Vascular Fluid Balance

Introduction

• Body fluid is composed of body water and the solutes dissolved within it.
  • Forms the bulk of the cell protoplasm.
  • Acts as the transport medium for metabolites.
  • Transports waste products to the exterior of the body.
• The electrolytes dissolved in body water are important.
  • Potassium, calcium and magnesium are required for normal cell function.
  • Sodium and chloride are important for the maintenance of extracellular osmotic pressure.
  • Bicarbonate contributed to regulation of the extracellular pH.
• 60 - 70% of the lean body weight is made up by body water.
  • This is about 5% less in females.

Distribution of body water

• Body fluid is comprised of aroung 60% intracellular fluid, and 40% extracelluar fluid.
  • Fluid is able to move between these states.
• Extracellular fluid can be further divided to:
  • Intravascular plasma (8%)
  • Transcellular fluid
    • Cerebo-spinal fluid
    • Occular humours
    • Serous fluid
    • Synovial fluid
    • Digestive juice
  • Interstitial fluid
    • Tissue fluid around cells.
• Transcelllar and interstitial fluid combined make up the other 32% of the 40% extracellular fluid.

Whole Body Fluid Exchange

• There is a balance between the interstitial fluid compartment and plasma.
  • This results in a controlled environment for the body cells.

Mechanisms of Fluid Balance

• There are three main mechanisms that regulate fluid balance between the body and the external environment.
  1. Mechanisms regulating renal water output
     • e.g. anti-diuretic hormone (ADH).
  2. Mechanisms regulating renal sodium and water balance, e.g.
     • Glomerular filtration rate.
     • The angiotension - aldosterone loop.
  3. Mechanism of thirst
     • Controlled by the central nervous centre and the baroreceptors (vascular pressure receptors).
• It is likely that the effect of natriuretic hormone on blood levels of sodium also plays a role.

• There is also a certain amount of internal fluid turnover, in the
  • Gut
    • Fluid leaves the blood to the digestive glands and organs. It is secreted to the gut lumen before reabsorption to the blood.
  • Kidneys
    • Fluid leaves the blood, passes through the glomerulus and tubules and rejoins the blood.
  • CNS
    • Fluid from the blood passes to the choroid plexus to the CSF, before later rejoining the blood.
  • Capillaries
    • Fluid from the blood joins the interstitial fluid before rejoining the blood via the lymphatics

Tissue Fluid Exchange

• Capillaries are a relatively closed system of vessels.
  • Metabolites diffuse through the endothelium into the tissue fluid .
• Metabolite diffusion and water exchange depends on:
  1. Hydrostatic pressure at the arteriolar end of the vascular network.
  2. Osmotic pressure of the plasma proteins relative to interstitial proteins, at the venous end of the vascular network.
  3. Osmolality of plasma compared to interstitial fluid as a result of changes in distribution of sodium, chloride and other ions.

Tissue Fluid Exchange At Rest

• At rest, fluid and solutes are forced out of the arteriolar end into the tissue spaces by hydrostatic pressure.
• The osmotic pressure of plasma proteins at the venous end exceeds venous hydrostatic pressure.
  • Fluid is reabsorbed.
  • The main plasma protein is albumin.
• Approximately equal volumes of fluids are exchanged.
  • A small surplus of fluid is supplied to the tissues.
    • This is absorbed by the lymphatic system.

Tissue Fluid Exchange in Activity States

• For example, in muscle action of glandular secretion.
• There is enhanced temporary formation of tissue fluid.
• This formation of tissue fluid results from:
  1. Vascular Events
     • Blood flow is increased as a result of:
       • Increases arteriolar pressure and cardiac output.
       • Capillary dilatation.
  2. Production of metabolic wastes
     • Osmotically active molecules are produced.
       • For example, lactic acid and urea.
• At the end of activity, the excess fluid may be:
  • Secreted (if glandular).
  • Metabolised/ voided.
  • Retained temporarily in tissues and slowly reabsorbed
  • Absorbed by lymphatics.

Abnormal States of Sodium and Water Balance

• Water and sodium are intimately related.
  • Imbalance of one tends to lead to upset in the other.
  • Cellular function is disrupted by imbalance.

Abnormal States of Potassium, Magnesium and Calcium Balance

Calcium

• Hypocalcaemia is most significant.

• Hypercalcaemia is less common.