Chapter 1: Introduction to Physiology – Structure, Function, and Homeostasis

Introduction to Physiology

Overview

Physiology is the study of how the human body functions, integrating knowledge from anatomy, biochemistry, physics, and other sciences. Understanding physiology requires an appreciation of the relationship between structure and function at multiple levels of biological organization.

How Does That Work?

Defining Anatomy and Physiology

• Anatomy: The study/science of body structures.

• Physiology: The study/science of body functions. Utilizes principles from other sciences (e.g., biochemistry, engineering, physics).

• Applied Physiology: Examines how structure and function are integrally related, often in the context of functional anatomy.

Hierarchy of Structural Organization

Levels of Organization

1. Chemical Level: Atoms combine to form molecules.

2. Cellular Level: Cells are made up of molecules and are the basic units of life.

3. Tissue Level: Tissues consist of groups of similar cells with a common function.

4. Organ Level: Organs are made up of different types of tissues.

5. Organ System Level: Organ systems consist of different organs that work together closely.

6. Organismal Level: The human organism is made up of many organ systems.

Cells & Tissues – A Refresher

Basic Units of Life

• Cells: Smallest living units of the body; over 200 types exist in humans.

• Tissues: Groups of cells functioning together for a common purpose.

• Four Major Tissue Types:

  • Nervous

  • Muscle

  • Epithelial

  • Connective

Nervous Tissue

Structure and Function

• Neurons: Specialized to transmit information via electrical signals. Functions include communication, control, sensory input, integration, and motor output.

• Neuroglia: Support cells that assist neuron function.

Muscle Tissue

Structure and Function

• Primary Function: Movement of the body and its parts.

• Secondary Functions: Thermoregulation, extensibility, elasticity, contractility, and excitability.

Epithelial Tissue

Structure and Function

• Arranged in sheets with an underlying basement membrane of connective tissue.

• Can be simple (one layer) or stratified (multiple layers).

• Cells may have different shapes (squamous, cuboidal, columnar).

• Main Functions: Covers or lines body surfaces and cavities.

Epithelium and Glands

• Exocrine Glands: Secrete products into ducts that lead to the external environment (e.g., sweat glands).

• Endocrine Glands: Release hormones directly into the bloodstream (e.g., thyroid gland).

Connective Tissue

Structure and Function

• Most abundant and diverse tissue type in the body.

• Main Function: Support and structural framework for other tissues and organs.

• Characterized by fewer cells and more extracellular matrix (fibers, ground substance).

Organs and Organ Systems

Integration of Tissues and Organs

• Organs are formed from multiple tissue types working together.

• Organ systems are composed of multiple organs with related functions.

• Some organs are shared between systems (e.g., the male urethra is part of both the urinary and reproductive systems).

Simplified View of the Human Body

Internal vs. External Environments

• A single layer of epithelial tissue separates the internal environment from the external environment.

• This barrier is essential for maintaining homeostasis and protecting internal conditions.

Semipermeable Membranes

Fluid Compartments

• Body fluids are separated by semipermeable membranes.

• Main compartments:

  • Total Body Water (TBW)

  • Intracellular Fluid (ICF)

  • Extracellular Fluid (ECF)

  • Plasma

  • Interstitial Fluid (ISF)

Body Fluid	Volume (L)
TBW	42
ICF	28
ECF	14
Plasma	3
ISF	11

Homeostasis

Maintaining Internal Stability

• Homeostasis is the maintenance of constant conditions in the internal environment (ECF).

• Key regulated variables:

  • Composition of ECF

  • Temperature of ECF

  • Volume of ECF

• All organ systems (except reproductive) contribute to homeostasis.

How Do We Maintain Homeostasis?

Feedback Mechanisms

• Negative Feedback: The primary mechanism for maintaining homeostasis. A change in a regulated variable triggers a response that opposes the change.

• Components:

  • Regulated variable

  • Sensor(s)

  • Integration center

  • Effector(s)

Example: Regulation of blood glucose by the pancreas.

Positive vs. Negative Feedback

Comparison

• Negative Feedback: More common; stabilizes physiological variables (e.g., blood glucose, body temperature).

• Positive Feedback: Less common; amplifies a response (e.g., LH surge and estrogen secretion in the menstrual cycle).

Practice Applying Your Knowledge

Sample Questions

• Hemorrhage is the loss of whole blood, which is approximately 55-60% plasma. Without new fluids entering the body, how can plasma volume be elevated toward normal levels to maintain homeostasis?

• What are some regulated variables that are challenged during exercise?

Additional info: During exercise, variables such as body temperature, blood pH, oxygen and carbon dioxide levels, and blood glucose are regulated to maintain homeostasis.