Final Exam Acid/Base

BODY FLUIDS

Water is most abundant body compound

Water is

•  60% of body weight in males

•  50% in females 

Variation in total body water is related to:

Total body weight 

• Fat content of body—the more fat the less water (adipose tissue is low in water content)

• Gender—female body has about 10% less water than male body 

• Age—in a newborn infant, water may account for 80% of total body weight. 

• In the elderly, water per pound of weight decreases (muscle tissue—high in water—replaced by fat, which is lower in water)

Intracellular fluid (ICF) 

2/3 of body  fluid

    Fluid inside cells

Serves as solvent to facilitate intracellular chemical reactions

Extracellular fluid (ECF) 

1/3 of body fluid

Fluids outside cells

• Interstitial fluid ( tissue fluid)

• Blood plasma

• Miscellaneous—lymph; joint fluids; cerebrospinal fluid; eye humors

Maintaining Water Balance

Water intake must equal water output

• Sources for water intake

• Ingested foods and fluids

• Water produced from metabolic processes

• Thirst mechanism ( hypothalamus) is the driving force for water intake

maintaiing water balence sources for water output

• Vaporization out of the lungs (insensible since we cannot sense the water leaving)

• Lost in perspiration

• Leaves the body in the feces

• Urine production

avarage intake per day

bev 60% 1500ml

foods 30% 750ml

metbolism 10% 250ml

avraeg output per day

feces 4% 100ml

sweat 8% 200ml

insensible looses via skin and lungs 28% 700ml

urine 60% 1500ml

Osmoreceptors 

• Sensitive cells in the hypothalamus 

• React to small changes in solute blood composition by becoming more active

• When activated, the thirst center in the hypothalamus is notified

• A dry mouth due to decreased saliva also promotes the thirst mechanism

Antidiuretic hormone (ADH) 

Prevents excessive water loss in urine 

• Causes the kidney’s collecting ducts to reabsorb more water

• increase BP

aldosterone

Regulates sodium ion content of ECF

• Sodium is the electrolyte most responsible for osmotic water flows

• Aldosterone promotes reabsorption of sodium ions 

• Remember, water follows salt!

•  increase BP

ANP (atrial natriuretic peptide) 

Secreted from atrium of the heart, acts on the kidney to promote sodium secretion so that it is excreted in the urine.

• Water follows salt!  decrease BP

Renin-angiotensin mechanism

• Mediated by the juxtaglomerular (JG) apparatus of the renal tubules

• When cells of the JG apparatus are stimulated by low blood pressure, the enzyme renin is released into blood

• Renin produces angiotensin II

• Angiotensin causes vasoconstriction and aldosterone release

• Result is increase in blood volume and blood pressure

Sodium

most abundant and important positively charged ion of plasma

Normal plasma level—142 mEq/L

• Average daily intake (diet)—100 mEq

• Chief method of regulation—kidney

• Aldosterone increases Na⁺ reabsorption in kidney tubules Sodium-containing internal secretions 

Dehydration

total volume of body fluids less than normal

• IF volume shrinks first

• ICF volume and plasma volume decrease

• dehydration occurs when fluid output exceeds intake for an extended period

Overhydration

total volume of body fluids greater than normal

• occurs when fluid intake exceeds output (giving excessive amounts of intravenous fluids or giving them too rapidly may increase intake above output)

Blood pH must remain between 

7.35 and 7.45 to maintain homeostasis

alkalosis

pH above 7.45

acidosis

pH below 7.35

physiological acidosis

pH between 7.0 and 7.35

Most ions originate as by-products of cellular metabolism, such as 

Phosphoric acid, lactic acid, fatty acids

• Carbon dioxide forms carbonic acid

• Ammonia ( nitrogen)

Acid -Base of blood controlled by:

1. Chemical  buffers -immediate

2. Respiration- short term

3. Renal System – most powerful, longer term

Chemical Buffer Systems

is a system of one or more compounds that act to resist pH changes when strong acid or base is added

Will bind H⁺ if pH drops or release H⁺ if pH rises

Three major buffering systems:

Bicarbonate buffer system

• Phosphate buffer system

• Protein buffer system

The Bicarbonate Buffer System

Mixture of carbonic acid (H₂CO₃) and sodium bicarbonate (NaHCO₃)

Carbonic acid 

is a weak acid that does not dissociate much in neutral or acid solutions

(remember this: CO2 + H2O ↔ H2CO3 ↔ H+ &  HCO3)

