BIOL 314 - Ch 24 Acid-Base Balance

Newborn water content

-75% of body weight is water

-Super high due to low fat and high metabolism

Young adult male water content

-55-60% water

Woman water content

-Slightly less due to typically higher fat content and lower muscle mass

Obese and elderly water content

-As low as ~45%

-Fat tissue contains less water, and aging reduces muscle mass and kidney function

Total body water

-TBW

-For a 70kg (150 lb) young male ~ 40 L

Major fluid compartments

-65% intracellular fluid (ICF)

-35% extracellular fluid (ECF)

ECF breakdown

-25% tissue (interstitial) fluid

-8% blood plasma and lymphatic fluid

-2% transcellular fluid "catch-all" category

Transcellular fluid

-Cerebrospinal fluid, synovial, peritoneal, pleural, and pericardial fluids

-Vitreous and aqueous humors of the eye

-Bile, and fluids of the digestive, urinary, and reproductive tracts

Fluid movement between compartments

-Water moves easily across cell membranes by osmosis

-Osmotic gradients don's last long due to fast water movement - balance is restored quickly

Osmotic shift

-ECF osmolarity rises -> water moves out of cell -> cell shrink

-ECF osmolarity falls -> water moves into cell -> cell swell

-Osmosis is driven by solute concentration, not just the amount of water

Electrolytes

-Key solutes

-They are the main solutes influencing water movement

-Sodium salts dominate in ECT

-Potassium salts dominate in ICF

Electrolytes bottom line

-Electrolytes play the principal role in governing the body's water distribution and total water content

Importance of pH homeostasis

-Maintaining pH balance is crucial for survival

-Body fluids must stay within a narrow pH range (7.35-.45 for blood)

Why balance matters

-Metabolism relies on enzymes, and they only work within certain pH ranges

pH shift can cause

-Denature enzymes

-Shutdown metabolic pathways

-Disrupt macromolecule structure/function (proteins and nucleic acids)

Consequences of imbalance

-Acidosis

-Alkalosis

Acidosis effects

-pH < 7.35

-CNS depression, confusion, coma

-This causes membrane hyperpolarization, nerve and muscle cells are hard to stimulate

Acidosis

-pH of EFC below 7.35

-H+ diffuses into cells and drives out k+, elevating K+ concentration in ECF

-H+ buffered by protein in ICF, so net result is cation loss

Alkalosis effect

-pH > 7.45

-Muscle spasms, convulsions, respiratory paralysis

-H+ diffuses out of cells and K+ diffuses in, membrane slightly depolarization, nerves overstimulated, muscles causing spasms, tetany, convulsions, respiratory paralysis

Alkalosis

-pH above 7.45

-A person cannot live for more that a few hours if the blood pH is below 7 or above 7.7

pH determining factor

-pH of a solution is determined solely by its hydrogen ions (H+)

Acid

-Any chemical that releases h+ in solution

Strong acid

-A stronger acid such as hydrochloric acid (HCL) ionizes freely

=gives up most of its h+

=Markedly lowers pH of a solution

Weak acid

-A weak acid such as carbonic acid (H2CO3) ionizes only slightly

=Keeps most H+ chemically bound

=Does not affect pH much

Bases

-Any chemical that accepts h+

Strong bases

-Such as the hydroxide ion (OH-), have a strong tendency to bind H+, markedly raising pH

Weak bases

-Such as the bicarbonate ion (HCO3-), bind less of the available H+ and have less effect on pH

Normal pH range

-7.35 - 7.45 is the normal pH range of blood and tissue fluid

Metabolism acid production

-Lactic acid from anaerobic fermentation

-Phosphoric acid from nucleic acid catabolism

-Fatty acids and ketones from fat catabolism

-Carbonic acid from carbon dioxide

Buffer

-Any mechanism that resists changes in pH

-Convert strong acids or bases into weak ones

Physiological buffer

-System that controls output of acids, bases, and CO2

Urinary system buffer

-Greatest quantity of acid or base

-Takes several hours to days to exert its effect

Respiratory system buffer

-Within minutes

-Cannot alter pH as much as the urinary system

Chemical buffer

-A substance that binds H+ and removes it from solution as its concentration begins to rise, or release h+ into solution as its concentration falls

-works in seconds

Major chemical buffers

-Bicarbonate, phosphate, and protein systems

-Amount of acid or base neutralized depends on the concentration of the buffers and the pH of the working environment

Bicarbonate buffer system

-A solution of carbonic acid and bicarbonate ions

reversable reaction

-Carbonic acid and bicarbonate ions participate in a reversible reaction

-The direction of the reaction determines whether it raises or lowers pH

Bicarbonate buffer system organs

-The bicarbonate buffer system coordinates with the lungs and kidneys to help control pH and CO2

-To lower pH, kidneys excrete HCO3-

-To raise pH kidneys excrete h+ and lungs excrete CO2

Phosphate buffer system

-A solution of HPO2/4- and H2PO4-

-As in the bicarbonate system, reactions that proceed to the right liberate H+ and decrease pH, and those to the left increase pH

Buffering in the ICF and renal tubules

-In these places phosphates are more concentrated and the buffer can function closer to its optimum pH of 6.8

-Constant production of metabolic acids create pH values from 4.5 to 7.4 in the ICF avg 7.0

Protein buffer system

-Proteins are more concentrated than bicarbonate or phosphate systems, especially in ICF

