Fluid and Electrolyte Balance

How much fluid is held intracellularly?

65%

How much fluid is held extracellularly?

35%

How much fluid is held in tissue (interstitial) fluid

• 25%

How much fluid is held in blood plasma/lymph fluid

8%

How much fluid is held transcellularly?

2%

How is fluid exchanged between compartments? 

• capillary walls and the plasma membranes 

how does water move from the digestive tract to the bloodstream?

• osmosis

How does water move from the blood to the tissue fluid?

• capillary filtration 

what happens if the osmolarity of tissue fluid rises

• water will move out of cells 

what happens if osmolarity of tissue fluid falls

• water will move into the cells

what determines the rate of osmosis from one compartment to abither? 

• concentration of solutes in each compartment (electrolytes) 

what plays a very principal role in governing the bodys water distribution and total water content? 

• electrolytes 

Fluid Balance

• occurs when daily gains and losses are equal and fluids are properly distributed in the body 

where do the gains of water come from

• metabolic

  • (produced from aerobic respiration, dehydration synthesis)

• preformed water 

  • ingested in water/food 

How much metabolic water is produced? 

• 200 ml/day 

how much performed water is consumed?

• food: 700 ml/day 

• drink: 1,600 ml/day 

what are the different routes of water loss

• 1,500 ml/day urine

• 200 ml/day feces

• 300 ml/day expired breath 

• 100 ml/day sweat

• 400 ml/day cutaneous transpiration 

Insensible water loss

• output through the breath and cutaneous respiration

• not usually aware of it 

Sensible water loss

• noticeable output

• urine and sufficient sweating 

Obligatory water loss

• output that is relatively unavoidable 

  • expired air, cutaneous transpiration, fecal moisture, min. urine output

What does dehydration do?

• reduced blood volume and pressure

• raises lood osmolarity 

osmoreceptors

• neuron that responds to changes in osmolarity of extracellular fluid

what do osmoreceptors respond to

• angiotensin II and rising osmolarity of extracellular fluid

what do osmoreceptors do in a state of dehydration

• communicate with hypothalamic neurons to produce ADH

• this promotes water conservation 

• communicate with cerebral cortex to produce sense of thirst

Osmoreceptors in hypothalamus

• Respond to angiotensin II produced when
  BP drops and also respond to rise in
  osmolarity of ECF
  • communicate with other
  hypothalamic neurons and with cerebral
  cortex

Hypothalamus produces antidiuretic hormone to do what

• promote water retention 

Cerebral cortex produces conscious sense of
thirst

• Intense sense of thirst with 2% to 3%
  increase in plasma osmolarity or 10% to
  15% blood loss
  • Salivation is inhibited with thirst
  • Sympathetic signals from thirst center to
  salivary glands

reasons why we salivate less when thirsty

• osmoreceptors lead to sympathetic output from hypothalamus (salivary glands inhibited) 

• dehydrated person has less capillary filtration 

what does long term satiation of thirst depend upon?

• water being absorbed from the small intestine and lowering osmolarity of the blood 

steps in long term satiation of thirst 

• Reduced osmolarity -> stops osmoreceptor response -> promotes capillary filtration -> makes saliva more abundant/watery

Mechanisms of short term satiation of thirst

• cooling/moistening of mouth temporarily satisfied an animal 

• Distension of stomach and small intestines

• Coolness, moisture, and filling of stomach 

How is flui output regulated?

• Only way to control water output significantly is through variation in urine volume

How can the kidneys regulate water and fluid output

• cannot replace water or electrolytes

• Can only slow rate of water and electrolyte loss until water and electrolytes can be ingested

What mechanisms controls water output?

• inked to adjustments in Na+
  reabsorption
  • As Na+ is reabsorbed or excreted, water follows

Volume depletion (hypovolemia)

• proportionate amounts of water and sodium
  are lost without replacement
  • Total body water declines, but osmolarity remains normal
  • Hemorrhage, severe burns, chronic vomiting, diarrhea, or Addison’s disease

Dehydration (negative fluid balance)

• Body eliminates significantly more water than sodium, so ECF osmolarity rises

• caused by lack of water input, diabetes, 

Hypotonic hydration

• water intoxication, positive fluid
  balance)
  • More water than Na+ retained or ingested
  • ECF becomes hypotonic
  • Can cause cellular swelling

Volume excess

• Both Na+ and water retained
  • ECF remains isotonic
  • Caused by aldosterone hypersecretion or renal failure

Fluid Sequestration

• TBW normal, drop in blood volume

  • edema

  • pleural effusion

  • hemmorage