Anatomy 8-17-26

Anatomy and Physiology, Grades 11 and 12

Homeostasis and Feedback Systems

Module 1  |  40 Minutes  |  Mr. Gibbs

At a Glance

• 0:00 to 0:08  Review quiz

• 0:08 to 0:11  Quiz collection and framing

• 0:11 to 0:17  Homeostasis and the anatomy of a control system

• 0:17 to 0:26  Negative feedback, three complete loops

• 0:26 to 0:32  Positive feedback

• 0:32 to 0:38  Activity, Build the Loop

• 0:38 to 0:40  Close

Materials: whiteboard, Build the Loop handout, one per pair. Leave the generic loop diagram on the board from minute 11 onward and do not erase it.

1. Review Quiz

8 minutes. Your quiz, your content.

• Collect promptly at 8 minutes. Do not extend it, since the rest of the period has no slack.

• Transition line rather than a debrief: tell them you will go over it next class. Spending five minutes on answers here costs you the positive feedback section entirely.

If the quiz covers directional terminology and levels of organization, you can bridge directly: those were tools for describing structure. Today is the first day the course is about function, and homeostasis is the organizing question for every remaining module.

2. Homeostasis and the Anatomy of a Control System

6 minutes. Build the generic loop on the board and leave it up all period.

• Define it precisely, and correct the loose version students arrive with: homeostasis is the maintenance of a relatively stable internal environment despite changes in external conditions. Emphasize relatively. It is dynamic equilibrium, not constancy. Nothing in the body sits still at a number.

• Introduce the regulated variable: the specific quantity being controlled. Core temperature. Blood glucose. Plasma pH. Mean arterial pressure. Plasma calcium. Each has its own loop and its own set point.

• Give the ranges, because narrowness is the whole point: core temperature near 37 degrees Celsius. Arterial pH between 7.35 and 7.45, and outside 6.8 to 7.8 is not survivable. Fasting glucose near 70 to 100 milligrams per deciliter. Plasma calcium about 8.5 to 10.5. These are not preferences, they are the operating limits of enzyme function.

The six components, and use the formal terms

• Stimulus. A change in the regulated variable that moves it away from the set point.

• Receptor, or sensor. Detects the change. Baroreceptors, thermoreceptors, chemoreceptors, osmoreceptors.

• Afferent pathway. Carries the signal inward, toward the control center.

• Control center, or integrating center. Holds the set point, compares the actual value to it, and generates an error signal. Usually the hypothalamus, the medulla, or an endocrine gland.

• Efferent pathway. Carries the command outward, toward the effector. Nerve, hormone, or both.

• Effector. The structure that acts. Muscle or gland, essentially always.

• Then close the circle: the response changes the regulated variable, the receptor detects the new value, and the loop informs itself. That return path is what makes it feedback rather than a one way command.

• Flag one honest complication now, so it does not derail you later: these components are not always separate structures. A pancreatic beta cell senses glucose, integrates, and secretes insulin. Receptor, control center, and effector in one cell. The model is a framework, not an anatomical rule.

3. Negative Feedback, Three Complete Loops

9 minutes. Walk all three onto the generic diagram. Do not summarize, trace.

• Define it by what it does: the response opposes the stimulus and drives the variable back toward the set point. The loop then shuts itself down as the error shrinks. This is the mechanism behind nearly every regulated variable in the body.

• Make the oscillation explicit: because correction begins only after deviation is detected, the variable never sits exactly at the set point. It swings around it in a narrow band. Draw the wavy line. Students who picture a flat line misunderstand the entire concept.

Loop 1, thermoregulation, in the cold

• Stimulus: falling core temperature. Receptors: peripheral thermoreceptors in the skin and central thermoreceptors in the hypothalamus.

• Control center: the preoptic area of the hypothalamus.

• Efferent and effectors: sympathetic output causes cutaneous vasoconstriction, reducing heat loss at the surface. Somatic motor output drives shivering, contraction whose only product is heat. Thyroid hormone and epinephrine raise metabolic rate for nonshivering thermogenesis.

• Ask the reverse case and let a student run it: too hot. Vasodilation, sweating, sudomotor sympathetic fibers. Same control center, opposite effectors.

Loop 2, blood glucose

• Rising glucose after a meal is sensed by pancreatic beta cells, which release insulin, promoting uptake into muscle and adipose tissue and glycogen synthesis in the liver.

• Falling glucose between meals is sensed by alpha cells, which release glucagon, driving glycogenolysis and gluconeogenesis in the liver.

