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Body Fluids and Cellular Compartments

Overview of Body Fluid Compartments

The human body is composed of various fluid compartments that are essential for physiological processes. Understanding the distribution and function of these fluids is fundamental in anatomy and physiology.

• Intracellular Fluid (ICF): The fluid contained within cells, making up about two-thirds of total body water.

• Extracellular Fluid (ECF): The fluid outside cells, including plasma and interstitial fluid, accounting for about one-third of total body water.

• Example: In a 72 kg adult, total body water is approximately 43 L, with about 28.6 L as ICF and 14.3 L as ECF.

Compartment	Fraction of Total Body Water	Volume (L)
Intracellular Fluid (ICF)	2/3	28.6
Extracellular Fluid (ECF)	1/3	14.3

Cell Structure and Function

Major Components of the Cell

Cells are the basic structural and functional units of life. Each cell contains several key components, each with specialized roles.

• Plasma (Cell) Membrane: The outer boundary of the cell, regulating interaction with the external environment and controlling the movement of substances in and out of the cell.

• Nucleus: The control center of the cell, directing cellular activities and containing genetic material (DNA).

• Cytoplasm: The region between the plasma membrane and the nucleus, containing organelles that perform specific functions.

• Organelles: Specialized structures within the cytoplasm, such as mitochondria (energy production), endoplasmic reticulum (protein and lipid synthesis), and Golgi apparatus (modification and packaging of proteins).

Plasma Membrane Structure

Composition of the Plasma Membrane

The plasma membrane is a selectively permeable barrier composed primarily of lipids and proteins, with a small amount of carbohydrates.

• Phospholipid Bilayer: Forms the fundamental structure, with hydrophilic (water-attracting) heads facing outward and hydrophobic (water-repelling) tails facing inward.

• Proteins: Embedded within or attached to the bilayer, serving as channels, carriers, receptors, or enzymes.

• Carbohydrates: Present in small amounts, mainly involved in cell recognition and signaling.

Types of Membrane Proteins

• Integral Proteins: Span the membrane and function as channels or carriers for transport.

• Peripheral Proteins: Attached to the membrane surface, often involved in signaling or maintaining cell shape.

• Channel Proteins: Allow specific ions or molecules to pass through the membrane.

• Carrier (Transporter) Proteins: Bind and transport substances across the membrane, either passively or actively.

• Receptor Proteins: Bind specific chemical signals (e.g., hormones, neurotransmitters) and initiate cellular responses.

• Enzymes: Catalyze chemical reactions at the membrane surface.

Membrane Transport Mechanisms

Passive Transport

Passive transport involves the movement of substances across the membrane without the use of cellular energy (ATP). Substances move down their concentration gradients.

• Simple Diffusion: Movement of lipid-soluble molecules (e.g., O2, CO2, fatty acids) directly through the phospholipid bilayer from high to low concentration.

• Facilitated Diffusion: Movement of larger or charged molecules (e.g., glucose, amino acids, ions) via channel or carrier proteins, still down the concentration gradient.

• Osmosis: Diffusion of water across a selectively permeable membrane from an area of low solute concentration (high water) to high solute concentration (low water).

Osmosis and Tonicity

• Isotonic Solution: Same solute concentration as the cell; no net water movement; cell size remains unchanged.

• Hypertonic Solution: Higher solute concentration than the cell; water moves out; cell shrinks (crenation).

• Hypotonic Solution: Lower solute concentration than the cell; water moves in; cell swells and may burst (lysis).

Active Transport

Active transport requires energy (usually from ATP) to move substances against their concentration gradients (from low to high concentration).

• Primary Active Transport: Direct use of ATP to transport molecules. Example: Sodium-potassium pump (Na+/K+ ATPase) moves 3 Na+ out and 2 K+ into the cell per ATP hydrolyzed.

• Secondary Active Transport: Uses the energy stored in the concentration gradient of one molecule (often Na+) to drive the transport of another molecule (e.g., glucose or amino acids) via symport (same direction) or antiport (opposite direction) mechanisms.

Equation for ATP hydrolysis:

$\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{energy}$

Vesicular Transport

Large molecules or particles are transported across the membrane via vesicles in processes requiring energy.

• Endocytosis: Uptake of substances into the cell by engulfing them in a vesicle. Includes phagocytosis (cell eating, for large particles) and pinocytosis (cell drinking, for fluids).

• Exocytosis: Release of substances from the cell when vesicles fuse with the plasma membrane.

• Example: Secretion of insulin from pancreatic beta cells via exocytosis.

Summary Table: Membrane Transport Mechanisms

Transport Type	Energy Required?	Direction	Example Substances
Simple Diffusion	No	High to Low	O2, CO2, fatty acids
Facilitated Diffusion	No	High to Low	Glucose, amino acids, ions
Osmosis	No	Water: Low to High solute	Water
Primary Active Transport	Yes (ATP)	Low to High	Na+, K+
Secondary Active Transport	Yes (indirect)	Low to High (for one substance)	Glucose, amino acids (with Na+)
Endocytosis/Exocytosis	Yes (ATP)	In/Out of cell	Proteins, hormones, debris

Key Terms and Definitions

• Diffusion: Movement of molecules from an area of higher concentration to lower concentration.

• Concentration Gradient: Difference in concentration of a substance between two areas.

• Homeostasis: Maintenance of a stable internal environment.

• Symport: Coupled transport of two substances in the same direction.

• Antiport: Coupled transport of two substances in opposite directions.

• ATP (Adenosine Triphosphate): The primary energy carrier in cells.

Additional info: Some explanations and table entries have been expanded for clarity and completeness based on standard Anatomy & Physiology curriculum.