chapter 27

Chapter 27Fluid, Electrolyte,

and Acid-Base Homeostasis

Copyright © John Wiley & Sons, Inc. All rights reserved.

The human body requires constant attention

to the methods of regulating body fluid.

These processes are necessary to maintain

required proportions of water and solutes

among body compartments

Water is by far the largest single component

of the body making up 55–80% of total body

mass (depending on age and sex). Filtration,

reabsorption, diffusion, and osmosis

continually exchange water and solutes

among these compartments

Body Fluids


In addition to its job of filtering and excreting

waste products from the blood, the kidneys

are also charged to take the lead in

maintaining the composition of water and

salts in the body’s various fluid

compartments

This makes the study of fluid

balance in this chapter a

fitting enjoiner to our

recent discussion of

renal function

Body Fluids


Body Fluids and Water Flow

Body Fluids(Interactions Animation)

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The fluid compartments of the body are all

contained in either the intracellular

compartment or the extracellular

compartment

Intracellular fluid is all fluid contained

inside cells, and comprises 2/3 of all body

fluids

Extracellular fluid is all fluid outside the

confines of the plasma membranes that

contain all intracellular contents. 1/3 of all

body fluid is contained in the extracellular

compartment

Fluid Compartments


While the intracellular fluids are all contained

within a single space (inside cells),

extracellular fluid is found in a number of

sub compartments

Most extracellular fluid (¾) is found between

the cells of the body (interstitial fluid)

The rest is found in the intravascular fluid

space (blood plasma – about ¼), with small

amounts existing as lymph, CSF, synovial

fluid, aqueous humor, endolymph and

perilymph, and pericardial fluid

Fluid Compartments


Babies are more

“wet” than adults,

with water

composing about

80% of total body

mass

Fluid Compartments


Normal fluid intake is through:

Ingestion of liquids and moist foods

(2300mL/day)

Metabolic synthesis of water during cellular

respiration and dehydration synthesis

(200mL/day)

Normal fluid loss is through:

The kidneys (1500mL/day)

Evaporation from the skin (600mL/day)

Exhalation from the lungs (300mL/day)

In the feces (100mL/day)

Fluid Balance


Fluid intake and output (I & O) are usually

balanced on a daily basis, despite the fact that

intake of water and electrolytes

are rarely proportional

The kidneys excrete

excess water through

dilute urine, or excess

electrolytes through

concentrated urine

Fluid Balance


Fluid Balance Animation

Fluid Balance(Interactions Animation)


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A number of feedback

mechanism contribute

to balance of daily fluid

inpu and output

Fluid Balance


Three hormones regulate renal Na+ and Cl–

reabsorption or excretion

Angiotensin II and aldosterone promote

urinary Na+ and Cl– reabsorption (and water

by osmosis) when dehydrated

Atrial natriuretic peptide (ANP) promotes

excretion of Na+ and Cl– followed by water

excretion to decrease blood volume

Fluid Balance


Na+ and Cl– balance is

regulated by 3

hormones

Aldosterone

Atrial natriuetic

peptide

Angiotension II

Fluid Balance


In addition to the hormones that regulate Na+

and Cl- homeostasis, antidiuretic hormone

(ADH) is a hormone that plays a major role in

directly regulating water loss in the collecting

ducts of the kidneys

Also known as vasopressin, ADH (from the

posterior pituitary) increases permeability of

the collecting ducts to water by promoting

insertion of aquaporin-2 into the principal

cells – producing a concentrated urine

Fluid Balance


Ions form when electrolytes dissolve and

dissociate. They have 4 general functions

Control osmosis of water between body

fluid compartments

Help maintain the acid-base balance

Carry electrical current

Serve as cofactors

Electrolytes


The term “milliequivalent” (mEq) is used to

measure the number of electrical charges

(electrolytes) in blood serum and other

solutions. Denoting the number of mEq per

liter of solution gives the concentration of

anions or cations in a given volume of

solution

One equivalent is the charge in 1 mole of H+

ions. A milliequivalent is simply 1/1000 of

an equivalent

◦ Sodium - 136-146 mEq/L

◦ Potassium - 3.5-5.0 mEq/L

Electrolytes


The size of a substance does not determine its

osmotic contribution – that is determined by

the number of milliequivalents. For example: 1 millimole NaCl = 2 mEq (1mEq of Na+ and

1mEq of Cl-

Electrolytes


As we have seen, osmotic forces have a

definite influence on movement of water

between body compartments. Osmotic

pressure exerted by proteins on either side of

the capillary membrane is called oncotic

pressure. It is not, however, the only force in

play - hydrostatic forces are another major

factor to consider

Net movement of fluids is controlled by all

forces favoring filtration minus all

forces opposing filtration

Starling Forces


The Starling equation compares the forces

at the arterial end of a capillary with those at

the venous end

Forces favoring filtration are the capillary

hydrostatic pressure (pressure against the

capillary wall) and the interstitial oncotic

pressure

Forces favoring reabsorption are the

plasma oncotic pressure (water-pulling)

and the interstitial

hydrostatic pressure

Starling Forces


Normal Starling forces favor a small amount of

fluid flowing out of the capillary which is

drained by the lymphatic system

Starling Forces


Edema occurs when excess interstitial fluid

collects, causing swelling in the tissues.

Edema occurs anytime filtration exceeds

reabsorption

The most important causes of edema are:

◦ increased blood pressure (increased blood

hydrostatic pressure)

◦ an increase in the capillary permeability

◦ a decrease in the concentration of plasma

proteins

◦ an obstruction in lymphatic drainage

Starling Forces


A major homeostatic challenge is keeping the

H+ concentration (pH) of body fluids at an

appropriate level. Because metabolic

reactions often produce a huge excess of H+,

failure of homeostatic mechanisms would

cause the pH of body fluids to quickly fall to a

lethal level

In a healthy person, chemical buffers, the

lungs, and the kidneys help maintain the

pH of systemic arterial blood between 7.35

and 7.45

Acid-Base Balance


1. Buffer systems act quickly to temporarily

bind excess H+ or OH -, sequestering (hiding)

the highly reactive ions until they can be

permanently excreted

2. By increasing the rate and depth of

breathing, CO2 is exhaled or retained, and

blood pH is corrected

3. Kidney excretion/reabsorption of acidic ions

(H+ and NH4+) or basic ions (HCO3 – or OH -) is

the slowest mechanism; but is the only way

to eliminate acids other than carbonic acid

Acid-Base Balance


Regulation of pH

Acid-Base Balance(Interactions Animation)


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Respiratory acidosis occurs whenever CO2

accumulates because of hypoventilation

Metabolic acidosis occurs whenever non-

respiratory acids accumulate, as seen in

diabetic ketoacidosis or aspirin overdose

Respiratory alkalosis occurs whenever too

much CO2

is lost because of hyperventilation

Metabolic alkalosis occurs whenever non-

respiratory acids are lost, which happens

infrequently

Acid-Base Imbalances


The homeostatic correction

for states of acidosis (which

are much more common and

serious than states of

alkalosis) are depicted in

this flowchart

Acid-Base Imbalances

chapter 27

Documents

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copyright john wiley

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