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Learning Outcomes
1. Outline the functions of water in the body and identify themain fluid compartments of the body.
2. Describe how fluid is gained and lost by the body.
3. Explain the difference between hypertonic, hypotonic and
isotonic solutions and explain the importance of plasmaproteins in controlling the movement of water between
fluid compartments.
4. Describe the homeostatic mechanisms that regulate fluid
balance and consider fluid balance issues related to age.
GML/Renal System/IHB/2009 2
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The Primary Functions of Water in
the Body: Provide a medium for transporting nutrients to cells
and wastes from cells, and transporting of blood cells
and substances eg. hormones, enzymes, platelets,
Facilitate cellular metabolism and cellular functions
Act as a solvent for electrolytes.
Help maintain normal body temperature
Facilitate digestion and promote elimination
Act as a tissue lubricant
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Fluid Sources:
Ingested liquids= 1500mls daily
Water in food =1000mls/day
Water from metabolic oxidation= 500mls/day
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The total amount of water in a man of
average weight (70 kg) is approximately40
litres(or 57% of his total body weight)
In a newborn, this may be as high as 75% of
total body weight (progressively decreasing)
Obesity the % of water in the body,
sometimes to as low as 45%
5
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Body fluid is contained within the cells (intra-
cellular) and outside the cells (extra-cellular)
Intra-cellular =
Extra-cellular can be further
subdivided into:
25 litres (or of total)
Extra-cellular = 15 litres (or of total)
Plasma =
Interstitial =(3rdspace)
3 litres
12 litres
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The cells are generally impermeable to
sodium
Most sodium that enters into the cells is
removed by a pump on the cell membrane
which exchanges it for potassium
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Electrolytes:
Ion:an atom or molecule carrying an electric charge
Electrolytes:substances capable of breaking into
electrically charged ions when dissolved in solution
Sodium Potassium
Calcium
Magnesium
Chloride
Bicarbonate
Phosphate
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Fluid and Electrolyte Movement:
Osmosis:
Diffusion:
Active transport:
Filtration:
Review the
defini tion forthese terms in
your books
and complete
this slide.
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Osmosis
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Definitions
Hypotonicfluid containing fewer solutes(therefore more water) than the cells eg. distilledwater, 0.5% saline (in extreme dehydration)
Isotonicfluids having the same soluteconcentration as cells do eg. 5% dextrose, 0.9%salineno damage to cells
Hypertonicfluids containing more solutes ordissolved substances,sometimes given foroedema to attract the fluid out of tissue spaces and
back into the bloodstream and excess to beexcreted by kidneys
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Movement of Water When the blood flows from arterioles to capillaries
(which are narrower) the hydrostatic pressure isincreased because of the decreased diameter of thevessel
Plasma is filtered out of the blood into the
interstitial fluid which bathes the cells (tissue fluid) The return of this fluid back into the blood is
essential to maintain the blood volume and it is theproteins in the blood which provides the osmotic
pressure to pull the plasma (water) back into thecapillary at the venous end (draw a picture!)
This is an example of how the body relies onconcentration gradients for movement of substances
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Homeostasis of Fluid BalanceAnti-Diuretic Hormone (ADH)
Osmoreceptors in the hypothalamus of brain reactto changes in blood composition such asdehydration where blood volume decreased.
Impulses sent to posterior pituitary gland in brain,which produces ADH*
ADH acts on DCTs and collecting ducts causingmore water to be reabsorbed back into the bloodresults in a more concentrated urineresults in anincrease in blood volume and BP
* Alcohol inhibits production of ADH and causeslarge volumes of dilute urine to be formed cerebral dehydration results in hung over feeling!
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Anti-Diuretic Hormone
Reduced water intake or increased water loss causesincreased osmotic pressure(solute concentration) of blood
Hypothalamus
Posterior Pituitary Gland
Anti-Diuretic Hormone (ADH)
Kidney
Increased water reabsorption by increasing the permeabilityof the distal convoluted tubules/collecting ducts
Increased blood volume and BPsmall volume ofconcentrated urine
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Homeostasis of Fluid Balance
Renin-Angiotensin System
Decrease in BP and Blood Volume (BV)stimulates kidneys to releases rennin
Rennin brings about production of angiotensin ll
(via a series of reactions) Angiotensin ll causes vasoconstriction leading to
increased BP
Angiotensin ll also stimulates adrenal glands (on
top of kidneys) to secrete ALDOSTERONEwhich increases Na+ and H20 reabsorption and
increased blood volume
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Renin-Angiotension-Aldosterone System
Low BP or Kidneys
Low BV
Renin (enzyme)
Angiotensin ll
Vasoconstriction Aldosterone (adrenals)
(Increased BP) Na+and H2O
reabsorption
Maintain BP and Blood Volume
By Altering Fluid Balance2000 Lippincott Williams & Wilkins Fudamentals of Nursing 4th Edition adapted
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Homeostasis of Fluid BalanceAldosterone
Aldosterone is a hormone secreted by the adrenal
cortex when Na+levels fall or K+levels rise
Causes active reabsorption of Na+ and secretion
of K+in DCT and collecting ducts 80% of Na+is passively reabsorbed in the PCT
Aldosterone alsoincreases water reabsorption in
the tubules since water follows Na+ back into the
bloodstreamwater follows salt!
Fluid and electrolyte balance fine tuned in DCT &
collecting ducts
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Regulation of Blood pH
Blood pH is determined by the H+
ionconcentration
Bicarbonate ions (HCO3) buffer pH by mopping
up excess H+ ions and make the blood less acidic
HCO3 + H+H2CO3(carbonic acid)
Fewer H+ions in solution ALKALINE
Kidneys regulate pH by excreting or reabsorbing
HCO3 and limits its availability to bind with H+
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So...........
When blood is too acidic ie. Lots of free floatingH+, the nephrons of the kidney REABSORB more
bicarbonate (HCO3) which binds to the H+making the blood more alkaline
When the blood is too alkaline, more bicarbonate(HCO3) ions are excreted in urine so that there arenot enough bicarbonate for the H+ to bind to,
making the blood more acidic
What does acidosis and alkalosis mean?
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