Compartmentation: Cells and Tissues – Human Physiology Study Notes

Compartmentation: Cells and Tissues

Overview

This chapter introduces the concept of compartmentation in human physiology, focusing on how the body is organized into functional and anatomical compartments, the role of biological membranes, and the structure and function of tissues.

Functional Compartments of the Body

Anatomical Cavities

The human body contains three major anatomical cavities that house vital organs and provide protection and separation.

• Cranial cavity: Contains the brain.

• Thoracic cavity: Contains the heart and lungs, separated by the diaphragm.

• Abdominopelvic cavity: Contains digestive, reproductive, and excretory organs.

Fluid-Filled Compartments

In addition to anatomical cavities, the body has several fluid-filled compartments essential for physiological function.

• Circulatory system: Blood vessels and heart chambers.

• Eyes: Chambers filled with aqueous and vitreous humor.

• Cerebrospinal fluid (CSF): Surrounds the brain and spinal cord.

• Pleural and pericardial sacs: Surround the lungs and heart, respectively.

Body Compartments and Membranes

Compartments are separated by biological membranes, which regulate the movement of substances and maintain distinct environments.

• Pericardial membrane: Surrounds the heart.

• Tissue membranes: Separate organs and cavities.

• Phospholipid bilayer: Forms the cell membrane, creating a barrier between the cell and its environment.

Lumens and External Environment

Lumens of Hollow Organs

The lumen is the interior space of any hollow organ. In some cases, the lumen is considered an extension of the external environment.

• Examples: Heart, lungs, blood vessels, intestines.

• Properties: May be filled with air or fluid; for some organs, the lumen is exposed to the external environment (e.g., digestive tract).

Body Fluid Compartments

Extracellular and Intracellular Fluid

Body fluids are divided into two main compartments:

• Extracellular fluid (ECF): Fluid outside cells, including plasma and interstitial fluid.

• Intracellular fluid (ICF): Fluid within cells.

These compartments are separated by cell membranes, which control the movement of water, ions, and other molecules.

Biological Membranes

Structure and Function

Biological membranes are primarily composed of lipids and proteins, forming a selective barrier between compartments.

• Physical isolation: Separates cell contents from the external environment.

• Regulation of exchange: Controls entry and exit of substances.

• Communication: Facilitates signaling between cells and their environment.

• Structural support: Maintains cell shape and integrity.

The fluid mosaic model describes the dynamic arrangement of lipids and proteins in the membrane.

Membrane Proteins

• Integral proteins: Span the membrane; involved in transport and signaling.

• Peripheral proteins: Loosely attached to the membrane surface.

• Transmembrane proteins: Extend across the bilayer.

• Lipid-anchored proteins: Covalently attached to lipids.

• GPI anchor: Glycosylphosphatidylinositol anchor for protein attachment.

• Lipid rafts: Microdomains rich in sphingolipids and cholesterol, involved in signaling.

Membrane Carbohydrates

• Glycocalyx: Protective layer formed by glycoproteins and glycolipids on the external surface of the cell.

Summary Table: Body Cavities and Fluid Compartments

Cavity/Compartment	Main Contents	Function
Cranial cavity	Brain	Protection, neural control
Thoracic cavity	Heart, lungs	Circulation, respiration
Abdominopelvic cavity	Digestive, reproductive organs	Digestion, reproduction
Extracellular fluid (ECF)	Plasma, interstitial fluid	Transport, nutrient exchange
Intracellular fluid (ICF)	Cytosol	Cellular metabolism

Key Terms and Definitions

• Compartmentation: The division of the body or cell into distinct regions with specialized functions.

• Lumen: The interior space of a hollow organ.

• Extracellular fluid (ECF): Fluid outside cells.

• Intracellular fluid (ICF): Fluid inside cells.

• Biological membrane: A selective barrier composed of lipids and proteins.

• Fluid mosaic model: Describes the structure of cell membranes as a mosaic of components that move fluidly.

Example: Membrane Transport Equation

The rate of diffusion across a membrane can be described by Fick's Law:

$J = -D \frac{dC}{dx}$

Where $J$ is the flux, $D$ is the diffusion coefficient, and $\frac{dC}{dx}$ is the concentration gradient.

Summary

• Body compartments are essential for organizing physiological functions.

• Biological membranes separate and regulate these compartments.

• Understanding compartmentation is fundamental to the study of human physiology.