-Acounts for 3/4 of all chemical buffering

-Proteins' buffering ability is due to certain side groups of their amino acids

Carboxyl

-(-COOH)

-Side groups release h+ when pH begins to rise

Protein amino buffer

-(-NH2) side groups that bind H+ when pH gets to low

Respiratory control

-The bicarbonate buffer system is the bases of the strong buffering capacity of the respiratory system the addition of CO2 to the body fluids raises the H+ concentration and lowers pH

-The removal of CO2 has the opposite effects

Respiratory neutralization

-Neutralizes 2-3 x as much acid as the chemical buffers can

Respiratory control aerobic metabolism

-CO2 is constantly produced by aerobic metabolism

-Normally eliminated by the lungs at an equivalent rate

-Increased CO2 and decreased pH stimulate pulmonary ventilation, while increased pH inhibits pulmonary ventilation

Renal control

-The kidneys can neutralize more acid or base that either the respiratory or chemical buffers

Renal control function

-Renal tubules secrete H+ into the tubular fluid

-Most bind to bicarbonate, ammonia, and phosphate buffers

-Bound and free H+ are excreted in the urine actually expelling H+ concentration by

Phosphate system

-Dibasic sodium phosphate is contained in glomerular filtrate

-Reacts with some of the H+ replacing a Na+ in the buffer which passes into the urine

-As urine becomes more acidic, more sodium ions enter the blood

Ammonia

-NH3

-Form amino acid catabolism acts as a base to neutralize acid

Respiratory acidosis

-Occurs when rate of alveolar ventilation fails to keep pace with the body's rate of CO2 production

-CO2 accumulates in the ECF and lowers its pH

-Occurs in emphysema where there is a severe reduction of functional alveoli

Respiratory alkalosis

-Results from hyperventilation

-CO2 eliminated faster than it is produced

Metabolic acidosis

-Increased production of organic acids such as lactic acid in anaerobic fermentation, and ketone bodies seen in alcoholism, and diabetes mellitus

-Ingestion of acidic drugs (aspirin)

-Loss of base due to chronic diarrhea, laxative overuse

Metabolic alkalosis

-Rare but can result from...

=Overuse of bicarbonates

-Loss of stomach acid

Compensated acidosis or alkalosis

-Either the kidneys compensate for pH imbalances of respiratory origin

-The respiratory system compensates for pH imbalances of metabolic origin

Uncompensated acidosis or alkalosis

-A pH imbalance that the body cannot correct without clinical intervention

Respiratory compensation

-Changes in pulmonary ventilation to correct changes in pH of body fluids by expelling or retaining CO2

-Works in min

Hypocapnia

-Deficiency of CO2

-Ventilation is reduced to allow CO2 to accumulate and thereby lowering pH

Hypercapnia

-Excess CO2

-It stimulates pulmonary ventilation eliminating CO2 and allowing pH to rise

Renal compensation

-Adjustment of pH by changing rate of H+ secretion by renal tubules

-Works in days, better at restoring normal pH

Renal acidosis

-Urine pH may drop to 4.5 due to excess h+

-Renal tubules increase rate of H+ secretion elevating pH in the body

Renal alkalosis

-Urine pH as high as 8.2 due to excess HCO3-

-Renal tubules decreased rate of H+ secretion and allow neutralization of bicarbonate, lowering pH in body

Fluid replacement

-Restoring and maintaining proper fluid volume, composition, and distribution is a significant problem in the treatment of seriously ill patients

Administration of fluids

-Replenish total body water

-Restore blood volume and pressure

-Shift water from one fluid compartment to another

-Restore and maintain electrolyte and acid-base balance

Alternative fluid replacement

-Enema

-Parenteral routes

Enema

-Fluid absorbed though the colon

Parenteral routes

-Fluid administration other than digestive tract

-Intravenous (I.V.) route is the most common

-Subcutaneous (sub-Q) route

-Intramuscular (I.M.) route

-Other parenteral routes

=interosseous cannulation

Normal saline

-Isotonic solution - same osmolarity as plasma

-Used to treat blood loss or dehydration

-Raise blood volume without changing osmolarity

-Limitation: large amounts needed, since much leaks into interstitial fluid

Hypernatremia

-Too much sodium

Hyperchloremia

-Too much chloride

Treating acidosis

-Low pH

-Treated with Ringer's lactate

-Contains lactate, which the liver converts to bicarbonate

Treating alkalosis

-High pH

-Treated with potassium chloride

-k+ helps cells take up H+, lowering blood pH

-Also replenishes potassium lost during vomiting or diuretics

Plasma volume expanders

-Hypertonic solutions or colloids that stay in the bloodstream rather than diffusing into tissues

-Create hypertonic environment in the blood

-Draw water from interstitial and intracellular spaces into the blood stream via osmosis

Hypotonic hydration

-Too much water dilutes body fluids -> cells swell

-Plasma expanders pull excess water out of cells, relieving swelling

-Rapid fluid shift

-Can restore blood volume and reduce cellular edema in minutes

Hypertonic hydration Ex

-hypertonic saline

-Albumin solutions

-Dextran or hetastarch (synthetic colloids)

Patients who cannot eat

-Isotonic 5% dextrose (glucose) solution

-Has protein-sparing effects: fasting patients lose as much as 70-85g of protein per day

=I.V. glucose reduces this by half

Renal insufficiency

-Given slowly through I.V. drip since kidneys might not excrete water fast enough (avoids hypotonic hydration)