FLUID, BUFFER, ACID-BASE BALANCE

Rondang Soegianto

2010

Fluid content in individuals vary due to

variability in amt of adipose tissue.

High body water lean

Low body water obese

Plasma > 90% H2O

Skin, muscle, internal organs 70-80% H O

Skeleton only 22% dry tissue

Fat 10% lowest

2

Intracellular fluid (ICF)

Extracellular fluid (ECF)

ICF compartment ~ 2/3 total body water

ECF: plasma = 1/5 of ECF

Interstitial fl ~ 4/5 of ECF

Minor ECF compartments:

- Lymph- Transcellular fluid: Cerebrospinal fl Interoccular fl Synovial fl Cardial, intrapleural, peritoneal fl Digestive juicesTranscelr fl does not affect fl balance of cellException: Vomiting, diarrhea

Metabolism and Acid-Base Balance

A-B balance related to

- Respiration

- Metabolism

Impaired A-B balance of ECF - Lactic acidosis

- Ketosis

Chemical Terms

Acidity = [H ] (proton)

Pure water [H ] = 10

[H ] > 10 = Acid

[H ] < 10 = Alkaline

pH = 7 [H+] = 10-7 M = 0.0001 mM

pH = 7.4 0.00004 mM

+

+ -7

-7

-7+

+

Note:

1. Absolute conc’s of H+ in organism much <<

other solutes

Ex. [Na ] inECF = 145 mM

2. pH units are logarithmic not linear scale

Rise of one unit pH = 10 fold rise in [H ]

+

+

pH of ECF (incl plasma) = 7.35 – 7.45

Outside this range: Acidosis <----- > Alkalosis

Compatible with life: pH 7.0 - 7.8

Exceeding this range may inhibit brain function

Source of Protons

1. Dietary

a. Phospholipids: hydrol. of phosph. acids

b. Sulphur containing amino acids

Cystein and metionine from proteins

Degrd’ion in liver sulphates and H

2. Respiratory ( ~ 12.5 moles/day)

CO + H O H CO + H + HCO+

+

2 2 2 3 3

-

3. Metabolic products- Lactic acid

- Ketone bodies

- ATP hydrolysis

- NAD+ reduction

Totally per day: ~150 moles proton

Reversal by body reactions no net charge

in protons

Body pH raised due to:

- Ingestion of weak acids (citric acid) as

- Na or K salts (from fruits)

- Hyperventilation (loss of CO )

- Vomiting (loss of HCl from stomach)2

Mechanism for pH adjustments

A. Buffer

B. Respiration

C. Renal acid secretion

A. BuffersWeak acid (binds added OH )

andConjugate base (binds added proton)

# Resists changes in pH when acid or base added

# Conjugate base = weak acid - proton Conj acid = weak base + proton

Blood buffer H CO : HCO

-

2 33

-

Salts Can Change pH of Blood or Lab Sol’n

- Acidic salts ex NH NO NH4+ is weak acid- Basic salts ex CH COO-Na CH3COO- is weak base- Neutral salts ex NaCL, KNO No weak acids or weak bases

4 3

3

3

Buffer Capacity

BC = Ability to consume added H+ and added OH

BC depends on - Buffer conc’n - pKaUseful buffering is at pH within pKa + 1 or pKa - 1

-

Examples of Buffers

a. Proteins. Have many ionizable groups

and pKa values.

Present in differing conc’ns.

H.Prot H + Prot

H.Prot H + Prot

Ex. Hb in erythrocytes (large amt)

H+ can penetrate eryth membrane

This buffering action influences plasma pH

+

+

+

-

b. Phosphates

One ionizable group, pKa = 6.8

Inorganic phosphates

H PO H + HPO

Organic phosphates (not much)

24- + 2-

4

c. Bicarbonates

H CO H + HCO , pKa ~ 3

Only slightly useful to buffering cap at pH=7.4

Important, since conc of H CO and HCO

can be regulated in response to pH change

Dynamic Buffer System

Active in respiratory system

2 3+

2 3

3

-

3

-

B. Respiration

H CO H O + CO

Enzyme: Carbonic anhydrase in RBC

and other tissues

Henderson-Hasselbalch eqn:

[salt]

pH – pKa + log _______________

[undiss. acid]

2 3 2 2

For carbonic acid: [HCO ]

pH = 3.0 + log ____________

[H CO ]

