The Human Body: An Orientation

Introduction

This section introduces the foundational concepts of anatomy and physiology, the organization of the human body, and the principles that govern its function and maintenance. Understanding these basics is essential for further study in Anatomy & Physiology.

Anatomy and Physiology

Definitions and Subdivisions

• Anatomy: The study of the structure of body parts and their relationships to one another.

  • Gross (Macroscopic) Anatomy: Study of large body structures visible to the naked eye (e.g., brain, kidneys, lungs).

  • Microscopic Anatomy: Study of structures too small to be seen without magnification (e.g., cells, tissues).

• Physiology: The study of the function of the body and how its parts work, often focusing on events at the cellular or molecular level.

  • Cardiovascular Physiology: Study of heart and blood vessel functions.

  • Neurophysiology: Study of nervous system functions.

• Principle of Complementarity: Structure and function are inseparable; function always reflects structure.

Levels of Structural Organization

Hierarchy from Simple to Complex

• Chemical Level: Atoms combine to form molecules.

• Cellular Level: Cells are the smallest units of living things; they vary in size and shape.

• Tissue Level: Groups of similar cells with a common function. Four basic tissue types:

  • Epithelium

  • Connective tissue

  • Muscle tissue

  • Nervous tissue

• Organ Level: Structures composed of at least two tissue types that perform specific functions (e.g., brain, liver, stomach).

• Organ System Level: Organs working together to accomplish a common purpose.

• Organismal Level: The sum total of all structural levels working together to keep us alive (a human being).

Organ Systems of the Human Body

Necessary Life Functions

Characteristics Required to Maintain Life

• Maintain Boundaries: Separation of internal and external environments (e.g., skin, cell membranes).

• Movement: Includes locomotion, movement of substances, and cellular movement.

• Responsiveness (Excitability): Ability to sense and respond to stimuli (e.g., withdrawal reflex).

• Digestion: Breakdown of ingested food for absorption and use.

• Metabolism: All chemical reactions in the body.

  • Catabolism: Breaking down substances into simpler components.

  • Anabolism: Synthesizing more complex substances from simpler ones.

• Excretion: Removal of wastes produced by metabolism and digestion.

• Reproduction: Cellular (mitosis) and organismal (offspring) reproduction.

• Growth: Increase in size and/or number of cells.

Survival Needs

Factors Required to Sustain Life

• Nutrients: Chemicals for energy and cell building (carbohydrates, proteins, fats, minerals, vitamins).

• Oxygen: Essential for oxidative reactions; survival without it is limited to a few minutes.

• Water: Most abundant chemical in the body (50–60% of body weight); necessary for chemical reactions.

• Normal Body Temperature: Around 98.6°F (37°C); deviations can disrupt metabolic reactions.

• Appropriate Atmospheric Pressure: Required for proper breathing and gas exchange in the lungs.

Homeostasis

Definition and Significance

• Homeostasis: The ability to maintain relatively stable internal conditions despite external changes; a dynamic state of equilibrium.

• Virtually every organ system contributes to homeostasis.

• Communication is critical, primarily via the nervous and endocrine systems.

Homeostatic Control Mechanisms

• Receptor: Sensor that monitors the environment and responds to stimuli by sending information to the control center.

• Control Center: Determines the set point, analyzes input, and determines the appropriate response.

• Effector: Provides the means for the control center's response to the stimulus.

Feedback Mechanisms

• Negative Feedback: The output reduces or shuts off the original effect of the stimulus, returning the variable to its ideal value. This is the most common feedback mechanism in the body.

  • Example: Regulation of body temperature, blood glucose by insulin.

• Positive Feedback: The response enhances or exaggerates the original stimulus, so the response is accelerated. Usually controls infrequent events.

  • Example: Labor contractions during childbirth, blood clotting.

Homeostatic Imbalance and Disease

• Most diseases are caused by homeostatic imbalance.

• As we age, control systems become less efficient, increasing risk of illness.

• Overwhelmed negative feedback mechanisms may allow destructive positive feedback to take over (e.g., heart failure).

• Severity of imbalance:

  • Moderate: Disorder or illness occurs.

  • Severe: Death may result.

Summary Table: Negative vs. Positive Feedback

Additional info: The concept of homeostasis is central to all of physiology and underlies the body's ability to adapt to internal and external changes. Disruptions in homeostasis are a key factor in the development of disease.