FLUID COMPARTMENT, FUNCTION & BALANCE

Prepared by, Dr. Nicole Seng Lai Giea

What so important? Loss of 10% -- disturbance of body

function Loss of 6-8% -- sensation of thirst,

headache and muscular incoordination, rise of body temperature

Loss of 20% -- delirium, coma, death

Variations in Water Content Variation due to Age Variation between Individuals (adipose

tissue) Habitat

Functions Transfer medium ( dissolved nutrients

and waste products) Secretion and excretion ( glandular

product) Temperature regulation Lubricant for body surface

Homeostasis of body water Removal or excess water (kidney,

alimentary, lung, skin, sweat, mammary glands)

All these mechanisms except lungs can be inhibited to enhance water conservation

Fluid Compartments

20% extracellular fluid

-5% plasma-15% interstitial

volume

40% intracellular fluid

Total body water (60% bodyweight)

Total body water Intracellular: 70% The cytosol has no single function the site of multiple cell processes Extracellular: 30% ECF is divided into several smaller

compartments (eg plasma, Interstitial fluid, digestive fluid and transcellular fluid)

1 gm = 1 ml; 1 kg = 1 liter; 1 kg = 2.2 lbs total body water: 55-75% (60%) of body

weight intracellular water: 30-40% of body weight extracellular water (plasma water +

interstitial water): 23-33% (30%) of body weight interstitial water: 15-25% of body weight plasma water: 5% of body weight blood volume: 8-10% of body weight (blood volume = plasma water + red blood

cell volume)  The ratio of ICF to ECF is 55:45.

major division is into Intracellular Fluid (ICF) and Extracellular Fluid (ECF), separated by the cell membranes

ECF -- Interstitial fluid  20% in total body fluid and interstices of

all body tissues  link between the ICF and the

intravascular compartment Oxygen, nutrients, wastes and chemical

messengers all pass through the ISF low protein concentration (in comparison

to plasma)  Lymph is considered as a part of the ISF.

The lymphatic system returns protein and excess ISF to the circulation

ECF -- Plasma 5% in total and it differs from ISF in its

much higher protein content and its high bulk flow (transport function)

Blood contains suspended red and white cells so plasma has been called the ‘interstitial fluid of the blood’

The fluid compartment called the blood volume is interesting in that it is a composite compartment containing ECF (plasma) and ICF (red cell water).

ECF -- Transcellular fluid 

Transcellular fluid (<1%) formed from the transport activities of cells. It is contained within epithelial lined spaces and produced by secretory cells

It includes CSF, GIT fluids, bladder urine, aqueous humour and joint fluid. The electrolyte composition of the various transcellular fluids are quite dissimilar

Aqueous humour thick watery substance filling the space between

the lens and the cornea Maintains the intraocular pressure Provides nutrition (e.g. amino acids and glucose) for

the avascular ocular tissues; posterior cornea, trabecular meshwork, lens, and anterior vitreous.

May serve to transport ascorbate in the anterior segment to act as an anti-oxidant agent.

Presence of immunoglobulins to defend against pathogens.

Provides inflation for expansion of the cornea and thus increased protection against dust, wind, pollen grains and some pathogens.

for refractive index.

CSF  clear, colourless, salty fluid that occupies

the subarachnoid space and the ventricular system around and inside the brain and spinal cord

In essence, the brain "floats" in it It constitutes the content of all intra-

cerebral (inside the brain, cerebrum) ventricles, cisterns, and sulci as well as the central canal of the spinal cord

It acts as a "cushion" or buffer for the cortex, providing a basic mechanical and immunological protection to the brain inside the skull

It is produced in the choroid plexus

Functions of CSF Buoyancy Protection: CSF protects the brain tissue

from injury when jolted or hit Chemical stability: CSF flows throughout

the inner ventricular system in the brain and is absorbed back into the bloodstream, rinsing the metabolic waste from the central nervous system through the blood-brain barrier

Prevention of brain ischemia

Joint fluid Synovial fluid is a viscous, fluid found

in the cavities of synovial joints with its yolk-like consistency 

the principal role is to reduce friction between the articular cartilage during movement

ECF – gut water  6-8% in total Ruminant and monogastric herbivore

with large ceca have higher in percentage

Regulation of fluid balance To maintain an ionic environment suitable for

the functioning of the various cells of body Components of Daily Obligatory Water

Loss Insensible loss: 800 mls Minimal sweat loss: 100 mls Faecal loss: 200 mls Minimal urine volume to excrete solute load:

500 mls Total: 1,600 mls Fluid input is from 2 major sources: External: Oral intake of fluids and food (and/or

IV fluids) Internal: Metabolic water production

Basic control system Sensors -these are receptors which

respond either directly or indirectly to a change in the controlled variable

Central controller -this is the coordinating and integrating component which assesses input from the sensors and initiates a response

 Effectors -these are the components which attempt, directly or indirectly to change the value of the variable.

Sensors The main sensors that are involved in

control of water balance in the body are:a) Osmoreceptorsb) Volume receptors (low pressure

baroreceptors)c) High pressure baroreceptors

Central controller  central controller for water balance is

the hypothalamus  The key parts of the hypothalamus

involved in water balance are:a) Osmoreceptors b)  Thirst centrec) OVLT & SFO (respond to angiotensin II)d)  Supraoptic & paraventricular nuclei (for

ADH synthesis)

Effector mechanisms The major effector mechanisms are:a) Control of Water Input : Thirst

Thirst is a mechanism for adjusting water input via the GIT

b) Control of Water Output : ADH & the KidneyADH provides a mechanism for adjusting water output via the kidney. Note that ADH is often called 'vasopressin' - this term refers to the vasoconstrictive properties of very large doses ('pharmacological doses') of the hormone

Fluid balance Plasma osmolality Three major effectors alter effective

circulating volume 1) The sympathetic nervous system, 2) angiotensin II, and 3) renal sodium excretion (Dog) colloid osmotic pressure: 25-

30mmHg Osmolarity: 280-310

mOsm/l

Stimuli to Thirst

The 4 major stimuli to thirst are:a) Hypertonicity: Cellular dehydration acts via

an osmoreceptor mechanism in the hypothalamus

b) Hypovolaemia: Low volume is sensed via the low pressure baroreceptors in the great veins and right atrium

c) Hypotension: The high pressure baroreceptors in carotid sinus & aorta provide the sensors for this input

d) Angiotensin II: This is produced consequent to the release of renin by the kidney (eg in response to renal hypotension)

ADH in the Hypothalamus & Posterior Pituitary The secretory granules containing the

ADH and neurophysin move down the axons (axonal transport) to the nerve terminals in the posterior pituitary from where they are secreted into the systemic circulation by a process of exocytosis (involving calcium)

Renal Actions of ADH ADH-dependent water permeability of

the collecting duct cells Aquaporin-2 is the protein which is the

vasopressin responsive water channel in the collecting duct

forms a channel which allows rapid water movement

In the absence of ADH, the apical membranes of the cells in the cortical and medullar collecting tubules have very low water permeability

In the presence of ADH, the cells are much more permeable to water. At maximal ADH levels, less then 1% of the filtered water is excreted (urine volume 500mls/day)

Feedback loop: Reabsorption of water reduces plasma [Na+] and this is detected by the osmoreceptors in the hypothalamus

Renal Water Regulation The major additional mechanisms which

act at the local renal level are: Glomerulotubular Balance Autoregulation Intrinsic Pressure-Volume Control System

Summary Importance and function Fluid compartment Fluid balance regulation