Introduction to Anatomy & Physiology

Definition and Scope

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

• Structure and function are inseparable: The form of a body part is directly related to its 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 common 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 the survival of cells and, consequently, the organism. Most body cells are not in direct contact with the external environment; instead, they rely on the internal environment (extracellular fluid) for life-sustaining exchanges.

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

• Intracellular fluid (ICF): Fluid within cells.

• ECF must be kept relatively stable for cell survival.

Homeostatically Regulated Factors

Critical Variables

Homeostasis involves regulating several key variables:

• Concentration of nutrients

• Concentration of O2 and CO2

• Concentration of waste products

• pH (acidity/alkalinity)

• Concentration of water, salts, and other electrolytes

• Volume and pressure of body fluids

• Temperature

Body Systems and Homeostasis

Roles of Major Systems

Each body system contributes to homeostasis by maintaining specific factors:

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

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

• Circulatory system: Transports substances throughout the body.

• Urinary system: Regulates volume, electrolyte composition, and pH; removes wastes.

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

• Nervous system: Controls rapid responses and higher functions (e.g., consciousness).

• Integumentary system: Protective barrier; temperature regulation.

• Muscular and skeletal systems: Support, movement, temperature regulation, calcium storage.

• Immune system: Defense against invaders and cancer cells.

• Reproductive system: Not essential for individual homeostasis, but for species perpetuation.

Homeostasis: Dynamic Steady State

Steady State vs. Equilibrium

Homeostasis maintains a dynamic steady state—a condition where variables fluctuate within a narrow range, but are not necessarily identical across compartments. This is different from equilibrium, where compartments would have identical compositions. Homeostasis often involves disequilibrium between compartments, but with no net movement of materials.

Homeostatic Control Systems

Components and Mechanisms

A homeostatic control system is an interconnected network that maintains internal variables at optimal levels. It must:

• Detect deviations from normal (sensor)

• Integrate information (integrating center)

• Make adjustments (effector)

There are two main types of regulation:

• Intrinsic controls: Local regulation within an organ (e.g., muscle adjusting O2 use during exercise).

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

Feedback Mechanisms

Negative and Positive Feedback

Feedback mechanisms are essential for maintaining homeostasis:

• Negative feedback: The response counteracts the initial change, stabilizing the variable (e.g., body temperature, blood glucose).

• Positive feedback: The response amplifies the initial change, moving the variable further from the set point (e.g., childbirth, blood clotting).

Feedforward mechanisms anticipate changes and initiate responses before a variable is affected (e.g., salivation before eating).

Components of a Feedback Mechanism

• Stimulus: Deviation from set point

• Sensor: Monitors conditions

• Integrating Center: Compares to set point

• Effector: Causes change to compensate

• Response: Moves system toward set point

Examples of Feedback Mechanisms

Negative Feedback Example

• Regulation of blood glucose: When blood glucose rises, insulin is released to lower it; when it falls, glucagon is released to raise it.

• Body temperature regulation: Sweating or shivering to maintain core temperature.

Positive Feedback Example: Childbirth

During childbirth, the head of the baby pushes against the cervix, stimulating nerve impulses to the brain. The brain signals the pituitary gland to release oxytocin, which increases uterine contractions, pushing the baby further against the cervix and amplifying the cycle until delivery is complete.

Disruptions in Homeostasis

Pathophysiology

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

Summary Table: Comparison of Negative and Positive Feedback

Key Equation: Cellular Respiration

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