Introduction to Physiology: Homeostasis, Control Systems, and Biological Rhythms

Introduction to Physiology

What is Physiology?

Physiology is the study of the normal function of living organisms and their component parts. It focuses on how organisms, organ systems, organs, cells, and biomolecules carry out the chemical and physical functions that exist in a living system.

• Definition: "Knowledge of nature"; the science of the functions of living things.

• Includes: All chemical and physical processes in the body.

• Relation to Anatomy: Physiology is closely related to anatomy (structure).

Mechanistic vs. Teleological Explanations

Physiology seeks to understand both why (teleological approach) and how (mechanistic approach) body processes occur.

• Teleological: Explains why a process occurs (its purpose or function).

• Mechanistic: Explains how a process occurs (the steps or mechanisms involved).

• Example: Why do red blood cells carry oxygen? Teleological: Because cells need oxygen and red blood cells bring it to them. Mechanistic: Oxygen binds to hemoglobin in red blood cells, which are pumped by the heart throughout the body.

Themes in Physiology

• Structure and Function: Closely related; structure determines function.

• Molecular Interactions and Compartmentation: Structure at the molecular and organ level allows for specialization and function.

• Energy: Living organisms need energy for making or breaking chemical bonds and for physiological processes.

• Information Flow: Coordinates body function via local or long-distance signaling (e.g., nervous and endocrine systems).

• Homeostasis: Maintains internal stability.

Homeostasis

Definition and Importance

Homeostasis is the ability of the body to maintain a relatively stable internal environment in response to changes in the external environment. It is essential for the survival of multicellular organisms.

• The internal environment is the extracellular fluid (ECF) that surrounds cells.

• Homeostasis involves compensatory mechanisms that act to return variables to their normal range of values.

• Failure to compensate can lead to disease or death.

• Example: Diabetes mellitus is a common disease of failed homeostasis (abnormal insulin action).

Process of Homeostasis

1. Internal environment can change due to external or internal factors.

2. Loss of homeostasis is sensed by the organism.

3. Physiological attempt to correct the internal environment.

4. Internal environment is maintained in a dynamic steady state (not chemical equilibrium).

Control Systems and Homeostasis

Control systems keep regulated variables within a set range of values around a set point, maintaining a stable internal environment.

• Regulated (physiological) variable: A quantity the body can measure and control (e.g., body temperature, blood pressure, heart rate).

• Set point: The value the variable is kept around (e.g., body temp set point: 98.7°F).

• Control system components:

  1. Input signal

  2. Controller/integrating center

  3. Output signal

Types of Control

• Local control: Restricted to the same cell or tissue involved; response is local.

• Reflex control: More complex; uses nervous and/or endocrine systems, and the integrating center is distant from the stimulus and response.

Reflex Pathway: The Response Loop

Stages of Response Loop	Examples
Stimulus: Change in regulated variable that moves out of its desirable range	Body temp. Set point: 98.7°F. If range 95.5°F - 100°F. Too cold → Body temp decreases e.g., 50°F
Sensor: Senses stimulus; sends input signal to integrating center when sensor is activated	Thermoreceptors
Input signal: Evaluates the info from the sensor; initiates output signal	How you get info from skin to brain (e.g., neurons)
Integrating center: Info gets sent here and figures out what the body must do. Compares stimulus to set point and figures out if it’s within or out of range.	Nervous system/Brain
Output signal: Info about the response sent to target	Hormones and neurons
Target (effector): Effector that acts on regulated variable. Final destination.	Muscle
Response: Action that affects the regulated variable and brings it back to the desired range	Shivering (increase in body temp.) Feedback tells you how often you need to go through the response loop to get back to set point.

The Feedback Loop

• Negative feedback: Maintains homeostasis by opposing/eliminating the stimulus.

  • Example 1: Stimulus = lower temp → Response = shiver → increase temp → stimulus eliminated.

  • Example 2: Stimulus = high blood pressure → Response = lower blood pressure → stimulus eliminated.

• Positive feedback: Does not maintain homeostasis. Response reinforces stimulus, producing a rapid, self-amplifying cycle. Must be stopped by an outside factor.

  • Example: Childbirth (uterine contractions push baby into cervix, stretching cervix more, which increases contractions until delivery).

Set Points and Variability

• Set points can change (e.g., fever raises body temp set point to destroy pathogens).

• When values are within range, no response is triggered.

• When values are out of range, a response loop is activated.

Biological Rhythms

Definition and Examples

Biological rhythms are repeating patterns of regulated variables that change predictably and create cycles of change within a period of time. These are called biological rhythms or biorhythms.

• Daily (circadian) rhythms: Most common in humans; repeat every 24 hours.

• Examples:

  • Body temperature is lowest in the early morning and peaks in the late afternoon.

  • Plasma cortisol is lowest during sleep and peaks shortly after awakening.

Set Point and Acclimatization

• Set points can change over time (e.g., fever, acclimatization to a new environment).

• Acclimatization: The natural adaptation of physiological processes to a given set of environmental conditions.

Summary Table: Negative vs. Positive Feedback

Feedback Type	Definition	Example	Effect on Homeostasis
Negative Feedback	Response opposes or removes the stimulus	Body temperature regulation, blood pressure control	Maintains homeostasis
Positive Feedback	Response reinforces the stimulus	Childbirth, blood clotting	Does not maintain homeostasis; must be stopped by outside factor

Key Equations

• Law of Mass Action: The rate of a chemical reaction is proportional to the product of the concentrations of the reactants. $Rate = k[A][B]$

Additional info: Some explanations and examples have been expanded for clarity and completeness based on standard academic context in physiology.