# Lung ventilation >> loss of CO2 >>

Blood H CO << pH >>

= Respiratory alkalosis

Ventilation << Resp acidosis

2 3

2 3

3

-

Quantitative Values of Buffer on Acid Load 1. Plasma protein 1% of total buffering of acid administration2. Erythrocytes ~6% (H+ penetrate RBC)3. ECF carbonic/bicarbonate system ~42%4. Intracellular protein ~51% EC H+ exchanges for IC Na (36%) and for IC K (15%) Some Na+ derived from bone apatite crystals In chronic acidosis bone resorption Ca and phosphate released

+

+

2+

Kidney in Acid-Base Balance ControlMaintains many solutes in plasma by adjustingrate of excretion in urine thru

- reabsorption from glomerular filtrate- secretion into urine

Glomer. Filtr.: high Na+ and bicarbonate (equals conc in plasma0

Loss of HCO pH of blood <<

Recovery: Protons secreted into lumen from tubular cells

In the lumen:HCO + H H CO H CO H O + CO CO2 diffuses back into tubuli

3+

2

2 3 22

-

3

-

Importance of Various Buffer Systems

* Protein buffer system primarily important intracellularly

* Hb buffer system buffers H+ generated from carbonic acid (H CO )

* The phosphate buffer system is an important urinary buffer

2 3

Line Order of Defense Mechanisms Against Changes in [H ]

1. Chemical buffer systems First line of defense. H not eliminated but incorporated into buffers

2. Respiratory system, second line of defense a. Works thru pulmonary ventilation Increase of arterial [H ] by metabolism stimulates resp. Center in brain stem >> pulm. ventilation

+

+

+

b. When arterial [H ] falls, pulm. Vent. <<

shallow breathing. Metab.

CO diffuses from cell blood, faster than CO removed

from

blood lungs. CO accuml in

blood restores [H ]

+

2

2

2+

3. Kidneys, third line of defense Most potent A-B regulating mechanism - vary removal of proton from any source - conserve or eliminate bicarbonate ion

Ex. Renal compensation for acidosis For each H to urine, new HCO returns to plasma to buffer another H

still remaining in body fluid

+

+3

-

Thus:

Kidneys are able to restore pH to normal

Respond continuously to pH change until

compensation is complete

Lungs can only adjust amt of CO that forms

H in the body

2+

1. CHEMICAL BUFFER

2. PHYSIOLOGICAL

BUFFER

MECHANCAL RESPIRATION

EXCRETION OF CO2

RENAL MECHANISM

EXCRETION OF H+

Kidneys Secrete Ammonium

H+ transp. actively from tubular cells tubular plasma. Capacity limited to urinary pH = 4.5H in tub fl must be buffered to << free HImportant urinary buffers:

1. Filtered phosphate buffer2. Secreted ammonia

+ +

Secreted H first buffered by phosph buffer.Phosph in tub fl comes from ingested phosph= dietary excessWith high H excretion, buff cap. of phosphExceeded. Kidney cannot respond by >> phosexcrt’n. Only phosph reabsorption subjectedto control mechanism.Next: Tubular cells secrete NH3NH + H NH urineNH synthesized from glutamine in tub. cells

+

+

3+

4

3

Acid-Base Balance

Ratio of [HCO ] : [CO ]

Normal = 20/1Resp acd < 20/1 CO >> Resp alk > 20/1 CO <<Met ac < 20/1 [HCO ] << Met alk > 20/1 [HCO ] >>

3-2

2

2

3-3

-

Compensation:

Resp Ac. Kidneys conserve filtered HCOResp Alk: Kidneys conserve H

Excrete more HCOMet Ac : Renal and chemical

- buffers take up H - lungs blow off CO - kidneys excrete H - conserve HCO

Met Alk: Liberation of HVentilation <<, CO retained in body fld

+

+

2

+

2

3

-

3-

3-

Causes of A-B Imbalance

Resp Ac : Hypoventilation Lung disease Depression of resp centr by drug or disease

Resp Alk. Fever, Anxiety, Aspirin poisoning

Met Ac Severe diarrhea, DM Strenuous exercise, Uremia (renal failure)

Met Alk Vomiting Ingestion of alkaline drugs

References:1. Biochemistry for the Medical Sciences E.A. Newsholme and A.R. Leech John Wiley & Sons, 1983 2. Biochemistry. A Foundation Peck Ritter Broks/Cole Publ Co, 19963. Human Physiology. From Cells to Systems L. Sherwood Brooks/Cole, 20044. Medical Biochemistry A.C Brownie, J.C. Kernohan Elsevier, 2005