentials1.3.5. Refractory Periods1.3.6. Propagation of an Action Potential1.4. The Synapse1.5. The Autonomic Nervous System1.5.1. Organization of the Nervous System1.5.2. Structural Organization of the ANS1.5.3. The SNS and the PNS1.5.4. The Enteric Nervous System1.5.5. Physiology of the ANS1.5.6. Neurotransmitters of the ANS1.5.7. Receptors of the ANS1.5.8. Actions of the Autonomic Nervous System1.5.9. Table of Actions for the SNS and PNS and Some Common DrugsModule 2.0. Skeletal Muscle and Special Senses2.1. Structural Organization of Skeletal Muscle2.2.1. Neuromuscular Junction, Excitation-Contraction Coupling2.2.2. Muscle Contractures and Cramps2.3. Whole Muscle Contraction, Fiber Type, Fatigue and Muscle Pharmacology2.3.1. Motor Units2.3.2. Factors that Influence the Force of Contraction2.3.3. Energy Source for Muscle Contraction2.3.4. Skeletal Muscle Fiber Types2.3.5. Fatigue2.3.6. Muscle Pharmacology2.4. Smooth Muscle2.4.1. Smooth Muscle Contraction2.5. Control of Body Movement2.5.1. Voluntary Control of Muscle2.5.2. Reflexes2.6. Taste and Smell2.6.1. Taste2.6.2. The Sense of Smell2.7. Vision2.7.1. Structure of the Eye2.7.2. Focusing Light on the Retina2.7.3. Converting Light to Action Potentials2.7.4. The Retina2.7.5. Phototransduction2.7.6. Receptive Fields2.8. Hearing and Equilibrium2.8.1. The Nature of Sound2.8.2. The Hearing Apparatus2.8.3. Sound Vibrations to Action Potentials2.8.4. The Sense of Balance and EquilibriumModule 3.0. Cardiovascular System3.1. Structure of the Heart3.1.1. Chambers and Circulation3.2. Cardiac Cell Action Potentials3.2.1. Action Potentials in Cardiac Muscle Cells3.2.2. Action Potentials in Cardiac Autorhythmic cells3.2.3. Cellular Mechanisms of Inotropy and Chronotropy3.3. Electrophysiology of Heart Muscle3.3.1. Heart Conduction System3.3.2. Electrocardiogram (ECG)3.3.3. Abnormal ECG - Current of Injury3.4. The Cardiac Cycle3.4.1. Cardiac cycle3.4.2. Cardiac Measurements and Pressure Volume Loops3.5. Blood vessels and Blood Pressure3.5.1. Arteries and Veins3.5.2. Capillaries3.5.3. Blood Pressure Regulation and Shock3.5.4. Capillary Exchange3.5.5. Myogenic and Paracrine Regulation of Vasoconstriction and Vasodilation3.6. Blood3.6.1. Composition of Blood3.6.2. Hematopoeisis3.6.3. Breaking Down Red Blood Cells3.6.4. HemostasisModule 4.0. Urinary and Respiratory Systems4.1. Function and Structure of the Kidney4.1.1. Urinary System Function4.1.2. Functional Anatomy of the Urinary System4.1.3. The Nephron: Functional Unit of the Kidney4.1.4. The Renal Corpuscle: Bowman's Capsule4.2. Physiology of Urine Production4.2.1. Filtration4.2.2. Renal Clearance4.2.3. Tubular Reabsorption4.2.4. Urine Concentration and Dilution4.2.5. Hormonal Regulation of Urine Production4.3. Acid/Base Balance4.3.1. Buffers4.3.2. Acid/Base Disturbances4.4. The Respiratory System4.4.1. Respiratory System Structure and Function4.4.2. Respiratory Membrane4.4.3. Respiratory pressures and Inspriation/Expiration4.4.4. Alveoli and Surfactant4.4.5. Pneumothorax4.4.6. Pressure-Volume Loops and the Work of Breathing4.5. Gas Exchange and Transport4.5.1. Gas Laws4.5.2. Partial Pressure Gradients in the Lung4.5.3. Alveolar Gas Equation4.5.4. Oxygen and Carbon Dioxide Transport in the Blood4.5.5. Alveolar Ventilation4.5.6. Ventilation/Perfusion Ratio4.6. Chronic Bronchitis and Emphysema4.6.1. Respiratory Control by the Medulla Oblongata4.6.2. Chemicals that Regulate VentilationModule 5.0. Digestive, Endocrine and Reproductive Systems5.1. Functional Anatomy of the Digestive System5.1.1. Layers of the Digestive Tract5.1.2. Enteric Nervous System5.1.3. Organs of the Digestive System5.2. Digestion5.2.1. Carbohydrates5.2.2. Proteins5.2.3. Lipids5.2.4. Lipoproteins5.3. Regulation of Digestive Secretions5.4. Endocrine System5.4.1. Overview of the Endocrine System5.4.2. Hormone Receptors5.4.3. Hormones of the Body5.4.4. Other Hormones: Melatonin and Pheromones5.5. The Hypothalamus and Pituitary Gland5.5.1. Structure and Function of the Hypothalamus and Pituitary Gland5.5.2. The Posterior Pituitary5.5.3. The Anterior Pituitary5.5.4. Growth Hormone5.5.5. Prolactin5.5.6. Thyroid Hormones5.5.7. Adrenal Hormones5.6. Pancreas5.6.1. Insulin and Glucagon5.6.2. Diabetes Mellitus5.7. Reproductive System Anatomy5.7.1. Female Reproductive Anatomy5.7.2. Male Reproductive Anatomy5.7.3. Sexual Development at Puberty5.7.4. Male Reproductive Endocrine Axis5.7.5. Spermatogenesis5.7.6. Female Reproductive System: Oogenesis5.7.7. Ovulation and Fertilization5.7.8. The Ovarian Cycle5.7.9. The Uterine Cycle5.7.10. PregnancyAppendix A. GenderAppendix B. The Placebo EffectB.2.1. The Placebo EffectB.2.2. Examples of the Placebo EffectB.2.3. How do Placebos Work?B.2.4. Are Placebos Ethical?B.2.5. How do we validate actual effectiveness of placebosB.2.6. Tips for evaluating scientific evidenceB.2.7. What about Faith Healings

