Cellular and Body Fluid Compartments in Human Physiology

Human Physiology Overview: Cellular and Body Fluid Compartments

Introduction to Levels of Organization

Human physiology is studied at multiple levels of biological organization, from the cellular level to the organism level. Understanding these levels is essential for grasping how the body functions as an integrated system.

• Cellular Level: The basic unit of life, such as a squamous epithelial cell, with a cell membrane that regulates entry and exit of substances.

• Tissue Level: Groups of similar cells, such as stratified squamous epithelium, performing specific functions.

• Organ Level: Structures composed of multiple tissue types, such as the esophagus.

• Organ System Level: Groups of organs working together, e.g., the digestive system.

• Organism Level: The complete human body, integrating all systems.

Body Fluid Compartments

Body fluids are distributed in distinct compartments separated by membranes, which are crucial for physiological processes and homeostasis.

• Tissue Membranes: Enclose organs and tissues, providing structural boundaries.

• Cell Membranes: Line the cell and act as selectively permeable barriers, controlling the movement of substances in and out of the cell.

Types of Body Fluid Compartments

• Intracellular Fluid (ICF): Fluid within cells; makes up approximately 2/3 of total body water.

• Extracellular Fluid (ECF): Fluid outside cells; comprises about 1/3 of total body water.

Subdivisions of Extracellular Fluid

• Interstitial Fluid: Fluid between cells, providing a medium for nutrient and waste exchange.

• Blood Plasma: The liquid component of blood, involved in transport of nutrients, gases, and waste products.

Membrane Structure and Function

The cell membrane is a critical structure that maintains the integrity of the cell and regulates its interactions with the environment.

• Phospholipid Bilayer: The fundamental structure of the cell membrane, composed of amphiphilic molecules with hydrophilic heads facing outward and hydrophobic tails facing inward.

• Selectively Permeable Barrier: Allows certain substances (e.g., water, small nonpolar molecules) to pass while restricting others (e.g., charged particles, large molecules).

Summary Table: Body Fluid Compartments

Compartment	Location	Proportion of Total Body Water	Main Components
Intracellular Fluid (ICF)	Inside cells	~2/3	Water, ions, proteins
Extracellular Fluid (ECF)	Outside cells	~1/3	Interstitial fluid, blood plasma
Interstitial Fluid	Between cells	Part of ECF	Water, ions
Blood Plasma	In blood vessels	Part of ECF	Water, proteins, nutrients

Key Concepts

• Homeostasis: The maintenance of stable internal conditions, including the balance of fluid compartments.

• Equilibrium: The state in which concentrations of substances are balanced across compartments.

Example

When a person becomes dehydrated, the proportion of water in the intracellular and extracellular compartments changes, potentially disrupting cellular function and homeostasis.

Additional info: The cell membrane's selective permeability is essential for processes such as nutrient uptake, waste removal, and signal transduction.