Body Fluid Compartments and Exchange

Body Fluid Compartments

• Body fluid is distributed between the intracellular fluid (ICF) and extracellular fluid (ECF) compartments
• ICF contains fluid within all the cells and comprises about 2/3 of total body water.
• ECF contains all fluids outside cells, including interstitial fluid and plasma, accounting for 1/3 of total body water.
• Transcellular fluid is a specialized type of ECF.
• ICF constitutes about 40% of total body weight.
• ECF constitutes about 1/3 of total body water.
• Interstitial fluid is about 3/4 of ECF
• Plasma is almost 1/4 of ECF

Fluid Exchange

• Plasma exchanges substances with interstitial fluid through capillary membrane pores.
• ICF and ECF are in continuous exchange.

Capillary Exchange of Fluids and Electrolytes

• Fluid and electrolyte exchange is primarily mediated by: diffusion and bulk flow.
• Diffusion allows movement of small molecules down concentration gradients.
• Bulk flow is mass movement of fluids and solutes due to pressure differences.

Capillary Pressure and Filtration

• Hydrostatic pressure pushes water out of the capillary - higher pressure results in more fluid leaving.
• Colloidal osmotic pressure pulls water into the capillary - higher protein concentration results in more fluid being pulled in.
• These opposing forces determine net filtration or reabsorption.

The Role of Lymphatic Capillaries

• More fluid exits the capillary at the arterial end via filtration than enters through absorption at the venous end, resulting in about 24 L filtered daily and 20.4 L reabsorbed.
• The excess fluid is collected and returned to the circulatory system via the lymphatic system.

Functions of Major Electrolytes (Sodium and Potassium)

• Sodium is responsible for resting membrane potentials, accounts for 90-95% of ECF osmolarity, and determines total body water and water distribution.
• Sodium gradient is a source of energy for cotransport of other solutes.
• Sodium-potassium pump generates body heat.
• Sodium bicarbonate (NaHCO3) buffers pH in ECF.

Functions of Potassium

• Major intracellular cation.
• Extracellular potassium influences skeletal and cardiac muscle activities.
• Essential for transmission of nerve impulses.
• Involved in acid-base balance, protein biosynthesis, and insulin secretion.

Clinical Abnormalities of Electrolytes Imbalance

Hyponatremia and Hypernatremia

• Sodium accounts for >90% of ECF solute, with normal level being 142 mEq/L.

Hyponatremia: Decrease in plasma sodium concentration

• Causes: Loss of sodium chloride from ECF (diarrhea, vomiting, Addison's disease), or excess water retention (dilutes sodium).

Hypernatremia: Increase in plasma sodium concentration

• Causes: Loss of water from ECF (dehydration), or excess sodium chloride added to ECF (excessive aldosterone secretion).

Edema

• Observable swelling caused by fluid accumulation in the interstitial space.
• Becomes evident when interstitial fluid volume increases by 2.5 to 3 liters.

Pathophysiological Mechanisms of Edema Formation

• Increased capillary filtration pressure: Increased fluid movement from capillaries to interstitial spaces due to increased pressure.
• Decreased capillary colloidal osmotic pressure: Decreased protein in blood, reducing osmotic pressure pulling fluid into the capillaries.
• Increased capillary permeability: Increased permeability of blood vessels allows proteins to leak into the interstitial space, increasing osmotic pressure there.
• Obstruction to lymph flow: Blockage of lymph drainage causes fluid to accumulate in interstitial spaces.

Decreased Capillary Colloidal Osmotic Pressure

• Plasma proteins exert osmotic force pulling fluid back into capillaries.
• Albumin is the most osmotically active plasma protein.
• Edema caused by decreased capillary colloidal osmotic pressure can be due to decreased production or abnormal loss of plasma proteins.

Factors Decreasing Capillary Colloid Osmotic Pressure

• Liver failure - impaired albumin synthesis
• Malnutrition/starvation - lack of amino acids for protein synthesis
• Kidney diseases - glomerular capillaries become permeable to plasma proteins
• Burn injury - injury and loss of plasma proteins

Increased Capillary Permeability

• Increased capillary pores or damaged capillary walls lead to increased permeability.
• Plasma proteins and other osmotically active particles leak into interstitial spaces.
• Accumulation of interstitial fluid due to increased tissue colloidal osmotic pressure.

Factors increasing capillary permeability

• Burn injury
• Capillary congestion

Obstruction of Lymph Flow

• Lymphatic system is essential for returning osmotically active plasma proteins to the circulatory system.
• Impaired lymph flow results in edema (lymphedema).

Factors causing lymphatic obstruction

• Malignant involvement of lymph structures
• Removal of lymph nodes
• Infection involving lymphatic channels and lymph nodes

Types of Edema

According to Distribution

• Localized edema: Occurs only in a specific part of the body.
• Generalized edema: Affects all body tissues.

According to Clinical Examination

• Pitting Edema: Indentation persists after pressure is released. Evaluated on a scale of 1 to 4.
• Non-pitting edema: Pressure does not produce persistent indentation, often caused by lymphedema or myxedema.

Homeostasis

• Body's attempt to maintain a constant internal environment.
• Requires constant monitoring and adjustments as conditions change.
• Dependent on control systems of the body.

Components of Control System

• Receptor: Receives information about environmental changes.
• Control Center: Processes information from the receptor.
• Effector: Responds to control center commands by opposing or enhancing the stimulus.

Negative and Positive Feedback

Negative Feedback

• System responds to change in a way that reverses the direction of change, maintaining homeostasis.

Positive Feedback

• Response amplifies the change in the variable.
• Less common than negative feedback.

Examples of Positive Feedback

• Nerve impulse generation: Threshold electric potential triggers action potential.
• Blood clotting: Activated platelets trigger further activation.

Harmful Positive Feedback

• Can be dangerous if not regulated.
• Example: High fever can increase metabolic changes, potentially leading to death.

Water Balance as an Example of Homeostasis

• Plasma osmolarity is the primary regulator of water balance.
• Dehydration causes increased plasma osmolarity.
• Increased osmolarity leads to water moving out of cells, causing initial cell volume reduction.
• Antidiuretic hormone (ADH) secretion is activated, leading to water retention and reducing plasma osmolality.