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.

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.

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

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.

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.

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.

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.

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

Uncommon.
When it does occur, it is an iatrogenic condition.
- For example, administering excess saline solutions in animals with impaired renal function.

Known as "salt poisoning".
Occurs sporadically in the U.K. as a group or herd problem in intensive systems.
Mainly affects pigs and poultry.
Caused by:
- Accidental ingestion of excess salt
  - Either by dietary contamination, or error.
- Deprivation of water on relatively high salt diets.
  - This is the more likely scenario.
Salt poisioning may be
- Peracute/ acute
- Subacute/ chronic

Peracute cases are often found dead.
- Only shock haemorrhages are seen at autopsy.
Acute cases present as abdominal pain, diarrhoea and vomiting.
- Death occurs in 24 - 48 hours.
- At autosy, may see:
  - Shock haemorrhages.
  - Gut congestion.
  - Fluid accumulations in body cavities.

For definitave diagnosis, histological examination is required.
On histological examination, the following may be seen:
- Perineuronal vasculation and cerebral neuronal necrosis.
  - Polio-encephalomalacia.
- Eosinophilic meningoencephalitis.
- Perivascular oedema in the brain.

Magnesium is important for muscular and nervous conduction.
Low Mg ⁺⁺ diets may induce acute hypo-magnesaemia in lactating cows.
- Seen as tremors and convulsions (tetany).
- For example, poor quality silage.
High K⁺ diets induce a relative Mg⁺⁺ imbalance.
- For example, lush pasture.
- Known as "grass staggers".
- Produces tetany and ataxia, and sometimes cardiac arrest.
- Only shock haemorrhages can be seen at autopsy; i.e. non-specific findings.

Potasstium is important for nervous and muscle conduction, especially in cardiac muscle.
The hypokalaemia is associated with diarrhoea and dehydration in calves.
- Animals die from cardiac arrest following ventricular dissociation.

Hypocalcaemia is most significant.
- "Milk fever" in lactating cattle.
- Eclampsia (parturient tetany) in bitches.
Hypercalcaemia is less common.
- Usually the result of dietary imbalance.
- Produces metastatic vascular calcification.