Feedback Response Loop

Homeostatic control systems, like the temperature example above, generally result in Feedback response loops. Feedback response loops start with a stimulus that changes a variable and ends with an effector that changes the variable. If the variable is changed in a way that brings it back towards set point, we call it negative feedback. We use the word negative to indicate that the resulting change in the variable is opposite of the initial change. In other words, if a stimulus were to cause the temperature variable to be increased to 99°F, the response of sweating would act to decrease the variable back to 98.6°F. Since the initial stimulus caused an increase in temperature and the resulting response was a decrease in temperature, opposite to the initial change, we call the whole process a negative feedback loop. Regulation of body temperature is only one of many examples of how the body maintains the constancy of the internal environment. Other negative feedback loops that regulate homeostasis include replenishment of oxygen by the lungs, the regulation of the pH of the blood at 7.4, and the regulation of blood glucose by insulin; however, keep in mind that there are many other examples.

Sometimes, the response to a stimulus results in a change to the variable that increases the deviation from the set point. This type of mechanism is called a positive feedback loop. Most of the time, positive feedback loops are the result of negative feedback systems that do not adequately correct the problem. For example, in response to a substantial loss of blood, the blood pressure would drop and the negative feedback response would be to increase the heart rate to help return blood pressure to normal. However, if the loss of blood was too great, the increase in heart rate might not be adequate to increase the blood pressure, and as a result, less blood would go to the heart. Since blood carries essential oxygen and nutrients, less blood to the heart would essentially starve the heart. This would result in loss of function and weaker contractions resulting in less blood being pumped, which would result in less blood to the heart and so on. Thus, because the negative feedback response (an increase in heart rate) was not adequate, the end result was that blood pressure continued to drop, causing an increased (positive) deviation from the set point. This situation would require intervention from a medical professional to save the individual.

There are a few examples where positive feedback mechanisms are good. For example, during childbirth, labor contractions are enhanced through positive feedback. This is the result of a hormone called oxytocin, which is released from the brain during labor contractions. Oxytocin enters the bloodstream from the brain and circulates through the blood to the uterus where it causes more powerful contractions. Contractions, of the uterus, push the baby's head downward which stretches the cervix. Stretch receptors in the cervix and uterus then send signals to the brain to release more oxytocin, and this positive feedback system continues until birth is accomplished. You may have heard of the drug Pitocin; this is a synthetic form of oxytocin that can be injected into expectant mothers to induce labor or assist contractions when the oxytocin system is not functioning naturally.

Some systems have adapted an anticipatory response called a feedforward control. For example, dehydrated humans in warm environments show low sweat rates but can be induced to sweat almost instantaneously when they drink water. The most widely known feedforward response is the salivation response that occurs with the sight, smell, or even the thought of food, inducing salvation and stomach acid secretion before food enters the mouth.

Feedback loops rarely operate in isolation but are almost always part of a larger network of systems that operate in a complex interplay with one another. Some of the loops can even be in competition with each other, sometimes making treatment options very complex. In addition, there is a hierarchy among the various feedback loops with the maintenance and balance of the brain having priority over everyone else. For example, the body will sacrifice bone for the essential ion calcium to ensure proper brain function. Two other important themes of homeostasis are redundancy and acclimatization. The maintenance of some variables is so crucial that the body does not entrust them to one system, opting instead to build in redundancy in case one system fails or is inadequate. The overall fitness of an organism can be determined by its ability to adapt to different physiological situations. For instance, acclimatization is observed in the long-term adaptation to high altitude in breathing rate and overall red blood cell numbers (discussed later). 

The practice of medicine then is to help individuals return to homeostasis when their own systems have become inadequate. Thus, stated another way, the discipline of medicine uses physiological parameters to establish reference states and then attempts to intervene to help return the parameters to that state.

Feedback Response Loop
Image created by JS at BYU-I 2013

Above is an image representation of a Feedback Response Loop. Notice that feedback loops can result in Negative or Positive Feedback. The red arrows in the top-left graph always show what would happen if the effector(s) always caused the variable to come back to set point (Negative Feedback). The red arrow in the right-hand graph (inside the cycle) shows what would happen if the effector(s) always caused the variable to go further and further from the set point (Positive Feedback).

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