Compartmentation: Cells and Tissues – Body Compartments and Membranes

About This Chapter

This chapter introduces the concept of compartmentation in human physiology, focusing on the organization of the body into functional compartments, the role of physiological membranes, and the structure and function of cells and tissues. Understanding these compartments is essential for comprehending how the body maintains homeostasis and coordinates complex physiological processes.

Body Cavities

Major Body Cavities

The human body contains several major cavities that house and protect vital organs. These cavities are separated by membranes and serve as distinct anatomical and functional regions.

• Cranial cavity: Encloses the brain within the skull.

• Thoracic cavity: Contains the heart and lungs, protected by the rib cage.

• Abdominopelvic cavity: Subdivided into the abdominal cavity (digestive organs) and pelvic cavity (reproductive and some excretory organs).

In addition to these, the body contains specialized fluid-filled compartments:

• Circulatory system: Blood vessels and heart chambers filled with blood.

• Eyes: Contain aqueous and vitreous humors.

• Cerebrospinal fluid (CSF): Surrounds the brain and spinal cord, providing cushioning and nutrient transport.

Diagram: Anatomical Body Cavities

The anatomical diagram (not shown here) illustrates the spatial relationships between the cranial, thoracic, abdominal, and pelvic cavities, as well as the membranes that separate them.

Lumens of Hollow Organs

Many organs in the body are hollow, with an internal space called a lumen. The lumen is the cavity or channel within a tube or tubular organ.

• Heart

• Lungs

• Blood vessels

• Intestines

• Bladder

Note: For some organs, the lumen is considered external to the body's internal environment (e.g., the lumen of the digestive tract is technically outside the body until substances are absorbed).

Functional Fluid Compartments

The body’s fluids are distributed into distinct compartments, each with specific functions and compositions.

• Extracellular fluid (ECF): Fluid outside the cells, subdivided into:

  • Plasma: The liquid component of blood.

  • Interstitial fluid: Fluid that bathes and surrounds tissue cells.

• Intracellular fluid (ICF): Fluid contained within cells.

Table: Major Body Fluid Compartments

Compartment	Location	Main Components
Plasma (ECF)	Blood vessels	Water, proteins, electrolytes
Interstitial Fluid (ECF)	Between cells	Water, electrolytes
Intracellular Fluid (ICF)	Inside cells	Water, proteins, organelles

Compartments Are Separated by Membranes

Membranes are essential for maintaining the integrity of body compartments. They regulate the movement of substances and provide structural boundaries.

• Pericardial membrane: Surrounds the heart, composed of layers of connective tissue and epithelial cells.

• Tissue membranes: Layers of cells supported by connective tissue, forming barriers between compartments.

• Cell membrane (plasma membrane): Phospholipid bilayer that encloses each cell, controlling entry and exit of substances.

Cell Membrane: Function

The cell membrane is a dynamic structure that performs several critical functions:

• Physical isolation: Separates the cell’s interior from the external environment.

• Regulation of exchange: Controls the movement of ions, nutrients, and waste products.

• Communication: Contains receptors and proteins for signaling with other cells and the environment.

• Structural support: Anchors the cytoskeleton and helps maintain cell shape.

Cell Membrane: Composition

The cell membrane is composed of a complex mixture of lipids, proteins, and carbohydrates, organized according to the fluid mosaic model.

• Lipids: Phospholipids (form bilayer), sphingolipids, cholesterol (modulates fluidity).

• Proteins: Integral (span the membrane), peripheral (attached to surface), and lipid-anchored proteins.

• Carbohydrates: Glycoproteins and glycolipids (involved in cell recognition and signaling).

Phospholipids can form micelles, liposomes, or bilayers in aqueous solutions, depending on their structure and environment.

Intracellular Compartments

The cell’s interior is organized into various compartments, each with specialized functions.

• Cytoplasm: Includes cytosol (fluid), inclusions (non-membranous particles), protein fibers, and organelles.

• Nucleus: Contains genetic material (DNA) and controls cellular activities.

Organelles and Inclusions

• Membranous organelles: Mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, peroxisomes.

• Inclusions: Ribosomes (free or attached), lipid droplets, glycogen granules (not surrounded by membranes).

• Protein fibers: Cytoskeleton (microfilaments, intermediate filaments, microtubules).

Cytoskeleton: Structure and Function

The cytoskeleton is a network of protein fibers that provides structural support, facilitates intracellular transport, and enables cell movement.

• Microfilaments (actin): Involved in cell shape and movement.

• Intermediate filaments: Provide mechanical strength (e.g., keratin, neurofilaments).

• Microtubules: Hollow tubes that form centrioles, cilia, and flagella; essential for cell division and intracellular transport.

Microtubule Functions

• Centrosome: Organizes microtubules and is important for cell division.

• Centrioles: Direct DNA movement during mitosis.

• Cilia: Move fluid across cell surfaces.

• Flagella: Propel cells (e.g., sperm) through fluid.

Motor Proteins

• Myosins: Responsible for muscle contraction.

• Kinesins and dyneins: Move vesicles and organelles along microtubules; dyneins also power cilia and flagella movement.

Mitochondria

Mitochondria are double-membraned organelles known as the "powerhouses" of the cell due to their role in ATP production.

• Structure: Outer membrane, highly folded inner membrane (cristae), and intermembrane space.

• Mitochondrial matrix: Contains enzymes and unique mitochondrial DNA.

• Function: Site of aerobic respiration and ATP synthesis.

Endoplasmic Reticulum (ER)

The ER is a network of interconnected membrane tubes involved in protein and lipid synthesis.

• Rough ER: Studded with ribosomes; site of protein synthesis and modification.

• Smooth ER: Lacks ribosomes; synthesizes fatty acids, steroids, and lipids; involved in detoxification and calcium storage in some cells.

Table: Comparison of Rough and Smooth ER

Feature	Rough ER	Smooth ER
Ribosomes	Present	Absent
Main Function	Protein synthesis and modification	Lipid and steroid synthesis, detoxification
Appearance	Granular	Smooth

Example: Liver cells have abundant smooth ER for detoxification, while pancreatic cells have extensive rough ER for enzyme production.

Additional info: The chapter also covers tissue remodeling and the integration of cells into tissues and organs, which are essential for understanding physiological function and pathology.