Introduction to Physiology and Homeostasis

Definition and Relationship to Anatomy

Physiology is the study of the functions of living things, focusing on the mechanisms underlying body processes. It is closely related to anatomy, which is the study of body structure. The relationship between structure and function is fundamental: anatomical features are designed to support physiological roles.

• Structure and function are inseparable: Each anatomical structure is suited for its specific physiological function.

• Tools used in physiology: Anatomy, chemistry, and physics.

Levels of Organization in the Body

Hierarchical Structure

The human body is organized into several levels, each building upon the previous:

• Chemical level: Atoms and molecules (e.g., molecules in cell membranes).

• Cellular level: Basic unit of life (e.g., stomach lining cells).

• Tissue level: Groups of similar cells performing a function (e.g., stomach wall tissues).

• Organ level: Structures composed of multiple tissue types (e.g., the stomach).

• Body system level: Groups of organs working together (e.g., digestive system).

• Organism level: The complete living being.

Concept of Homeostasis

Definition and Importance

Homeostasis is the maintenance of a stable internal environment within narrow limits, essential for cell survival and function. In multicellular organisms, most cells are not in direct contact with the external environment; instead, they rely on the internal environment (the fluid surrounding cells) for life-sustaining exchanges.

• Extracellular fluid (ECF): Fluid outside cells, including plasma and interstitial fluid. Acts as a buffer zone between the external world and intracellular fluid (ICF).

• Intracellular fluid (ICF): Fluid within cells.

• ECF must be kept stable for proper cell function.

Critical Variables Regulated by Homeostasis

• Concentration of nutrients

• Concentration of O2 and CO2

• Concentration of waste products

• pH (acidity/alkalinity)

• Concentration of water, salts, and electrolytes

• Volume and pressure

• Temperature

Organization of the Body: Internal vs. External Environment

• External environment: Air, lumen of body cavities.

• Internal environment: ECF and ICF.

• Body systems work together to maintain homeostasis by regulating the internal environment.

Dynamic Steady State vs. Equilibrium

Definitions

• Dynamic steady state: Materials move between compartments, but there is no net change in composition.

• Equilibrium: Compartments have identical composition (rare in physiology).

• Homeostasis maintains a dynamic steady state, not equilibrium.

Body Systems and Homeostasis

Roles of Major Body Systems

• Digestive system: Obtains nutrients, water, and electrolytes; eliminates undigested food.

• Reproductive system: Not essential for homeostasis, but necessary for species survival.

• Circulatory system: Transports substances throughout the body.

• Respiratory system: Obtains O2, eliminates CO2, helps regulate pH.

• Nervous system: Controls rapid responses, higher functions (consciousness, memory).

• Urinary system: Regulates volume, electrolytes, pH; removes wastes.

• Endocrine system: Regulates processes via hormones, especially those requiring duration.

• Integumentary system: Protective barrier, temperature regulation.

• Muscular and skeletal systems: Support, movement, heat generation, calcium storage.

• Immune system: Defense against invaders, tissue repair.

Homeostatic Control Systems

Components and Function

A homeostatic control system is an interconnected network that maintains a given factor at an optimal level. It must:

• Detect deviations from normal (sensor)

• Integrate information (integrating center)

• Make adjustments (effector)

Types of Controls

• Intrinsic controls: Local regulation by an organ's own cells/tissues (e.g., muscle O2 during exercise).

• Extrinsic controls: Systemic regulation, usually by nervous and/or endocrine systems.

Feedback Mechanisms

Negative Feedback

Negative feedback is the primary mechanism for maintaining homeostasis. The response counteracts the initial stimulus, shutting off the response loop. This stabilizes the variable.

• Examples: Body temperature, blood pressure, blood glucose regulation.

Positive Feedback

Positive feedback amplifies the initial change, moving the variable further from the set point. It is less common but important in certain situations.

• Examples: Childbirth, blood clotting, heat stroke.

Feedforward Mechanisms

Feedforward responses are made in anticipation of a change, preparing the body before the variable is altered.

Components of Feedback Mechanisms

• Stimulus: Deviation from set point.

• Sensor: Monitors conditions.

• Integrating Center: Compares conditions to set point.

• Effector: Causes change to compensate for deviation.

• Response: Moves system toward set point.

Disruptions in Homeostasis

Pathophysiology

When homeostatic mechanisms fail, it can lead to pathophysiology—abnormal functioning associated with disease. Severe disruptions may result in death.

Summary Table: Comparison of Feedback Types

Key Equation: Cellular Respiration

Cells require a continual supply of nutrients and O2 to generate energy: