FLUID COMPARTMENT,
FUNCTION &
BALANCE
Prepared by, Dr. Nicole Seng Lai Giea
What so important?
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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
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Variation due to Age
Variation between Individuals (adipose tissue)
Habitat
Functions
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Transfer medium ( dissolved nutrients and waste
products)
Secretion and excretion ( glandular product)
Temperature regulation
Lubricant for body surface
Homeostasis of body water
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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
Total body water
(60%
bodyweight)
40% intracellular fluid
Total body water
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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.
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major division is into Intracellular Fluid (ICF) and
Extracellular Fluid (ECF), separated by the cell
membranes
ECF -- Interstitial fluid
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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
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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
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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
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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
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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
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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
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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
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6-8% in total
Ruminant and monogastric herbivore with large
ceca have higher in percentage
Regulation of fluid balance
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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
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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
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a)
b)
c)
The main sensors that are involved in control of
water balance in the body are:
Osmoreceptors
Volume receptors (low pressure baroreceptors)
High pressure baroreceptors
Central controller
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a)
b)
c)
d)
central controller for water balance is
the hypothalamus
The key parts of the hypothalamus involved in
water balance are:
Osmoreceptors
Thirst centre
OVLT & SFO (respond to angiotensin II)
Supraoptic & paraventricular nuclei (for ADH
synthesis)
Effector mechanisms
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a)
b)
The major effector mechanisms are:
Control of Water Input : Thirst
Thirst is a mechanism for adjusting water input via
the GIT
Control of Water Output : ADH & the Kidney
ADH 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
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Stimuli to Thirst
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a)
b)
c)
d)
The 4 major stimuli to thirst are:
Hypertonicity: Cellular dehydration acts via an
osmoreceptor mechanism in the hypothalamus
Hypovolaemia: Low volume is sensed via the low
pressure baroreceptors in the great veins and right
atrium
Hypotension: The high pressure baroreceptors in
carotid sinus & aorta provide the sensors for this input
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
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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
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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
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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
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The major additional mechanisms which act at the
local renal level are:
Glomerulotubular Balance
Autoregulation
Intrinsic Pressure-Volume Control System
Summary
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Importance and function
Fluid compartment
Fluid balance regulation