Physiology Chapter 15: Derangements of Body Fluids

Fluid and Haemodynamic Disorders

Homeostasis

• The term "homeostasis" was coined by Claude Bernarde in 1949 to describe the state in which the interstitial fluid that bathes the cells and the plasma maintain the normal morphology and function of the cells and tissues of the body.
• Homeostasis is maintained by living membranes with varying permeabilities, such as vascular endothelium and the cell wall, which play an important role in exchange of fluids, electrolytes, nutrients, and metabolites across the compartments of body fluids.

Body Fluid Compartments

• The body fluid compartments are divided into two main compartments: intracellular fluid (ICF) and extracellular fluid (ECF).
• ICF comprises 33% of the body weight, while ECF comprises 27% of the body weight.
• ECF is further divided into four subdivisions:
  • Interstitial fluid (including lymph fluid) - 12% of body weight
  • Intravascular fluid (blood plasma) - 5% of body weight
  • Mesenchymal tissues (dense connective tissue, cartilage, and bone) - 9% of body weight
  • Transcellular fluid (secretions of secretory cells) - 1% of body weight

Water and Electrolyte Balance

• Water is the principal and essential constituent of the body, making up 50-70% of the body weight.
• Electrolytes, such as sodium, potassium, calcium, magnesium, and phosphate, are present in the body fluids.
• The concentration of electrolytes is different in intracellular and extracellular fluids.
• The normal composition of the internal environment consists of water, electrolytes, and nutrients.

Acid-Base Balance

• An acid is a molecule or ion that can give off a hydrogen ion (H+ ion donor), while a base is a molecule or ion that can take up a hydrogen ion (H+ ion acceptor).
• The body regulates acid-base balance through buffer systems, pulmonary mechanisms, and renal mechanisms.
• Buffers are substances that have weak acids and strong bases and limit the change in H+ ion concentration to the normal range.

Pressure Gradients and Fluid Exchanges

• Osmotic pressure is the pressure exerted by the chemical constituents of the body fluids.
• Hydrostatic pressure is the pressure of fluid in capillary and interstitial fluid.
• Effective oncotic pressure is the difference between the higher oncotic pressure of plasma and the lower oncotic pressure of interstitial fluid.
• Effective hydrostatic pressure is the difference between the higher hydrostatic pressure in the capillary and the lower tissue tension.

Derangements of Body Water

• Oedema: abnormal and excessive accumulation of "free fluid" in the interstitial tissue spaces and serous cavities.
• Dehydration: abnormal loss of water from the body.
• Overhydration: abnormal gain of water by the body.

Oedema

• Oedema can be caused by decreased plasma oncotic pressure and hypoproteinaemia, increased hydrostatic pressure in the capillary, lymphoedema, tissue factors, and increased capillary permeability.
• Oedema can occur in the interstitial space or in body cavities, such as the peritoneal, pleural, or pericardial cavities.### Oedema
• Oedema is the accumulation of excess fluid in the interstitial space
• It can be pitting or non-pitting, depending on the underlying cause
• Pitting oedema is characterized by a depression or pit that remains after pressure is applied to the skin
• Non-pitting oedema does not leave a pit, and is often seen in myxoedema, elephantiasis, and other conditions

Transudate vs Exudate

• Transudate is a type of oedema fluid with low protein content (less than 1 gm/dl) and low specific gravity (less than 1.015)
• Exudate is a type of oedema fluid with high protein content (more than 2.5 gm/dl) and high specific gravity (more than 1.018)
• Transudate is typically seen in cardiac and renal disease, while exudate is seen in inflammatory conditions

Mechanisms of Oedema

• Decreased plasma oncotic pressure: results in shift of fluid from blood vessels to interstitial space
  • Causes: hypoproteinaemia (albumin levels below 2.5 gm/dl), protein-losing enteropathy, and liver disease
• Increased capillary hydrostatic pressure: results in increased fluid shift from blood vessels to interstitial space
  • Causes: cardiovascular disease, constrictive pericarditis, and passive congestion
• Lymphatic obstruction: results in accumulation of fluid in interstitial space
  • Causes: removal of axillary lymph nodes, tumours, and filariasis
• Tissue factors: alterations in oncotic pressure of interstitial fluid and tissue tension can contribute to oedema
• Increased capillary permeability: allows proteins and fluid to leak from blood vessels into interstitial space
  • Causes: inflammation, anaphylaxis, and certain drugs and chemicals
• Sodium and water retention: results in increased fluid retention in extravascular compartment
  • Causes: intrinsic renal mechanisms, extra-renal mechanisms, and ADH release

Examples of Oedema

• Localised oedema: lymphatic oedema, inflammatory oedema, allergic oedema, pulmonary oedema, cerebral oedema, etc.
• Generalised oedema (anasarca or dropsy): often seen in cardiac disease, renal disease, nutritional deficiencies, and protein-losing enteropathy