Chapter 1 of 5 - Physiology Course
Introduction to Human Physiology
Physiology explains how organs and systems work together. This chapter frames Homeostasis, feedback control, and how later chapters on membranes, heart, lungs, and kidneys fit into clinical reasoning.
For the USMLE, shelf exams, and wards, thinking in physiological terms helps you predict compensation - for example how respiratory and renal mechanisms buffer acid-base disturbances.
The baroreflex and other reflex arcs oppose sudden drops in blood pressure. The material below pairs diagrams with the vocabulary examiners expect.
What Is Physiology?
Physiology bridges anatomy and clinical medicine: it describes mechanisms - transport across membranes, electrical activity, pump function, ventilation-perfusion - that explain signs, symptoms, and treatment responses.
Levels of analysis include molecular (receptors, channels), cellular (excitation-contraction coupling), organ (cardiac output, GFR), and integrative regulation.
A practical habit: move downward from a clinical problem (hypotension, hypoxemia, hyperkalemia) to the failing mechanism.
Move upward from a molecular change (channelopathy, receptor blockade) to expected vitals and labs.
Later chapters follow that path: membranes enable excitability; the cardiovascular system distributes perfusion; the lungs set blood gas tensions; the kidneys set long-term fluid and electrolyte balance.
Regulation usually involves sensors, integrating centers (neural or endocrine), and effectors that compare signals to set points and restore the variable.
Negative feedback dominates stable vitals; the figures below show how major systems chain together to support cellular metabolism.
Quick Check
Which feedback pattern most commonly stabilizes blood pressure and body temperature?
Homeostasis and Feedback
Homeostasis is the stability of the internal environment. Negative feedback opposes change and stabilizes a regulated variable.
Positive feedback amplifies a process (for example oxytocin during labor) and is less common for steady-state variables.
In exams, negative feedback is the default for stable core temperature, mean arterial pressure, plasma osmolality, and blood glucose - all held near set points by loops that tune effector activity.
Positive feedback appears when the stem describes self-reinforcement - platelet plugs, oxytocin and uterine contractions - until an external brake ends the cycle.
Feed-forward mechanisms (for example cephalic-phase responses) exist, but board questions usually reward clear sensor-integrator-effector arcs.
| Concept | Example in the body |
|---|---|
| Set points | Target value for temperature, glucose, or mean arterial pressure |
| Sensor | Chemoreceptors, baroreceptors, osmoreceptors |
| Effector | Heart, vessels, kidneys, respiratory muscles |
The response opposes the original error signal; the loop settles when the variable returns close to the set point.
Controlled variable drifts (stimulus)
Example: MAP falls or core temperature rises.
Sensor detects deviation
Baroreceptors, thermoreceptors, osmoreceptors, chemoreceptors.
Afferent signal to integrator
Often CNS or endocrine axis compares signal to set point.
Effector activation or inhibition
Heart rate, vascular tone, sweating, ADH release, etc.
Response opposes the stimulus
Blood pressure or temperature moves back toward normal.
Loop dampens (negative feedback complete)
Reduced error signal lowers effector drive.
Each step depends on the prior one; failure at any link produces predictable clinical patterns (hypoxemia, azotemia, shock).
Respiratory system
Ventilation and diffusion set alveolar and arterial blood gases.
Cardiovascular system
Cardiac output and distribution deliver blood to capillary beds.
Capillary exchange
Nutrients, gases, and wastes cross between plasma and interstitium.
Cellular uptake / use
Membrane transporters and metabolism match supply to demand.
Renal system
Filters plasma, reabsorbs needed solutes and water, excretes the rest.
Negative feedback opposes change: sensors detect deviation from a set point, and effectors return the variable toward normal - core to temperature, glucose, and blood pressure regulation.
Fill in the Blank
The maintenance of stable internal conditions despite external change is called________.
What You Will Learn
- Membrane transport & resting potential - gradients, channels, Na/K-ATPase
- Cardiovascular physiology - cardiac cycle, output, pressures
- Respiratory physiology - ventilation, diffusion, gas transport
- Renal physiology - filtration, reabsorption, fluid balance
Practice now
- Turn your physiology PDF into a matching game - upload notes and play on lorea.app.
- Challenge a classmate to the feedback-loop game - free, no sign-up.
- Turn a physiology PDF into MCQs - instant question bank from lecture slides.
- Open the full study guide - one-page review of the whole course.
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