Compartmentation: Cells and Tissues – Study Notes for Anatomy & Physiology

Compartmentation: Cells and Tissues

Introduction

This chapter explores the organization of the human body into compartments, the structure and function of biological membranes, and the foundational tissue types. Understanding compartmentation is essential for grasping how physiological processes are specialized and regulated within the body.

Major Body Cavities and Compartments

Anatomical Body Cavities

The human body is organized into distinct anatomical cavities, each housing specific organs and structures.

• Cranial cavity: Contains the brain and spinal cord.

• Thoracic cavity: Enclosed by the rib cage; includes the pleural sacs (surrounding the lungs) and the pericardial sac (surrounding the heart).

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

Example: The heart is located within the pericardial sac of the thoracic cavity.

Functional Body Cavities

Body fluids are distributed in two main compartments:

• Extracellular fluid (ECF): Accounts for about 1/3 of total body fluid. Includes interstitial fluid (surrounds most cells) and blood plasma (fluid inside blood vessels).

• Intracellular fluid (ICF): Makes up about 2/3 of total body fluid. Found within all cells and can be further subdivided into organelle-specific compartments (e.g., nucleus, mitochondria).

Additional info: The separation of ECF and ICF is critical for maintaining homeostasis and proper cellular function.

Advantages and Disadvantages of Compartmentation

Compartmentation allows biological contents to differ from one another, enabling specialization and organization.

• Advantages:

  • Specialization of function (e.g., mitochondria for energy production).

  • Grouping of similar molecules (e.g., enzymes of the TCA cycle within mitochondria).

• Disadvantages:

  • Creates barriers that can hinder transport of molecules (e.g., gas transport, electrolyte movement).

  • Cells require specialized mechanisms for transport across compartments.

Biological Membranes

Definitions and Types

The term membrane can refer to:

• Tissue membranes: Line body cavities (e.g., mucous membranes, peritoneum).

• Cell membranes: Composed of a phospholipid bilayer, forming the boundary of cells.

Functions of the Cell Membrane

The cell membrane performs several essential functions:

• Physical isolation: Acts as a barrier separating the cell from its environment.

• Regulation of exchange: Controls entry and exit of ions, nutrients, and gases.

• Communication: Surface proteins (receptors) recognize and respond to external signals.

• Structural support: Provides attachment points for the cytoskeleton and forms cell junctions.

Membrane Structure and Components

Biological membranes are primarily composed of lipids and proteins, with the ratio varying by cell type.

• Lipids:

  • Phospholipids (form bilayers, micelles, or liposomes)

  • Cholesterol (modulates fluidity)

  • Sphingolipids (involved in signaling and structure)

• Proteins:

  • Integral proteins (span the membrane; removal disrupts membrane integrity)

  • Peripheral proteins (attached by non-covalent bonds; removal does not disrupt membrane)

  • Transmembrane proteins (a type of integral protein that loops across the membrane multiple times)

  • Lipid-anchored proteins (covalently bonded to one side of the membrane)

• Carbohydrates: Attached to lipids or proteins, serving as physical barriers or immune markers (e.g., blood types).

Example: Blood type antigens are glycosylated proteins on the surface of red blood cells.

Separation of Compartments by Membranes

Membrane Barriers

Compartments are separated by membranes, which can be:

• Cellular: Phospholipid bilayer surrounds each cell.

• Tissue: Membrane layers such as the pericardial membrane (heart) or peritoneum (abdominal cavity).

Additional info: These barriers are essential for maintaining distinct environments and enabling specialized functions within organs and tissues.

Summary Table: Major Body Cavities

Cavity	Main Contents	Membrane
Cranial	Brain, spinal cord	Meninges
Thoracic	Lungs, heart	Pleural sac (lungs), pericardial sac (heart)
Abdominal	Digestive organs	Peritoneum
Pelvic	Reproductive organs	Peritoneum

Key Equations

Fluid compartment distribution:

• Extracellular fluid (ECF): $ECF = \frac{1}{3} \times \text{Total Body Fluid}$

• Intracellular fluid (ICF): $ICF = \frac{2}{3} \times \text{Total Body Fluid}$

Conclusion

Compartmentation and biological membranes are fundamental to the organization and function of cells and tissues in the human body. Understanding these concepts provides a foundation for further study of tissue types, organ systems, and physiological processes.