Bicarbonate ions (HCO₃^–) react with strong acids to change them to weak acids

HCl + NaHCO₃  H₂CO₃ + NaCl

strong acid   weak base       weak acid           salt

Carbonic acid dissociates in the presence of a strong base to form a weak base and water

NaOH + H₂CO₃  NaHCO₃ + H₂O

strong base   weak acid          weak base         water

Respiratory Regulation of H⁺

Respiratory and renal systems are physiological buffering systems

• Act more slowly than chemical buffer systems
  but have more buffering power

• Respiratory system eliminates CO₂ (an acid)

• A reversible equilibrium exists in blood

       CO₂ + H₂O ↔ H₂CO₃ ↔ H⁺ + HCO₃^–

• During CO₂ unloading, reaction shifts to left
  (and H⁺ is incorporated into H₂O)

• During CO₂ loading, reaction shifts to right
  (and H⁺ is buffered by proteins)

Respiratory Regulation of H⁺ (cont.)

If P_CO2 in blood rises (hypercapnia) activates medullary chemoreceptors

• Causes increased respiratory rate and depth

Rising plasma H⁺ (acidosis) activates peripheral chemoreceptors

• Causes increased respiratory rate and depth

Both cause more CO₂ to be removed from the blood, pushing reaction to left, which reduces H⁺ concentration

Hypoventilation

leads to respiratory acidosis

Hyperventilation

leads to respiratory alkalosis

Renal Regulation

Chemical buffers cannot eliminate excess acids or bases from body

• Lungs eliminate volatile carbonic acid by eliminating CO₂

• Kidneys eliminate nonvolatile (fixed) acids produced by cellular metabolism (phosphoric, uric, and lactic acids and ketones) to prevent metabolic acidosis

• Kidneys also regulate blood levels of alkaline substances; renew chemical buffers

Renal Regulation pt2

Kidneys regulate acid-base balance by adjusting amount of bicarbonate in blood by either:

• Conserving (reabsorbing) or generating new HCO₃^–  

• Excreting HCO₃^–

Generating or reabsorbing one HCO₃^– is same as losing one H⁺

Excreting one HCO₃^– is same as gaining one H⁺

Renal regulation of acid-base balance depends on kidney’s ability to secrete or retain H⁺

• To reabsorb bicarbonate, kidney must secrete H⁺

• To excrete excess bicarbonate, kidney must retain (not secrete) H⁺

Summary of Renal Mechanisms of Acid-Base Balance

When blood pH rises (Alkalosis)

• Bicarbonate ions are excreted 

• Hydrogen ions are retained by kidney tubules

When blood pH falls ( Acidosis)

• Bicarbonate ions are reabsorbed

• Hydrogen ions are secreted

Urine pH varies from 4.5 to 8.0

Abnormalities of Acid-Base Balance

All imbalances are classed as either respiratory or metabolic

Respiratory acidosis and alkalosis

• Caused by failure of respiratory system to perform
  pH-balancing role

• Single most important indicator is blood PCO2

Metabolic acidosis and alkalosis

• All abnormalities other than those caused by P_CO2
  levels in blood

• Indicated by abnormal HCO₃^– levels

Respiratory Acidosis and Alkalosis

Most important indicator of adequacy of respiratory function is P_CO2 level.

• CO2 normally  35–45 mm Hg 

P_CO2 above 45 mm Hg: respiratory acidosis

Common cause of acid-base imbalances

• Due to decrease in ventilation or gas exchange
  (examples: emphysema, pneumonia, cystic fibrosis)

• CO₂ accumulates in blood; blood pH drops

P_CO2 below 35 mm Hg: respiratory alkalosis

• Common result of hyperventilation, often due to stress or pain

• CO₂ is eliminated faster than produced

Metabolic acidosis; 

Low blood pH and HCO₃^–

Causes:

• Ingestion of too much alcohol (converts to acetic acid)

• Excessive loss of HCO₃^– (example: persistent diarrhea)

• Accumulation of lactic acid (exercise or shock), ketosis in diabetic crisis, starvation, and kidney failure

Metabolic alkalosis 

Indicated by rising blood pH and HCO₃^–

• Much less common than metabolic acidosis

Causes include vomiting of acid contents of stomach or intake of excess base

• Example: overingestion of antacids

• Acidosis

• Blood pH below 6.8 causes depression of CNS, which can lead to coma and death

Alkalosis

• Blood pH above 7.8 causes overexcitation of nervous system, leading to muscle tetany, extreme nervousness, convulsions, and death, often from respiratory arrest