Anatomy & Physiology Lecture Outline: Week #3b

• Fluid and Electrolyte Balance

LSc 107 Anatomy and Physiology Spring 1999

Thibodeau Ch 29

Homeostasis applies to the fluid volume, and to the electrolyte concentrations of the body fluids - fixed normal range of values for both - mechanisms for keeping them within that range

Total Body Water

• 40 - 60% of total weight
• Decreases throughout life
• Varies with body composition, age, gender, weight

Body fluid compartments (Fig. 29-1 & Table 29-1)

ICF - fluid inside cells 33-38% total body weight

ECF - constant environment for cells, and transports substances to and from cells

• plasma - 4%
• interstitial fluid and lymph - 10 - 15%
• other fluids (cerebrospinal fluid, synovial fliuds etc)

Water balance of the body (Table 29-3)

Water in:

• drinks (1500ml)
• food (700ml)
• water of metabolism (200ml)

Water out:

• lungs (350ml)
• sweat - variable
• diffusion of water through skin, transpiration (350ml)
• urine (1400 ml)
• feces (200 ml)

Regulation of input

Sensation of thirst (Fig. 29-7)

• osmoreceptors of hypothalamus
• dry mouth
• "Social drinking" ie without physiological need

Regulation of output

• Water loss from skin, and loss from lungs and GI tract can not be reduced
• only flexible category of output is the urine volume
• Urine volume must be adjusted to compensate for variation in intake and variations in sweat loss
• Output = Input

 

How is urine volume controlled?

Main mechanisms are ADH and Aldosterone (recall notes from Ch 28)

Fig. 29-6, 28-8

• to understand total body water balance - control of thirst and output
• to understand ADH mechanism
• to understand aldosterone mechanism
• to understand homeostasis of plasma osmolality (concentration)
• to understand regulation of Na⁺ and K⁺ in body fluids
• to understand body's response to fluid deprivation (dehydration or hemorrhage)

 

Electrolytes in body fluids

What is an electrolyte? Compound which breaks up into charged particles in solution

• Anions neg. charge
• Cations pos. charge

Comparison of intracellular and extracellular electrolytes (Fig. 29-3)

• Plasma and interstitial fluid similar except blood slightly larger total of ions and more proteins
• Intracellular fluid different chemical composition

NOTE - the electrolyte contents of the body fluids determines water distribution and movement between them

Movement between the circulating plasma and the interstitial fluid (IF) (Fig. 29-10)

• separated by capillary wall which is practically impermeable to proteins
• blood hydrostatic pressure tends to force fluid out of capillaries into IF
• blood osmotic pressure tends to draw fluid back in (from IF to blood)
• IF hydrostatic pressure tends to force fluid out of IF into capillaries
• IF osmotic pressure tends to draw fluid back in (from capillaries into IF)
• rate of fluid exchange is determined by hydrostatic and osmotic pressure of both fluids
• At the arteriolar end the net effect is - fluid forced out of capillaries
• At the venous end, hydrostatic pressure is less, - fluid returns to capillary
• Any fluid not returned becomes lymph

Normally:

Osmotic pressure (protein and electrolyte content of blood), stays same in arteries and veins

Hydrostatic pressure (higher at one end than the other) responsible for fluid movement

But, when osmotic pressure reduced (due to loss of protein, retention of electrolytes in the blood) or increased hydrostatic pressure (backed up from a failing heart) - greater vol of fluid forced into tissues from blood to IF, leads to edema

Movement between the ECF and ICF (Fig. 29-13)

separated by the plasma membrane- critical in controlling ICF composition

Forces acting across membrane:

• hydrostatic pressure (similar on both sides, does not cause movement)
• main factor: osmotic pressure (main intracellular electrolyte (K⁺) and large proteins inside cell, main extracellular electrolyte (Na⁺), concentration gradients across membrane)

Therefore it is the distribution of Na⁺ and K⁺ which is mainly responsible for fluid distribution between the ICF and the ECF, and any change in distribution of Na⁺ or K⁺ will cause movement of water by osmosis into or out of the cell

Fig. 29-15

Dehydration (Box 29-4)

 

Regulation of Na⁺ and K⁺ in body fluids

• aldosterone: regulates Na⁺ reabsorption by renal tubules, when body Na⁺ is low (eg. excess sweating) then virtually sodium-free urine (Note: drinking will not replace the Na⁺, unless a special electrolyte fluid is drunk, rather than fruit juice or water)
• ADH: regulates amount of water reabsorbed by renal tubules
• K⁺ is the major intracellular ion, can not get out for the same reason Na⁺ can not get in. Serum levels of K⁺ are therefore low.

 

Fluid and Electrolyte disorders

Hypovolemia: dehydration Fig. 29-9

Inadequate fluid in ECF, particularly in circulating compartment

• GI losses
• excess sweat
• Fluid shift into a different compartment (during disease states)

Hypervolemia

Fluid overload

• Heart failure
• Kidney failure
• Liver failure

Water intoxication

ECF is dilute - water passes by osmosis into cells (because serum sodium levels are diluted), especially brain causing mental changes

Hypernatremia: excess serum sodium causes cellular dehydration

Hyponatremia: loss of Na⁺ - severe signs and symptoms from NS

Hypokalemia: loss of K⁺, most common from diuretics, GI losses, causes weakness

Hyperkalemia: excess K⁺ in serum, caused by kidney failure, causes cardiac standstill

 

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