• Name the pattern, because it recurs everywhere: antagonistic control. Two opposing effectors give far finer regulation than one effector that can only be turned on or off. They will see this again with calcitonin and parathyroid hormone, and with sympathetic and parasympathetic tone.

Loop 3, blood pressure, and this one is the most clinical

• Stimulus: a fall in mean arterial pressure, for instance on standing quickly.

• Receptors: baroreceptors in the carotid sinus and aortic arch. Afferent pathways: glossopharyngeal and vagus nerves.

• Control center: the cardiovascular center in the medulla oblongata.

• Efferent and effectors: increased sympathetic output raises heart rate and contractility and causes vasoconstriction. Reduced parasympathetic tone permits heart rate to rise.

• Give the clinical hook: orthostatic hypotension is this loop responding too slowly. The lightheadedness on standing is the interval before correction arrives. They have all felt this loop fail briefly.

4. Positive Feedback

6 minutes. Precision matters here, because the name misleads.

• Define it by what it does, not by whether it is desirable: the response amplifies the stimulus and drives the variable further from its starting point. Positive does not mean beneficial. It means the loop reinforces itself.

• State the structural requirement: a positive feedback loop cannot end itself. Something outside the loop must terminate it. Ask what the terminating event is for each example before you name it.

Three physiological examples

• Parturition. Cervical stretch is detected, the hypothalamus signals the posterior pituitary to release oxytocin, uterine contractions intensify, the fetus is pushed further against the cervix, stretch increases. Terminating event: delivery.

• Hemostasis. Activated platelets release mediators that activate additional platelets, and thrombin generation catalyzes further thrombin generation. Terminating event: the vessel is sealed, and plasma anticoagulants confine the process to the site of injury.

• The action potential. Sodium entry depolarizes the membrane, depolarization opens more voltage gated sodium channels, more sodium enters. Terminating event: inactivation gates close. Tell them this is the mechanism behind Module 6 and that they are meeting it early on purpose.

• Add lactation and the LH surge in one line each if the room is following well. Cut both if it is not.

When positive feedback is pathological

• Name two, briefly, because it makes the concept real: in circulatory shock, falling cardiac output reduces coronary perfusion, which weakens the myocardium, which further reduces output. In severe hemorrhage, falling pressure reduces perfusion of the tissues that would otherwise compensate.

• The generalization worth writing down: positive feedback is useful when an event needs to run rapidly to completion and stop. It is lethal when nothing stops it.

Two refinements, thirty seconds each

• Set points are adjustable. Pyrogens raise the hypothalamic set point and the body then defends the higher temperature, which is why a febrile patient shivers while already hot. The system is functioning correctly at a new target.

• Not all control is reactive. Feedforward, or anticipatory, control acts before the variable changes. Heart rate rises at the start of exercise before any metabolic signal has accumulated, and insulin release begins with the sight and taste of food.

5. Activity, Build the Loop

6 minutes. Pairs. One scenario per pair, handout with a blank loop.

• The task: label all six components for the assigned scenario, name the regulated variable, and classify the loop as negative or positive with one sentence of justification.

• Scenarios to distribute: dehydration and ADH release. Rising plasma calcium. A drop in blood oxygen at altitude. Blood clotting after a cut. Body temperature during a fever. Blood pressure after standing.

• Where they will get stuck, and it is productive: identifying the control center. Students name the effector twice and leave the integrating center blank. Push on it.

• Share out, final two minutes: take only the fever and the clotting scenarios. Fever is negative feedback around a raised set point, which nearly everyone miscalls. Clotting is positive, and ask them to name the terminating event.

Collect as a formative grade. Grade the control center and the justification, not the neatness of the diagram.

6. Close

2 minutes.

• State the through line for the year: every system studied in this course will be examined for how it contributes to homeostasis. Ask that question of every module and you will be answering the majority of exam questions before they are asked.

• One sentence on disease, since it reframes the whole course: most pathology is a homeostatic loop failing. Diabetes is the glucose loop. Hypertension is the pressure loop. Aging is the progressive narrowing of the range across which these loops can still correct.

• Name the reading and the due date. Dismiss by procedure.

Objectives

• Students will define homeostasis as dynamic equilibrium and identify regulated variables with their physiological ranges.

• Students will name the six components of a homeostatic control system and identify each within a given scenario.

• Students will trace complete negative feedback loops for thermoregulation, blood glucose, and blood pressure, naming specific receptors, control centers, and effectors.

• Students will distinguish negative from positive feedback by mechanism rather than by desirability, and identify the terminating event of a positive feedback loop.

• Students will explain how a raised set point during fever is consistent with normal negative feedback.