Water, Electrolyte and Acid-Base Balance
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| Extracellular Fluid Compartment – 35%, | Tissue (interstitial) fluid – 25% : Blood plasma and lymph - 8% Transcellular fluid - 2%
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| Water Gain and Loss | A person is in a state of water balance when daily gains and losses are equal. We gain
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| Water gain comes from two sources | Metabolic water & Preformed Water
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| Water loss comes from several sources | Urine, Feces, Expired air, Sweat, Cutaneous Transpiration
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| Different types of water output (loss) | Insensible water loss & Obligatory Water Loss
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| Effects of Dehydration and the Physiological Response | a)Dehydration reduces blood volume/BP and increases blood osmolarity.
b)The anterior nucleus of the hypothalamus has a neural pool called the Thirst Center
that responds to multiple signs of dehydration:
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| Effects of Dehydration and the Physiological Response | i)Angiotensin II is produced in response to decreasing BP.
ii)Antidiuretic hormone (ADH) is released in response to rising blood osmolarity.
iii)Signals from osmoreceptors – which are neurons in the hypothalamus that monitor
the osmolarity of the ECF.
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| Long term satiation of thirst depends on absorbing water from the small intestine and lowering the osmolarity of the blood. | a)Reduced osmolarity stops the osmoreceptor response, promotes capillary filtration and
makes the saliva more abundant and watery. But these changes require 30 minutes or
longer to take effect – and it would be impractical if one had to drink this long
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| Cooling and moistening of the mouth | rats drink less if their water is cool vs. warm., simply moistening the mouth temporarily satisfies and animal even if the water is drained from its esophagus before it reaches the stomach.
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| Distension of the stomach and small intestine | if a dog is allowed to drink drained from its esophagus but its stomach is inflated w/ a balloon – its thirst is temporarily satisfied. If the water is drained away but the stomach is not inflated, satiation does not last as long.le the water is
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| Regulation of Output | The only way to significantly control water output is through variations in urine
volume.
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| Regulation of Output | Changes in urine volume are usually connected to adjustments in sodium reabsorption
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| Regulation of Output | Antidiuretic Hormone (ADH) a)This hormone provides a means of controlling water output, independently of sodium.
b)In true dehydration, blood volume declines and sodium concentration increases. The
increased osmolarity of the blood stimulates the hypoth
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| Changes in urine volume are usually connected to adjustments in sodium reabsorption | As sodium is reabsorbed or excreted, proportionate amounts of water accompany it.
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| Changes in urine volume are usually connected to adjustments in sodium reabsorption | The total volume of fluid remaining in the body may change, but its osmolarity remains stable.
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| Changes in urine volume are usually connected to adjustments in sodium reabsorption | c)Controlling water balance by controlling sodium is best understood in the context of electrolyte balance.
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| Antidiuretic Hormone (ADH) | This hormone provides a means of controlling water output, independently of sodium.
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| Antidiuretic Hormone (ADH) | b)In true dehydration, blood volume declines and sodium concentration increases. The increased osmolarity of the blood stimulates the hypothalamic osmoreceptors, which then stimulate the posterior pituitary to release ADH – which has the following effect
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| Antidiuretic Hormone (ADH) | the kidneys reabsorb more water and produce less urine.
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| Antidiuretic Hormone (ADH) | ii)Sodium continues to be excreted, so the ratio of sodium to water in the urine increases. By helping the kidneys retain water, ADH slows down the decrease in blood volume and the rise in its osmolarity.
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| Antidiuretic Hormone (ADH) | if blood volume/BP are too high or blood osmolarity is too low – then ADH release is inhibited.
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| E. Disorders of Water Balance | The body is in a state of fluid imbalance if there is an abnormality of total volume,concentration or distribution of fluid among the compartments.
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| Volume Depletion (Hypovolemia) | a. occurs when proportionate amounts water & sodium are lost w/out
replacement. Total body water declines osmolarity remains normal.
b. Volume depletion occurs in cases of hemorrhage, severe burns & chronic
vomiting or diarrhea. * addisons disease
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| Dehydration (Negative Water Balance) | a. This occurs when the body eliminates significantly more water than sodium, so
the ECF osmolarity increases.
b. Causes of dehydration include lack of drinking water, diabetes mellitus, ADH
hyposecretion (diabetes insipidus), profuse sweating, overuse
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| The most serious effects of fluid deficiency are | Circulatory shock due to loss of blood volume and Neurological dysfunction due to dehydration of brain cells.
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| Two types of fluid excesses | Volume Excess & Hypotonic Hydration (Water intoxication or positive water balance)
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| Volume Excess | In this case, both sodium and water are retained and the ECF remains constant.Causes may be due to hypersecretion of aldoseterone or renal failure.
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| Hypotonic Hydration (Water intoxication or positive water balance) | Loss of a large amount of water and salt through urine/sweat – then replacing this by ingestion of plain water. Without a proportionate intake of electrolytes, water dilutes the ECF, makes it hypotonic and causes cellular swelling.
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| Hypotonic Hydration (Water intoxication or positive water balance) | Hypersecretion of ADH – this would stimulate excess water retention as
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| Most serious effects of fluid excess | Pulmonary edema & Cerebral edema
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| Edema | this is the abnormal accumulation of fluid in the interstitial spaces causing swelling of a tissue.
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| Hemorrhage | here blood that pools/clots in the tissue is lost to the circulation.
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| Pleural effusion | this may be caused by lung infections and may result in fluid accumulating in the pleural cavity.
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| Fluid Compartments | Body water is distributed among several fluid compartments – areas separated from
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| Intracellular Fluid Compartment | 65%
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| Water | Water is continually exchanged b/w compartments by way of capillary walls and
plasma membranes. From the tissue fluid, water may be reabsorbed by the capillaries, osmotically
absorbed into cells or taken up by the lymphatic system, which returns blood
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| Osmosis | from one fluid compartment to another is determined by the concentration of
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| electrolytes | The most abundant solutes are electrolytes – especially sodium salts in the ECF and
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| Electrolytes | Electrolytes play the main role in regulating the body’s water distribution/total water
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| Urine | Water lost in the urine accounts for 1,500 mL/day
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| Feces | Water eliminated in the feces accounts for 200 mL/day.
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| Expired air | This accounts for 300 mL/day.
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| Sweat | Accounts for loss of 100 mL/day.
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| Cutaneous Transpiration | Accounts for loss of 400 mL/day.
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| Antidiuretic Hormone (ADH) | Sodium continues to be excreted, so the ratio of sodium to water in the urine
increases. By helping the kidneys retain water, ADH slows down the decrease in
blood volume and the rise in its osmolarity.
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