Body Fluids, Electrolytes, and Homeostasis: Study Notes

Body Fluids, Electrolytes, and Homeostasis

Introduction

The regulation of body fluids and electrolytes is essential for maintaining homeostasis in the human body. This topic covers the distribution of water in various compartments, mechanisms of exchange, and the physiological processes that balance intake and output.

Session Learning Outcomes

• Identify what is kept in homeostasis in a physiological setting

• Define water intake and output

• Calculate compartment fluid volumes

• Describe diffusion and bulk flow as mechanisms of exchange between fluid compartments

Homeostasis of Body Fluids

Definition and Importance

Homeostasis refers to the maintenance of a stable internal environment. In physiology, this includes the regulation of body fluid volumes, electrolyte concentrations, and osmotic balance.

• Key regulated variables: Water volume, sodium, potassium, chloride, and other electrolytes.

• Mechanisms: Renal function, thirst response, hormonal control (e.g., ADH, aldosterone).

• Example: The body maintains plasma osmolality within a narrow range to ensure proper cell function.

Water Intake and Output

Balancing Water Gain and Loss

Water balance is achieved by matching intake with output. The body gains water through ingestion and metabolic production, and loses water via skin, lungs, urine, and feces.

• Water Gain:

  • Food and drink: ~2.2 L/day

  • Metabolic production: ~0.3 L/day (from cellular respiration)

• Water Loss:

  • Skin (insensible loss): ~0.9 L/day

  • Lungs: ~0.9 L/day

  • Urine: ~1.5 L/day

  • Feces: ~0.1 L/day

• Equation for Water Balance:

  • Intake (2.2 L) + Metabolic Production (0.3 L) - Output (0.9 + 1.5 + 0.1 L) = 0

• Example: During dehydration, urine output decreases to conserve water.

Body Fluid Compartments

Distribution of Total Body Water

Total body water (TBW) is distributed among different compartments, each separated by selective membranes.

• Major Compartments:

  • Intracellular Fluid (ICF): Fluid within cells; ~28 L (about 2/3 of TBW)

  • Extracellular Fluid (ECF): Fluid outside cells; ~14 L (about 1/3 of TBW)

• Subdivisions of ECF:

  • Interstitial Fluid: ~11 L (3/4 of ECF)

  • Plasma: ~3 L (1/4 of ECF)

• Formulas:

  • $\text{Total Body Water (TBW)} = 0.6 \times \text{body weight (kg)}$

  • $\text{ICF} = 0.4 \times \text{body weight (kg)}$

  • $\text{ECF} = 0.2 \times \text{body weight (kg)}$

• Example: For a 70 kg adult, TBW ≈ 42 L, ICF ≈ 28 L, ECF ≈ 14 L.

Compartment Barriers and Solute Distribution

Different solutes are distributed based on the permeability of compartment barriers.

• Plasma membrane: Separates ICF from ECF; permeable to water, glucose, urea.

• Capillary endothelium: Separates plasma from interstitial fluid; permeable to water, NaCl, but not to large proteins like albumin.

• Solute Distribution:

  • Water, Glucose, Urea: Can cross plasma membrane

  • NaCl: Restricted to ECF

  • Albumin: Restricted to plasma

Table: Distribution of Major Solutes Across Body Fluid Compartments

Solute	Intracellular Fluid	Interstitial Fluid	Plasma
Water	Yes	Yes	Yes
Glucose	Yes	Yes	Yes
Urea	Yes	Yes	Yes
NaCl	No	Yes	Yes
Albumin	No	No	Yes

Mechanisms of Exchange Between Fluid Compartments

Diffusion and Bulk Flow

Exchange of water and solutes between compartments occurs via diffusion and bulk flow.

• Diffusion: Movement of molecules from high to low concentration; important for small solutes and gases.

• Bulk Flow: Movement of water and solutes together due to pressure gradients; occurs across capillary walls.

• Example: Oxygen diffuses from blood to tissues; plasma proteins remain in the vascular compartment due to size.

Osmosis Across a Semi-permeable Membrane

Principles of Osmosis

Osmosis is the movement of water across a semi-permeable membrane from an area of lower solute concentration to higher solute concentration.

• Driving force: Osmotic gradient created by solute concentration differences.

• Equation for Osmotic Pressure:

  • $\Pi = iMRT$

  • Where $\Pi$ = osmotic pressure, $i$ = van 't Hoff factor, $M$ = molarity, $R$ = gas constant, $T$ = temperature (K)

• Example: Water moves into a cell placed in a hypotonic solution, causing it to swell.

Effect of Tonicity on Cellular Structure

Tonicity describes the ability of a solution to cause a cell to gain or lose water.

• Isotonic: No net movement of water; cell volume remains constant.

• Hypotonic: Water enters the cell; cell swells and may burst.

• Hypertonic: Water leaves the cell; cell shrinks.

• Example: Red blood cells in hypertonic saline shrink due to water loss.

Movement Across Capillary Walls: Filtration and Reabsorption

Starling Forces

Fluid movement across capillary walls is governed by hydrostatic and oncotic pressures, known as Starling forces.

• Capillary Hydrostatic Pressure (Pc): Pushes fluid out of capillaries.

• Interstitial Hydrostatic Pressure (Pi): Pushes fluid into capillaries.

• Capillary Oncotic Pressure (πc): Pulls fluid into capillaries (due to plasma proteins).

• Interstitial Oncotic Pressure (πi): Pulls fluid out of capillaries.

• Net Filtration Pressure (NFP) Equation:

  • $\text{NFP} = (P_c - P_i) - (\pi_c - \pi_i)$

• Example: At the arterial end of a capillary, filtration predominates; at the venous end, reabsorption occurs.

Table: Typical Starling Forces Across Capillary Walls

Location	Hydrostatic Pressure (mm Hg)	Oncotic Pressure (mm Hg)
Arterial End	32	25
Venous End	15	25
Interstitial Fluid	2	0

Additional info: Academic context and equations have been added to clarify mechanisms and provide self-contained explanations.